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ABSTRACT

THE PARTICIPATION OF SUBMERSED VASCULAR HYDROPHYTES IN THE
PHOSPHORUS BUDGET OF A WASTE STABILIZATION LAGOON

By

David Craig Mahan

In Pond h of the Belding, Michigan wastewater system submersed
vascular hydrophytes aid in the removal of phosphorus from the water
during the growing season of the plants. Through their uptake of this
element the hydrophytes contained an estimated h0% of the phosphorus
trapped by the pond at the time of their maximum biomass on August 15.
As a result of their decline in biomass and release of phosphorus
after this date, they contained only 20% of the total trapped phosphorus
when the study was terminated in early fall.

The vascular hydrophyte community present during this study was
dominated by Ceratophyllum demersum. This and the other plant species
that were present showed luxury consumption of phosphorus, with levels
in the plant tissues generally over 1.0% of the ash-free dry weight.

Due to their active growth and resulting large biomass, the
macrophytes influenced the water chemistry of the pond. The oxidizing
environment that they promoted enhanced the precipitation of inorganic
phosphates. The presence of the plants also served to physically
stabilize the water column which would promote sedimentation of both
inorganic and organic phosphorus containing materials. Sedimentation

accounted for 80% of the phosphorus removal during this study.

THE PARTICIPATION OF SUBMERSED VASCULAR HYDROPHYTES IN THE
PHOSPHORUS BUDGET OF A WASTE STABILIZATION LAGOON

BY

David Craig Mahan

A THESIS

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

MASTER OF SCIENCE

Department of Fisheries and Wildlife

197u

‘.
~
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l'

“3

ACKNOWLEDGMENTS

I would like to thank Dr. C. D. McNabb for serving as my major
professor and for his continued assistance throughout this project.
I especially appreciate his sincere desire to help me grow, personally
and professionally.

The help of Drs. N. R. Kevern and F. M. D'Itri in serving on my
graduate committee and in reviewing this thesis is also appreciated.

It is almost impossible to single out specific graduate students.
for recognition. I am very thankful for the friendship and help
that many in this department have provided. Special thanks must go
to Doug Bulthuis, John Craig, Bob Glandon, Jane Kotenko, Jerry Lisiecki,
and Fred Payne who assisted me in many ways on this project.

I would espcially like to thank my wife, Barb, for doing so much
to make the last two years pleasant ones.

The work upon which this thesis is based was supported in part
by funds provided by the E.P.A.-O.W.P. Training Grant WP-26h and
T—900331, the Michigan Agricultural Experiment Station at Michigan
State University, and the United States Department of Interior,

Office of Water Resources Research, Grant A-073-MICH.

ii

TABLE OF CONTENTS

INTRODUCTION .

DESCRIPTION OF STUDY SITE.

METHODS. . . . . . . . . .

RESULTS. . . .

DISCUSSION .

CONCLUSIONS. . . .

BIBLIOGRAPHY . . . . . .

APPENDIX . . . . . . . .

iii

10
13
35
h?
1:9
53

Table

LIST OF TABLES

Hydrologic data (values x 103 liters) for Pond h
of the Belding wastewater system during the growing
season of 1973.

Summary of chemical and physical measurements taken
on nine twenty-four hour surveys of Pond h, June-
September, 1973.

Biomass and net productivity (ash free dry weight)
of vascular hydrophytes in Pond h as estimated from
curves fitted to the hydrophyte sampling data.

Gross primary production for vascular hydrophytes
and phytOplankton as determined with plexiglass

enclosures.

Phosphorus concentrations (mg/l as P) as taken from
curves fitted to the data of Table A—3.

Total quantity of phosphorus in the vascular hydro—

phytes of Pond h as derived from curves fitted to the

data for ash—free biomass and tissue concentration
(Table A-5) (also given) for dates of sampling in
1973.

Phosphorus budget (in Kg P) for Pond h established
with values from curves fitted to the data for
phosphorus levels in the water and vascular hydro-

phytes.

Cumulative totals of differences (in Kg P) from
TmfleS.

Coefficients of partitioning of phosphorus in
Pond h.

Biomass of vascular hydrophytes.

Summary of chemical and physical measurements in
plexiglass enclosures on the dates shown in Table 2.
The dawn values are shown first followed by those

at dusk.

iv

1h

18

2O

32

33

53
Sh

Table

A-3

Total phosphorus and dissolved orthophosphate con-
centrations (both as mg/l P) in the water expressed
as three-point means.

Total quantities of water and phosphorus flowing in
and out of Pond h over the time periods shown and
total pond volume and phosphorus in the pond on the
given dates.

Phosphorus as percent of the ash-free dry weight in
the vascular hydrOphytes.

Percentage ash of dry—weight in the vascular hydro-
phytes.

Y—intercept and coefficients fOr X-variable to n-
powers of polynomial regression curves fit to growing
season data (6/6-9/26) for components of Pond h.

Ranked means and results of Duncan's multiple range
test (p=0.05) for biomass measurements and phos-
phorus in vascular hydrophytes.

57

58

59

61

LIST OF FIGURES

The system of ponds receiving untreated wastewater
from the city of Belding, Michigan.

Rates of growth for Ceratophyllum demersum in Pond
h of the Belding system.

Regression of tissue-P on ambient concentration of
orthophosphate for Ceratophyllum demersum during
growth.

Regression of tissue-P on ambient concentration of
orthophosphate for Lemna minor (*) and Potamqgeton
spp. (0) during growth.

 

Biomass (o) of the plant community and total amount
of phosphorus (u) held by the community during the
growing season.

vi

16

2h

26

INTRODUCTION

In fresh waters that have not experienced gross pollution phosphorus
is often the limiting factor for aquatic productivity (Bahr, et al.,
1973; Vollenweider, 1968). This is due to its relatively high
requirement by living things in relation to its low abundance in the
environment. Man concentrates large amounts of phosphorus as a
result of his cultural activities with much of this often dispersed
into natural waters. The release of phosphorus into the aquatic
environment has resulted not only in a loss of this valuable element
to non-available storage but also in a deterioration of water quality.
Today, this excessive fertilization of natural waters is one of the
most significant causes of water quality problems in North America
(Lee, 1970).

Various schemes are now being implemented to reduce this cultural
enrichment, an example being waste stabilization ponds. Tertiary
waste stabilization ponds are designed to function as manageable sinks
for the objectionable material found in municipal effluents (McNabb and
Tierney, 1972). These systems usually consist of a series of cells,
progressing from anaerobic to facultative to aerdbic units. The
aerobic final cells are often dominated by various species of vascular
hydrophytes with these plants having considerable influence on the
physical—chemical milieu of the cell.

One of the objectives of these pond systems is to reduce the

amount of phosphorus contained in the waste water. This reduction

1

takes place mainly through the sedimentation of phosphorus containing
organic material and inorganic chemical compounds and complexes and
the accumulation of phosphorus in vascular plant tissues as a result
of orthophosphate uptake by epidermal foliage and by the roots. In
general, sediments serve as a phosphorus sink with the net flux of
phosphorus from the water to the sediments (Lee, 1973). The overall
effect of the plants on phosphorus in the water, however, is not so
well known.

The primary objective of this study was to determine the degree
to which submersed vascular hydrophytes participate in the phosphorus
budget of an aerobic waste stabilization pond during the summer

growing season.

DESCRIPTION OF STUDY SITE

This study was carried out in the fourth pond of a five pond
series of waste stabilization lagoons located in the west-central
Michigan community of Belding (Figure l). Belding is a residential
city of 5,000, discharging domestic and industrial waste to the
treatment system. The cells in the series can be characterized as
anaerobic (Pond l), facultative (Ponds 2 and 3), and aerobic (Ponds
h and S) (Bulthuis, 1973). A flow of water through the system is
promoted during the growing season by irrigation of adjacent land
from Pond 5, and from the loss of water by seepage through the bottom
of Pond 5 (Annett, et al., 1973). The ponds are ordinarily drawn
down in the spring and fall for storage in subsequent months. This
outflow is discharged to the nearby Flat River.

