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This is to certify that the
dissertation entitled

THE RATIO OF RESPIRATION TO PHOTOSYNTHESIS IN
LAKE SUPERIOR AND THE NORTH PACIFIC OCEAN:
EVIDENCE FROM STABLE ISOTOPES OF 02

presented by

Mary Elizabeth Russ

has been accepted towards fulfillment
of the requirements for the

Doctoral degree in Environmental Geosciences

 

 

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' Major Professor’s Signature

 

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Date

MSU is an Affirmative Action/Equal Opportunity Institution

THE RATIO OF RESPIRATION TO PHOTOSYNTHESIS IN LAKE SUPERIOR AND
THE NORTH PACIFIC OCEAN: EVIDENCE FROM STABLE ISOTOPES OF 02

By

Mary Elizabeth Russ

A DISSERTATION
Submitted to
Michigan State University
in partial fitlfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Department of Geological Sciences

2003

ABSTRACT

THE RATIO OF RESPIRATION TO PHOTOSYNTHESIS IN LAKE SUPERIOR AND
THE NORTH PACIFIC OCEAN: EVIDENCE FROM STABLE ISOTOPES OF 02

By

Mary Elizabeth Russ

A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on
the fraction of 02 saturation and the isotopic composition of 02, was undertaken to
evaluate spatial and temporal variations in the trophic status of a large oligotrophic
' fi’eshwater lake. The lake was predominantly net heterotrophic fiorn April to October
2000 (R:P ratios: 1.2-2.5). Uniform R:P ratios of ca. 1.5 with depth and across the lake in
April 2000 and 2001 revealed the homogeneity of the water column during spring. A
brief period of net autotrophy was observed during summer thermal stratification in 2000
and 2001, and surveys showed this condition to be prevalent and lake-wide in August
2001 (R:P ratios: 0.5-0.9). Strong net autotrophy (R:P ratios: 0.6) was found near
Duluth, Minnesota and suggested the potential for the formation of mesotrophic
conditions within areas of increased nutrient loadings from urbanization. Respiration and
photosynthesis were shown to exert a strong control on 02 gas exchange within Lake
Superior, as evidenced by significant correlations between R:P ratios and 02 gas
exchange during periods of net heterotrophy and autotrophy. This observation was
unexpected since [02] in the lake appears to be dominated by atmospheric 02 gas
exchange, given that the fraction of 02 saturation is continuously near levels expected for

equilibration with the atmosphere. Furthermore, the relationship between the biological

and physical 02 fluxes may enable the use of R:P ratios to calculate 02 gas exchange and
ultimately estimate C02 fluxes between lakes and the atmosphere.

To evaluate spatial and temporal variations of net heterotrophy and autotrophy
within the North Pacific ocean, R:P ratios for a six station transect comprising the
oligotrophic North Pacific subtropical gyre (NPSG) to the eutrophic eastern tropical
North Pacific (ETNP) were determined. Lower R:P ratios in the summer within the core
of the NPSG (R:P ratios: 1.1) as compared to other seasons (R:P ratios: 1.2-l.3) likely
reflected a reduction of organic carbon influx fi'om below the pycnocline and an increase
in new production supported by N2 fixation. Within the NPSG, the 1998/2000 La Nifia
conditions resulted in lower R:P ratios within the upper 100 m (1.1) than during a weak
El Nifio event in 1990 (R:P ratio: 1.2) from a potential reduction of organic carbon
consumption within the euphotic zone. Despite these variations in R:P ratios, the NPSG
was net heterotrophic (R:P ratios: 1.1). Net autotrophy at the fiinge of the NPSG may
provide an external organic carbon source within the NPSG that is transported via
horizontal transport and the downwelling conditions within this region. Net heterotrophic
conditions also prevailed throughout the ETNP, indicating that expected high rates of
photosynthesis were coupled with equally high rates of respiration. Gross 02 production
calculated fi'om 170-02 data was suflicient to generate net autotrophic conditions within
the NPSG and ETNP, given that either external organic carbon inputs decrease or
nutrients, from a physical perturbation of the water column, increase. Due to a
decoupling in respiration and photosynthesis during storm events within the ETNP, a
shift toward a balance in respiration to photosynthesis was the result of the physical

redistribution of both organic carbon and nutrients rather than net autotrophy.

