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This is to certify that the
dissertation entitled
OXYGEN DYNAMICS IN PERIPHYTON COMMUNITIES
AND ASSOCIATED EFFECTS ON
PHOSPHORUS RELEASE FROM LAKE SEDIMENTS

presented by
RICHARD GRAY CARLTON

has been accepted towards fulfillment
of the requirements for

Ph.D. degree in Zoology

ugnw

Major professc»

Date 12 September 1986

 

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OXYGEN DYNAMICS IN PERIPHYTON COMMUNITIES
AND ASSOCIATED EFFECTS ON
PHOSPHORUS RELEASE FROM LAKE SEDIMENTS

By

Richard Gray Carlton

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Zoology

1986

 

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ABSTRACT
OXYGEN DYNAMICS IN PERIPHYTON COMMUNITIES
AND ASSOCIATED EFFECTS ON
PHOSPHORUS RELEASE FROM LAKE SEDIMENTS
By

Richard Gray Carlton

Periphyton is typically a heterogeneous assemblage of filamentous

and single celled photoautotrophic and heterotrophic microorganisms

suspended in a mucopolysaccharide matrix which they produce. By
definition, the assemblage is attached to a substratum such as rock,
sediment, or plant in an aquatic environment. Periphyton communities
are generally thin (< 1 mm thick) and, when exposed to sufficient light
and nutrients, very productive. Therefore, microtechniques with high
spatial and temporal resolution are required in order to define
metabolic interactions among the heterotrophic and autotrophic
constituents, and between periphyton and its environment.

This study used oxygen sensitive microelectrodes with tip diameters
of < 30 pm to investigate the effects of photosynthesis and respiration
on the oxygen dynamics of several diverse periphyton communities both in
situ and in laboratory microcosms. A novel flow—through system that
utilized 32? radiotracer and that permitted manipulation of the
velocity, flushing rate, and oxygen concentration of overlying water was

developed to investigate the role of photosynthetic oxygen production on

 

 

the phosphorus dynamics in lake sediments colonized by epipelic
periphyton.

Direct measurements with oxygen microelectrodes demonstrated that
the distribution of dissolved oxygen in periphyton communities varied
diurnally and was markedly different among periphyton types. During
illumination, photosynthesis in periphyton resulted in oxygen
supersaturation in microzones that Were subsaturated or anoxic during
darkness. The fate of oxygen produced within periphyton depended on the
relative rates of production and consumption, the diffusion character—
istics of the periphyton, physical and chemical interactions with the
substratum, and the transport rate across the boundary layer, which was
affected significantly by water currents. During constant environmental
conditions, steady state equilibria of oxygen distribution occurred
within periphyton, but seldom was equilibrium with the surrounding water
found.

Data obtained with the flow—through system showed that epipelic
algae mediated the release of phosphorus from lake sediments to
overlying water via the daily formation and breakdown of the oxidized
microzone. During daylight, the surficial sediments became oxidized,
and some of the phosphorus diffusing from deeper sediment layers was
trapped. During darkness the microzone became anoxic and phosphorus was
released at an accelerated rate. The short-term result was a diel
fluctuation in the efflux rate of phosphorus. More importantly,
however, over the period of weeks to months, the daily photosynthetic
activity of epipelic periphyton resulted in the conservation of

phosphorus in the sediments.

 

 

For Pam...

who taught me love and inspired me to do better.

ii

 

 

I am deeply grateful to Dr. Robert G. Wetzel for being given the
opportunity to pursue my goals as a limnologist. His guidance and
encouragement, blended with his tolerance and patience, made my
graduate research a productive and pleasurable experience. My
colleagues Richard Losee, William Taylor, and JoAnn Burkholder were of
great assistance in many phases of my research, especially in the field
work. Dr. Michael J. Klug was very generous with his laboratory
facilities and equipment, and provided excellent constructive
criticism, both for my research and for my dissertation. I am thankful
to my other committee members, Dr. William E. Cooper and Dr. Thomas R.
Burton, for their contributions to my graduate program. Dr. Robert
Moeller was helpful in field and lab work and contributed ideas which
were instrumental in the formation of my hypotheses and proposed
research. Dr. Michael Coveny was always ready with quick, but
thoughtful, answers to my many questions. Last, but not least I want
to thank Anita Johnson for her smile, her sense of humor, and her

friendship.

 

 

TABLE OF CONTENTS

LIST OF FIGURES ................................................ vi
INTRODUCTION .............. ...... ............................... 1

CHAPTER I. DISTRIBUTIONS AND FATES OF OXYGEN IN
PERIPHYTON COMMUNITIES ..........................

.p

Introduction ................................. 5
Methods ........ ............................. 6
Microelectrodes ........... ........... ..... 6
Epiphytic periphyton ...................... 8
Epilithic periphyton ...................... 1O
Epipelic periphyton ....................... 1O
Epizoic periphyton ............... ......... 11
Results ... ...... . ......... ................... 11
Epiphytic periphyton . ..... ................ 11
Epilithic periphyton .. ........ . ........... 18
Epipelic periphyton ....................... 18
Discussion . ..................... ............. 21
References ............. ........ ........... .n. 30

CHAPTER II. PHOSPHORUS FLUX FROM LAKE SEDIMENTS: EFFECT
OF EPIPELIC ALGAL PHOTOSYNTHESIS . ..... .. ....... 33

Introduction ................................. 34
Experimental procedures ........ .............. 35
Results ... ..... .............................. 42
Discussion ................................... 52
References ...................... . ..... ....... 57

CHAPTER III. A BOX—CORER FOR STUDYING METABOLISM OF
EPIPELIC MICROORGANISMS IN SEDIMENT UNDER
IN SITU CONDITIONS .......... ....... ............. 60

Introduction ..... . ........... ................ 61

Description of apparatus ........ ...... ....... 61
References . ....... ........................... 64

iv

 

CHAPTER IV.

CHAPTER V.

SYMBIOSIS BETWEEN SALAMANDER EGGS AND GREEN

ALGAE: MICROELECTRODE MEASUREMENTS INSIDE

EGGS DEMONSTRATE EFFECT OF PHOTOSYNTHESIS

0N OXYGEN CONCENTRATION .........................
Introduction .................................
Methods ......................................
Results ......................................
Discussion ...................................
References ...................................

SYNOPSIS ........................................

LIST OF REFERENCES ..............................

 

 

LIST OF FIGURES

Figure

CHAPTER I

1

Short term steady state oxygen distributions at ingreasing
light intensities D = dark, numbers are uEinst m s ) in
periphyton epiphytic on a leaf of Potamogeton illinoensis.
Vertical scale is distance from leaf surface. Arrow
indicates surface of periphyton layer.......................

 

A: Short term (.““‘.) and long term (.--'.) steady state
0 microprofiles in periphyton epiphytic on a lgaf_pf
Potamogeton illinoensis exposed to 20 uEinst m s .

 

B: Oxygen microprofiles measured during darkness (H)
and at an irradiance of 20 uEinst m“ s_ (.""'.) in
periphyton epiphytic on a leaf of Scirpus subterminalis,
and above a bare section of leaf at the same irradiance
(.---.). C: Oxygen microprofiles in epilithic periphyton
during darkness (H) and after 1 h (.‘"".? and 2 h ~'

 

(0—-0) of illumination with 40 uEinst m‘ s . Vertical
scale is distance from leaf or rock surface. Arrows
indicate surfaces of periphyton ............ . .......... ......

Oxygen microprofiles in sediments colonized by epipelic
microalgae. A: During darkness «D--C», and after 1 h
(H), 8 h (Ar—i , and 10 h (H) illumination
with 10 uEinst m_ s ; overlying water not stirred.

B: During darknes ( ), and after 8 h of illpmination
with 10 uEinst m" s ), 30 uEinst m‘2 s“ ( ),
and 45 uEinst m_ s_ ( ); water stirred. Vertical
scale is distance from sediment—water interface.............

    
   

1

Oxygen concentration profiles in and above sediments
colonized by epip51194Dichotomosiphon tuberosus illuminated
with 46 uEinst m s '. D = 12 h of darkness, numbers are
cumulative time of irradiance in hours. Vertical scale is
distance from sediment—water interface. Arrow indicates

 

surface of periphyton.......................................

vi

Page

13

 

 

5 Oxygen concentration change through time during illumination
following darkness in f ur_periphyton communities: epilithic
exposed to S uEinst m s ), epiphytic exposed to
492uEipst m s ), epizoic exposed to 105 uEi st_1
m s (O—O), and epipelic exposed to 20 uEinst m 3

CHAPTER II

1 Diagram of apparatus used to isolate surficial sediment
section on a membrane. Scheme flows clockwise from upper
left. After sampling sediment with the piston corer
(I.D. = 38 mm) the hole in the membrane and the upper
chamber are aligned and the sample is pushed upward to the
desired height. The membrane is pulled across severing the
sediment, the lower section of which is discarded. Then
the base plate is secured and the piston is installed.......

2 Microprofiles of oxygen concentration in sediments priOr to
their use in the flow-through system; after 6 h of darkness
(solid line), after 2 h of illumination(dashed line), and
after 6 h ofzillpmination (dotted 1%ne). Light intensity =
30 uEinst m s ; temperature = 17 C; equilibrium air
saturation = 302 umol 02 L ...................... ... .......

3 Oxygen concentrations through time at depths of 0.2 and
1.0 mm below the sediment surface in a sediment section
isolated on a membrane within a flow—through chamber.
Black barg represent dark period. Light intensity = 30
uEinst m S , temperature = 17 C, equilibrium air
saturation = 302 umol 02 L— ................................

4 Distribution of pH in sediments used for flow-through
experiments; after 6 h of darkness (black line) and aftep
6 h of illumination (hatched line) with 30 uEinst m- s—
at a temperature of 17 C. Error bars are one standard
error of the mean on either side of the mean (n = 3)........

5 Phosphorus flux experiment with time course of 32F release
(CPM) from sediments exposed to a 6:6 LD cycle. Light
ingensity during illumination of chambers A & B = 30 uEinst
m— s_ , C = control (permanently dark). Temperature = 17°C.
Bars designate dark period.............. ........... .........

6 Phosphorus flux experiment with time course of 32P release
(CPM) from sediments exposed to a 6:6 LD cycle. Light
ingensity during illumination of chambers A & B = 30 uEinst
m s . Temperature = 11 C. Bars designate dark period...

7 Phosphorus flux experiment with time course of 32P release
(CPM) from sediments exposed to a 12:12 LD cycle. Light
intensity during illumination of chambers A & B = 30 uEinst
m‘2 s- Black bars indicate dark periods for chambers A &
B; the control (C) was permanently dark. Round symbols on

38

45

46

47

48

49

 

 

data points designate period during which gases in flow-
through water were in atmospheric equilibrium; square
symbols start at point where [O J was manipulated in the
flow-through water. Diagonally hatched bars at bottom
indicate periods when flow—through water was superoxygenated;
bars with vertical hatching indicate anoxic flow—through
water. All three chambers received same quality of water.
Temperature = 17 C..........................................

In situ distribution of soluble reactive phosphorus in
interstitial water in sediments of Lawrence Lake in a plot
which was permanently shielded from light (black line) and
in control plots which received normal light intensities
(hatched line). Error bars are one standard error of the
mean on either side of the mean (n = 4 for control, n = 3
for dark plot)..............................................

CHAPTER III

1

Perspective diagram of the box—corer with the upper and
lower doors partially closed. Note the protrusion of side
walls on the left to accomodate the vertical travel of the
flexible door. The support rod at the lower left maintains
rigidity where the door flexes. ..... ...... ...... . ...... .....

Sediment core sample within the box—corer held within the
transport caddy (caddy lid removed) .............. ... ........

Comparison of microprofiles of oxygen concentration measured
in situ (Lawrence Lake, Michigan; water depth = 1.9 m) and
in a box—corer sample of the same sediment in the laboratory
after 12 h darkness followed mgy 513 h and 9.0 h of
illumination at 150 uEinst m . Water temperature was
9.00 C in all cases. ................. ...... .................

Microprofiles of oxygen concentration in and above pelagic
sediments sampled at 10—m water depth in Lawrence Lake,
Michigan, with the box—corer and maintained in the labor—
atory.O Water temperature was 140 C for the upper curve,

and 110 C fo thg three lower curves. Light intensity was
10 uEinst m Note change in depth scale at sediment—
water interface......... ...................... . ............ .

CHAPTER IV

1

Oxygen concentration (umol 0 1-1) in and around eggs of A.
macalatum after 3 hours of darkness in uncirculated water

at 17 C. 100% air saturation = 302 uM 02...................

Oxygen concentration (umol 0 l_1) in and around eggs Of A.

maculatum after 3 hours darkness in continuously aerated

water at 190 C. 100% air saturation = 290 uM O2 ......... ....

viii

50

62

62

63

63

68

68

 

Oxygen concentration (umol 0 1—1) over time within

illuminated eggs of A. maculatum after 3 hours darkness.

Light on at time zero. Curve A: Egg mass in hypoxic water

(Fig. 1) at 17°C. Light turned off after 34 min. elapsed

time. Curve B: Same egg mass in continuously aerated water

at 190C. The steps in Curve B were caused by tail movements

by the embryo which mixed and broke down the oxygen gradient

in the egg fluid............................................ 68

ix

 

INTRODUCTION

Periphyton is typically a heterogeneous assemblage of filamentous
and single celled photoautotrophic and heterotrophic microorganisms
suspended in a mucopolysaccharide matrix which they produce. By
definition, the assemblage is attached to a substratum in an aquatic
habitat. Periphyton communities are commonly found attached to
sediments (epipelic), rocks (epilithic), plants (epiphytic), and animals
(epizoic). The biological diversity of periphyton communities and the
variety of habitats in which periphyton occurs are well documented. The
relationships among the microbial constituents of periphyton, and
between periphyton and its environment are, however, poorly studied.
Recent advances in microelectrode technology together with novel uses of
isotope tracer methods are enabling detailed investigations of the
metabolism of periphyton and of the role of periphyton in the nutrient
dynamics of aquatic ecosystems.

Dominant features of the physical structure of periphyton
communities that vary greatly within and among periphyton types include
the thickness of the layer, the ratio of living to dead and inorganic
material in the layer, the nature of the substratum, and the extent of
development of the matrix. Each characteristic has an effect on the
diffusive properties of the periphyton layer. The relevance lies in the
fact that the transfer of solutes within periphyton occurs by diffusion

in the interstitial water. Because diffusion is a slow process,

 

 

wherever the rates of production and consumption of a compound are not
balanced, the compound will either accumulate or become depleted; this
is important because diffusion and uptake rates of solutes and
substrates are concentration dependent.

Before the development of microelectrodes for environmental
research, knowledge of the dynamics of oxygen, pH, sulfide and other
compounds in periphyton and sediments was based on changes measured in
the adjacent aqueous mileau. For example, a rock or small plant with
attached periphyton would be placed in a chamber and incubated for a
time, during which the concentrations of various solutes would be
periodically measured. However, within the periphyton layer, some of
the oxygen that is produced photosynthetically is consumed by
heterotrophs, and some of the carbon dioxide produced by microbial
respiration is consumed by autotrophs. Furthermore, chemical
equilibrium seldom exists between the interstitial water of periphyton
and the adjacent aqueous environment. Therefore, measurements made in
the water surrounding a periphyton community only provide approximations
of the metabolic activity of the community. As a result of
photosynthesis in periphyton, oxygen can accumulate to concentrations
far exceeding atmospheric equilibrium. During darkness the same
microzones become anoxic because the rate of diffusion of oxygen into
the layer from the overlying water is exceeded by the oxygen demand of
the community. The implications for microbial metabolism in periphyton
are manifold, but few studies of these phenomena exist.