Pond A has an average surface area of approximately 3.0 hectares
and a working depth of 2.0 meters. The sediment is composed of
approximately 10 centimeters or less of organic matter underlain with
native clay. The pond is dominated by large growths of submersed
vascular plants with only occasional algal blooms occurring. The
dominant hydrophyte species during the summer of 1973 was Ceratophyllum

demersum. Potamogeton foliosus, Potamogeton berchtoldii, and Lemna

 

 

minor were also present in relative abundances that were quantified.

Three other species, Potamogeton zosteriformis, Lemna trisulca, and

 

Cladophora fracta (a filamentous Chlorophycean alga) were occasionally

 

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The water budget from this study is shown in Table 1. From the
seasonal totals it can be seen that water entered the pond from the
Pond 3 outflow and by rainfall. Pond 3 effluent provided 95% of the
incoming water with rainfall being a relatively minor contribution.
Seepage and transpiration accounted for the changes in pond volume
not explained by the flow and rainfall data with the net result being
a small loss from the pond due to these factors. Thus, it would appear
that Pond h is well sealed at the bottom. This is well documented
by the close agreement of the net inputs and outputs to the pond as
shown in Table 1.

In looking over the hydrologic budget the most noticeable trend
is one of uniformity. Average inflow was 12.2 million liters per
week with a pond volume replacement time of h.5 weeks. The outflow
averaged 11.7 million liters per week, 96% of the inflow. Flow rates
remained quite constant during the study, being somewhat higher during
the first four weeks, probably the end of spring runoff. Since Belding
is a residential city and the flow into Pond h is moderated by the
earlier ponds, this rather constant flow might be expected.

A summary of the chemical and physical parameters that were
monitored in this study is given in Table 2. Water temperatures
responded rather quickly to air temperature changes though differences
between surface and deeper water temperatures were quite small. As
would be expected at this latitude, lower temperatures of the range
were characteristic of the May and late September sampling dates
while higher temperatures generally occurred in mid—June through
early September.

The hydrogen ion concentration exhibited large fluctuations in

the daily and seasonal readings. The median values show that pH levels

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Table 2. Summary of chemical and physical measurements taken on nine
twenty-four hour surveys of Pond h, June-September, 1973.
Depth Mean Range Mean Range
Temperature (°C)

uAMb hPMC
Surface 22.3 15.0-26.5 2h.1 16.5-28.0
1.0 m 22.0 15.5-2h.2 23.1 16.3-26.0
Bottom 21.1 15.3-2h.2 21.6 15.3-25.0

pH

hAMb hPMC
Surface .1 7.3-9.7 9.6 8.5-9.9
1.0 m .o 6.3-9.6 9 6 8.5-9.7 ,
Bottom .5 6.7-8.8 8. 7.2-9.3

Dissolved Oxygen (mg/1)

8AMb 8PM°
Surface 8.7 5.2-12.h 12.h 8.h-17.8
1.0 m. 8.0 5.2-12.h 12.2 8.h-l6.0
Bottom .9 0.2-5.h 2. 0.8-h.8

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8AMb 12 Noonc
Surface 28h 102-51h h83 292-7hh
1.0 m 223 26-h63 h28 226-566
Bottom 86 -96-367 227 67-h02

 

8‘The pH values are medians rather than means.

bLowest values occurred most consistently at this time.

cHighest values occurred most consistently at this time.

8

were quite high which in natural waters is often indicative of a
large plant biomass (Ruttner, 1963).

As with pH, the level of oxygen in Pond h was usually influenced
by photosynthesis and respiration. During the first month of the
study when plant biomass was relatively small, diurnal oxygen
fluctuations at a given depth were not nearly as pronounced as those
occurring later in the summer. The very high, supersaturated oxygen
levels characteristic of mid summer were not present then either.

Redox potentials also experienced diurnal fluctuations due to
the changes in pH and dissolved oxygen. With the exception of the
sediment interface, redox measurements were generally above 200 mV,
the critical level for the resolubilizing of the iron-phosphates
(Graetz, et al., 1973).

Light intensity had a mean seasonal value of 75% and 13% of the
surface light at 0.25 meter and 2.0 meters respectively. At 0.25 meter
the percent light remaining was fairly stable throughout the season,
however, at 2.0 meters light underwent a general decline after mid—June
and was reduced to less than 20% remaining after late July. This
reduction of light during the last two months of the study was a

result of the large hydrophyte biomass and a bloom of Microcystis s2,

 

which took place.

Aerobic lagoons are characterized by oxidation of organic materials
and nutrients with this oxidizing environment maintained by the photo-
synthetic activities of the plants (Cooper, 1966). In view of the
results in Table 2, this pond can be classified as an aerdbic, oxidizing
environment for the period of the study. Reducing conditions were con-

sistently approached only at the sediment interface with low dissolved

oxygen and redox potential levels occurring for just a short time

during the day.

METHODS

This study took place over an eighteen week period from'May 23,
1973 to September 27, 1973 with nineteen sampling trips being made
to the study site. Nine of the sampling trips were twenty—four hour
sampling surveys and took place on the dates shown in Table 1. These
twenty-four hour trips consisted of: 1) measurements of dissolved
oxygen, pH, redox potential, temperature, and light extinction;

2) water and plant samples; 3) measurements of flux of water in the
pond. Water chemistry and temperature profiles were done every four
hours, beginning on Wednesday at noon and ending Thursday at noon.
Flow measurements and water samples of the influent and effluent of
the pond were also taken at these four hour intervals. Sampling of
the pond water and the macrophytes was done on Thursday afternoon just
before returning to the laboratory. The remaining ten sampling trips
took place on alternate Thursdays in this eighteen week period and
consisted of the above mentioned measurements and samples with the
exception of the macrophyte sampling.

Temperature and oxygen were measured with a Yellow Springs
Instrument Model 51 automatic compensating prdbe. Redox potential
and pH values were determined with a Beckman Model SBL and SEX,
respectively. Flow—rate measurements were made with a Gurley current
meter. Volume of input and output was calculated from the flow-rate
and the cross sectional area of the conduits. Light extinction was
measured with a submarine photometer.

lO

11

Water sampling in the pond was done by lowering an acid washed
one-liter bottle below the surface of the pond from a boat. Initially
another sample was taken near the bottom but this additional sample
was discontinued when phosphorus concentrations were found to be almost
identical at the two locations. Samples from the inflow and outflow
were taken from the bottom of the conduits between ponds with a sampler
specially constructed to open in the stream of flow within the conduits.
A single sample was taken on the short term trips while an approximately
one—liter sample was composited from the four-hour samples on the basis
of flow on the twenty-four hour trips.

A portion of the water samples was filtered through a 0.h5 u Milli-
pore filter for dissolved orthophosphate analysis upon return to the
laboratory with the remainder of the sample refrigerated. The refriger-
ated samples were digested in a hzl mixture of concentrated nitric—
sulfuric acid for total phosphorus analysis.

Sampling for submersed macrophytes was done at random locations
in the pond but varied according to the occurrence of the species.

Most sampling was done by dropping a weighted cylinder (0.1135 me) to
the bottom and collecting the plants inside. When floating mats of
the submersed plants occurred, the cylinder was raised into the mat

from below. Lemna minor, a floating macrophyte, was collected after

 

randomly placing a 25 cm2 sampler in the mat of this species. The
total amount of plant material from these samples was placed in
individual plastic bags. Each sample was then washed and separated
by species in the laboratory. The plants were dried in a forced-draft
oven at 80°C and weighed. Five grams of plant tissue were digested

in a Bethge distillation apparatus with a 5:1 nitric-perchloric acid

l2

mixture. Total phosphorus measurements were made on diluted sub-
samples from this digestion with mean values for each species on a
specific date established by replicate analyses.

A11 phosphorus determinations were done by the vanadomolybdophos-
phoric acid method as described in Standard Methods (APHA, AWWA, WPCF,
1971). Absorbance was measured on a Bausch and Lomb Spectronic 20
colorimeter with a blue filter at MOO mu.