ACKNOWLEDGMENTS

I realize that a person can not live in a vacuum, no matter how comforting that
concept may be to me. I wish, therefore, to attempt to acknowledge those people who
assisted me in this achievement of my degree. First, I need to extend a word of
appreciation to my committee. Each member brought their wisdom and knowledge, as
well as a personal quality to the committee. Dr. Michael Klug brought the experience,
Dr. Stephen Hamilton the sanity and Dr. Phanikumar Mantha the life line. I would also
like to acknowledge, Dr. Grahame Larson, who, though not on my final committee, came
through temporarily when I needed a helping hand. Lastly, my advisor, Dr. Nathaniel
Ostrom, our complex relationship of give and take has provided the development of a
sense of self and outward confidence to defend my opinions in a mature manner.

I would like to thank the captains and crew of the RV Lake Guardian, RV
Laurentian and RV Roger Revelle. The professionalism and seasoned experience of all
involved allowed me to focus on my research and not worry about what may be lurking
over the next wave. Furthermore, I would especially like to acknowledge Dr. Noel Urban
at Michigan Technological University, and Dr. Brian Popp and Dr. David Karl at the
University of Hawaii, each of these people has added to my knowledge base and offered
both encouragement and respect for my fledging ideas and scientific career.

Over the course of my time at MSU, I have learned that some times the best
answer to a problem is found within the listening ear of a good confidant. For this role in
my education I would like to recognize Stephanie Bour, Kim Frendo, Hasand Gandhi,

Terri Rust and Nat Saladin. A special award needs to be given to Amanda Field and

iv

Michelle Gedeon. These two women have listened to my endless rantings and ravings
and through it all keep me sane and focused. I could never have made it through without
Amanda’s Friday lunches or Michelle’s good-natured bluntness. I admire both of them
and hope they realize how important they have been in my life.

I would like to recognize my natural parents, Joseph and Charlotte Dueweke, and
my nurture parents, George and Bernadine Russ. My parents conceived me with the
innate abilities of rational thought, stubbornness, tenacity and a persistence that kept me
breathing. Together my parents taught me to have faith in God, and instilled in me the
belief that during times of complete despair and isolation I was never alone, and that
strength and comfort were only a prayer away. My parents-in-law offered an
unconditional belief in my ability to obtain whatever I set my mind to do. Their
encouragement and exuberance were much appreciated. Further, I would like to thank
my brother, Stephen Dueweke, who first taught me to look at all sides of an argument.

Finally, somehow I must place into words all the gratitude, admiration and
passion I feel for my husband, Steven Russ. Steve had the strength, courage and
confidence in himself and our marriage to provide me with an opportunity to pursue a
dream. There are few men that have such qualities, and for some unknown reason I have
been blessed and honored to have this man as my husband for the past 22 years. Though
the miles have kept us physically apart most of the last five years our hearts have kept us
emotionally together. The ultimate last note must be given to our little one, Erasmus. He
has taught me to take each day as it comes and to appreciate the now, for in essence the
present is all we really have to experience. Little did I know when I started this journey

called life that I would learn so much from a schnauzer.

TABLE OF CONTENTS

List of Tables ................................................................................................................... viii
List of Figures ................................................................................................................... ix
Chapter 1: Temporal and spatial variations in R:P ratios in Lake Superior ..................... 1
Background ............................................................................................................ 1
Methods .................................................................................................................. 3
Study site and sample analysis ................................................................... 3
Determination of 02 gas exchange ............................................................ 6
Determination of R:P ratios ....................................................................... 6
Results .................................................................................................................... 9
HN transect (April through October 2000) ................................................ 9
West-East transect (April and August 2001) ........................................... 11
Discussion ............................................................................................................ 13
Seasonal variations in R:P ratios .............................................................. 13
Ecological implications - Duluth, Minnesota (Station SUZZB) ............... l9
Ecological implications - the microbial loop and climate change ........... 20