The productivity of most freshwater ecosystems in North America is
limited by the availability of phosphorus. Both the concentration and

the regeneration rate of phosphorus in a lake are crucial to the primary

 

 

 

 

productivity of the system. In many lakes the regeneration and
subsequent release of phosphorus from the sediments (internal loading)
is a major fraction of the annual loading of phosphorus to the water
column. Numerous studies concerned with eutrophication reversal and
lake restoration have addressed the phenomenon with the goal of finding
ways to reduce the efflux of phosphorus from the sediment. When the
bottom water of a lake becomes anoxic the oxidized microzone at the
sediment—water interface becomes chemically reduced. The reduction of
phosphorus—containing Fe (III) compounds, such as iron hydroxyphosphate,
a precursor to hydroxyapatite, releases the phosphate which does not
precipitate with Fe (II). The result is a marked increase in the rate
of phosphorus release to the overlying water. Measurements with oxygen
microelectrodes have demonstrated, however, that sediments colonized by
epipelic algae are, on the average, more intensely oxidized than
sediments not receiving sufficient light for epipelic algal
photosynthesis. Therefore, a primary goal of this study was to define
the oxygen dynamics of lake sediments on a diel basis and determine the
effect of epipelic algal photosynthesis on the efflux of phosphorus to

the water column.

 

 

CHAPTER I

DISTRIBUTIONS AND FATES 0F OXYGEN

IN PERIPHYTON COMMUNITIES

 

INTRODUCTION

The biological diversity of periphyton communities and the variety
of habitats in which epiphytic, epilithic, epipelic, and epizoic
periphyton occur are well known (Lund 1942; Hutchinson 1975; Round 1981;
Wetzel 1983). In recent years the relationships among the constituents
of periphyton, and between periphyton and its environment have received
increased attention (e.g., Allen 1971; Gruendling 1971; Kairesalo 1977;
Gone 1982; Bjdrk-Ramberg 1983; Cuker 1983; Losee and Wetzel 1983; Reuter
et al. 1986). Because periphyton is a heterogeneous assemblage of
autotrophic and heterotrophic organisms, investigation of the internal
metabolism of the community is difficult. Before the development of
oxygen microelectrodes for environmental research (reviewed by Revsbech
and Jérgensen 1986), knowledge of oxygen production and consumption in
periphyton communities was based on measurements made in the surrounding
aqueous milieu (e.g., Hunding 1973; Schindler et al. 1973). However,
Revsbech and Jnrgensen (1983) and Lindeboom et al. (1985) showed that
such measurements underestimate oxygen production and consumption in
periphyton communities, because chemical equilibrium seldom exists
between the interstitial water of the periphyton and the adjacent
aqueous environment. Results from measurements with oxygen
microelectrodes in investigations of periphyton have demonstrated that
the distribution and fates of oxygen in periphyton communities are
controlled by four dominant factors: i) the extent of photosynthetic
oxygen production, 11) the diffusion characteristics of the
microenvironment, which are governed by water currents and the physical
structure of the periphyton community, iii) physical interactions with

the substratum on which the periphyton grows, and iv) respiratory

 

 

6
activity of the microflora within the periphyton and its

microenvironment.

METHODS
Microelectrodes

Two types of polarographic oxygen sensitive microelectrode were
constructed for this study. The simpler electrode (the design of
Revsbech 1983) consisted of a fine platinum wire encased in glass and
gold plated at the exposed tip (cathode surface). Cathode diameters
ranged from 2 to 10 um with overall electrode diameters of 7 to 20 pm in
the sensing region. Construction and use are further described in
Jergensen et al. (1979) and Revsbech et al. (1981). This cathode—type
(Revsbech and Jérgensen 1986) of oxygen microelectrode requires a
separate reference electrode (anode), which must be immersed in the same
fluid. Two common characteristics of polarographic sensors are i) they
drift for at least a few minutes after the polarizing voltage is first
applied, and ii) their calibration changes unpredictively if the couple
becomes depolarized, which occurs when electrical continuity is not
maintained within the solution between the cathode and the anode.
Calibration of the electrode couple must therefore be made in the same
fluid body where the measurements are to be made.

The second type of electrode consisted of the same basic components
discussed above except that the cathode and anode are encased within an
electrolyte-filled, glass housing with a tip diameter ranging from 15 to
40 um (Revsbech and Ward 1983). This micro—Clark style of electrode
maintains constant electrical continuity between the cathode and anode

via the electrolyte solution. Therefore, the electrode can be moved

 

from one fluid to another without depolarization, which permits
calibration and experimental measurements to be made in numerous

solutions. However, because 0 must diffuse through a membrane and then

2
through ca. 5 pm of electrolyte, the response time is slower than the
cathode-type of electrode.

Calibration of the cathode-type of electrode was necessarily
accomplished in the same solution where the measurements were made.
This calibration required determining electrode response
(picoamperes/[02]) at two known oxygen concentrations in order to
establish the slope and y-intercept of the linear calibration curve.
The chemical microsensor (Transidyne General, Ann Arbor, Michigan) used
supplied the polarizing voltage of -0.75 V and was calibrated to read
out in percent saturation (percent of atmospheric equilibrium
concentration). The meter was set to read 100% when the electrode tip
was positioned ca. 100 um below the air—water interface. Zero percent
was taken as the asymptotic minimum of microprofiles measured in known
anoxic media, such as sediments (Revsbech and Jdrgensen 1983).

The micro—Clark electrode allowed more versatility for experimental
use and in calibration options. The meter was calibrated at 0 and 100
percent of air-saturation while the electrode was positioned in either
nitrogen-saturated or air—saturated water, respectively, at temperatures
equal to the experimental systems. This capability was necessary when
measurements were to be made in systems where at least two known oxygen
concentrations did not occur (essentially all periphyton assemblages
except epipelic). Therefore, many of the data from epipelic communities
were obtained with the cathode—type of electrode and most other

measurements were made with the micro—Clark style.

 

 

8

Aside from breakage during use, the electrodes had a typical life
span of ca. 1 to 2 months; therefore, numerous electrodes were used
during this study. The electrodes were positioned in the laboratory
with micromanipulators, which permitted accurate movement and alignment
with a resolution of < 50 um. A waterproofed fiberscope (American
Optical) was used for initial alignment of the electrode tip at an
interface appropriate for the periphyton under study (Carlton and Wetzel
1985). Readings were made at 0.2—mm intervals (except above the
sediment—water interface); all microprofiles shown are means of
duplicate determinations. The high reproducibility of microprofile
measurements has been demonstrated by Jdrgensen and Revsbech (1985).
Percent saturation values were converted to oxygen concentration ([02])
as micromoles 02 l_1 to simplify comparison of microprofiles measured at
different temperatures. All light intensity measurements were made with
a LiCor model LI-185 meter with a cosine corrected quantum sensor

(Lambda Instruments).

Epiphytic Periphyton

Periphyton attached to submersed aquatic plants was the most
difficult to handle. Extreme care had to be taken during sampling and
laboratory handling in order to ensure the structural integrity of the

epiphyte-macrophyte complex. Samples of Potamogeton illinoensis and

 

Scirpus subterminalis with representative midsummer epiphytic periphyton

 

development were retrieved using SCUBA from the littoral of Lawrence
Lake, Michigan (described in Rich et al. 1971). The samples were
maintained in aerated, 17°C—1ake water for < 4 days before measurements

were made. The plants were exposed to 40—80 uEinst mfl2s-1 of

 

 

photosynthetically active radiation (PAR) on a 12:12 LD cycle. An
identical mecury vapor lamp, which could be raised or lowered to alter
intensity, was used for illumination during microelectrode measurements
on these samples.

Excised leaves were transferred without emersion to a shallow pan
and securely suspended between two rubber stoppers in approximately 1
liter of lake water. This arrangement permitted contact of the bulk
water with all surfaces of the leaves in the region where measurements
were made. The water was continuously aerated and mixed with an
airstone bubbler located ca. 20 cm distant from the leaf.

Oxygen concentration microprofiles were measured by visually
aligning the electrode tip at the surface of the periphyton and then
advancing incrementally through the layer, with readings taken at
various intervals. Upon reaching the leaf surface an erratic output
signal was received (thus revealing the thickness of the periphytic
layer). This symptom was verified by observing electrode response when
the tip was lowered onto bare sections of leaf surface.

The glycocalyx (mucopolysaccharide) in epiphytic periphyton tended
to foul the electrode tips after measuring a few profiles. This
interference decreased electrode sensitivity and invalidated previous
calibration by altering membrane characteristics (Hale 1983).
Microscopic examination showed that layers of material would form over
the electrode tip. Dipping the electrode in 3 N HCl quickly removed the
fouling material. This problem was also encountered during measurements

in epilithic periphyton, but not in other periphyton communities.

 

Epilithic Periphyton

Algae attached to rocks was simplest to sample because of its
secure attachment to the substratum. Rocks used for this study were of
sedimentary origin (ca. 5-10 cm diam.), and were encrusted with a
carbonate layer at the base of the periphyton; they were taken from the
eulittoral of Gull Lake, Michigan (described in Moss 1972).
Measurements were made in essentially the same manner as in the
epiphytic periphyton, except that detection of the substratum interface

with the electrode tip was a more delicate and critical operation.

luznr‘

Light was supplied as was done for epiphytic periphyton.

Epipelic Periphyton

All samples of periphyton growing on sediments were taken from
Lawrence Lake by a SCUBA-diver using the box corer—microcosm method
described in Carlton and Wetzel (1985), except that the acrylic
transporting caddy was replaced with an aluminum box with a watertight,
hinged lid. This device sampled > 700 cm2 of sediment surface area with
a volume > 29 liter (sediment + overlying water). Sediment profile
samples were 15 to 20 cm thick with 20 to 25 cm of overlying water. The
corer design permitted transport of the sample to the laboratory and
transfer to an acrylic sleeve in an aquarium filled with lake water,
with essentially no disturbance of the sediment profile or of conditions
at the sediment—water interface. The aquaria were maintained at
environmental temperatures by a circulating temperature controller which
could also be used to provide circulation over the core samples. These
sediment "microcosms" were exposed to ambient light levels and

photoperiods which were simulated with height—adjustable banks of

 

 

timer-controlled, fluorescent lamps (Luxor, natural light simulating).
This apparatus permitted observations and measurements to be made

throughout simulated diurnal periods.

Epizoic Periphyton

Eggs of the spotted salamander Ambystoma maculatum appear green due
to the presence of symbiotic chlamydomonad algae inhabiting the egg
envelope. Egg masses containing late term embryos were collected from a
shallow woodland pond in southeastern lower Michigan and maintained for
10 days in an aquarium containing continuously aerated well water at
15-1700. The eggs were exposed to indirect sunlight and full—spectrum

1 . '
. Mlcroelectrode measurements were

fluorescent light for ca. 12 h d—
made inside eggs containing live embryos. Prior to the measurements the
egg mass was isolated from light for 3 h. An electrode was inserted
into an egg and then the system was exposed to 105 uEinst m—2 s_1.

Oxygen concentration within the egg was monitored for 34 min. Other

pertinent information can be found in Bachmann et al. (1986).

RESULTS
Epiphytic periphyton

The periphyton community present on leaves of Potamogeton
illinoensis during this study consisted of a ca. 2—mm—thick layer of
algae, bacteria, particulate detritus, and calcium carbonate suspended
in a mucoid matrix with high water content (Losee and Wetzel 1983).
Algal cell numbers were dominated by the genera Scenedesmus (40%),
Fragilaria (9%), Achnanthes (9%), Gomphonema (6%), and Pinnularia (6%).

Steady state oxygen concentration microprofiles measured in the

 

 

12
periphyton layer are shown in Fig. 1. After each increase in
illumination, a steady state microprofile occurred in ca. 25 min; this
time was similar to that observed by Sand—Jensen et al. (1985) for
epiphytic periphyton on Potamogeton crispus. During darkness the rate
of oxygen consumption in the layer exceeded the rate of diffusion into
the periphyton from the surrounding water, producing a weak negative
gradient. Illumination of the assemblage with 7 uEinst m_2s_1 for 0.5 h
produced a very slight increase in oxygen concentration. At steady

state under 20 uEinst m—23—1

there was essentially no gradient of [02],
indicating that oxygen production was balanced by respiratory
consumption within the layer, and that there was no net diffusion of 02.
Further increases in intensity of PAR resulted in increased [02]
throughout the periphyton layer, with the maximum change occurring at
the leaf surface, i.e. from 97 NM in darkness to 842 uM at 80 uEinst
m-2s_1. In terms of extent of atmospheric saturation these
concentrations represented a change from 33 to 290 percent.

Oxygen concentration microprofiles were also measured in periphyton

epiphytic on Scirpus subterminalis. This periphyton community was

 

thinner (ca. 0.8 mm), less calcareous and less dense with algal cells
than the community on P. illinoensis, and allowed a greater fraction of
the incident irradiation to reach the leaf surface. Very little
difference was observed between microprofiles measured on colonized and
uncolonized regions of the leaf after 8 h of illumination at 20 uEinst
m‘2s_1 (Fig. 2B). After several hours of darkness [02] at the leaf

surface beneath the periphyton layer was depleted to only 228 uM.

OXYGEN (pmoI-L“)

 

O 200 400 600 800
l l l

2.8 _

. eEfi:

2.0 -

 

MHAJMETERS

0.8 —

 

 

 

 

Figure 1. Short term steady state oxygen distributions at increasing
light intensities (D = dark, numbers are uEinst m_2 5—1) in periphyton

epiphytic on a leaf of Potamogeton illinoensis. Vertical scale is

 

distance from leaf surface. Arrow indicates surface of periphyton

layer.

 

MHJJMETERS

OXYGEN (pmoI-L-1)

 

 

 

 

 

 

 

O 200 400 O 200 400 O 200 400 600
I I I I l ‘-.'.I
2.8 A B C 3‘,
'. \
:_ I
—> 8 ‘o‘
I
2.4 - 3 o i _
El 5 I
: -‘ '
it ,r
2.0 — —> '9; 4 ,’ a
El 1'
5,1 9 4
: I , I
51 E '
‘6- :1 r’ r' - .
i l 5' / ‘
:it ,6 I
:' ‘l II
1 .‘ I
12 ~ ? a f r _
é 1: p" I'
\ II
0 8 - A \c\ —)— g' ;l ~
3 \ 3 I
.‘ I. .5 ’
I f T
\ : l
0.4 '— ' . O ‘ -—I
\ ,- I
\ _- l
.5 K 9" 7'
LEAF i \\ LEAF I ROCK
Figure 2. A: Short term (0 """ C) and long term (.--.) steady state 02

microprofiles in periphyton epiphytic on a leaf of Potamogeton
illinoensis exposed to 20 uEinst m"2 s-1. B: Oxygen microprofiles

measured during darkness (H) and at an irradiance of 20 uEinst m—2

(.-----C) in periphyton epiphytic on a leaf of Scirpus subterminalis,
C:

 

-1
s
and above a bare section of leaf at the same irradiance (."".).

Oxygen microprofiles in epilithic periphyton during darkness (H)

and after 1 h (0 ----- O) and 2 h (0--0) of illumination with 40 uEinst
Vertical scale is distance from leaf or rock surface. Arrows

—2 —1
m s .

indicate surfaces of periphyton.

 

OXYGEN (pmol-L")

 

 

 

0 200 400 600 0 200 400 600 800
l l l I l I
A B

2— _
(I)
E SWI

—>

I...
Lu
2
2‘ 2“ ‘
2

4 ~ UNSTIRRED STIRRED —

6

 

 

 

Figure 3. Oxygen microprofiles in sediments colonized by epipelic
microalgae. A: During darkness (H), and after 1 h (H), 8 h
(H), and 10 h (H) illumination with 10 pEinst m“2 s";

overlying water not stirred. B: During darkness (H), and after 8 h
—1
_ —1 - -2
of illumination with 10 nEinst m 2 s (H), 30 “Em“ "1 s

(H), and 45 pEinst m"2 s-1 (H); water stirred. Vertical

scale is distance from sediment—water interface.