In an attempt to quantify the effect of the macrophytes upon the
chemical milieu and primary productivity of the pond specially con-
structed enclosures were used. These enclosures were made of 0.32
centimeter plexiglass with dimensions of 0.61 meter by 0.61 meter by
1.53 meters and had tight fitting lids made of the same material which
floated on the water surface. They were placed at a 1.1 meter water
depth in various locations in the pond with determinations of dissolved
oxygen, pH, and redox potential made at dusk, dawn, and dusk for a
twenty-four hour period. On the dates when the enclosures were put
in the pond and monitored two of the structures were placed over a
sample of the Q, demersum population and two other structures were
placed over sediments cleared of macrophytes. Gross oxygen production
was estimated by addition of the oxygen lost from the enclosure from
dusk to dawn to the oxygen gained in the enclosure from dawn to dusk
of the following evening. Oxygen production by the vascular hydro-
phytes was taken as the amount remaining after oxygen production in
the open enclosures was subtracted from that produced in the hydro-
phyte containing enclosures. Gross primary production in terms of
g carbon/m2/day was estimated by multiplying the oxygen production by

a factor of 0.312 (Vollenweider, 1969).

RESULTS

The results from the macrOphyte sampling are shown for the
different species of the plant community in Table l. A one way analysis
of variance was done for each species (Potamogetons were combined) with
Duncan's Multiple Range Test run if treatment effects were present
(Table A-8). Logarithmic transformations were carried out on all
biomass data prior to statistical analysis since means were positively
correlated with variances (Sokal and Rohlf, 1969). Variances were
also checked with the F-max test to insure homogeneity before analysis
of variance was done.

The major vascular plant species in Pond h during this study was
Q, demersum (Table 3). It generally accounted for over 90% of the
plant biomass. When vascular plant growth began, this species covered
95% of the pond bottom and it maintained this cover over the entire
season. Wet growth rates were determined for the plant species by
measuring changes in standing crop biomass (g/m2) with a maximum growth
rate of 3.0 g/m2/day observed for g, demersum between July h and 18.
Its maximum estimated biomass occurred on August 15 with 5100 Kg of
organic weight present in the pond, a standing crop of 169 g/m2. The
results of the analysis of variance show that there was a highly signifi-
cant change in biomass over the summer (Table A—8). A real change in
biomass such as this is often a characteristic of rapidly expanding

biological populations.

13

1h

Table 3. Biomass and net productivity (ash free dry weight) of vascular
hydrophytes in Pond h as estimated from curves fitted to the
hydrophyte sampling data.

 

 

 

 

 

 

Species: Ceratophyllum Potamogetonb Lemna
.QEEEEEEE. E22: minor
Date Kga g/m2/day Kga g/m2/day Kga
6/6 362 1&1 ---
1.5 0.2
6/20 987 210 1
2.8 0.2
7/h 216A 279 159
3.0 -O.5
7/18 3&16 155 238
2.3 -l 0
8/1 h336 25 81
1.9
8/15 5101 19%
—O.8
9/5 ASYY A9
-1.8
9/26 3368 18

 

aAntilog of estimated mean + 32/2 from ideal curve with exception of
L, minor values which are field estimates.

b2, foliosus and P, berchtoldii combined.

 

15

Another way of expressing the biomass data which serves to better
show the suitability of the environment for a plant species is biomass
doubling time (Tierney, 1972). This involves comparisons of the time
required for doubling plant biomass from the beginning of growth to
the inflection point on the seasonal growth curve. It can be seen in
Figure 2 that the Q, demersum biomass doubled almost four times during
the summer. As environmental conditions in the pond changed and growth
slowed down, the doubling times became longer. The shortest doubling
time represents the maximum unlimited rate of growth for a plant
species in a particular environment (McNabb and Tierney, 1972). In
Pond h the doubling time of 10 days between June 6 and 16 represents
the maximum rate of growth for Q, demersum.

Potamogeton foliosus and P, berchtoldii made up a sizable portion

 

 

of the plant biomass early in the summer but were eliminated after
August 1. Since the two pondweeds are quite similar and must be
differentiated taxonomically by reproductive structures, they could
not be accurately separated until they formed seeds, just prior to
vegetative extinction of the populations. Therefore, for the purpose
of analysis their material was lumped together. When identification
was possible, it was found that most of the Potamogeton biomass con—
sisted of P, foliosus. The pondweed biomass did not change significantly
during its occurrence which is indicative of the small maximum growth
of 0.2 g/m2/day being observed. The largest Potamogeton s22, biomass
realized was on July A with 279.5 Kg organic weight found at 13.3 g/m2.
The doubling of biomass took approximately 28 days, indicating a poor
coupling of the pondweeds to the environment present during this study.
The other important vascular plant species in this pond was

Lemna minor, common duckweed. Although it occasionally covered up to

 

l6

 

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asap anon sou-.030

 

 

 

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_ _ _ _ 4 fl _ a

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17

1 hectare of the surface of the pond, it never made up a large
portion of the plant biomass. A maximum.biomass of 237.6 Kg was
achieved by L, ming£_on July 18. During its initial growth the
duckweed biomass doubled rapidly with the pond apparently providing
optimum habitat (Table 3).

As previously mentioned, plexiglass enclosures were used to
obtain an estimate of the relative contribution to the primary
production of the pond by the vascular hydrophytes and phytoplankton
and to determine any effects of the hydrophytes on the water chemistry
of the pond by looking at water chemistry differences between the two
types of enclosures. Dissimilarities between the enclosures and the
pond which should be kept in mind are the absence of mixing and gas
exchange in the enclosures due to their structure.

The calculated gross production rates for each of the dates
along with mean values for a seven week period are given in Table h.
The enclosure results show a higher primary production rate for the
vascular plants during this time period, their average being almost
60% of the total production. Since relatively few areas of the pond
were free of vascular plants, the enclosure without vascular plants
may have over estimated the phytoplankton primary production in the
pond. If that is true, the subtraction of this amount from the
production in the g, demersum containing enclosure to determine
vascular plant production would have resulted in an under estimation
of submersed vascular plant production. Therefore, from the results
Obtained, vascular hydrophytes appear to be the largest contributor
to primary production in Pond h at this time of year.

Water temperature differences were slight between the two types

of enclosures as would be expected, however, there were slightly

18

Table A. Gross primary production for vascular hydrophytes and
phytoplankton as determined with plexiglass enclosures.

 

 

Vascular Hydrophytesa Phytoplankton
Date g 02/m2/dayb g C/m2/day g 02/m2/dayb
7/19 n.78 1.h9 7.53
8/2 9.10 2.8h 3.92
8/16 8.73 2.72 8.21
9/5 17.3h 5.hl 8.05
Mean 9.99 3.12 6.93

 

aProduction estimate in terms of the standing crop biomass of
C, demersum on the given data within the pond.

b
Mean value for two enclosures.

19

higher temperatures in the enclosures as compared to the pond around
them (Table A—2). Although pH was similar in the two types of
enclosures down to 0.5 meter, it was higher in the vascular hydro-
phyte enclosures at the bottom. In the pond, pH was lower than that
in the enclosures. Dissolved oxygen changes were quite different in
the two kinds of enclosures with higher oxygen levels at dusk in the
vascular plant enclosures and higher levels at dawn in the phyto-
plankton enclosures. These differences are a result of the greater
productivity in the vascular hydrophyte enclosures. The oxygen con-
centrations were alike in the pond and in the vascular plant enclosures
at dusk indicating a similarity of the two environments. At dawn the
pond water oxygen content was similar to that in the phytoplankton
enclosures. If the Q, demersum containing enclosures did approximate
the pond environment, rates of respiration should have been nearly
alike. However, oxygen could not diffuse from the atmosphere into
the enclosures at night due to their covers, so lower values would
have resulted. Redox potentials were quite similar in all three
situations with the exception of the dawn pond readings which were
higher than those of the enclosures.