Relationship between 02 gas exchange and R:P ratios in an oligotrophic
lake ........................................................................................................... 22
Conclusions .......................................................................................................... 24

Chapter 2: Temporal, spatial and storm related changes in R:P ratios in the North Pacific

ocean ................................................................................................................................ 61
Background .......................................................................................................... 61
Methods ................................................................................................................ 63

Determination of R:P ratios ..................................................................... 65

Determination of gross 02 production ..................................................... 68

Results .................................................................................................................. 69
Transect - station ALOHA to S6 (May 25 to June 22) ............................ 69

Storm events - SS and S6 ......................................................................... 71

Discussion ............................................................................................................ 73
Transect fiom the NPSG to the ETNP (station ALOHA to S6) .............. 73

Pre-storm conditions along the transect (statiOn ALOHA to S6) ............ 76

Storm events - SS (June 14-16) and S6 (June 19-22) .............................. 78

References ........................................................................................................................ 96

vii

LIST OF TABLES

Table 1.1: Latitude and longitude coordinates, maximum depth and distance to shore for
all stations along the HN and West-East transects. Note: Mid-station HNO70 was
sampled in June instead of l-INO90. ................................................................................. 27

Table 2.1: Average R:P ratios for the upper 100 m of the water column for past and
current cruises, and before, during and after the storm events at stations SS and S6. ..... 80

Table 2.2: The calculated gross 02 production for stations ALOHA, S3, 85 and S6.
Station S6 was sampled after the storm event. ................................................................ 81

viii

LIST OF FIGURES

Figure 1.1: Locations of the stations comprising the West-East and HN (insert) transects
in Lake Superior. .............................................................................................................. 28

Figure 1.2a: Temperature as a function of depth for all stations along the HN transect in
April through June 2000. Note: Mid-station HNO70 was sampled in June instead of
HNO90. ............................................................................................................................. 29

Figure 1.2b: Temperature as a fiinction of depth for all stations along the HN transect in
July and August 2000. ...................................................................................................... 30

Figure 1.2c: Temperature as a fiinction of depth for all stations along the HN transect in
September and October 2000. .......................................................................................... 31

Figure 1.3a: Chlorophyll fluorescence as a function of depth for all stations along the
HN transect in April through June 2000. Note: Mid-station HNO70 was sampled in June
instead of HNO90. ............................................................................................................ 32

Figure 1.3b: Chlorophyll fluorescence as a function of depth for all stations along the
HN transect in July and August 2000. ............................................................................. 33

Figure 1.3c: Chlorophyll fluorescence as a fiinction of depth for all stations along the
HN transect in September and October 2000. ................................................................. 34

Figure 1.4a: The fiaction of 02 saturation as a fiinction of depth for all stations along the
HN transect in April, May and June 2000. Note: Mid-station HNO70 was sampled in
June instead of HNO90. .................................................................................................... 35

Figure 1.4b: The fraction of 02 saturation as a fimction of depth for all stations along
the HN transect in July and August 2000. ....................................................................... 36

Figure 1.4c: The fiaction of 02 saturation as a fiinction of depth for all stations along the
HN transect in September and October 2000. ................................................................. 37

Figure 1.5a: The isotopic composition of 02 as a fiinction of depth for all stations along
the HN transect in April, May and June 2000. Note: Mid-station HNO70 was sampled in
June instead ofHNO90. .................................................................................................... 38

Figure 1.5b: The isotopic composition of 02 as a function of depth for all stations along
the HN transect in July and August 2000. ....................................................................... 39

ix

Figure 1.5c: The isotopic composition of 02 as a fiinction of depth for all stations along
the HN transect in September and October 2000. ............................................................ 40

Figure 1.6a: Ratios of respiration to photosynthesis (R:P ratios) as a function of depth at
all stations along the HN transect in April through June 2000. Note: Mid-station HNO70
was sampled in June instead of HNO90. .......................................................................... 41

Figure 1.6b: Ratios of respiration to photosynthesis (R:P ratios) as a fiinction of depth at
all stations along the HN transect in July and August 2000. ........................................... 42