 

OXYGEN (umol-L")

O 100 200 300 400
I l

70

 

l

MHJJMETERS

SVVL—a»

 

 

 

 

 

Figure 4. Oxygen concentration profiles in and above sediments

colonized by epipelic Dichotomosiphon tuberosus illuminated with 46

uEinst m—2 s_1. D = 12 h of darkness, numbers are cumulative time of

 

irradiance in hours. Vertical scale is distance from sediment-water

interface. Arrow indicates surface of periphyton.

 

 

 

 

 

800 —
-.J 600 —
6
E _
3
Z _.
m 400
o
>- ...
X
o
200 —
I I l I
O0 20 40

TIME AFTER FIRST ILLUMINATION (min)

Figure 5. Oxygen concentration change through time during illumination
following darkness in four periphyton communities: epilithic exposed to
80 IlEinst In_2 s-1 (H), epiphytic exposed to 40 uEinst m"2 5—1

. . - —2 -
(H), eplZOlc exposed to 105 )lElnSt m s 1 (H), and epipelic

exposed to 20 uEinst m_2 5—1 (H).

 

Epilithic Periphyton

The periphyton used in this part of the study was composed largely
of filamentous diatoms and green algae, which had received 80—120 uEinst
mm2s-1 of irradiance for ca. 6 h in its natural environment. At that
time [02] throughout the entire 2.6—mm—thick epilithic layer was in
excess of 400% of atmospheric saturation (i.e. > 1200 pH 02); ebullition
was abundant. The periphyton was then shielded from light in the
laboratory. After 6 h of darkness the metabolic activity of the
community had produced suboxic conditions throughout the periphyton
layer and anoxia occurred at the rock surface (Fig. 20). Subsequent
illumination with 40 uEinst m'-23_1 resulted in an increase in [02] at
the rock surface to 84 and 248 uM after 1 and 2 h of illumination,
respectively. Increasing the irradiance to 80 uEinst m-'2s_1 resulted in

bubble formation and extreme supersaturation throughout the profile, as

had been observed when the sample was first retrieved.

Epipelic Periphyton

At the time of sampling, the epipelic periphyton of the profundal
in Lawrence Lake was dominated by diatoms, in particular Navicula.
Maximum midday irradiance at that depth in the lake ranged from 10 to 20

—1

pEinst m_2s depending on weather conditions. Water currents

immediately above the sediments in the lake were negligible; therefore,
no circulation was supplied in the laboratory. After 12 h of darkness
[02] was < 10 uM at the sediment—water interface (SWI) and 02 was absent
below 1.0—mm depth into the sediment (Fig. 3A). After 1 h of

2 ~1

illumination with 10 uEinst m_ s , 02 concentration at the SWI

increased to 203 uM, but did not penetrate farther into the sediment.

 

19

A total of 8 h of illumination resulted in a marked increase in [02],
which was maximal at the SWI (467 pH), and in penetration of oxygen to a
depth of 5.5 mm. After 10 h of illumination, [02] at the SWI had
increased to 542 uM, with a penetration depth of 6 mm. Based on the
[02] gradient above the SWI at that time the boundary layer was
apparently thinner than after the previous dark period. It is possible
that convection currents occurred during illumination and reduced the
thickness of the boundary layer. This phenomenon has been observed in
other experiments where the overlying water was not intentionally
circulated (Carlton, unpublished).

In order to assess the effects of currents the water in the
microcosm was subsequently circulated, generating a current velocity of

1 above the sediments (determined by observing suspended

ca. 0.5 cm s-
particles). After 12 h of darkness [02] at the SWI was 136 uM, and the
boundary layer was ca. 0.5 mm thick (Fig. BB). On three successive
days, the system was exposed to 10, 30 and 45 uEinst m_23_1,
respectively, with a 12—h dark period each night; microprofiles were
measured after 8 h of illumination (Fig. 33). An illumination intensity

-1

of 10 uEinst m-2s resulted in only slight oxygen supersaturation at

the SWI (330 uM 02), with maximum oxygen penetration of 1.8 mm. At 30

and 45 uEinst m-zs_1

the maximum [02] occurred at a depth of 0.2 mm in
the sediment, rather than at the SWI, and oxygen penetrated to 2.4 and
3.4 mm, respectively. These distributions differ greatly from those in
the unstirred situation (Fig. 3A).

During summer, sediments between the 5.5 and 7—m depth contours in

Lawrence Lake become heavily colonized by a dense population of

Dichotomosiphon tuberosus. This branched, filamentous, green alga

 

 

20
anchors itself with rhizoids which penetrate several millimeters into
the sediment. During the period of maximum biomass the filaments
extended 4~8 cm above the SWI. Oxygen dynamics were investigated in
microcosms in the laboratory. The 6-cm—thick periphyton layer was
exposed to 43 pEinst m"2s—1 on a 12:12 LD cycle for 5 d before
measurements were made. After 12 h of darkness anoxic conditions
existed for 15 mm above the sw: (Fig. 4). Brief (15 min) illumination
produced an increase in [02] at the SWI from O to 90 pH. However, after
4 and 10 h of illumination the [02] at the SWI increased only to 96 and
115 uM, respectively. After a total of 14 h of illumination [02] at the
SWI had increased to 283 pM (90% saturation) and slight supersaturation
existed above 10 mm above the SWI. At that time oxygen had penetrated
to a depth of 4.5 mm which was just 1.0 mm deeper than after only 15 min
of illumination. Following these measurements the light was
extinguished and 4 h later the distribution of oxygen in the periphyton
was virtually identical to the profile measured after 4 h of
illumination (data not shown).

The change in [02] through time at a single point in each of four
different periphyton communities after exposure to light is shown in
Fig. 5. In each case the periphyton was isolated from light for 3—6 h
prior to illumination, which was supplied at intensities typical of the
habitats from which the samples were taken. The [02] near the base of a
2—mm—thick layer of epilithic periphyton exposed to 80 uEinst mn2s_1
increased from O to 302 uM during the first 5.5 min of illumination and
continued to rise, although at a slower rate, until bubble formation
became visible after ca. 45 min. The change in [02] in the middle of a

2—mm—thick layer of epiphytic periphyton on a leaf of g. illinoensis was

 

21
measured during illumination with 40 uEinst m_2s_1. The [02] increased
in 18 min from an initial value of 102 uM to an asymptotic maxmimum of
395 uM. At the surface of a layer of epipelic microalgae (in a
microcosm similar to that used for the data in Fig. 3A; overlying water
not stirred) exposed to 20 pEinst m_25_1, [02] increased from 86 to 289
uM (100% sat.) in 32 min. After 50 min of illumination the [02] was 380
uM and was still increasing. The fourth curve in Fig. 5 shows the
effect of photosynthesis by epizooic algae living symbiotically in the
egg wall of the spotted salamander Ambystoma maculatum (for further
details see Bachmann et al. 1986). During illumination with 105 pEinst
mu2s-1 the [02] inside an egg containing a live embryo increased from 63

uM to an asymptotic maximum of 476 uM in 32 min.

DISCUSSION

The distribution of oxygen in periphyton is a function of the rate
of oxygen production, the rate of oxygen consumption by respiration and
chemical oxidation, and the rates of diffusion into and out of the
system. The direct effects of each of these functions upon oxygen
concentration can be evaluated from oxygen microprofile data collected
under different irradiance intensities and through time. Interactions
between the productive and consumptive processes in periphyton have not
yet been addressed quantitatively, but it is known that oxygen
consumption in sediments varies in a nonlinear fashion with oxygen
concentration (Hunding 1973; Lindeboom et al. 1985; Revsbech et al.
1986). The diffusive flux of oxygen at the surface of the periphyton
layer can be calculated using Fick's first law of diffusion (Crank

1983):

 

 

22
[1] J = -1)S ' (dC/dx)

where flux J is equal to the product of the diffusion coefficient DS in
and the concentration gradient dC/dx across the boundary layer adjacent
to the periphyton. The slope of the linear gradient within the boundary
layer is defined by i) the oxygen concentration of the bulk surrounding
water, ii) the thickness of the boundary layer, and iii) the oxygen
concentration at the surface of the periphyton. The boundary layer
thickness results largely from the viscosity and velocity of the aqueous
medium, but it can also be affected by the topography of the periphyton
(Vogel 1981). The [02] at the surface of the periphyton is a function
of the production, consumption, and diffusion of oxygen within the
periphyton layer. These rates are difficult to determine when a
gradient exists within the periphyton layer. However, when the gradient
equals zero no net diffusion occurs and the rates of photosynthesis and
respiration are balanced, providing a situation in which photosynthesis
and respiration can be determined (method described by Sand—Jensen et
al. 1985). Gradients in which dC/dx = 0 have not been observed in other
types of periphyton. Periphyton epiphytic on healthy lacunate
hydrophytes are unique in that their substratum produces oxygen when
sufficiently illuminated, and it possesses internal gas spaces (lacunae)
which provide for rapid transport of gases within the plant (the
diffusion of oxygen in air is ca. 10,000 times faster than in water
[Vogel 1981]), and between the plant and the aqueous environment
(Westlake 1978). Therefore, the concentration of oxygen and other gases

in epiphytic periphyton (particularly near the base of the community)

 

23
can be significantly affected by photosynthesis, respiration, and gas
transport within the host plant. As part of another study in Lawrence
Lake, it was observed that in situ lacunar oxygen concentrations in

stems of Potamegeton praelongus (a species morphologically similar to g.

 

illinoensis) at depths of 5 to 5 m changed from 25.9 _+_ 1.4 (S.E.) 2:
(v/v) in early morning to 30.9 :_1.23 % in midafternoon, (Carlton and
others, unpublished), but direct effects on [02] in epiphytic periphyton
have not been established.

Solute flux in epilithic periphyton can only occur horizontally
within the layer and vertically between the periphyton and the
surrounding aqueous environment. This functional difference between
epiphytic and epilithic periphyton communities resulted in markedly
different oxygen distributions during darkness and after periods of
illumination (Figs. 1 and 2C, respectively). After 6 h of darkness
oxygen was fully depleted at the rock surface, but in periphyton
epiphytic on B. illinoensis the [02] was 98 uM at the leaf surface; in
this study anoxic conditions did not occur in any of the epiphytic
periphyton samples. After 2 h of illumination at 40 uEinst m—23-1
oxygen was still subsaturated for 1 mm above the rock surface (Fig. 2C),
but in the epiphytic periphyton oxygen was supersaturated throughout the
steady state microprofile measured at 40 uEinst m_2s-1 (Fig. 1). This
comparison is, however, confounded by the inability to separate the
effects of different diffusivities and photosynthetic efficiencies of
the communities. The epilithic periphyton was adapted to higher light
intensities, which was demonstrated by the fact that [02] near the base
of the epilithic layer increased from < 10 to 100 uM in 1 h during

2

exposure to 40 uEinst m- s-1 (Fig. 2C), but when exposed to 80 uEinst

 

24
m-2 s—1 a similar change in concentration occurred in ca. 2 min (Fig.
5).

The structural relationships between epipelic periphyton and its
substratum differs markedly from that of epiphytic and epilithic
periphyton. Except among habitats characterized by quiescent water and
high light intensities, where thick algal mats can develop, there often
is no obvious demarcation between the epipelic periphyton community and
the sediment. Conversely, epiphytic and epilithic periphyton are easily
distinguishable from their substrata. Therefore, the prefix "epi-" can
be an inappropriate misnomer, and "endopelic" may be a more suitable
term for many situations. The semantics, however, are of less
importance than recognition of the functional relationship of these
microflora to their environment.

Comparison of the distribution of oxygen in different periphyton
communities reveals some of the functional differences that result from
the structural relationships between periphyton and their respective
substrata. In epiphytic and epilithic periphyton [02] exceeded 100%
saturation at the base of the community after a period of illumination
(Figs. 1 and 2). However, regardless of the extent of photosynthetic
activity of epipelic periphyton, metabolic activity within the
substratum always resulted in total depletion of oxygen at some depth
below the depth of maximum 02 concentration (Fig. 3A and 3B). Direct
measurement of photosynthetic oxygen production in epipelic periphyton

has shown that 0 is produced throughout the upper few millimeters in

2
high light situations (Jfirgensen et al. 1983; Revsbech and Ward 1984),

and in the upper few tenths of a millimeter under low light conditions

(Carlton and Wetzel, unpublished). The effects of photosynthesis and

 

25
respiration on the flux and distribution of oxygen are described
mathematically by a modified version of Fick's second law of diffusion

(Crank 1983; Revsbech et al. 1981):
[2] dC(x)/dt = Ds ' (dZC/dxz) + [P(x) — R(x)]

where C(x) is the oxygen concentration, Ds is the diffusion coefficient
(assumed to be constant within the sediment), P(x) is the rate of oxygen
production, and R(x) is the rate of oxygen consumption, at depth x.

The penetration depth of oxygen is affected greatly by the relative
magnitudes of oxygen production and consumption in the sediment profile,
i.e. P(x)Ix and R(x)'x, respectively. During darkness when P(x)|x = 0,
oxygen diffuses from the overlying water, through the diffusive boundary
layer, and into the sediment. Therefore, in theory, although the
diffusion coefficient of 02 in sediment is ca. 30% lower than in the
boundary layer (Revsbech et al. 1986), if no consumptive processes
existed (i.e. R(X)Ix = O) the gradient dC/dx would eventually equal zero
throughout the sediment and the boundary layer. In reality, the
magnitude of R(x) in each stratum (x) varies as a function of the
biomass and types of organisms present, the concentration of oxidizable

substrates (e.g., H S or reduced carbon compounds), the temperature, and

2
the concentration of oxygen. While the interdependence of these factors
is obvious, the dominating role of photosynthesis by epipelic periphyton
can only be appreciated by comparing the distribution of oxygen during

darkness and after illumination (e.g., Fig. 3A). This formation during

day and deterioration during night of an oxidized microzone at the

sediment—water interface produces a situation that has previously been

 

 

 

26
thought to be a seasonal rather than a diel phenomenon (Mortimer 1941,
1942; Gorham 1958). Associated effects on microbial metabolism and
chemical reactions in sediments are largely unknown. However, it has
been determined that the diel fluctuation of [02] in sediment colonized
by epipelic algae causes a diel fluctuation in the release of phosphorus
from the sediment to the overlying water (Carlton and Wetzel,
unpublished).

A portion of the oxygen produced in epipelic periphyton
photosynthesis diffuses through the diffusive boundary layer to the
overlying water. The flux rate is inversely proportional to the
thickness of the boundary layer, which decreases with increasing
velocity of the overlying water (Vogel 1981). The increased flux rate
of 02 from illuminated sediment overlain by moving water results in
decreased oxygen penetration and lower [02] in the sediment; this is
demonstrated by comparing the microprofiles in Fig. 3A (overlying water
not stirred) and Fig. 3B (free stream velocity of water above the SWI
was ca. 0.5 cm s-1). (Boundary layer thicknesses are not accurately
presented due to sampling resolution above the SWI). After 8 h of
illumination at 10 uEinst m-2 s_1, the 02 content in the sediment was

2 vs. 1.80 X 10-2 umol cm—2 in unstirred vs. stirred

conditions, respectively; at 45 uEinst m"2 s_1 under stirred conditions,

9.74 X 10'2 umol cm”

the sediment oxygen content (10.29 X 10_2 umol cm_2) was only slightly
greater than at 10 uEinst m—2 s_1 without stirring. During darkness the
transport of oxygen into the sediment was enhanced by currents in the
overlying water (Fig. 3B), but under stagnant water the sediment was

virtually anoxic and a stronger negative gradient existed above the SWI

(Fig. 3A). The effects of currents on periphyton oxygen dynamics are

 

27
even more pronounced in streams, where water velocities can exceed 50 cm
s_1. Even in highly productive epilithic stream periphyton the
concentration of oxygen is not significantly higher than in the moving
water because of rapid transport across the thin boundary layer. But,
if the current is halted oxygen accumulates rapidly in the periphyton
layer and ebullition quickly ensues. Therefore, the use of
microelectrodes to investigate oxygen dynamics in highly productive
periphyton in high—flow—rate environments is limited.