When considering phosphorus uptake by vascular plants, it is
important to establish the phosphorus status of the water in which
the plants were growing. As will be more fully explained in the
following section on the phosphorus budget, the phosphorus concentra-
tions in the water were subjected to a regression analysis in order
to establish lines and/or curves best estimating phosphorus in the
water over the summer (Table 5). The best fits for influent phosphorus

(both total phosphorus as P and dissolved orthophosphate as P) were

20

Table 5. Phosphorus concentrations (mg/l as P) as taken from curves
fitted to the data of Table A—3.

 

 

Dissolved '
Sample Date Total P Ortho-P
Inflow 6/6 3.h9 3.1h
6/20 3.27 2.87
7/h 3.05 2.59
7/18 2.83 2.31
8/1 2.62 2.03
8/15 2.u0 1.75
9/5 2.07 1.3h
9/26 1.7h 0.92
Pond 6/6 2.65 2.12
6/20 2.02 1.60
Y/h 1.h6 1.16
7/18 1.06 0.8h
8/1 0.81 0.6h
8/15 0.73 0.56
9/5 0.91 0.66
9/26 1.h5 1.0h
Outflow 6/6 3.1a 2.39
6/20 2.87 1.82
7/h 2.59 1.37
7/18 2.31 1.03
8/1 2.03 0.82
8/15 1.75 0.72
9/5 1.3h 0.81
9/26 0.92 1.16

 

21
straight lines of negative slope showing a gradual decline in incoming
phosphorus over the summer. The actual value for total phosphorus
in the influent was 3.66 mg/l on May 23 and declined to 1.66 mg/l on
September 27, a reduction of 55%. Dissolved orthophosphate was 3.h1
mg/l at the beginning of the study and ended up at 0.91 mg/l, a 73%
reduction. Over the duration of the study, 501 Kg of phosphorus came
into the pond.

The effluent total phosphorus and dissolved orthophosphate levels
almost mirrored one another over the season. However, their curves
were very different from those of the inflow indicating a marked effect
on phosphorus in the water as it passed through the pond. The outflow.
values actually measured at the beginning of the summer (3.35 mg/l
total-P, 3.15 mg/l ortho—P) and at the end (1.65 mg/l total-P, 1.16
mg/l ortho-P) were close to influent values, but over the majority of
the study they were quite different. From June 6 to September 5 total
phosphorus in the outflow was at least 0.90 mg/l less than the inflow
with the greatest differences on July 18 and August 1 of 1.60 mg/l.
The differences between influent and effluent dissolved orthophosphate
were similar over this time period also since the curve was similar to
that of total phosphorus as mentioned above. A total of 386 Kg of
phosphorus left the pond during the study. This was 77% of the total
influent.

In the pond water the phosphorus concentrations were similar to
those found in the effluent water. Total phosphorus in the pond was
2.65 mg/l on June 6 and 0.73 mg/l on August 15. Dissolved orthophos-
phate was 2.12 mg/l and 0.56 mg/l on the same respective dates.
Generally the pond water values were somewhat lower than those of

the outflow.

22

Phosphorus levels in the plant tissues expressed as a percentage
of the ash-free dry weight were determined over the summer as shown in
Table 6. The results were handled statistically like those for biomass
sampling, with the exception of transformations which were not required.

The mean total phosphorus in Q, demersum varied significantly
over the summer (Table A-8), ranging from 1.37 to 2.2h% phosphorus.

The highest mean came on June 20 during the species exponential growth
phase while the lowest value was found when the plant biomass was
declining at the end of the season. A plant sample taken from the
pond on December 13 during winter senescence had the lowest phosphorus
content of 1.09%. In Figure 3 a significant linear regression

(I) < .05) of phosphorus in the plant tissue on the dissolved ortho-
phosphate concentration in the water is shown. With the exception of
the first date when seasonal growth was beginning and the last date
when biomass was quickly declining, these points fit a straight line
quite well. These changes in plant phosphorus in accord with changes
in water concentrations suggest a relationship which will be dis—
cussed in a following section.

In Potamogeton spp., phosphorus means ranged from 1.h5 to 1.81%
with significant differences being found between sampling dates (Table
A-8). These means appear to be related to the state of activity of
the plants and the changes in water concentrations in a manner similar
to Q, demersum. Phosphorus in L, mlngg also changed significantly over
the summer with means ranging from 0.63 to 1.39%. The highest
phosphorus level was found on June 20 when the duckweed was growing
most rapidly while the lowest value came on September 5. Again there

appears to be a relationship between phosphorus in the plant and that

T

23

able 6. Total quantity of phosphorus in the vascular hydrophytes
of Pond h as derived from curves fitted to the data for
ash-free biomass and tissue concentration (Table A-5)
(also given) for dates of sampling in 1973.

 

 

 

 

 

P as % P a

Ash-Free Held in P Uptake
Date Dry Wt. Kg
Ceratophyllum demersum
6/6 1.51 5.5 26.7
6/20 2.2h 22.1 16.1
7/h 1.90 h1.l 11.7
7/18 1.63 55-7 19.2
8/1 1.69 73.3 15.2
8/15 1.66 8h.7 __
9/5 1.59 72.8 __
9/26 1.37 h6.1
Potamogeton §22,b
6/6 1.70 2.h 20.3
6/20 1.81 3.8 15.h
7/h 1.75 h.9 __
7/18 1.60 2.5 __
8/1 l.h5 0.h
Lemna minor
6/20 1.33 0.1 10.8
7/h 1.06 1.7 5.1
7/18 0.86 2.0 __
8/1 0.71 0.6 __
8/15 0.63 1.2 __
9/5 0.62 0.3 __
9/26 0.75 0.1

 

aP uptake during species growth expressed

b

P, foliosus and L, berchtoldii combined.

 

in mg P/g ash-free

dry weight.

2h

 

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Q.N

25
in the water with the tissue phosphorus declining rapidly with decline
of phosphorus in the pond water. The linear regressions for Potamogeton
m. and L. REESE shown in Figure h were both significant (.05 < p <
.10).

In Table 6 the total estimated phosphorus content of the vascular
plant species is shown. Due to its large biomass and relatively
high concentration in its tissues, 9, demersum was the dominant plant
species in terms of total kilograms of phosphorus in the plant biota.
With the exception of the first month of the study, the Potamogeton
gpp, contained only a small part of the vascular plant phosphorus.

L, miggg never contained a significant amount of phosphorus.

Figure 5 is a representation of the total plant community biomass
and its phosphorus content during this study. On August 15 the maximum
amount of 86 Kg of phosphorus in the plants corresponded with the maximum
plant biomass of 5300 Kg. The relative amount of increase in these
two curves is similar, phosphorus increased seven times while biomass
increased nine times. As the plants were growing, they took up phos-
phorus from their environment. It was noted earlier that with each
individual species phosphorus uptake was greatest in the early part
of the seasonal growth as phosphorus content per unit weight of plant
tissue gradually declined after this high rate of uptake. This de-
creased uptake is shown by the somewhat smaller relative increase in
phosphorus content as compared to biomass. Since the two curves are
not parallel, these two parameters are not directly related. The
decline in phosphorus uptake relative to organic weight increase may
.have been due to the decreased phosphorus in the water which would have

resulted in less phosphorus being available to the plants.

26

 

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27

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28

As shown by the water budget (Table 1), changes in the pond
volume are mainly due to the inflow from Pond 3 with rainfall, evapora-
tion, seepage, and transpiration causing only minor changes. There-
fore, the phosphorus entering Pond h is mainly from Pond 3 and this
phosphorus either remains in the pond or passes out in the effluent.
The amounts of phosphorus in the system components on a given date are
not actual field determined values. For the purpose of formulating a
phosphorus budget and looking at general trends in the system, lines
sf best fit were determined with a polynomial regression program on
a CDC 6500 computer. Curves were fitted with increasingly higher
degree polynomials until a reasonable approximation of the data was
obtained (Table A-7). The amounts of change in phosphorus in a system
component between two sampling dates are denoted as differences. These
differences were used to calculate the phosphorus budget for the pond.