Figure 1.6:: Ratios of respiration to photosynthesis (R:P ratios) as a fimction of depth at
all stations along the HN transect in September and October 2000. ............................... 43

Figure 1.7: Temperature as a fiinction of depth for all stations along the West—East
transect in April and August 2001. .................................................................................. 44

Figure 1.8: Chlorophyll fluorescence as a fiinction of depth for all stations along the
West-East transect in April and August 2001. ................................................................. 45

Figure 1.9: The fraction of 02 saturation as a function of depth for all stations along the
West-East transect in April 2001. .................................................................................... 46

Figure 1.10: The isotopic composition of 02 as a fiinction of depth for all stations along
the West-East transect in April 2001. .............................................................................. 47

Figure 1.1 1: The R:P ratios as a fiinction of depth for all stations along the West-East
transect in April 2001. ...................................................................................................... 48

Figure 1.12: The fi’action of 02 saturation as a function of depth for all stations along the
West-East transect in August 2001. ................................................................................. 49

Figure 1.13: The isotopic composition of 02 as a function of depth for all stations along
the West-East transect in August 2001. ........................................................................... 50

Figure 1.14: The R:P ratios as a function of depth for all stations along the West-East
transect in August 2001. .................................................................................................. 51

Figure 1.15: R:P ratios as a function of temperature for the western and central sections
(df= 17, or = .05) and the eastern section (df= 9, ct = .05) in August 2001. ................... 52

Figure 1.16a: The fi'action of 02 saturation as a flinction of the isotopic composition of
02 during April, May and June 2000, within the mixed layer along the HN transect. The
locus of the fraction of 02 saturation equal to 1.00 and the isotopic composition equal to
0.7 %o represents a system at atmospheric 02 saturation. Data points within quadrant II
represents R:P ratios greater than 1.0 or net heterotrophy, within quadrant III R:P ratios
equal to 1.0 or a balance of respiration to photosynthesis, and within quadrant IV R:P
ratios less than 1.0 or net autotrophy. .............................................................................. 53

Figure 1.16b: The fi'action of 02 saturation as a function of the isotopic composition of
02 during July and August 2000, within the mixed layer along the HN transect. The
locus of the fi'action of 02 saturation equal to 1.00 and the isotopic composition equal to
0.7 %o represents a system at atmospheric 02 saturation. Data points within quadrant II
represents R:P ratios greater than 1.0 or net heterotrophy, within quadrant III R:P ratios
equal to 1.0 or a balance of respiration to photosynthesis, and within quadrant IV R:P
ratios less than 1.0 or net autotrophy. .............................................................................. 55

Figure 1.16c: The fiaction of 02 saturation as a fiinction of the isotopic composition of
02 during September and October 2000, within the mixed layer along the HN transect.
The locus of the fraction of 02 saturation equal to 1.00 and the isotopic composition
equal to 0.7 %o represents a system at atmospheric 02 saturation. Data points within
quadrant II represents R:P ratios greater than 1.0 or net heterotrophy, within quadrant III
R:P ratios equal to 1.0 or a balance of respiration to photosynthesis, and within quadrant
IV R:P ratios less than 1.0 or net autotrophy. .................................................................. 57

Figure 1.17: 02 gas exchange as a function of R:P ratios during periods of net autotrophy
(df = 2, or = .05), periods of respiration equal to photosynthesis (df = 7, ct = .05), and
periods of net heterotrophy (df = 7, a = .05) for all depths within the mixed layer for all
stations along the HN transect in April through October 2000. Note: For the delineation
of each period see Figures 1.16a-c. . ................................................................................. 59

Figure 2.1: The location of the stations comprising the EPREX transect. Note: The
shaded area approximates the NPSG. .............................................................................. 82

Figure 2.2: Sigma 9 as a function of depth for all stations along the EPREX transect
fi'om May 24 to June 28, 2000. ........................................................................................ 83

Figure 2.3: Fluorescence as a function of depth for all stations along the EPREX
transect from May 24 to June 28, 2000. ........................................................................... 84

Figure 2.4: [NOg'] as a function of depth for all stations along the EPREX transect fiom
May 24 to June 28, 2000. ................................................................................................. 85