The physical structure of the epipelic alga 2. tuberosus resulted
in markedly divergent oxygen dynamics compared to those of epipelic
microalgae. Although the filaments were not sufficiently dense to alter
the diffusion coefficient as sediments do, the macro-structure of the
alga produced a 60—mm—thick stagnant boundary layer above the SWI which,
by virtue of its volume, also provided a sink for oxygen produced there.
The time course of oxygen distribution in sediment colonized by_2.
tuberosus (Fig.4) contrasted markedly with that of diatom dominated
sediments (Fig. 3) where the advance of the oxic zone was slow, but
continuous during the illumination period. It is likely that the oxygen
distribution during illumination of_2. tuberosus resulted from
translocation of oxygen from the thalli to the rhizoids at the onset of
photosynthesis, but that during the ensuing photoperiod the rate of
translocation did not exceed the net oxygen demand of the rhizosphere.
The role of rhizoids in a variety of rhizophytic algae is discussed by
Raven (1981), and recently, ammonium uptake by rhizoids with subsequent

translocation to thalli has been demonstrated in Caulerpa cupressoides,

 

a marine siphonous green alga (Williams 1984). However, physiological

studies of solute translocation in 2. tuberosus are unknown, and

 

28
therefore no further conclusions concerning internal oxygen dynamics of
the alga can be made.

During photosynthesis in four distinctly different periphyton
communities [02] increased most rapidly in epilithic periphyton and
slowest in epipelic periphyton (Fig. 5). Although the epizoic
periphyton received the highest irradiance intensity the [02] there
increased more slowly than in epilithic and epiphytic periphyton. These
results are congruous with what would be expected, because in the
epipelic and epizoic periphyton the thickness of the algal layer was
only a few tenths millimeter, which was quite small relative to the
volume of the adjacent sinks (i.e. the boundary layer and subtending
sediment for epipelic algae, and for epizoic algae the vitelline fluid
and the gelatinous mass in which the eggs are suspended), where
considerable oxygen demand existed via sediment microbial metabolism or
respiration of the salamander embryo. Therefore, oxygen produced in
epipelic and epizoic periphyton was probably more quickly diluted or
consumed. Conversely, in the relatively thick epilithic periphyton
layer there was but one path for the escape of oxygen, and the [02]
increased most rapidly there. The diffusion regime and [02]
distributions in epiphytic periphyton was intermediate to that of the
others. Oxygen could diffuse from the layer in two directions, but the
subtending leaf probably did not supply as great an oxygen demand as did
sediments on the epipelic periphyton.

A general understanding of photosynthetic oxygen production and
oxygen dynamics in a limited variety of periphyton systems has been
developed in recent years, primarily as a result of the increased

availability of microelectrode technology. In this comparative study we

 

29
specifically addressed the physical differences in the structure of
periphyton communities and their substrata, and described the attendant
functional aspects which govern the distribution and fates of oxygen.
The results show i) that through the diurnal period periphyton of all
types act alternately as sources and sinks for oxygen, and ii) that the
substrata for epiphytic, epilithic, epipelic, and epizooic periphyton
vary greatly in their function in the oxygen dynamics of their
respective communities. Of equal importance were the observations that
the structural modification by periphyton of their microenvironments can
cause significant reduction in the transport rates of oxygen between
their substrata and the aqueous environment, with the result that
microzones which are anoxic during darkness can become hyperoxic after
exposure to light. Observations on epipelic microalgae revealed that
even at very low irradiances photosynthesis was sufficient to
significantly oxidize surficial sediments, but that the extent of this
effect was moderated by the presence of currents. The dominating role
of diffusion, and the chemical disequilibrium between the interstitial
water of the periphyton layer and the surrounding aqueous environment
were also demonstrated. In conclusion, it is apparent that periphyton
communities are more complex than is commonly realized, and it is
suggested that the interrelatedness of structure and function be

considered in future investigations of periphyton ecology.

30
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reference to the bottom deposits. J. Ecol. 30:235—283.

Mortimer, C. H. 1941. The exchange of dissolved substances between mud
and water in lakes. I & II. J. Ecol. 29:280—329.

Mortimer, C. H. 1942. The exchange of dissolved substances between mud
and water in lakes. III & IV. J. Ecol. 30:147—201.

Moss, E. 1972. Studies on Gull Lake, Michigan. I. Seasonal and depth
distribution of phytoplankton. Freshwat. Biol. 2:289—307.

Raven, J. A. 1981. Nutritional strategies of submerged benthic plants:
the acquisition of C, N and P by rhizophytes and haptophytes. New
Phytol. 88:1-30.

Reuter, J. E., S. L. Loeb, and C. R. Goldman. 1986. Inorganic nitrogen
uptake by epilithic periphyton in a N-deficient lake. Limnol.
Oceanogr. 31:149—160.

Revsbech, N. P. 1983. In situ measurement of oxygen profiles of
sediments by use of oxygen microelectrodes, p. 265—273. In E.
Gnaiger and H. Forstner [eds.], Polarographic Oxygen Sensors.
Springer Verlag, New York.

Revsbech, N. P., B. B. Jorgensen, and O. Brix. 1981. Primary
production of migroalgae in sediments measured by oxygen
microprofile, H CO fixation, and oxygen exchange methods.
Limnol. Oceanogr. 28:717—730.

Revsbech, N. P., and B. B. Jorgensen. 1983. Photosynthesis of benthic
microflora measured with high spatial resolution by the oxygen
micrOprofile method: capabilities and limitations of the method.
Limnol. Oceanogr. 28:749—756.

 

32

Revsbech, N. P., and D. M. Ward. 1983. Oxygen microelectrode that is
insensitive to medium chemical composition: use in an acid
microbial mat dominated by Cyanidium caldarium. Appl. Environ.
Microbiol. 45:755—759.

Revsbech, N. P., and D. M. Ward. 1984. Microelectrode studies of
interstitial water chemistry and photosynthesis activity in a hot
spring microbial mat. Appl. Environ. Microbiol. 48:270—275.

Revsbech, N. P., and B. B. Jorgensen. 1986. Microelectrodes: their use
in microbial ecology. Adv. Microb. Ecol. (in press).

Revsbech, N. P., B. Madsen, and B. B. Jorgensen. 1986. Oxygen
production and consumption in sediments determined at high spatial
resolution by computer simulation of oxygen microelectrode data.
Limnol. Oceanogr. 31:295-304.

Rich, P. H., R. G. Wetzel, and N. G. Thuy. 1971. Distribution,
production and role of aquatic macrophytes in a southern Michigan
marl lake. Freshwat. Biol. 1:3—21.

Round, F. E. 1981. The ecology of algae. Cambridge University Press,
Cambridge.

Sand—Jensen, K., N. P. Revsbech, and B. B. Jorgensen. 1985.
Microprofiles of oxygen in epiphyte communities on submerged
macrophytes. Mar. Biol. 89:55-62.

Schindler, D. W., V. E. Frost, and R. V. Schmidt. 1973. Production of
epilithiphyton in two lakes of the Experimental Lakes Area,
northwestern Ontario. J. Fish. Res. Ed. Can. 50:1511—1524.

Vogel, S. 1981. Life in moving fluids. Willard Grant, Boston.

Westlake, D. F. 1978. Rapid exchange of oxygen between plant and
water. Verh. Internat. Verein. Limnol. 20:2363—2367.

Wetzel, R. G. [ed.]. 1985. Periphyton of freshwater ecosystems. Junk
BV, The Hague.

Williams, S. L. 1984. Uptake of sediment ammonium and translocation in
a marine green macroalga Caulerpa cupressoides. Limnol. Oceanogr.

29:374—379.

 

 

 

 

CHAPTER II

PHOSPHORUS FLUX FROM LAKE SEDIMENTS:

EFFECT OF EPIPELIC ALGAL PHOTOSYNTHESIS

33

 

34

INTRODUCTION

Microbial metabolism in aquatic sediments regenerates inorganic
phosphate which accumulates in the sediment interstitial water forming
concentration gradients. Subsequent diffusive transport to the
overlying water can be retarded by a number of processes which either
temporarily or permanently immobilize phosphate. Mortimer (1941, 1942)
demonstrated that the presence of an oxidized microzone at the sediment
surface inhibited phosphorus release, but that a decrease in redox
potential of the microzone following the onset of anoxic conditions in
the overlying water stimulated the reduction of iron (III), thus
releasing phosphate bound in hydrous oxides and gels at the sediment
surface. This key role of oxygen has been substantiated in numerous
studies in a variety of lake and sediment types (Kamp—Nielsen 1974,
Patrick and Khalid 1974, Frevert 1980). Also identified as factors
affecting the rate of P flux from sediments are pH, temperature,
ebullition, bioturbation, nitrate concentration, microbial activity, and
macrophyte root exudates (reviewed by Bostrum et al. 1982, and by Ryding
1985). However, because of the diversity of lake sediment
characteristics, few broadly applicable generalizations can be
formulated. Increased temperature, by virtue of its effect on rates of
microbial oxygen consumption in sediments, has been universally shown to
result in increased P flux (e.g., Kelderman 1984, Kamp—Nielsen 1975).
Conversely, pH and nitrate concentration have been demonstrated to exert
both positive and negative effects on P efflux rates (Andersen 1975,
Andersen 1982, Bostrum 1982, Bostrum 1984, Tiren and Pettersson 1985).
For obvious reasons short-term P release is typically enhanced by

ebullition and bioturbation.

 

35

Germane to this study are the effects of pH and oxygen
concentration ([02]) in surficial sediments. Even at low light
intensities (<50 uEinst m_2 s_1) photosynthesis by epipelic microalgae
can be sufficient to produce marked changes in the distribution of pH
and oxygen in surficial sediments (Carlton and Wetzel in press; Revsbech
et al. 1983). The shift from oxic conditions in the upper few
millimeters during day to nearly anoxic conditions at night results in
daily formation and breakdown of the oxidized microzone, a phenomenon
that was previously thought to be seasonal in lake sediments (Mortimer
1941, 1942; Gorham 1958). Although the diel change in pH as a result of
epipelic algal photosynthesis is not as spatially extensive as the
change in [02], the magnitude of the increase is significant. We show
here that i) during periods of illumination, sediments colonized by
epipelic periphyton released less P than during darkness, and ii) the
metabolic activity of epipelic periphyton produced a conserving
mechanism for phosphorus in lake sediments, thus reducing the net flux

of P to the overlying water.

EXPERIMENTAL PROCEDURES

The periphyton—sediment samples used in this study were obtained
from Lawrence Lake, a hard water, oligotrophic, P—limited lake in
southwest lower Michigan. The samples were taken at a depth of 8 m by a
SCUBA diver using a box corer which sampled a surface area of > 700 cm2
with a combined height of 42 cm for the sediment and overlying water
(Carlton and Wetzel 1985). The samples were transferred without

disturbance to laboratory aquaria and maintained as short—term

microcosms in which the water was gently stirred and aerated by a stream

 

36
of rising air bubbles, and temperature and photoperiod could be
controlled. In all microcosms and subsequent flow-through experiments
the illumination intensity was 30.: 5 uEinst m-2 s—1.

The effects of photosynthesis and sediment microbial respiration on
the distribution of oxygen and pH in the sediment were determined with
high spatial resolution at various times during the simulated
photoperiod using microelectrodes constructed by R. Carlton. The oxygen
microelectrodes were the micro—Clark style with a cathode diameter of 3
to 7 pm and an outer diameter of 15 to 40 pm in the sensing region
(Revsbech and Ward 1983). A chemical microsensor (Transidyne General,
Ann Arbor) supplied polarizing voltage (- 0.75 V) and processed the
electrode output signal. Calibration was accomplished with volumes of 0
and 100% air—saturated lake water at the temperature of the experimental
systems. The pH electrode (Thomas 1978) had a diameter of ca. 100 pm in
the sensing region. A Beckman pHi 55 ph meter and buffer solutions of
known pH were used to perform a two—point calibration of the electrode.
During measurements in the microcosms the microelectrodes were
positioned with a micromanipulator (0.05—mm resolution), and were
aligned initially at the sediment—water interface by remotely viewing
position through a water proofed fiberscope (American Optical).
Measurements were made at 0.2—mm intervals.

The algal species assemblage of the periphyton community was
determined by removing the thin layer with a micro—siphon that was
positioned with a micromanipulator. Samples were preserved with acid
Lugol's solution and then settled before observation with an inverted
microscope. Samples for chlorophyll a content of the periphyton were

obtained with the micro—siphon by removing ca. 1.5 mm of the upper

 

37

sediment layer within a 1-cm-high plastic ring (area = 1.65 cm2) placed
on the sediment surface. The microflora-sediment slurry was
immediately passed through a 47—mm-diameter glass fiber filter (Gelman
GF/F, retention size 0.7 um), that was immediately frozen at -20°C, and
later stored at -7OOC. The filters were ground in a 9:1 mixture of
chilled acetone and water in a tissue grinder, and then centrifuged.
The pheophytin—corrected chlorophyll a concentration was determined
spectrophotometrically by the method of Strickland and Parsons (1972).

The phosphate flux experiments were conducted in flow—through
chambers which permitted manipulation of i) the diffusive efflux rate of
phosphate across the sediment—water interface, ii) the volume and
flushing rate of the overlying water, iii) the oxygen concentration of
the overlying water, iv) the photoperiod and light intensity to which
the epipelic algae was exposed, and v) the temperature of the system.
Each experimental unit consisted of an 8—12—mm—thick layer of sediment
and its intact periphyton isolated on a supported nylon membrane

(Ultipor N 0.45 pm pore size) which separated the upper flow—through

66'
chamber from the lower artificial pore water (APW) chamber. In a test

3-
4

material was found to be superior to teflon and polycarbonate

for permeability of P0 in flow-through chambers without sediment the

Nylon66
membranes. Periphyton-sediment samples were transferred from the
microcosms to the chambers by the method depicted in Fig. 1. The
cylinders of the flow—through chambers (O.D. = 45 mm, I.D. = 38 mm,
height = 50 mm) and the O—ring—sealed piston (diam. = 37 mm, height = 60
mm) were constructed of translucent acrylic plastic. The base of the

flow—through chamber, and the cylinder and base of the APW chamber

 

 

38

      
    
  

NYLON SCREEN

 

 

 

MEMBRANE

 

 

 

FLOW CHANNELS

 

 

 

MINI

 

BASE PLATE

  
   

MEMBRANE GAS ESCAPE
CHANNEL

 

Figure 1. Diagram of apparatus used to isolate surficial sediment
section on a membrane. Scheme flows clockwise from upper left. After
sampling sediment with the piston corer (I.D. = 58 mm) the hole in the
membrane and the upper chamber are aligned and the sample is pushed
upward to the desired height. The membrane is pulled across severing
the sediment, the lower section of which is discarded. Then the base

plate is secured and the piston is installed.

 

39

(volume = 1.0 L) were 12.8-mm-thick, opaque acrylic. The volume of
water above the sample was adjusted to 15 ml by inserting the piston to
a predetermined level. The flushing water entered and left the
flow—through chamber via channels inside the piston; a three—way valve
mounted on the inlet to each chamber allowed switching among different
source reservoirs. The space above the sediment was divided
horizontally by a 1-mm—mesh nylon screen on which a magnetic stir bar (2
x 8 mm) rested. A second, slightly larger stir bar was suspended in the
APW chamber on a thin nylon line 2 cm below and 1 cm to the side of the
lower face of the membrane. Each experiment typically utilized three
units that were submersed in a temperature—controlled water bath; two
units (A & B) were exposed to simulated photoperiods, and the third
(control, C) was shielded from light. The gentle motion of the stir
bars, which was induced by a magnet rotating at 60 rpm between the three
chambers, decreased the thickness of boundary layers at the
sediment—water interface and at the lower surface of the membrane, and
decreased streaming of the flow—through water between the inlet and
outlet ports. A septum at the top of the inlet channel in the piston
permitted removal of gas bubbles from the flow-through chamber; a septum
on the top of the APW chamber was used for removal of bubbles from the
APW and for the injection of 32PO43-. J

The flow—through water was prepared by filtering water from
Lawrence Lake (SRP concentrations < 2.0 ug L—1) through a series of
filters (10 um, 1 um, 0.45 pm, and 0.22 um). The desired oxygen
concentration was obtained by vigorous sparging with either air, oxygen,
or nitrogen for > 4 h. The pH (7.9) and alkalinity (4.4 meq L'1) of the

water did not change significantly during sparging. After a given

 

 

40
treatment the water was transferred to a flexible polypropylene
container (Cubitainer) which, during the experiment, was suspended in
the bath and which collapsed as the water was removed, thus preventing
gas exchange with the atmosphere. Problems of gas exchange through the
wall of the container were minimized both by its immersion in the bath
and by changing the reservoir fluid daily.