Referring to Table 7, column 2, the change in Kg of phosphorus
in the plants over a time interval was determined by multiplying the
estimated biomass of the plants in Kg ash-free dry weight by the
percent phosphorus in the ash-free dry weight and subtracting the
previous estimate from it. Column h, the change in total phosphorus
1n the pond water, was found by multiplying the mg/l concentration of
phosphorus by the total volume of the pond and subtracting this from
the previous estimate. Column 5, total phosphorus trapped in the
pond over time, was calculated by subtracting the average effluent
amount multiplied by concentration from the average influent amount
multiplied by concentration. Column 6, total phosphorus going to
the plants or sediments over time, was found by subtracting the change

in *otal phosphorus in the pond (column A) from the amount of phosphorus

29

Table 7. Phosphorus budget (in Kg P) for Pond h established with values
from curves fitted to the data for phosphorus levels in the
water and vascular hydrophytes.a '

 

 

Pond Inflow- Plants & Sedi-
Date Plants Diff. Water Diff. Outflow Sediments ments
Column 1 2 3 A 5 6 7

6/6 7.9 155.2
18.0 -38.2 15.6 53.8 35.8

6/20 25.9 177.0
21.8 -32.7 9.6 h2.3 20.5

7/h h7.7 8h.3
12.5 -25.9 15.1 h1.0 28.5
7/18 60.2 58.h .
lu.0 -1h.3 20.0 3h.3 20.3

8/1 7h.2 hh.l
11.7 - u.6 16.7 21.3 9.6

8/15 85.9 39.5
-12.8 8 6 22.8 1h 2 27 0

9/5 73.1 u8.l
-26.8 28.7 15.h -13.3 13.5

9/26 h6.3 76.8
Study 38.u -78.h 115.2 193.6 155.2

Total

 

aBudget calculations explained on previous page.

30
trapped in the pond (column 5). Column 6 minus column 2 gave column 7,
the total phosphorus going to the sediments between two dates.

The total phosphorus content (column 1) and the estimated amount
of net phosphorus flux (column 2) in the vascular hydrophyte community
are shown in Table 7. Over the time of this study there was a net
accumulation of phosphorus in the plants; approximately 20% of the total
phosphorus trapped by either plants or sediments. If the large release
of phosphorus by the vascular plants during the last six weeks had not
occurred, this amount would have been substantially greater.

Columns 3 and A show the change in total phosphorus in the pond
water during the study. This part of the system showed a trend
opposite to that of the plants with a net phosphorus loss from this
compartment over the season. Only during the last six weeks when the
plant release of phosphorus was taking place was there any increase in
the amount of phosphorus contained in the pond water. Between June 6
and August 15, phosphorus in the water decreased 75% (from 155.2 Kg to
39.5 Kg).

The difference between influent and effluent phosphorus is shown
in column 5. The percent of the phosphorus coming into the pond that
went out through the outflow decreased over the summer (Table A-h).

The important function of this column, however, is in combination
with column h to give column 6, the amount of phosphorus trapped in
the pond by the other two components (plants and sediments) over time.
Just by itself column 5 is ambiguous since it does not account for
changes in the pond water.

The amount of phosphorus going to the plants and sediments (column

6) was a positive quantity until the final three weeks of the study.

31
The net amount was almost 90% of the 501 Kg of phosphorus that came
into the pond during the study. The amount trapped during each time
interval decreased throughout the summer with this decrease related
most closely to the changes of phosphorus in the pond water (column A).

Column 7 is an estimate of the total phosphorus being sedimented
over time. No clear trends seem evident except there is a net
deposition of phosphorus to the sediments, not only over the entire
study period but also over each individual time interval. On September
26, of the cumulative total phosphorus going to the sediments and
plants, the sediments accounted for 80%.

To aid in an explanation of the results a second table was
constructed with the data. Table 8 is a listing of the cumulative
totals added over each time period. In this way the net changes in
the various components of the budget over time can be observed.

The amount of phosphorus trapped or released by the sediments and
plants can be used as a divisor for the cumulative amounts of phos-
phorus trapped in each of these two components (columns 2 and 7). Over
the first 70 days of the sampling period, h0% of the phosphorus trapped
in the plants and sediments went to the plants. Looking at the totals
for the entire period, this percentage changed markedly with only 20%
of this trapped phosphorus in the plants.

Another way of looking at the budget is shown by Table 9. Parti—
tioning coefficients express all the components in terms of the total
influent phosphorus with this total coming into the pond identified
as one unit. Phosphorus processed in this system can be thought of as
the total amount of phosphorus handled and is equal to influent phos-

phorus minus stored phosphorus (the change in the pond water phosphorus

32

Table 8. Cumulative totals of differences (in Kg P) from Table 5.

 

 

Pond Inflow- Plants & Sedi- % in
Date Plants Water Outflow Sediments ments Plants Sedimentsa
b

Column 2 h 5 6 7
6/6

18.1 -38.2 15.6 53.8 35.7 33.6 66.h
6/20

39.8 -70.9 25.2 96.1 56.2 h1.5 58.5
7/h

52.3 -96.8 no.3 137.2 8h.8 38.2 61.8
7/18

66.h -111.1 60.h 171.5 105.1 38.7 61.3
8/1

78.0 -115.7 77.1 192.8 11h.7 no.5 59.5
8/15

65.2 -107.2 99.9 207.0 1h1.8 31.5 68.5
9/5

38.h -78.h 115.3 193.6 155.2 19.8 80.2
9/26

 

aColumns 2 and 7 were divided by column 6 to determine these percentages.

Column numbers from Table 5.

33

Table 9. Coefficients of partitioning of phosphorus in Pond h.a

 

 

b Sedi- % in % in
Time Processed ments Plants Outflow Plants Sediments
6/6 - 7/3 1.hh 0.35 0.25 0.8h 17 2h
6/6 - 8/1u 1.33 0.32 0.22 0.78 17 2h
6/6 - 9/26 1.16 0.31 0.08 0.77 7 27

 

aThese coefficients were calculated by taking the total input to the pond
over the time period as 1.00 unit.

bProcessed equals inflow minus storage with storage being the change in
total Kg phosphorus in the pond water.

c% in plants equals the percent of processed phosphorus contained by the
plants. % in sediments equals the percent of processed phosphorus in

the sediments.

3h

over time). An amount of phosphorus processed greater than that
coming in results when phosphorus is lost from the water column.
Since there was a net loss of phosphorus from the water over the
summer, processed phosphorus was greater than unity. The effectof
the phosphorus loss from the plant tissue over the final six weeks
is shown by the substantial decrease in the amount of processed
phosphorus in the plants. The sediments seemed to be a rather
constant factor over the entire study as demonstrated by the similar
amounts of processed phosphorus that was sedimented over the three

time intervals.

DISCUSSION

Phosphorus is absorbed by submersed hydrophytes in relation to its
availability with concentrations in the plant tissue often far above
the critical level needed for maximum growth (Gerloff and Krombholtz,
1966). Luxury consumption of the element is especially evident in
highly enriched environments such as Pond h. The highest means reported
in this study are at least twice as high as the maximum values ordinarily
observed in field studies. In a review of aquatic plant mineral
composition, Boyd (1967) reports phosphorus levels ranging from 0.1 to
0.6% by dry weight. Other investigators have also indicated phosphorus
tissue levels generally within this range (Boyd, 1970b; Caines, 1965;
Gerloff and Krombholtz, 1966, Stake, 1968). Only Adams et a1. (1971),
Fish and Will (1966), and Tierney (1972) have observed phosphorus
levels above 1.0% of the dry weight with all three of these studies
being conducted in nutrient-rich environments.