Figure 2.5: [02] as a fiinction of depth for all stations along the EPREX transect fi'om
May 24 to June 28, 2000. ................................................................................................. 86

Figure 2.6: The fi'action of 02 saturation as a function of depth for all stations along the
EPREX transect fi'om May 24 to June 28, 2000. ............................................................. 87

Figure 2.7: The isotopic composition of 02 as a firnction of depth for all stations along
the EPREX transect from May 24 to June 28, 2000. ....................................................... 88

Figure 2.8: Ratios of respiration to photosynthesis (R:P ratios) as a function of depth at
all stations along the EPREX transect from May 24 to June 28, 2000. ........................... 89

Figure 2.9: Sigma 9, fluorescence and [N03] as a function of depth prior to and after
the storm at SS from June 14 to 16, 2000. ....................................................................... 90

Figure 2.10: Sigma 6, fluorescence and [N03] as a fiinction of depth prior to, during and
after the storm at S6 from June 19 to 22, 2000. .................... . ........................................... 91

Figure 2.11: [02], the fraction of 02 saturation and the isotopic composition of 02 as a
function of depth prior to and after the storm at SS from June 14 to 16, 2000. ............... 92

Figure 2.12: [02], the fraction of 02 saturation and the isotopic composition of 02 as a
function of depth prior to, during and after the storm at S6 from June 19 to 22, 2000. ..93

Figure 2.13: Ratios of respiration to photosynthesis (R:P ratios) as a fimction of depth
for storm events prior to and alter the storm at SS fi'om June 14 to 16, 2000 and prior to,

during and after the storm at S6 from June 19 to 22, 2000. ............................................. 94
Figure 2.14: The A170 values of 02 for stations ALOHA, S3, S5 and S6. .................... 95

xii

Chapter 1

TEMPORAL AND SPATIAL VARIATIONS IN R:P RATIOS IN LAKE SUPERIOR

Background

Perhaps the most firndamental measure of whole lake metabolism is the balance
between the rates of respiration and primary production (R:P). In addition, since
respiration in excess of primary production results in the net production of C02, the
balance between these processes also controls the flux of this important greenhouse gas
to the atmosphere from lakes (Cole et al., 1994; del Giorgio et al., 1997). In a closed
ecosystem with no external inputs, all of the organic matter generated by primary
production may potentially be metabolized by respiration and the rates of these two
processes may thus be equal. Natural systems, however, are rarely in balance. R:P ratios
less than 1.0 are commonly the result of excessive nutrient loading and occur when
autochthonous organic carbon production exceeds that which can be readily metabolized
(Odum and Prentki, 1978; del Giorgio and Peters, 1993). This excess organic carbon is
either buried or lost fi'om the system by outflowing water. Respiration may exceed
photosynthesis in response to allochthonous inputs of organic carbon from rivers or
groundwater (Odum and Prentki, 1978; Cole et al., 1989; del Giorgio and Peters, 1993;
Cole et al., 2002). Spatial and temporal variation in R:P ratios result because external
inputs of organic carbon and nutrients may not be rapidly distributed evenly across a lake
and there may be a time variant decoupling of primary production and respiration in

response to thermal stratification.

Within the last 10 years R:P ratios in lakes have been shown to correlate with
phosphorus and chlorophyll concentrations, and this observation suggests that the
primary controls on R:P in lakes are nutrient levels or the magnitude of phytoplankton
biomass (del Giorgio and Peters, 1994). In this manner, R:P ratios provide a foundation
for evaluating the trophic state of a lake, in which eutrophic systems are characterized by
R:P ratios less than 1.0 and oligotrophic systems by values greater than 1.0. The balance
between respiration and photosynthesis is not, however, simply affected by nutrient
levels but also responds dynamically to external inputs, internal recycling and physical
controls such as thermal structure. Since low levels of primary production characterize
oligotrophic systems, the delicate balance between primary production and respiration is
expected to be particularly sensitive to alterations within these environments.