The artificial pore water was prepared by mixing several liters of
lake water and sediment (ca. 3:1 v01. ratio) continuously for 3 days
under a nitrogen purged head space. The slurry was allowed to settle
for several seconds before transferring 1.0 L to each APW chamber. The
concentration of soluble reactive phosphorus in each APW chamber was

32PO 3-

adjusted to 50 ug P043_—P L_1, and 3-5 mCi 4 were added.

A peristaltic pump downstream of the flow—through chambers
continuously pulled flushing water through at a rate of 7.5 ml h—1, thus
flushing each unit once per 2 h. Each effluent stream from that pump
was continuously sampled by a second peristaltic pump at a rate of 0.8
or 1.6 ml h_1; this liquid was collected in 20—ml liquid scintillation
vials mounted on the deck of a fraction collector timed to advance at
intervals of 1 or 2 h. Therefore, 12 or 24 samples were obtained from
each flow—through unit each day.

Light was supplied to units A and B through fiberoptic wands. The
timer—controlled source was a projector lamp (Sylvania EJY), and the
light was diffused at the distal end of the wand by passage of the beam
through a piece of nylon membrane material. The intensity of irradiance
incident on sediment within a flow—through chamber was measured in units
assembled without sediment with a LiCor model LI—185 meter with a cosine

corrected quantum sensor (Lambda Instruments). Tests were conducted

 

 

 

‘—

41
which showed that no temperature changes occurred in the chambers as a
result of illumination.

The radioactivity was determined in each vial by adding ca. 14 ml
of aqueous counting coctail and counting for 20 min in a Beckman LS-8000
liquid scintillation counter. Quench was insignificant and the counting
efficiency was essentially 100%. Background radiation (52 cpm) was
subtracted from sample counts before plotting.

The effect of periphyton photosynthesis on the distribution of
P043— in surficial sediments of Lawrence Lake was determined by
comparing the distribution of SRP in surficial sediments isolated from
light with that in unshaded plots. Shortly after the spring ice thaw a
6 m x 7 m area of sediment at a depth of 9 m in Lawrence Lake was
shielded from illumination by suspending a sheet of black polyethylene
on thin wood posts ca. 0.6 m above the sediment surface. Two nearby
control plots each consisted of an area (2 m x 2 m) covered by
transparent polyethylene sheet that was similarly suspended.

Downwelling light was only slightly attenuated by the clear sheet, which
was cleaned periodically of sedimented seston and periphyton. Light

intensity within the darkened plot was < 0.1 uEinst m-2 s-1 when ca. 30

uEinst m_2 s_1 reached the sediment surface in the control plots.
Sediment pore water in the plots was sampled with dialysis samplers
(Hesslein 1976) which were inserted by a SCUBA—equipped diver 2 weeks
after installation of the plots. Each sampler consisted of an acrylic
plastic body with four 8—ml chambers spaced at 1.7—cm vertical
intervals. The surface area of the nylon membrane (described

previously) over each chamber was 5 cm2. The chambers were filled with

purified (Millapore Q) water, and the membranes were secured in a manner

 

42
which excluded bubbles from the chambers. Then the samplers were
deoxygenated by immersion for 2 d in a bath of purified water that was
continuously sparged with N2. The samplers were left in situ for 50 d,
with two in each control plot, and three in the dark plot. Just prior
to removal from the sediment the face of each sampler was covered with a
thin acrylic plate to inhibit oxygenation of the samples which could
affect subsequent SRP analysis (Bray et al. 1973). SRP was determined
using the method of Murphy and Riley (1962) within 1 h by withdrawing
4 ml from each chamber and diluting it with 4 ml of 0.2 N HC1 (to
achieve a higher final acidity; John 1970) before adding the mixed
reagent. A color blank was measured on the remaining 4—m1 in each cell
by the same procedure, except ascorbic acid was not added to the mixed
reagent.

In order to determine if algal oxygen production and subsequent
oxygen dynamics in sediments were altered during preparation of the
sample and assembly of the flow—through chamber, a chamber was
constructed that permitted insertion of an oxygen microelectrode. A
simulated flow—through experiment (no 32P) was performed with a
photoperiod of 6:8 LD. Measurements were obtained at 0.2 and 1.0 mm

below the SWI.

RESULTS

In each of the flow—through experiments reported here, the
CaCOB—rich sediment was colonized by a thin periphyton community which
was dominated by pennate diatoms, especially of the genera Navicula and
Nitzschia. Pheophytin—corrected chlorophyll a content of the periphyton

was 30.2 :_8.3 mg m“2 (pooled mean : s.d.; n = 9, 3 samples per

 

43
experiment).

Except for minor differences due to temperature, the temporal and
spatial distributions of oxygen were similar in all of the sediment
microcosms used. Fig. 2 is presented as an example. After 6 h of
darkness [02] at the SWI was < 50% saturated, and the sediment was
anoxic below 0.8 mm. Illumination for two hours resulted in"
supersaturated conditions in the upper 0.9 mm, and in increased oxygen
penetration. After 6 h of illumination the zone of supersaturation
extended to a depth of 1.9 mm, and oxygen penetration occurred to a
depth of 4.0 mm. Under similar light and temperature conditions in
similar sediment microcosms (taken from the same area of the lake) where
the overlying water was not stirred, oxygen maxima were greater and
oxygen penetrated farther into the sediment (Carlton and Wetzel, in

in

press). The temporal and spatial distributions of 02

membrane—mounted sediment were similar to that in the microcosms (Fig.
3). Measurement of pH microprofiles was possible only for the third
experiment. During darkness a negative gradient of pH occurred across
the SWI, but after 6 h of illumination a positive heterograde existed in
the thin photosynthetic layer, with a maximum pH of 8.9 (Fig. 4).

In the first flow—through experiment (temperature = 17°C, influent
water saturated with air) two chambers (A & B) were exposed to a 12:12
LD photoperiod, and a third was kept dark as a control. The efflux rate
of 32F oscillated in a pattern coincident with the photoperiod (Fig. 5).
Although the magnitude of the effect was different in the two replicates

f 32P efflux always increased during darkness and decreased

32

the rate 0
during illumination. There was no diel oscillation in P efflux in the

dark control.

 

 

44
The experiment was repeated with identical conditions except the

2P

temperature was lowered to 11°C (Fig. 6). Although the background 3
efflux rate was similar to the first experiment at 17°C, no obvious diel
pattern existed. This absence of diel periodicity of P efflux was
observed in two similar experiments at 10 and 120C (Carlton and Wetzel,
unpublished).

The third experiment was performed identically to the first for
three days during which the diel pattern occurred in the illuminated
treatments (Fig. 7). At the end of the third day, when the light was
extinguished, the air—saturated water influent to all three flow-through
chambers was replaced by water saturated with pure oxygen (45—50 mg

32F efflux in all three

L‘1). This treatment resulted in a decrease in
flow—through chambers; the greatest effect occurred in the dark control.
After 16 h the oxygen—supersaturated water was replaced with
deoxygenated water and the 32P efflux rate increased in all three
chambers. These manipulations were repeated again with similar results.
After 12 h of anoxic influent water, chambers A and B were again exposed
to light (for the first time in 54 h), but with no obvious effect. The
deoxygenated water was maintained for two more photoperiods, during
which no response to illumination occurred in chamber A, but in B a
response was apparent. The 32P efflux rate in the control (never
exposed to light) remained nearly constant until after 52 h of treatment
with anoxic flow—through water it increased rapidly to the highest level
observed in any chamber during that experiment. However, upon switching

32

to superoxygenated water the P efflux in the control decreased

precipitously. Similar decreases occurred in replicates A and B, and no

 

 

 

45

OXYGEN (% saturation)

 

 

 

 

 

O 50 100 150 200
1 I l I
UJ /\;\..‘ ....
g 0 \=\ ohl:....
U. \ 1
,I 1'
D ,I‘ ‘9
‘0 , x 6
1- ,z’ s
z I,» _y
E 1 - ,r’ I" r
a ’(/ ‘c
% ’1'” ...... ...
2 ," ‘ '
O r -r
E 2 ~;’ .1" _
U) f :-
E f ,n"
E ' ..-"
Z ..-"
:i 3 — - .—
3 ..."
r"
i
i'
4 1 I 1

Figure 2. Microprofiles of oxygen concentration in sediments prior to
their use in the flow—through system; after 6 h of darkness (solid
line), after 2 h of illumination(dashed line), and after 6 h of
illumination (dotted line). Light intensity = 30 uEinst m_2 8.1;

temperature = 17°C; equilibrium air saturation = 302 umol O2 L~1~

 

 

46

 

/—1mm

OXYGEN(% SATURAWON)

  

 

 

   

 

‘B-GB-B ————,— 8713137887”, I I net
0 10 20 30 4O

TIN1E (h)

Figure 3. Oxygen concentrations through time at depths of 0.2 and 1.0

mm below the sediment surface in a sediment section isolated on a

membrane within a flow—through chamber. Black bars represent dark

Light intensity = 30 uEinst m~2 s-1, temperature = 17°C,

equilibrium air saturation = 302 umol 02 L—1-

period.

 

 

47

 

 

 

 

 

 

ptl
6 7 8 9 10
1 l l |
UJ
22 0
U.
o:
:3
09
+—
UZJ
:5 1" _.
E3
UJ
0)
IE
0
E 2— _
U?
a:
DJ
l...—
UJ
2
i 3— ~
:5
4 1 1 1 |

Figure 4. Distribution of pH in sediments used for flow—through

experiments; after 6 h of darkness (black line) and after 6 h of

_ -1
illumination (hatched line) with 30 uEinst m 2 s at a temperature 0f

17°C. Error bars are one standard error of the mean on either side of

the mean (n = 3)-

 

 

48

“P FLUX FROM SEDIMENT 17°C

 

2000
.1

M HF-wvfl'xw “Ker/k‘Xf-‘K/

—__n____

MW \W

E 1000 ‘
U : AM“¢I\IA‘

; v A

. new...» skim -
: \vx‘\/\vi\w

0 10 20 30 40 50 80 7O 80 90

O

 

 

 

 

TIME (11)

Figure 5. Phosphorus flux experiment with time course of 32P release

(CPM) from sediments exposed to a 6:6 LD cycle. Light intensity during
illumination of chambers A & B = 30 uEinst m—2 s_1, C = control

(permanently dark). Temperature = 17°C. Bars designate dark period.

 

 

 

49

“P FLUX FROM SEDIMENT 11°C

1,000

500

CPM
O

   

500
flail/w MAW “FM

0 10 20 so 40W 60

 

TIME (h)

Figure 6. Phosphorus flux experiment with time course of 32F release

(CPM) from sediments exposed to a 6:6 LD cycle. Light intensity during
1

illumination of chambers A & B = 30 uEinst m_2 s_ . Temperature =

11°C. Bars designate dark period.

 

 

50

"P FLUX FROM SEDIMENT 17°C

 

l I I I I I I I l I I I

CPM

 

 

 

g} .
1
5‘
2. ,

 

 

 

 

H "“Xa.a“”3KJ _
HULIHIHLLII
l I I l l I 1 I I I l
U 40 80 120 160 200 240
NME(m

32

Figure 7. Phosphorus flux experiment with time course of P release

(CPM) from sediments exposed to a 12:12 LD cycle. Light intensity
during illumination of chambers A & B = 30 uEinst m-2 s_1. Black bars
indicate dark periods for chambers A & B; the control (C) was
permanently dark. Round symbols on data points designate period during
which gases in flow—through water were in atmospheric equilibrium;
square symbols start at point where [02] was manipulated in the flow-
through water. Diagonally hatched bars at bottom indicate periods when
flow—through water was superoxygenated; bars with vertical hatching

indicate anoxic flow—through water. All three chambers received same

quality of water. Temperature = 17°C.

 

SOLUBLE REACTIVE PHOSPHORUS

 

 

(pg - L")
120
Lu
0
< —
u.
n:
3
a)
[_-
2
”J -1
E
0
LL!
a)
5 ~
0
_J
Lu
m
(0
o: _
UJ
’—
LU
E
F—
2 ..
LL]
0
6 I l l I I

 

 

 

Figure 8. In situ distribution of soluble reactive phosphorus in
interstitial water in sediments of Lawrence Lake in a plot which was
permanently shielded from light (black line) and in control plots which
received normal light intensities (hatched line). Error bars are one
standard error of the mean on either side of the mean (n = 4 for

control, n = 3 for dark plot).

 

 

52
further illumination effects were observed.

The distributions of soluble reactive phosphorus in sediment
interstitial water in the unshaded (control) and permanently darkened
plots, at a depth of 9 m in Lawrence Lake, were markedly different (Fig.
8). The concentrations and gradients of SRP in the unshaded plots were
similar to previous determinations in that region of the lake (Carlton
unpublished; R. Moeller, personal communication). In comparison, in the

1 and

upper 1.7 cm of sediment in the darkened plot SRP was 3 pg L-
virtually no gradient existed. Below 1.7 cm the slopes of the gradients
were similar, but the concentration of SRP was lower throughout the
darkened sediment profile. Visual examination of sediment during
retrieval of the interstitial water samplers revealed that the shaded

sediments had not developed the coloration evident on the sediment

surface in the unshaded plots and elsewhere in the lake.

DISCUSSION

The increase in the efflux rate of 32P from sediment in the
flow—through chambers during dark periods probably resulted from the
cessation or reversal of one or more chemical or biological processes
that had immobilized phosphorus diffusing upward through the sediment
during the illumination period. Several chemical processes are known to
affect phosphate mobility in sediments as a function of [O ] and [OH-J,
which we have demonstrated to fluctuate greatly in surficiil sediments
exposed to a light—dark cycle. The rate of formation of iron hydroxide
is first order with respect to [02], and second order with respect to

[OH—J (Stumm and Morgan 1981). When the sediment surface is oxidized

iron (III) hydroxides can bind phosphate as a precursor to the formation

 

 

53
of hydroxyapatite (Syers et al. 1973). However, some investigators have
reported increased phosphate flux at high pH and speculated that
hydroxide ions competed with phosphate for binding sites on the oxidized
iron gels, thus increasing the mobility of PO43_ (Jacoby et al. 1982).
Conversely, by virtue of its effect on calcite formation in hard water,
increased pH in overlying water has been shown to reduce the net flux of
P from calcareous sediments via coprecipitation (Otsuki and Wetzel 1972)
with and adsorption to the surfaces of calcite crystals (Gunatilaka
1982). Gunatilaka also showed that the adsorption reaction was rapid
and that the maximum phosphate adsorption occurred at pH 9, which is
approximately the value we observed in surficial sediment after 6 h of
epipelic algal photosynthesis (Fig. 4). The decreases in oxygen
concentration and depth of penetration in sediments which occur during
darkness (Fig. 2 and 3) result in breakdown of the oxidized microlayer
and lowering of the redox potential. The attendant chemical reduction
of oxidized iron gel complexes releases phosphate (Syers et al. 1973).
Phosphate adsorbed to calcite can also desorb as the pH in the microzone
drops during darkness.