In looking over different studies describing phosphorus levels
in vascular hydrophyte tissues it is evident that differences between
species occur. As seen in Table A this was true for the species
occurring in Pond h in 1973. These differences suggest alternate ways
of obtaining phosphorus among the plant species. Two Obvious dissimi-
larities of the plants in this study are their locations of growth

and anatomical structure. Since Lemna minor floats on the surface

 

with short, unattached roots, its source of nutrition (other than CO2)

must be the pond water. With submersed macrophytes, nutrients are

35

36
obtained from both the water and the substrate, though there is con-
siderable disagreement on this. Den Hartog and Segal (196M), for
example, consider that submerged plants are almost completely dependent
on the aquatic medium for their nutrition, absorbing nutrients through
the epidermis of their foliage. According to Bristow and Whitcombe
(1971), however, the substrate (via the roots) is most important in
plant nutrition. DeMarte and Hartman (l97h) and McRoy and Barstow
(1970) have noted 32F uptake in both the roots and the plant shoots.
Denny (1972) suggests a range among aquatic plants from emergent
plants that rely entirely on the roots for nutrient supply to rootless
plants that rely solely on their shoots for absorption. He also
suggests that the absorption site may alter, depending on relative
nutrient levels in substrate and solution. Since 9, demersum is

rootless, its major source of phosphorus nutrition is probably the

ambient water. The two Potamogetons sampled have well developed

 

roots (the roots of P, foliosus are not very extensive), therefore
they obtain nutrients from both the water and substrate. As indicated
in Figure h the association between tissue and ambient phosphorus

is such that much of the pondweed's luxury consumed phosphorus may
have come from the water.

In the context of this study the importance of phosphorus accumula—
tion and fluctuation in the plant tissues is the effect this has on
phosphorus in the other two main components of the pond and the effect
on the budget as a whole. A net uptake of phosphorus by the hydro—
phytes will result in lower amounts in the water and sediments while
a net release will have the opposite effect. Phosphorus concentrations

in the water underwent a gradual decline over the summer resulting

37
in a net loss of phosphorus from the pond water. Total phosphorus
in kilograms in the water was negatively correlated (r = -.97) to
the plant biomass. This would indicate that the uptake of phosphorus
by the developing plant biomass may have influenced the decline of
phosphorus in the water.

Although phosphorus levels in all plant species decreased after
their early, high levels, the total phosphorus in the plant community
increased to a maximum on August 15. The increase in plant biomass
overshadowed the decrease in tissue content with the total kilograms
of phosphorus in the plants highly correlated (r = .99) to the total
plant biomass. Though uptake (mg P/g plant) was higher at the beginning
of the summer, Table 9 shows that the relative amount of processed 9
phosphorus in the plants was almost identical for the first and second
time periods. Apparently, the plants maintained a constant rate of-
phosphorus uptake in relation to the concentration of phosphorus in
the water. The season's total of only 7% of the processed phosphorus
in the plant tissues shows the marked effect on the water and sediment
components of the phosphorus release during the last six weeks. If
sampling had continued, the amount of processed phosphorus in the
plants would have been expected to decline even further with the
sediments and the water being virtually the only reservoirs for net
trapped phosphorus.

By looking at the cumulative totals in the phosphorus budget
(Table 6) it can be seen that the plants maintained a hold on about
h0% of the trapped phosphorus up to August 15, containing an estimated
78 kilograms. Since most of the phosphorus in the major plant species,

9, demersum, probably came from the water, much of the phosphorus in

38
the plants must have come from the current growing season inputs.
Thus, while the submersed plants were actively growing in Pond h, a
large amount of phOSphorus trapped in the pond was trapped solely as
a result of its absorption into their tissues.

The role of submersed hydrophytes in the phosphorus balance in
fresh waters has not received much attention in limnological writing.
Various scientists have noted decreasing soluble phosphorus levels
in the water due to phytoplankton uptake (Ruttner, 1963), however,
this association with ambient phosphorus and vascular plants has
seldom been expressed. In a study of Linsley Pond, Conneticut,

Cowgill (1968) noted a sharp decline in total phosphorus in the

water coincident with the developing submersed vegetation and suggested
this decline was due to the vegetation and that this was probably a
normal occurrence in the pond. Rigler (1956), in attempting to account
for 32F added to the littoral zone of an experimental lake, concluded
that the major loss was to the littoral zone rather than precipitation
to pelagic sediments. This 32F loss to the littoral zone was presumably
due to vascular hydrophytes there. In a one year study of the Belding
pond system, Annett et a1. (1973) observed a mean phosphorus level of
.59 ppm at the system outflow from July 21 to October 12, 1969. They
reported that the concentrations and variability during this time were
considerably lower than the rest of the year. This would roughly
coincide with the time of year when the submersed hydrophytes most
strongly influence the system.

Although the total kilograms of phosphorus did increase in the

plants through August 15, this amount would have been substantially

larger if the percent of phosphorus in the plant tissues had not

39

declined. Since the major source of the plant phosphorus was probably
the pond water, this decline in phosphorus in the plant tissues is
reasonable due to the decline in concentration in the water.

Another reason for phosphorus decline in the plant tissues is
the possible relationship between tissue phosphorus and metabolic
activity. Coinciding with decreased tissue levels was a slowdown
in growth rate as seen not only by the decreased production rates
(g/m2/day) but also by the increased doubling times. Relative to
the critical levels established by Gerloff and Krombholtz (1966),
tissue phosphorus was more than adequate for maximum growth. Thus,
it was not a phosphorus limitation which caused this decrease in growth.
In several similarly enriched environments, Tierney (1972) Observed

that the rate of phosphorus accrual by g, demersum and Elodea canadensis

 

was fairly close to the rate of biomass synthesis.

Seasonal changes in tissue phosphorus have been noted in the
literature indicating a decline in submerged and emergent plant with
aging (Boyd, 1969, 1970a; Stake, 1968; Caines, 1965) or indiscriminant
variation (Gerloff and Krombholtz, 1966). Caines (1965) and Boyd (1969)
found the greatest net absorption of phosphorus during the early growth
of the plants with much of luxury consumed phosphorus being later
translocated to new growth. Phosphorus levels in vascular hydrophytes
are generally associated with fertility of the habitat (Adams et al.,
1971; Caines, 1965; Gerloff and Krombholtz, 1966), however, a relation-
ship between the tissue and water has seldom been demonstrated. Tierney
(1972) and McNabb and Tierney (1972) have noted a significant high
correlation between tissue phosphorus and ambient soluble phosphorus

concentrations among several study sites. They also found the plants

ho
grown in an environment high in external phosphorus had higher
seasonal variations than those from less fertile habitats.

Besides the reduction of phosphorus by tissue uptake, the vascular
hydrOphytes may be expected to exert other influences on phosphorus
cycling in the pond. One influence which has been briefly mentioned
is that of the plants on the water chemistry, specifically the
dissolved oxygen, pH, and redox potential. Where they are the dominant
plant form, macrophytes are closely associated with the diurnal oxygen
changes in lakes and ponds (Sculthorpe, 1967). During the day plants
are actively engaged in photosynthesis and release oxygen, thereby
increasing the dissolved oxygen in the water. At night though, no
oxygen is produced and dissolved oxygen in the water decreases due
to plant (and other community members) respiration. Macrophytes
store some of the oxygen produced in their lacunal air spaces for use
in respiration as night (Hartman and Brown, 1966) which does serve
to moderate the amount of diurnal fluctuation, especially on days
of low light intensity. The presence of these diurnal fluctuations
is evident from the high and low means shown in Table 2. Since macro—
phytes were such a major part of the pond biota, these fluctuations
may have been largely due to their presence. In small and/or shallow
lakes, the effect of vascular hydrophytes on diel oxygen fluctuations
can be very great (Davies, 1970; Wetzel, 1965). As plant biomass in
a system is increased more pronounced oxygen fluctuations may be
expected. In general, greater dissolved oxygen changes did occur in
mid summer when the biomass was largest with conditions of over 200%
saturation occasionally found in dense macrOphyte stands.

The addition of oxygen to the water of Pond h by the plants

would directly affect the solubilities of many of the phosphorus

hl
compounds in the water. Most phosphorus not organically held exists
in cationic compounds and complexes (Frink, 1967). These compounds
are relatively insoluble in oxygenated waters at pH and redox potential
ranges normally encountered, but under anaerobic reducing conditions
they become more soluble. Since significant concentrations of Fe,
Mn, and other metal species are found in this pond (Bulthuis, 1973;
Lisiecki, l97h), dissolved oxygen levels are important in considering
available phosphorus in the system.