The main objective of this study is to quantify temporal and spatial variations in
the R:P ratios of Lake Superior, the largest of the Laurentian Great Lakes. Low
phytoplankton biomass and production, the presence of a deep chlorophyll maximum
(DCM), and phosphorus limitation of algal production define Lake Superior as an
oligotrophic system (Matheson and Munawar, 197 8; M01] and Stoermer, 1982; Guildford
et al., 1994). A variety of land-use activities (urban, forest, agriculture, and wetlands)
within the drainage basin (Matheson and Munawar, 1978; Phillips, 1978; Weiler, 1978;
Robertson, 1997) result in variation in the magnitude of allochthonous organic carbon
influx and nutrient levels across the lake. The dimictic nature of Lake Superior affords
an opportunity to evaluate the response of R:P to both complete vertical water column
mixing and thermal stratification (Bennett, 1978a). In addition, the relatively short

mixing time of a few years and the long flushing time of 177 years (Matheson and

Munawar, 1978; Bennett, 1978b) result in a rapid distribution of organic carbon and
nutrients across the lake, and a slow return to preconditions following episodic influx
events. All these factors indicate that Lake Superior is an ecosystem sensitive to external
loadings, and consequently the magnitude of R:P ratios is likely to vary markedly on
temporal and spatial scales.

Fluxes of 02 in Lake Superior are strongly controlled by atmospheric gas
exchange in addition to community respiration and phytoplankton photosynthesis. Lake
Superior, with a large surface area (82,100 kmz), and low rate of primary productivity
(Putnam and Olsen, 1966; Fahnenstiel and Glime, 1983; Fee et al., 1992), maintains [02]
at or near levels in equilibrium with the atmosphere at all times. Little variation in [02]
occurs seasonally or with depth, and saturation levels are rarely below 80 % or above
110% (Weiler, 1978; Matheson and Munawar, 1978), which reflects the predominance of
atmospheric gas exchange as the primary control on [02]. Previous studies using 6180-02
have been largely focused in eutrophic systems that exhibit a wider range of 02 saturation
values (Quay et al., 1995; Wang and Veizer, 2000). A second objective of this study,
therefore, is to utilize 880-02 and the fraction efo2 saturation to evaluate the
relationship between atmospheric ()2 flux and R:P ratios under oligotrophic conditions in
which atmospheric equilibration largely controls 02 concentrations.

Methods
Study site and sample analysis

To provide detailed temporal and spatial profiles of R:P ratios in Lake Superior,

two transects were studied (Figure 1.1). Sampling along the Houghton North (HN)

transect was performed on the RV Laurentian in conjunction with Michigan

Technological University and the Keweenaw Interdisciplinary Transport Experiment in
Superior (KITES) program (chmac2.chem.mtu.edu/KITES/kites.html). Three stations,
one coastal and two offshore, were located northwest of the Keweenaw Peninsula (Figure
1.1, Table 1.1). These stations were sampled monthly from April to October 2000 at five
depths: the surface (~5 m), three depths bracketing the chlorophyll maximum, and
approximately 5 or 10 m above the bottom. In April and August 2001, a West-East
transect was sampled aboard the RV Lake Guardian, during the Environmental Protection
Agency’s biannual monitoring survey of the Great Lakes (http://www.epa.gov/glnpo/
monitor.html). Seven stations in April and eight stations in August comprised the West-
East transect that extended from Duluth, Minnesota, to outside Whitefish Bay (Figure
1.1, Table 1.1). Each station was sampled at the surface (between 1 and 4 m), three
samples within the epilimnion, thermocline, and upper hypolimnion, and two samples at
depth (5 and 10 m above bottom). Water column samples for all the stations, from both
transects, were analyzed for [02], 6180-02, and the 8‘80-H20.

A SeaBird Electronics CTD profiler determined the temperature and fluorescence
as a function of depth within the water column. Water was collected using 5 L lever-
action Niskin samplers on the HN transect, and a rosette containing twelve 8 L Niskin
samplers on the West-East transect. A modified Winkler method was used to determine
[02] (Carpenter, 1965; Emerson et al., 1999). The analysis of6180-H20 was performed
by Mountain Mass Spectrometry in Evergreen, Colorado via a MultiPrep and reduction
furnace system designed and developed at the facility.