If the mechanisms producing the observed diel P flux variation were
strictly chemical in nature the pattern at 170C should also have existed
at 11°C, but with slightly lower magnitude because of the temperature
(Kelvin) dependence of chemical reactions. Because a small decrease in
temperature resulted in a virtual absence of the phenomenon, it is
apparent that the dominant mechanism was biological. Algae are known to
absorb excess phosphate under certain conditions and store it as
polyphospate (e.g., Shapiro 1968). However, we know of no studies

showing subsequent release of the phosphate as a result of darkness.

 

 

54
Conversely, Levin and Shapiro (1965) showed that the microorganisms

(e.g., the bacterial genera Zooglea, Escherichia, Bacillus,

 

Flavobacterium, and Pseudomonas, and some ciliated protozoans) in

 

activated sludge in sewage treatment plants absorbed excess phosphate
when the suspension was aerated, and then released it when aeration was
stopped and the system became anoxic. The reaction was shown to be
rapidly reversible, temperature dependent, independent of the external
phosphate concentration, and did not involve cell death or growth.
Shapiro et al. (1967) reported that it was not oxygen concentration, but
redox potential which must be depressed to a sufficiently low level to
induce the release, but this was refuted by Randall et al. (1970) who
reported,that the release began when [02] reached zero, and that the
lowering of redox potential lagged by 40 to 60 min.

The conditions for induced uptake and release of phosphate by
microorganisms are extant in Lawrence Lake sediments. Bacteria and
protozoans are common in aquatic sediments (Kuznetzov 1970).
Photosynthesis by epipelic periphyton produces a diel fluctuation of
[02] from zero to ca. 200% saturation in surficial sediments (Fig. 2).
Therefore, the periodicity of P flux observed in Lawrence Lake sediments
is possibly a result of biological uptake and release of phosphate
induced by diel changes in [02] and redox potential resulting from
epipelic algal photosynthesis.

The 32P—P043_ injected into the artifical pore water may have
undergone biological transformations prior to diffusion through the
sediment layer. However, it is unlikely that significant diffusive
fractions were converted to biologically unavailable forms (Bostrum

1984; Cembella et al. 1984). Furthermore, as several nutrient loading

 

 

55
models have shown, lake phytoplankton productivity responds to total P

inputs (e.g., Dillon and Rigler 1974). Therefore, the flux of 32

P from
sediments in the flow—through chambers represents release of phosphorus
compounds that probably can stimulate phytoplankton productivity in
P—limited systems. Because we do not know the molecular forms involved
or the specific activities of the individual pools, we hesitate to

32P flux . However, the

quantify net P flux based on the observed
existence of the phenomenon implies that the net release of P from
sediments is reduced by the presence of epipelic periphyton. Analysis
of the interstitial water in the darkened and unshaded plots in Lawrence
Lake demonstrated that the concentration of SRP in sediments colonized
by an epipelic microflora capable of oxygenic photosynthesis was
significantly greater than in permanently darkened sediments (Fig. 8),
indicating that conserving mechanisms operating in the unshaded plots
were absent in the darkened plot.

Photosynthesis by periphyton epipelic on lake sediments modifies
the oxygen and pH conditions of interstitial water in surficial
sediments, even at low irradiance intensities. Subsequent chemical or
biological reactions that are sensitive to [02] or pH mediate the
mobility of phosphorus compounds in the sediment interstitial water,
thus affecting the release rate of P to the overlying water. The net
effect is the conservation of P in the sediment, i.e. P efflux is
reduced when an active epipelic algal community is present. This
conclusion implies that epipelic periphyton not only contributes to
whole-lake primary productivity, but that it may reduce the productivity
of phytoplankton by conserving P in the sediments. The ecology of

epipelic periphyton in lakes is largely unknown. However, the results

 

 

56
of this study have demonstrated that the function of epipelic periphyton
in lake ecosystems is of much greater importance than is presently
realized. Further studies of the effect of periphyton on nutrient
cycles and flux rates in lake sediments are essential, and will perhaps

reveal information which can be used in lake management and restoration.

 

 

57
REFERENCES

Andersen, J. M. 1975. Influence of pH on release of phosphorus from
lake sediments. Arch. Hydrobiol. 76:411—419.

Andersen, J. M. 1982. Effect of nitrate concentration in lake water on
phosphate release from the sediment. Water Res. 16:1119—1126.

Bostrum, B. 1982. Effect of nitrate concentration in lake water on
phosphate release from the sediment. Water Res. 16:1119—1126.

Bostrum, B. 1984. Potential mobility of phosphorus in different types
of lake sediment. Int.Rev. ges. Hydrobiol. 69:457—474.

Bostrum, B., M. Jansson, and C. Forsberg. 1982. Phosphorus release
from lake sediments. Arch. Hydrobiol. Beih. Ergebn. Limnol.
18:5—59.

Bray, J. T., O. P. Bircker, and B. N. Troup. 1973. Phosphate in
interstitial waters of anoxic sediments: oxidation effects during
sampling procedure. Science 180:1362—1364.

Carlton, R. G., and R. G. Wetzel. 1985. A box corer for studying
metabolism of epipelic microorganisms in sediment under in situ
conditions. Limnol. Oceanogr. 30:422-426.

Carlton, R. G., and R. G. Wetzel. Distributions and fates of oxygen in
periphyton communities. Can. J. Bot. (in press).

Cembella, A. D., N. J. Antia, and P. J. Harrison. 1984. The
utilization of inorganic and organic phosphorus compounds as
nutrients by eukaryotic microalgae: a multidisciplinary
perspective: part 1. CRC Critical Rev. Microbiol. 10:317—389.

Dillon, P. J., and F. H. Rigler. 1974. The phosphorus—chlorophyll
relationship in lakes. Limnol. Oceanogr. 19:767—773.

Frevert, T. 1980. Dissolved oxygen dependent phosphorus release from
profundal sediments of Lake Constance (Obersee). Hydrobiologia
74:17-28.

Gorham, E. 1958. Observations on the formation and breakdown of the
oxidized microzone at the mud surface in lakes. Limnol. Oceanogr.
3:291—298.

Gunatilaka, A. 1982. Phosphate adsorption kinetics of resuspended
sediments in a shallow lake, Neusiedlersee, Austria. Hydrobiologia
91:293—298.

Hesslein, R. H. 1976. An in situ sampler for close interval pore water
studies. Limnol. Oceanogr. 21:912—914.

Jacoby, J. M., D. D. Lynch, E. B. Welch, and M. A. Perkins. 1982.
Internal phosphorus loading in a shallow eutrophic lake. Water Res.

 

 

 

58
16:911-919.

John, M. 1970. Colorimetric determination of phosphorus in soil and
plant materials with ascorbic acid. Soil Sci. 109:214-220.

Kemp—Nielsen, L. 1974. Mud—water exchange of phosphate and other ions
in undisturbed sediment cores and the factors affecting the exchange
rates. Arch. Hydrobiol. 73:218-237.

Kamp—Nielsen, L. 1975. A kinetic approach to the aerobic
sediment—water exchange of phosphorus in Lake Esrom. Ecol. Model.
1:153-160.

Kelderman, P. 1984. Sediment—water exchange in Lake Grevelingen under
different environmental conditions. Neth. J. Sea Res. 18:286-311.

Kuznetsov, S. I. 1970. The microflora of lakes and its geochemical
activity. Editred by C. H. Openheimer. University of Texas Press,
Austin. 503 pp.

Levin, G. V., and J. Shapiro. 1965. Metabolic uptake of phosphorus by
wastewater organisms. J. Water Poll. Cont. Fed. 37:800-821.

Mortimer, C. H. 1941. The exchange of dissolved substances between mud
and water in lakes. I & II. J. Ecol. 29:280—329.

Mortimer, C. H. 1942. The exhange of dissolved substances between mud
and water in lakes. III & IV. J. Ecol. 30:147—201.

Murphy, J., and J. P. Riley. 1962. A modified single solution method
for the determination of phosphate in natural waters. Anal. chim.
Acta. 27:31—36.

Otsuki, A. and R. G. Wetzel. 1972. Coprecipitation of phosphate with
carbonates in a marl lake. Limnol. Oceanogr. 17:763—767.

Patrick, W. H. Jr., and R. A. Khalid. 1974. Phosphate release and
sorption by soils and sediments: effect of aerobic and anaerobic
conditions. Science 186:53—55.

Randall, C. W., D. W. Marshall, and P. H. King. 1970. Phosphate
release in activated sludge process. J. San. Eng. Div. 96:395—408.

Revsbech, N. P., B. B. Jorgensen, and T. H. Blackburn. 1983.
Microelectrode studies of the photosynthesis and 02, H S, and pH
profiles of a microbial mat. Limnol. Oceanogr. 28:1062—1074.

Revsbech, N. P., and D. M. Ward. 1983. Oxygen microelectrode
insensitive to chemical composition of the medium and its use in an
acid microbial mat dominated by Cyanidium caldarium. Appl. Environ.

Microbiol. 45:755-759-

Ryding, S.—O. 1985. chemical and microbiological processes as
regulators of the exchange of substances between sediments and water

 

59
in shallow eutrophic lakes. Int. Rev. ges. Hydrobiol. 70:657—702.

Shapiro, J. 1967. Induced rapid release and uptake of phosphate by
microorganisms. Science 155:1269-1271.

Shapiro, J. 1968. Studies on the natural factors affecting phosphate
absorption and its utilization by algae. Water Res. 2:21-23.

Shapiro, J., G. V. Levin, and H. Zea G. 1967. Anoxically induced
release of phosphate in wastewater treatment. J. Water Pol. Cont.
Fed. 39:1810—1818.

Strickland, J.D.H., and T. R. Parsons. 1972. A practical handbook of
seawater analysis. 2nd ed. Bull. Fish. Res. Ed. Can. 167, 310 pp.

Stumm, W., and J. J. Morgan. 1981. Aquatic chemistry. An introduction
emphasizing chemical equilibria in natural waters. 2nd ed. Wiley &
Sons, New York.

Syers, J. K., R. F. Harris, and D. E. Armstrong. 1973. Phosphate
chemistry in lake sediments. J. Environ. qual. 2:1—14.

Thomas, R. C. 1978. Ion-sensitive intracellular microelectrodes, how
to make and use them. Academic Press, London.

Tiren, T., and K. Pettersson. 1985. The influence of nitrate on the
phosphorus flux to and from oxygen depleted lake sediments.
Hydrobiologia 120:207—223.

 

 

CHAPTER III

A BOX-CORER FOR STUDYING METABOLISM OF

EPIPELIC MICROORGANISMS IN SEDIMENT

UNDER IN SITU CONDITIONS

6O

 

 

Limnol. Ormnogr.. 30(2), I985, 422—426
© 1985, by the American Socier of Limnology and Oceanography, Inc.

A box corer for studying metabolism of epipelic microorganisms

in sediment under in situ conditions‘

Abstract—The device described here permits
sampling ofa combined sediment and overlying
water volume of about 29 liters (cross-sectional
area > 700 cmz) with minimal disturbance ofscd-
iment structure. Vertical microprofiles ofoxygcn
concentration measured in littoral sediments be-
fore (in situ) and after sampling with the box corer
showed good agreement. These data are com-
pared with oxygen microprofiles measured in pe-
lagic sediments sampled with the box corcr and
kept in a laboratory under in situ conditions.
Oxygen production by epipelic algae exposed to
very low quantum flux resulted in marked diurnal
phasing of oxygen above and within the sedi-
ments.

In recent years increased emphasis has
been placed on the importance of investi-
gating sediment function undcr natural con-
ditions. Because ofthe difficulty ofperform-
ing experiments in situ, samples are often
brought into the laboratory and maintained
for short periods; piston corers are usually
adequate for this retrieval procedure (re-
viewed by Wright et al. 1965). A properly
used piston corer can retrieve an undis-
turbed column of sediment and overlying
water with little disturbance of the sedi-
ment-water interface. These conditions are
necessary in studies of, for example, benthic
algal productivity, sediment microbial me-
tabolism, and solute diffusion. Large di-
ameter cores may produce less “edge effect”
and the increased sediment surface area can
be advantageous for many reasons. Sedi-
ments at great depth are often sampled with
large box corers which are maneuvered with
winches (Hessler and Jumars I974; Bouma
and Marshall 1964). However, no presently
available hand-held device can sample a
large surface area (>500 cm?) and volume
of sediment (and overlying water) without

' This work was supported by the U.S. Department
ofEncrgy (DE-AC02-76EV01599, COO-1599-245) and
National Institutes of Health, Division of Research
Resources (BRSG Grant 2—SO7 RR07049-15). Contri-
bution 546, W. K. Kellogg Biological Station of Mich-
igan State University.

disturbing the sediment profile or destroy-
ing conditions at the interface. We describe
here a box-corer sampling method, com-
pare oxygen microprofiles measured in lit-
toral sediments before (in situ) and after
sampling with the box corer, and present
data from pelagic sediments that were sam-
pled with the box corcr and maintained as
a microcosm. We thank N. P. Revsbech for
his tutelage in the skills and knowledge nec-
essary for construction of oxygen micro-
electrodes.

In the past several years knowledge con-
cerning the dynamics of dissolved oxygen
in freshwater and marine sediments has been
increased greatly, largely as a result ofwork
by Revsbech and others using oxygen—sen-
sitive microelectrodes (Revsbech I983;
Revsbech ct al. 1980, I981, I983; Reimers
et al. 1984). Some of this work was per-
formed in situ in very shallow water, but
sediments underlying deeper water were
sampled with corers and analyzed in a lab-
oratory. In situ measurements are impor-
tant, but often not possible. Further, it can
be advantageous or necessary to control en-
vironmental conditions. Therefbre, to en-
hance the versatility of oxygen microelec-
trodes and improve the efficiency of
obtaining accurate microprofile measure-
ments, we have developed the following
methodology.

The box corer is made of 6-mm-thick
acrylic plastic and has a flexible aluminum
lower door and a flat 3—mm—thick acrylic
upper door, each ofwhich slides in grooves
milled in the box walls (Fig. 1). The lower
tambour door is made of 0.5—mm alumi-
num flashing, strengthened with 1.6-mm-
thick aluminum crossbars. As this flexible
door is pushed downward in the grooves,
its leading edge is guided through the sed—
imcnts around the lower edge of the end
wall and across the bottom of the box. In-
side dimensions of the box corer are 25- X
28-cm horizontal by 42-cm vertical, giving
a total sample volume (sediment + water)

61

 

 

Perspective diagram of the box corer with
the upper and lower doors partially closed. Note the
protrusion of side walls on the left to accommodate
the vertical travel of the flexible, tambour door. The
support rod at the lower left maintains rigidity where
the door flexes.

Fig. l.

of >29 liters. The Meiostecher of Thiel
(1983) also uses a sliding flexible door, but
this unit has a cross—sectional area of <25
cm2 and was designed for subsampling large
box corers.

Our box corer is deployed with SCUBA;
while in the water the corer can be operated
and maneuvered by one person. A sample
is taken by pushing the open box the desired
distance into the sediment, sliding the top
door in to isolate the sample from distur-
bance, and then inserting the lower door
into the grooves and pushing it until the box
is sealed. The loaded eorer is transported in
a caddy, a watertight acrylic box that has
inside dimensions about 1 cm larger than
the box corer (Fig. 2). The caddy serves the
purposes ofredueing stress on the lower door
and not permitting air into the top of the
sample, which would allow sloshing and
aeration of the water during transport. To
facilitate later removal ofthe box corer from
the caddy, we secure two nylon straps with
100ps in the ends in the caddy before loading
the corer (Fig. 2). The mass of the sealed
caddy is about 65 kg. Two persons can easily
maneuver the unit on land after securing
two l—m-long handles to the side ofthe cad-

62

   
  
  

f.

.r VII 3*.4

~‘<.. .‘_‘ ‘L‘IT . -':

x corcr
held within the transport caddy (caddy lid removed).

dy (not shown). We also have made smaller
versions that can be lifted by one person
into a small boat.