It has been reported by Mortimer (1971) that at dissolved oxygen
concentrations at the sediment surface of over 2.0 mg/l little nutrient
release takes place, however, below this level major amounts of phos-
phorus may be released. From the study results, dissolved oxygen was
always above 2.0 mg/l in the water column with the exception of some
measurements at the sediment-water interface. Even at the interface
the consistently lowest time of the day (8 AM) had a mean of 1.9 mg/l.
Due to the generally high oxygen levels in this pond it seems reasonable
to conclude that sedimentation of phosphorus compounds was aided by
this factor.

Since all but an average of 2h% of the plant biomass decomposes,
macrophyte decomposition does exert considerable pressure on the
dissolved oxygen content of the water (Jewell, 1971). Although some
decomposition occurs throughout the growing season, it is most pro-
nounced in the fall when the plants die or return to a small over-
wintering biomass. A large drop in dissolved oxygen could resolubilize
much phosphorus at this time, both in the upper few centimeters of
the sediments and much of that in the decomposing plant tissue. Although

plant biomass was declining in the pond over the last six weeks of the

h2
study, pronounced drops in dissolved oxygen were not found. However,
phosphorus levels did increase in the pond water during this period
possibly indicating a release of phosphorus from the macrophytes.

Abundant plant growth in the water has a definite effect on the
hydrogen ion concentration. Plants often use up all of the free C02
in the water and much of the bicarbonate resulting in a fairly high
alkalinity. In extreme conditions, with luxuriant submersed vegeta-
tion, the decomposition of bicarbonate can proceed up to the formation
of hydroxide with the pH being raised up to about 11 (Ruttner, 1963).
Free 002 is returned to the system at night due to respiration, re-
sulting in a diurnal change in pH values. From the median values
given in Table 2, it is evident that diurnal changes occurred in the'
pond, although these fluctuations are shown more clearly in daily
values (see Lisiecki, 197A). That relatively high pH values occurred
is also apparent with both the diurnal changes and high values con-
siderably influenced by the plant biomass.

Besides the organically bound phosphorus, probably the other major
amount of phosphorus precipitation was through the loss of Fe, Al, Ca,
and other cation—phosphate complexes and compounds. In Wisconsin
lakes, Armstrong et a1. (1971) found that the amount of inorganic
phosphorus in the sediments was most closely related to the
amount of hydrated Fe oxides and hydrous oxides. The solubility
dependence of these cation—phosphate complexes and compounds on pH was
mentioned earlier. Above pH 7 their solubilities are generally in-
creased while their ability to bind phosphate ions is decreased

(Stumm and Morgan, 1970). This means that the high pH values in Pond A

may have aided in maintaining the relatively high phosphorus levels

h3

in the water. The crucial question, though, is the phosphorus/cation
ratios which existed in the pond since high cation levels would have
resulted in large inorganic phosphorus sedimentation, even at these
high pH levels. Bulthuis (1973) found a mean level of 0.1 mg/l Fe and
0.03 mg/l Mn in Pond 3 water. Since phosphorus in Pond h was at least
six times greater than this Fe concentration in 1973, high levels of
inorganic phosphorus would tend to remain in the water.

It is likely that, in natural environments, the pH of micro-
environments such as those around macrophytic surfaces is raised well
above pH 9 with photosynthetic activity (Otsuki and Wetzel, 1972).
This would cause coprecipitation of phosphates and carbonates thus
reducing phosphorus levels in the water. Although alkalinity levels‘
are moderately high in Pond h (McNabb, unpublished data), the macrOphyte
surface appeared to have very little marl on them when microscopically
examined. Thus, this coprecipitation probably did not have a large
effect on phosphorus removal from the water.

Closely associated with the changes in pH and dissolved oxygen
in the water are the changes taking place in the redox potential. It
has been shown that vascular hydrophytes do influence redox potentials
(Wium-Andersen and Andersen, 1972) and this could be expected to
exert control on the metal-phosphate complexes and compounds in the
water. Generally, redox levels in the pond were such that an oxidizing
environment was found. The high levels of oxygen in the pond water
as a result of macrophyte photosynthesis would serve to maintain this
oxidizing environment. The oxidized forms of Fe and Mn which are more
insoluble than the reduced forms would predominate in the pond water
resulting in phosphate precipitation. Below 200 mv, the ferric com-

pounds dissolve due to reduction (Graetz, 1973) with phosphate being

hh

released. Redox potentials were quite constant over the summer with
mean values at the sediment interface of 86 mv at 8 AM (consistent low)
and 227 mv at 12 noon (consistent high). In the water column above
the interface, values were generally over 200 mv.

The surfaces of macrophytes may provide a means for phosphorus
to be taken out of the water. Many phosphorus containing compounds
and complexes adsorb to surfaces quite readily and macrophytes pro-
vide an increased surface area where this adsorption could take place.
The surface of vascular plants also provides an ideal substrate for
epiphytic algae. The algae may attain such biomass that they can
obscure the nutritional response of the macrophytes (Fitzgerald, 1969).
The epiphyte cover, in general was very sparse on the macrophytes
during this study. This scarcity of epiphytes is consistent with
observations on the macrophytes in this wastewater system from previous
years (McNabb and Tierney, 1972).

Another effect of the vascular hydrophytes on the phosphorus
in the pond was the greater stability of the water column resulting
from the presence of the plants. Due to its openness, the pond is
quite subject to wind action. Somewhat indicative of this added
stability of the water column is the occurrence of L? mig93_throughout
the final two-thirds of the study. According to Duursma (1967),
agitation of sediments can serve to increase phosphorus release from
the sediments since diffusion coefficients are at least a factor of
ten lower for mud than solutions. Phosphorus containing seston and
the insoluble cation—phOSphate compounds and complexes mentioned
earlier would tend to precipitate out easier in calmer water. Since

80% of the phosphorus trapped in the pond during this study went to

145

the sediments, the stabilizing effect and the aid in maintenance of
van oxidizing environment in the pond may have been the most important
functiOn of the submersed hydrophytes (as far as phosphorus removal)
in the pond.

Until recently, it has generally been assumed that the exchange
of nutrients across the sediment—water interface is the only significant
pathway of nutrient recycling between these two compartments. Vascular
hydrophytes were thought to contribute to this exchange only after
their death when they had become part of the sediment compartment.
The studies of DeMarte and Hartman (197A), McRoy and Barsdale (1970),
Bristow and Whitcombe (1971), and McRoy et a1. (1972) have shown that
these plants play an important role in nutrient cycling in aquatic
systems by releasing elements through the shoots to the surrounding
water. According to Foehrenbach (1969), Zostera released substantial
amounts of inorganic phosphate to the water. McRoy and Barsdale (1970)
found that 33% of the phosphate absorbed by the roots of eelgrass
(Zostera) was excreted by the foliage into the surrounding sea water.
DeMarte and Hartman (197A) have also reported substantial release of

32F by the shbots of Myriophyllum exalbescens and that the amount re-

 

leased could be increased by injury to the test plants. These studies
do not report whether the amount of phosphorus release would vary
according to phosphorus levels in the plant tissue or phosphorus
concentrations in the ambient water.

From the studies of these scientists, it seems appropriate to
hypothesize that phosphorus was released by the vascular hydrophytes
to the water of Pond h. The overall magnitude of this release and

its effect on phosphorus levels in the other system components cannot

 

I'll‘ll‘llllj‘lll'l‘ll

116
be determined. It is possible that through their release of phos-
phorus to the water, the plants maintain some kind of equilibrium
between the phosphorus level in the water and their tissues. The

apparent response of tissue phosphorus to changes in concentrations

in the water may be evidence for this.

CONCLUSIONS

The habitat of the pond during this study was best suited to
g, demersum since this species virtually dominated the vascular hydro—
phyte community. All the macrophyte species which were quantitatively
sampled showed luxury consumption of phosphorus with levels in the
plant tissues being some of the highest reported in the literature.
In general the submersed species contained almost twice the amount
of phosphorus on an ash—free dry weight basis as the floating species
(L, mingg). The access of the submersed hydrophytes to the sediments
may explain this difference.