For our purposes we transport the sealed
caddy to a laboratory and transfer the box
corer with a rope hoist (hooked through the
straps) to an aquarium partially filled with
lake water. As the box corer is lowered, the
water rises to the top ofthe aquarium. First,
the small amount of mud lying outside the
box above the curved portion of the lower
door (Fig. 2) is siphoned ofl". The bottom
door is then removed. We allow any slightly
disturbed external sediment to settle, re—
move the top door, and then insert a tight
fitting 42—cm—long sleeve (3-mm-thick
acrylic) into the top of the box corer (itself
a sleeve with the doors removed) and push
it to the bottom. The box corer is then with-
drawn vertically, without disturbance to the
sample, for later use. The resulting system
consists of a rectangular solid volume of
sediment, with its overlying water intact,
enclosed by a system in which environ-
mental conditions can be regulated.

Temperature in the systems was con-
trolled with an external circulating cooler
which was also used at times to simulate

 

 

 

 

 

 

E
e
V 4 r 1 1 1 1 1 r 1 1 1
D:
‘“ l
._
< 2'—
z
' L)
2~ ‘ j > 2
L 53h-
4 - x _.
%
A /D//a A
g 6 A *m snu a
e /
m /c(/
2 8 - r
_1
=1
z
e 10~ »
Z
‘5 ‘2 X/fr/ z ,
Q Q
"’ 14 2 7
/'/11
D
1 >—
16 I (7’ _.
av
[al/ 8 A
20 ,# 1 1 1 1 1 a 1 1
200 400 600 500 1000
"M 0:
Fig. 3. Comparison ofmicroprofrles ofoxygen con-

centration measured in situ (Lawrence Lake, Michigan;
water depth, 1.9 m) and in a box-corer sample of the
same sediment in the laboratory after 12 h ofdarkness
followed by 5.3 and 9.0 h ofillumination at 150 uEinst
m’2 5". Water temperature was 9.0°C in all cases.

water currents. Light was supplied from
either lluorescent (Luxor, natural light sim-
ulating) or mercury vaoor lamps, with neu-
tral density shadeclot11(Chicopee, Ball Seed
Co.) used to reduce light intensities to in
situ values. Light intensity at the sediment~
water interface was determined with a LiCor
model LI- 185 meter with a cosine quantum
sensor.

Microprofiles of dissolved oxygen were
measured with polarographic oxygen-sen—
sitive microelectrodes (Revsbech and Ward
1983) with an outer tip diameter <30 um
and a cathode diameter < 10 pm. This type
has an internal reference electrode, which
allows the unit to be moved from one so-
lution to another without depolarizing the
cathode. A Transidyne General chemical
microsensor supplied polarizing voltage
(—0.75 V) to the electrodes and displayed
the output signal. Calibration in the labo-
ratory was performed with volumes ofO and
100% oxygen—saturated lake water equal in
temperature to the system. Field calibration

63

 

A

      

D 24h (STIRRED)

/"i

 

—+—WATER(CENT1METER$)

   

5

0|

   

SEDlMENT (MILLIMETERS)

"1'ddi'sxfiairion"" ' "

 

 

 

l l l
200 300 400 500

11M 0.

Fig. 4. Microprofiles of oxygen concentration in
and above pelagic sediments sampled at 10-m water
depth in Lawrence Lake, Michigan, with the box corcr
and maintained in the laboratory. Water temperature
was 14°C for the upper curve, and l 1°C for the three
lower curves. Light intensity was 10 uEinst m" 5".
Note change in depth scale at sediment-water interface.

was accomplished by assuming 100% sat~
uration in the open water near the lake sur-
face, and taking‘ ‘”2ero as the asymptotic
minimum at the bottom ofthe oxygen mi-
croprofile. Readings of percent saturation
were later converted to pmol O2 liter‘I (Figs.
3 and

Because of their fragility, these micro-
electrodes must be positioned carefully us-
ing a micromanipulator. For in situ mea-
surements the micromanipulator had to be
submerged, stabilized on a framework above
the sediment, and operated by a SCUBA
diver; because of the difficulty of working
with microelectrodes while under water,
readings were taken at intervals of only 1.0
mm through the sediment.

For samples in laboratory systems the mi-
cromanipulator was mounted on a deck
above the water, and a glass rod with a mi-
croelectrode secured at the bottom was at-
tached to the micromanipulator. Measure-
ments were taken at 0.5- or 1.0-mm intervals
(we have easily made measurements at 0.2-
mm intervals). A watcrproofed Fiberscope
(American Optical) was used to view the

 

 

electrode tip position for alignment at the
sediment-water interface. Accuracy with this
method was _+_0.05 mm. We found this
alignment technique to be essential, partic-
ularly when positioning microelectrodes at
low light intensity or beneath epipelic lila-
mentous macroalgae, where the interface
could not be seen from outside the aquar-
ium. We could not locate the sediment~
water interface accurately by searching
downward with the electrode for the initial
change in oxygen concentration (the meth-
od described by Revsbech et al. 1980 and
Reimers et a1. 1984).

Data from Lawrence Lake, Michigan, lit-
toral sediments covered with a dense, 8—10—
mm-thick, algal-bacterial mat are presented
in Fig. 3 (water depth, 1.9 m). Oxygen mi-
croprofiles measured before (in situ) and af-
ter sampling with the box corer were sim—
ilar. The in situ measurements were made
on 26 April 1984, at 1400 hours, when ir-
radiance at the interface was 480 uEinst m‘2
5". Although this intcnu.ty could not be du-
plicated in the laboratory, a proximally lo-
cated mercury vapor lamp supplied 150
uEinst rn‘2 s"'; on 27 April, after 9.0 h at
this intensity the oxygen concentration 1.0
mm below the seditnent—water interface was
nearly equal to the observed in situ value
of928 11M 02. The lower oxygen production
and penetration into the sediment in the
laboratory system probably resulted from
insufficient irradiance at the interface and
hydrostatic pressure differences. Bubble for—
mation in the mat was observed in the lake
and the laboratory (note supersaturation in
surficial strata, Fig. 3), but this presented no
problems for microelectrode measurements
(cf. Revsbech and Jorgensen 1983).

Pelagic sediments at a depth of 10 m were
also sampled with the box corer. The epi-
pelic periphyton community on these sed-
iments consisted ofa thin film of diatoms
and microflagellates. In the laboratory, the
system was maintained at 1 l°—l4°C, with a
14:10 LD cycle at 10 pEinst m“2 s". Figure
4 shows the eflect of water currents above
the sediment and the relationship between
sediment oxygen distribution and quantum
flux to the sediment—water interface. Down-
ward transport of oxygen was accelerated
by water currents above the sediment (two

64

upper curves). Ten hours ofdarkness under
unstirred water produced an oxygen micro-
profile (not shown) similar to the curve la-
beled “D 28 h." The lower two curves show
the advance of oxygen into the sediment
through time when illuminated; these mi-
croprofiles show that sediments exposed to
even very low irradiance intensities can dai-
ly become supersaturated with oxygen and
nightly become anaerobic, except for the
upper 1—2 mm.

The box corer permits sampling of un-
consolidated'sediments with minimal dis-
turbance to the sediment-water interface and
the sediment profile. The sample is readily
held asa laboratory microcosm under the
original in situ or other controlled condi-
tions. The large sediment surface area pro-
vides ample space for the equipment nec-
essary for measuring oxygen microprofiles.
The technique should provide samples ap-
propriate for numerous applications, e.g.
studies ofbiogeochemical cycling and solute
flux, interactions among benthic popula-
tions, and periphyton productivity and nu-
trient dynamics.

Richard 0. Carlton
Robert G. Wetzel

W. K. Kellogg Biological Station
Michigan State University
Hickory Corners 49060

References

BOUMA, A. H. , AND N. F. MARSHALL. 1964. A meth-
od for obtaining and analysing undisturbed ocean-
ic sediment samples. Mar. Geol. 2: 81—99.

HESSLER, R. R., AND P. A. JUMARS. 1974. Abyssal
community analysis from replicate box cores in
the central North Pacific. Deep-Sea Res. 21: 185-
209.

REIMERS, C. E., S. KALuORN, S. R. EMERSON, AND K.
H. NEALSON. 1984. Oxygen consumption rates
in pelagic sediments from the Central Pacific: First
estimates from microelectrode profiles. Geochim.
Cosmochim. Acta 48: 903—910.

Revsnrcu, N. P. 1983. In situ measurement ofoxygen
profiles of sediments by use of oxygen microelec-
trodes, p. 265—273. In E. Gnaiger and H. Forstner
[eds], Polarographic oxygen sensors: Aquatic and
physiological applications. Springer.

,AND B. B. JoRGENSEN. 1983. Photosynthesis
of benthic microflora measured with high spatial
resolution by the oxygen microprofile method: Ca-
pabilities and limitations of the method. Limnol.
Oceanogr. 28: 749—756.

—, ~——~, T. H. BMCKBURN, AND Y. COHEN.

 

 

 

 

 

 

65
I983. Microelectrode studies ofthe photosynthe- position: Use in an acid microbial mat dominated
sis and 0,, H18, and pH profiles of a microbial by Cyanidium caldarium. Appl. Environ. Micro-
mat. Limnol. Oceanogr. 28. 1062— 1074. biol.45: 755—759.
-—-, ND O.BR1x. 1981. Primary pro- T111151, H. 1983. Meiobenthos and nanobenthos of
duction of microalgae 1n sediments measured by the deep sea, p. 167—230. In G. T. Rowe [ed.],
oxygen microprofile, H"CO, fixation, and oxy- The sea, v. 8. Wiley-lntersciencc.

gen exchange methods. Limnol. Oceanogr. 26: 717— WRIGHT, H. E., E. J. CUSHrNo, AND D. A. LIVINGSTONE.
730. 1965. Coring devices for lake sediments, p. 494—
, J. SoRENssN, T. H. BLACKBURN, AND J. P. 520. In B. Kummel and D. Raup [cds.], Handbook
LOMMOLT. 1980. Distribution of oxygen in ma- of paleontological techniques. Freeman.

rine sediments measured with microelectrodes.

Limnol. Oceanogr. 25: 403-411 .

, AND D. M. WARD. 1983. Oxygen microelec- Submuted: 28 December 1983
trode that is insensitive to medium chemical com- Accepted: 10 October 1984

 

CHAPTER IV

SYMBIOSIS BETWEEN SALAMANDER EGGS AND GREEN ALGAE:

MICROELECTRODE MEASUREMENTS INSIDE EGGS DEMONSTRATE

EFFECT OF PHOTOSYNTHESIS ON OXYGEN CONCENTRATIONS

66

 

 

Symbiosis between salamander eggs and green algae: microelectrode measurements
inside eggs demonstrate effect of photosynthesis on oxygen concentration1

b’lARILYN D. BActtMANN
Dcpurlmrnt(If/111111101Ecology, Iowa Slate Unicorn/y. Ann's, IA, U.S./1. 5001/
AND
RK ”Aim (1. CArtr,r0N, JoANN M. lionKtrot.oF.11. AND RonERT G. WErzEL
W. K. Kellogg Biological Slution. Michigan State University, Hickory Corners, MI, U.S.A. 49060
Received December 17, 1985

BActtMANN. M. D., R. G. CARLION, J. M. BURKMOlDER, andR. G. WETZEL I986. Symbiosis between salarnandercggs and
greenJ algae. microelectrode measurements inside eggs demonstrate effect of photosynthesis on oxygen concentration.
Can 1.:64 IS 6—1583.

E 501' the ospotted s. lamander. Ambvrlloma Imrculalllm are usually green because ofs the presence of symbiotic,
chlamydomonad algae wha h inhabit , reac egg. . '1 “3'; M. sed to measure the effect of
algal photosynthesis on oxygen concentration inside eggs and within the gelatinous matrix surrounding them During darkness,
oxygen became severely depleted within the eggs but upon exposure to light oxygen concentrations increased rapidly.
Photosynthetic oxygen produ cution by the chlamydomonads exceeded respiratory consumption by the em ryo—algae complex
and led to oxygen supersaturation inside eggs. even when water surrounding the egg mass was almost anoxi c.

 

BActtMANN, M. D., R. G. CARLTON, J. M. BURKMOLDER at R. G. WETZEL. 1986. Symbiosis between salamander eggs and
green algae: microelectrode measurements inside eggs demonstrate effect of photosynthesis on oxygen concentration.
Can. J. Zool. 64: 1586-1588

Les oeufs de la salarnandre Ambysmnrrr nrnrnlrrlnnr sont ordinairement rendtts verts par la présence d'algues chlamydomo-
nades syrnbiotes qui occupent l'enveloppe de chaque oeuf. Une electrode sensible :1 l'oxygene a servi 21 mesurer les effets de la

photosyntltese des algues sur la concentration d'oxygcnc :1 l'intérienr dcs oeufs et dans la matrice gelatineuse qui les entoure. A

l'obscurite, in concentration d'oxygene contenuc diins les oeufs baisse considémblcment, mais de retour 1 la lunriere. ellc

augmente de nouvcau rlapidcnient La production d oxygcne due .1 la photosynthcsc par les algucs exccde la consommation

 

entourc la masse d'oeufs devient prcsque anoxique.

Introduction

The eggs of the spotted salamander, Ambystoma mncn/nnmr
Shaw (Cattdata: Ambystomatidae), usually are bright green
because of the chlamydomonad algae inhabiting the middle
layers of each egg envelope. First reported by Orr (1888), this
unusual symbiotic association between green algae and the eggs
of a vertebrate was shown by Gilbert (1942, 1944) to result in
lower mortality and faster development ofembryos and in larger
hatchling size than embryos developing without algae. We
demonstrate here in contradiction to an earlier report of
Hutchinson and Hammett (1958), that photosynthetic oxygen
production by the algal syrnbionts exceeds the total respiratory
demand within the egg mass, even when the surrounding water
is nearly anoxic.

[Eggs of A. mnc'nlutmn are often found in shallow woodland
ponds which are hypoxic because of microbial metabolism of
deposits of leaves and other detritus (Collins and Wilbur 1979).
Because premetamorphic A. mnculultmr larvae are oxygen
conformers. their metabolic rates decline and development
slows under reduced oxygen tension (Branch and Taylor 1977).
Therefore, embryos developing in hypoxic environments could
bcnclrt from an association with algae capable of photosynthe-
siririg at rates sufficient to increase oxygen levels within the
egg. 111 this study we used an oxygen—sensitive microelectrode
to directly measure oxygen concentration ([02!) within eggs and
their surrounding gelatinous matrix. Experiments were per-
formed with and without light and with manipulation of the
degree of oxygenation of surrounding water.

 

IContribution No. 560. W. K. Kellogg Biological Station, Michigan
State University, and No. 70. College of Agriculture, lowa State
Universrty.

Printed in (Laud-1 lrnpnme In (mid.

'intérieur des oeufs méme lorsquel' eau qui

[Traduit par la revue]

Materials and methods

Egg masses with late-term embryos of A. nmrnlalmn were collected
in May from a woodland pond in southeastern lower Michigan and
maintained in aquaria containing continuously aerated well water at
l5—l7°C. During the 10 days before the experiment the eggs received
indirect sunlight and full-spectrum fluorescent light for approximately
12 h da “'

To determine algal cell density three eggs were randomly selected
and their embryos were excised. The egg envelopes were separately
ground using a loose-fitting tissue grinder and the suspensions were
settled using the inverted microscopy method of Ulcrmohl (Lund et al.
I958) ()ne field- width strip per chamber was counted at 1.600x Two
perpendicular strips were counted in one chamber to determine
uniformity of sell in

icros scale measurements of oxygen concentration were made with
a polarogtaplric oxy-gen sensitive microelectrode (internal reference
style; Revsbech and Ward l983) with an outer tip diameter of 30 um
and a cathode diameter of7 am. This electrode experienced drift less
111111137011", had a 95% response time of <2 5 for a 200 11M change in
[01]. and was insensitive to water currents. A Transidync General
chemical microsensor supplied polarizing voltage (-0.75 V) to the
electrode, and displayed the output signal. Calibration was accomp-
lished using volumes of() and 100% air-saturated water at temperatures
equal to those of the experimental systems. The fragile microelectrode
was positioned with a micromanipulator. Unless stated otherwise,
measurements were made at l7— |9"C in eggs containing live embryos.
Aerated systems were circulated by air from an air stone

n egg mass containing ca. ()0 eggs at Harrison stage 40 (near
hatching) was immersed 111 100 mL of air saturated water in a 250 mL
beaker and shielded from light for 311.11 17°C without further aeration.
The initial measurements in and around the eggs were then made under
very dim light (5- 10 1113 m‘zs ') which was unavoidable Following
these measurements, the electrode tip was situated in the center of an
egg located approximately 3 mm inside the outer edge of the gelatinous
matrix and the system was illuminated. A mercury vapor lamp supplied

67

 

 

 

 

i

FIG. 1 Oxygen concentration (micromolles of 02 per litre) itr and
...