In Pond h of the Belding wastewater system submersed vascular
hydrophytes aid in the removal of phosphorus from the water during
the growing season of the plants. Through their active uptake of the
element the hydrophytes contained an estimated h0% (17% processed
phosphorus) of the phosphorus trapped by the pond at the time of
their maximum biomass on August 15. As the biomass declined after
this date much of the accumulated phosphorus was re—introduced to the
sediments and the water. When the study was terminated the plants
contained only 20% (7% processed phosphorus) of the phosphorus trapped
by the plants and sediments.

Due to their active growth and resulting large biomass, the hydro-
phytes influenced the water chemistry of the pond. The oxidizing

environment present in the pond enhanced the precipitation of inorganic

A7

118

cation-phosphate compounds and complexes. The presence of the sub-
mersed vascular plants also served to physically stabilize the water
column which would promote sedimentation of both inorganic and organic
phosphorus containing materials. Sedimentation accounted for 80% of

the phosphorus removed by the pond during this study.

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APPENDIX

5h

 

 

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wmmnmmH om: 0.:Huw.HH N.MH m.mum.> m.w m.mmum.mm o.mm mosmHSm
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54m7m: mmm ®.mum.o m.m :.m-:.> m.> o.mmnm.mm m.:m soupom
wmmnomm as: m.mH-m.w m.HH m.m-m.m m.m o.mmum.:m m.mm a m.o
m:m-mmm NJ: m.mHuH.m H.mH m.mnm.m m.m o.>mu:.:m m.mm moomnsm
momumm- mm o.u-:.o w.m o.muo.m >.> m.:mnw.mm 0.:N soupom
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mHQon noHSOmo> HSoQHHs moHSmOHosm
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Ea em Ema so an Gov dame

 

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oH GaoSm moped 039 so moHSmOHooo mmononHQ GH mpsosoHSmooE HsOHmhnm one Hoofiaoso mo thEESm .mu< wands

55

 

 

 

 

Table A-3. Tota1.phosphorus and dissolved orthophosphate concentrations

(both as mg/l P) in the water expressed as three-point means.
Total Phosphorus Dissolved Orthophosphate

Date Inflowb Outflowb Pondc Inflowb Outflowb Pondc

5/23 3.h7 3.16 -- 3.0h 2.92 --

5/31

6/6 3.29 3.05 2.65 2.96 2.79 2.12

6/1u

6/20 3.31 1.9h 2.18 3.00 1.69 1.76

6/28

7/h n.06 1.33 1.35 3.60 1.22 1.08

7/12

7/18 2.85 1.06 0.77 2.h8 0.91 0.56

7/26

8/1 2.3M 1.08 0.97 1.76 0.92 0.82

8/9

8/15 2.00 0.99 0.79 1.22 0.7% 0.67

8/23

8/30

9/5 1.85 1.37 1.02 1.1M 0.9M 0.68

9/13

9/20

9/26 2.01 1.5M 1.36 1.17 1.07 0.99

 

aTabled values are means of samples taken on date listed, preceding date,

and following date, except for the first and last in the series which

are two point means.

bFrom samples composited over twenty-four hours.

cFrom surface samples taken randomly in the pond at noon.

56

Table A-h. Total quantities of water and phosphorus flowing in and out
of Pond h over the time periods shown.and total pond volume
and.phosphorus in.the pond on.the given dates.

 

Total Volume (106L) Total Amount (Kg)

 

 

 

Date Inflow Outflow Pond Inflow Outflow Pond
6/6 58.6 155.2
26.9 25.1 91.1 75.5
6/20 57.9 117.0
22.h 22.h 70.8 61.2
7/h 57.8 8h.3
' 20.3 18.2 59.6 uh.5

7/18 55.1 58.u
25.h 22.7 69.3 h9.3

8/1 5h.h hh.l
25.1 2h.5 63.1 h6.3

8/15 514.1 39.5
3u.1 3u.5 76.1 53.3

9/5 52.8 h8.l
37.6 37.1 71.6 56.2

9/26 53.0 76.8

 

 

57

Table A-5. Phosphorus as percent of the ash-free dry weight in the
vascular hydrophytes.

 

 

 

 

 

Date Mean (I) 3y s2

Ce rat ophyllum deme rsum

6/6 1.51 .0200 .00h0 7
6/20 2.20 .0970 .0630 7
7/h 1.96 .0750 .0560 10
7/18 1.56 .0750 .0560 10
8/1 1.71 .1100 .0900 10
9/5 1.58 .073h .0550 10
9/26 1.38 .0351 .0900 10
12/13 1.09 .0200 .0400 10
Potamogeton spp.a

5/23 1.38 .0hh7 .0100 5
6/6 1.50 .0685 .02u0 5
6/20 1.99 .0860 .0370 5
7/h 1.81 .1212 .0733 5
7/18 1.u3 .0721 .0260 5
8/1 1.51 .0632 .0200 5
Iemna minor

6/20 131+ .1319 .0691; L.
7/h 1.0M .101u .0517 5
7/ 18 0. 93 .0300 . 00% 5
8/1 0.63 .0223 .0025 5
8/15 0.63 .0173 .0015 5
9/5 0.65 .01h1 .0010 5
9/26 0.7M .0173 .0013 5

 

aP_. foliosus and

 

P. berchtoldii combined.

58

Table A-6. Percentage ash of dry—weight in the vascular hydrophytes.

 

 

 

 

 

Species Date Mean (Y) si 32 n
Ceratgphyllum 5/23 17.h3 .67 2.22 5
demersum 6/6 l8.’+5 .31+ .59 5
6/20 29.68 1.97 19.h6 5

7/h 26.10 1.62 13.07 5

7/18 22.38 1.72 1h.75 5

8/1 21.60 1.1u 6.65 5

8/15 18.78 .73 2.6A 5

9/5 20.h6 2.66 35.27 5

9/26 20.85 1.31 8.62 5

12/13 21.71 1.72 1h.83 5

Potamogeton spp.a 5/23 18.09 1.69 1’4.25 5
6/6 20.37 2.85 no.7h 5

6/20 22.27 1.30 8.h3 5

7/h 22.01 2.7h 37.h9 5

7/18 18.79 n.5u 61.93 3

8/1 19.00 1.33 8.82 5

lemma. minor 5/23 13.81} -- -- 1
6/20 22.82 1.18 2.76 2

7/h 21.35 1.20 7.20 5

7/18 20.78 .83 3.h8 5

8/1 19.09 .h2 .87 5

8/15 20.h3 .37 .67 5

9/5 19.hh .39 .76 5

9/26 19.87 .29 .h2 5

 

 

aP. foliosus and P. berchtoldii combined.

 

59

upemes_awe

 

 

 

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60

 

 

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be . HGOOV baa. manna.

61

Table A-8. Ranked means and results of Duncan's multiple range test
(p =0.05)a' for biomass measurements and phosphorus in
vascular hydrophytes .

 

Log:LO _C_. demersum biomass (g ash-free dry weight/sampler).D
AOV = 12.63***

1.11225 1.02018 1.017%7 .98305 .78111 .h3109 .oh170
8/1 7/18 9/5 9/26 7/h 6/20 6/6

 

 

Phosphorus in _C_. demersum (% ash-free dry weight) AOV = 12.22***

2.20 1.96 1.71 1.58 1.56 1.51 1.38
6/20 7/h 8/1 9/5 7/18 6/6 9/26

 

Phosphorus in Potamoggton 532,0 (% ash-free dry weight) AOV = 8.90***

 

1.99 1.81 1.51 1.50 1.h3 1.38
6/20 7/h 8/1 6/6 7/18 5/23

 

 

Phosphorus in L. minor (% ash-free dry weight) AOV = 18.52***

1.3h 1.0h 0.93 0.7M 0.65 0.63 0.63
6/20 7/h 7/18 9/26 9/5 8/1 8/15

 

 

 

aLines connect means not significantly different at the 95% level.
AOV values are the analysis of variance mean square statistics.
*** = significant differences at the 99.9% level.

bSize of vascular hydrophyte sampler was 0.1135 m2.

GP, foliosus and P. berchtoldii combined.