 

at 17°C. 100%A air saturation= 302 MM 01.

105 1113 111—2 s'I of photosynthetically available radiation (approxi-
mately 8% of full sunlight in summer) as measured by a Licor quantum
photometer with a cosine sensor. After [02] stabilized at a maximum
within the eggs the system was darkened arid rrtotritorcd for another
hour. Latcrthc sanre egg mass was transferred to 11 4- L container which
was contr 11111111st aerated and mixed 'lhsi. system was darkened for3b
after u hrcli the initial [02] measurements were made in total darkness.
To provide comparative data, even though we lacked algae-free
clutches ofA. macalatum, we measured [01] in similar-sized eggs ol~
Amlnxrtmnn Iigrinrun (a closely related species which olterr coexists
with A. Inuculrtlum; Collins and Wilbur I979), which did not possess
an algal symbiorrt. Ambystoma Iigrinnnr adults collected in southwest~
errr lower Michigan were allowe to mate and the egg rrrasses were
reared in the laboratory under the sarrre conditions as tl1ccggs sof A.
Illiltllltlllllll. An A (igrinnnr egg mass containing ca 40 embryos at
Harrison stage 40 was held rn darkness 1n continuously aerated w. rter
for 3 l1, and then used for [02] measurements as for A. llltlt'ltltlllltll.

Results

'l he algae were identified as a species of CIr/rmtytIonronus.
During the experiments an average egg contained ca. 8. 001
algal cells. lli- and quadri-tlagellate round and ovoid cells
ranged from 120 to 900111n‘biovolume. Round vegetative cells
averaged 3600 11m". Algal population density per egg averaged
5.39 X 105 cells cm '2 with an average egg surface area of 1.54
c1112.

Oxygen concentration in watersurrounding the A. muculritrun
egg mass was initially in equilibrium with the atmosphere (302

M ()2), bttt rrretabolic activity during the 3—h dark period
produced an IOII gradient in the unstirred system between the
water surface (172 MW) arid an egg within the gelatinous matrix
(52 11/11) (Fig. 1). Mean [01] in two eggs at this point was 57
11M, bill this value was slightly elevated (as are the initial points
on curve A, Fig. 3) as 11 result ot'sliglrt photosynthesis from the
unavoidable ditu liglrt. Subsequent exposure of the egg mass to
light produced an increase in [02] within an egg capsule from (13
to 4711 tall! in 32 111111 (Fig. 3 curve A). One hour after
eliminating the light, [02] within the. egg w;rs‘)11.M, while [02]
in the gelatinous matrix and surrounding water were 1(101111d 91
11M, respectively.

Later, after 3 l1 ofdat‘kncss in continuously aerated water, the
concentration oft); inside four e111bryovoccnpied eggs averaged
31 i 14 11M (mean : SD). A strong gradient oi decreasing [02]
(110111 291) to <51) 11/”) again existed between the open water
and the interior of eggs (Fig. 2), except that in this case [02]
values within the gelatinous matrix were considerably higher

68

 

1 10cm J
\\

\V
FIG. 2. Oxygen concentration (micromoles of 02 per litre) in and

I, nun-u;

 

  
     
 

 

 

water at 19°C. 100% air saturation = 290 1.1M 02.
"V” I I l l
LIGHT OFF (A) B
500 _ a
.fe...

11
6
E
3‘

100% SATURAttoN —
2
Id
£9
>-
X

0 ~.
.~ _
.-
.‘ A
..I.
v‘
5.-
“I
\
.1
U l l l l “
20 40 60 80 t o

 

TIME (min)

3.0xygen concentration (micromoles of 01 per litre) over time
w itliiGn illuminated eggs of A. mucululrmt after 3 h ofdarkness. Light on
at time zero. Cu urve A: egg mass in hypoxic water (11g. 1) at 17°C.
Light turned offafter 34 111111 elapsed time. Curve B: same egg mass in
continuously aerated water .11 l9“C. The steps in curve B were caused
by tail movements by the errrbryo which mixed and broke down the
oxygen gradient 111 the egg fluid

than in the previous unaerated system (Fig. 1). During 64 nrin of
illumination, [02] inside one egg increased from 34 to 540 11/1!
(Fig. 3, curve B).

The mean [021m A. Iigtinmn eggs was [021' 1511M after 3 h
of darkness in aerated water. Exposure to light produced no
change 111 102]. in this study and another (Anderson et al. [971),
algae were found only in dead A. Iigrr'nrmr eggs. Insertion ofthe
microelectrode through egg capsules of A. tigriimm was
considerably nrore difficult than for A. maculalrun and eventu-
ally resulted in breakage of the electrode tip.

 

 

Discussion
Our data show that photosynthetic oxygen production by the
chlamydomonads in tie egg envelope exceeded the total
respiratmy demand of the egg—algae complex, Oxygen concen<
trations increased rapidly inside eggs when they were exposed
to light, even when the surrounding water was hypoxic (Fig. 3,
curve B). This result contradicts the assumptions and conclu-
sions of Hutchinson and llammen (I958), who reported that
there was no surplus of photosynthetically produced oxygen
available to the embryo and that oxygen could not be responsible
for the benefits to the embryo reported by others (e.g., Gilbert
I942, I944)
Premetamorphic ambystornatids are oxygen conformers,
c , their respiration rate varies directly with [02]. yet they are
tolerant of temporarily hypoxic conditions. Adolph (I979)
reported that A. macalatum embryos lived several hours without
oxygen and up to 24 h at low oxygen partial pressures
(equivalent to <50 11M 02 in our study). Adolph (I979) also
presented data that indicated that A. macalatum may be more
tolerant of anoxia than A. Iigrimmr. In our study, [02] in A.
(r'grr'rrrmr eggs was approximately three times greater than [02]
in A. macalatum eggs after 3 It of darkness in aerated water.
This difference could have been a result of different numbers
and metabolic rates of embryos, respiration by algae, or
differences in diffusive characteristics of the egg envelopes 11nd
gelatinous matrices. Ilowever, given the conclusions ofAdolpb
(I979) this difference may be moot in terms ofembryo survival,
although of interest from an evolutionary perspective. (Ion-
versely, the difference in [Ozl between A. IIIttt‘u/rlltlm and A.
rigrinrrm after illumination may be important. The ability of
oxygen conformers to increase oxygen consumption and meta-
bolic rate at higher [02] may confer an advantage to A.
mucululrrm in illuminated, hypoxic envirormtcnts.
any animal species exist in hypoxic environments. but few
are exposed to hyperoxia such as that occurring in A. Irrrrr'ulurrrnr
eggs; even fewer experience the diurnal changes in l 2]
depicted 111 Fig. 3. The relationship between the algae 11nd
embryos is undoubtedly more complex than is presently
realized. Further research could investigate the function of pll
inside eggs which may vary because of photosynthetically
induced changes in II' and Oil", or the possibility that the
gelatinous matrix acts as an oxygen storage capacitor for A.
"rum/11111111 embryos. Obviously much more c1111 be accomp-

69

lished using microelectrodes, which are extremely versatile
and appropriate for field measurements (Carlton and Wetzel
1985)

Acknowledgements

We are grateful to A. J. Johnson for technical assistance, and
.I. Petranka, D. J. Hall, V. H. Hutchinson, C. R Goldman, and
anonymous reviewers for helpful comments 011 the manuscript.
This research was supported in part by the U.S. Department of
Energy (EY-76-S-02-1599, COO-l599-245).

ADot PH E. F. I979. Development of dependence on oxygen in em-
bryo salamandcrs. Am. J. Phy siol. 236: R282—R29I.

ANDERSON. J. D. D. D. IIASSINGER, nndG. H. Dxutmr'uz. I97].
Natural mortality of eggs and larvae of Ambystoma rr‘grr'rrrrm.
Lcology, 521CIIO7~ III7..

BRANC11,L. 11 II. TAYLOR. I977. Physiological and
behavioml responses of larval spotted salamanders (Ambystoma
n111c1rlrrrum)to various concentrations ofoxygen. Comp. Biocltern.
Physiol. A, 58; 269—274.

CARLTON, R. IG.. and R. G. Werch. I985. Abox- eorer for studyrng

nsitu

 

conditions. Limnol. Oceanogr. 30: 42 2—4

COLLINS, J. P., H. M. WthUR. I979. BreeGdinghabits and habitats
of the amphibians of the Edwin S. George Reserve. Michigan, with
notes on the local distribution of fishes. Occas. Pap. Mus. Zool.
Univ. Mich. 686: 4.

Git LII r. T. P. W. I942. Observations on the eggs of Ambystoma
Inrri‘ulmrtm with especial reterence to the green algae found within
the egg envelopes. Ecology. 23: 2l5—227.

-——l 9.44 The alga— egg relationship in Arrrbyxlmna manila/11m, a
case of symbiosis. Ecology. 25: 366 —369.

Ilu1r111Nsor~1, V. II, d.C S. HAMMEN. I958. Oxygen utrlrratron 111
the symbiosis of e1i1bi1y os ofthe salamander. Amlnrlorim "1111' rrlulrurr
and the alga. 0017/11/11rmtblystwttulix. Biol. Bull. (Woods Hole.
Mass), “5:4 43 489

LUND, J. W. G., C. KIPI.ING, and E. D. LECREN. I958. 'Ihc inverted
microscope method of estimating algal numbers and the statistical
basis ol estimations by counting. Hydrobiologia, ll: 43—.l70

ORR. I.I I888 Noel on the development of amphibians. chiefly
concerning the central nervous system;w with addi tio nal observations
on the hypopl11ysis, mouth, and the appendages and skeleton of the
head Q. J. c.Sei. New Ser. 219: 29 5— 324

Rsvsriscn. N. PI ,and I). M WARD. 93. Oxygen microelectrode
that rs insensitive to medium chemicail composition: use in an aci
microbial 1n11t dominated by Cymrrduurr caldarium. Appl. Environ.
Microbiol 45.7

 

 

CHAPTER V

SYNOPSIS

7O

 

 

71

Although periphyton is widely recognized as an essential component
of primary production in streams, it has historically been relegated a
minor role in the metabolism of lakes. Over twenty years ago Wetzel
(1964) showed that periphyton in a shallow lake supplied the majority
of primary production on an annual basis. Few additional quantitative
studies exist, but in lakes with low nutrient content and high water
transparency (i.e. those in which much of the basin lies within the
euphotic zone) periphyton can contribute significantly to whole—lake
primary production.

Relative to the plankton, the ecology of periphyton is poorly
understood. Traditional studies evaluating ecological interactions of
the planktonic community such as competition for nutrients or light are
lacking for lake periphyton. Most research on lake periphyton has
simply attempted to enumerate the algal species composition on
substrata that could be easily sampled from the surface of the water A
recent opinion of some limnologists is that lakes classified as
oligotrophic based on the water transparency and the areal chlorophyll
and nutrient concentrations of the water column may actually be closer
to mesotrophic when the biomass and primary productivity of the
epipelic algae are considered.

Perhaps most enigmatic is the nearly total lack of information
about the nutrient dynamics of epipelic periphyton. Though it is
widely recognized that lake sediments are responsible for internal
loading of nutrients, no published research has attempted to elucidate
the role of sediments in the nutrient dynamics of epipelic periphyton.
Carlton (1981) showed that epipelic algae of the littoral of Castle

Lake, California, had the potential to absorb all of the ammonium

 

72
diffusing from subtending sediment. Other studies in the same lake
demonstrated high nitrogen fixing capability of epilithic periphyton
(Reuter et al. 1986) and the mediation of nitrification and
denitrification in sediments by epipelic algal oxygen production
(Paulsen 1986).

The development of microelectrode technology for environmental
research by N. P. Revsbech (reviewed by Revsbech and Jdrgensen 1986)
has enabled scientists for the first time to make direct measurements
of the distribution of oxygen in periphyton and sediments (as well as
in many other microenvironments and ecosystems e.g., salamander eggs,
rice soils, foraminiferans, insect haemolymph and gut tracts, leaf
packs, and partially decomposed, water—logged wood). By making
measurements through time, while manipulating light intensity and
oxygen concentration in the overlying water, one can determine rates of
photosynthesis and respiration in periphyton and sediments (methods
described in Chapter I). Use of these techniques in the initial phase
of this study demonstrated that periphyton communities of all types
were extremely dynamic with respect to oxygen. Even under low light

2 s—1) a thin layer of diatoms on the

intensities (e.g., 1O uEinst m—
surface of sediments generated sufficient oxygen to increase the
penetration depth of oxygen from < 1 mm to > 6 mm into the sediment and
produce oxygen supersaturation near the sediment—water interface.
Because of the difficulty of working in situ with microelectrodes,
the box corer/microcosm system (presented in Chapter IV) was designed.
This apparatus allowed manipulation of several relevent environmental

factors and gave ready access to a large surface area of undisturbed

sediment. Up to twelve box corer samples were maintained as microcosms

 

 

73
under simulated environmental conditions. The extensive documentation
of the daily formation and breakdown of the oxidized microzone at the
surface of Lawrence Lake sediments led to a working hypothesis:
If the seasonal formation and breakdown of the oxidized
microzone results in respective decreases and increases in
the flux of phosphorus from lake sediments, then perhaps the
diel oxygen dynamics resulting from epipelic algal
photosynthesis can cause a diel fluctuation in the efflux
rate of phosphorus.
This hypothesis was investigated using sediments from Lawrence Lake
(Chapter III), with the result that epipelic algal photosynthesis did
indeed mediate the efflux rate of phosphorus on a diel basis. An in
situ experiment in Lawrence Lake that compared the distribution of
phosphate in sediment interstitial water inside and outside shaded plots
demonstrated that oxidation of the surficial sediment microzone also
resulted in long—term conservation of phosphorus in sediment. This
result implies that the presence of a photosynthetically active epipelic
periphyton community may be advantageous by reduction of internal
loading to the overlying water column. Furthermore, the phenomenon may
be important in maintaining oligotrophic conditions in the water column
of transparent lakes. In essence, a positive feedback mechanism is
invoked: nutrients regenerated in sediments are conserved there either
by utilization by the periphyton or some metabolically induced
immobilizing process, with the result that phytoplankton production is
reduced and water transparency is maintained, thus allowing sufficient
light to continue to reach the sediment surface and support the epipelic
community.
For the feedback mechanism to continue to operate as a regulator of

internal loading, the external nutrient loading must remain low.

 

 

74
Otherwise, phytoplankton production and biomass increase and water
column transparency decreases, short—circuiting the feedback mechanism.
Human activities in drainage basins have resulted in increased external
loading to thousands of lakes, and we may never know how important
epipelic periphyton was to the maintenance of oligotrophic lakes. Few
lakes exist that have not been adversely affected by humankind.
However, if this feedback mechanism can be substantiated in some of the
remaining pristine lakes, perhaps a new awareness of the delicacy of

lake ecosystems will emerge.

 

 

 

 

LIST OF REFERENCES

 

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