Collection of water samples for determination of the isotopic composition of 02

and R:P ratios followed the protocol of Emerson et al. (1991;1999). Samples were

collected in pro-evacuated 200 mL glass vessels fitted with high vacuum stopcocks. Prior
to use, 1 mL of saturated HgCl2 was added to each vessel and dried, to eliminate
biological activity following water collection. Immediately before and after samples
were collected, the vessel inlet was flushed with C02 to displace air. Upon returning to
the laboratory, headspace gases were equilibrated at a constant temperature water bath (~
24 °C) for at least 4 hours, under continuous rotation. After equilibration, water was
removed by vacuum until 1 mL remained in the vessel and inlets were flushed again with
C02 to prevent air contamination from potential leakage across the stopcock seals.
Determination of 6180-02 was accomplished using a gas chromatograph
interfaced to a stable isotope ratio mass spectrometer (Roberts et al., 2000). The sample
vessel was connected to an inlet system on the gas chromatograph that consisted ofi in
series, an ascarite trap to remove water and C02, a 3 mL gas sampling loop between two
Valco sampling valves (one 6 port and one 4 port), a vacuum isolation valve, and a
vacuum pump. Initially, the inlet system was completely evacuated before closing the
isolation valve separating the inlet system from the vacuum pump. The stopcock on the
vessel was opened and sample gas was allowed to equilibrate for 10 seconds within the
inlet system. Upon rotation of the Valco valves, sample gas was carried by Helium flow
onto a 5 m by 1/8” OD molecular sieve 5 A GC column, and N2 and 02 were separated in
time. Any residual water or C02 entering the GC column was efficiently trapped onto the
molecular sieve column and removed later by heating. The effluent of the gas
chromatograph was routed to the mass spectrometer and sample isotopic ratios were
determined by comparison to a reference pulse of previously characterized pure O2 tank

standard. Stable isotope ratios for O are expressed in per mil (%o) notation:

8180:I(Rsamplo/Rstandard)’1Im1000 (1)
where R is the ratio of 180 to 160. All 8’80-02 values are expressed with respect to air,
which is enriched in ‘80 by 23.5 is. relative to VSMOW, resulting in a 5‘80 value of
VSMOW, with respect to air, of -23.5 %o.
Determination of 02 gas exchange
The rate of air/water gas exchange was determined from the following

relationship (Emerson et al., 1995):
For = - G02 ([02] - [02] est) (2)
where F02 is the air/water gas exchange rate, G02 is the gas transfer coeflicient, [O2] is

the concentration of 02 in the water column, and I02]sat is the saturation concentration of

O2 (Benson and Krause, 1984; Garcia and Gordon, 1992).

Gas transfer coefficients (G02) were calculated from wind speeds using the
empirical relations of Clark et al. (1995), with daily wind speeds determined by
averaging hourly data generated from Buoy number 45006 (47° 19.06’ N, 39° 51.56’ W)
and compiled by the National Data Buoy Center (http://www.nodc.noaa.gov/BUOY/bgl.
html).

Determination of R:P ratios

Photosynthesis, respiration and gas exchange at the air/water interface control the
concentration and isotopic composition of 02 (Bender and Grande, 1987; Quay et al.,
1995). These three processes are represented by the following equation:

d[02]/dt=P—R+Fo2 (3)

where d[02]/dt is the change in the concentration of 02 over time, P is the rate of
photosynthesis, and R is the rate of respiration.

Air/water gas exchange is generally the primary process controlling the isotopic
composition and concentration of 02 within the water column. The 8180-02 in the
atmosphere is defined as 0 %o with respect to air. A small fractionation effect during
dissolution results in 180 enrichment of 02 in surface waters by approximately 0.7 960
(Knox et al., 1992). The 02 produced during photosynthesis is derived from water
(Stevens et al., 1975; Guy et al., 1993), and the 8‘80-H2O with respect to the air standard
in Lake Superior was determined to have an average isotopic value of -3 1 .5 a; 0.1 %o (n =
30). Photosynthesis, therefore, not only increases [02], but also results in a decrease in
6180—02. In contrast, during respiration, 02 is consumed and the residual 02 pool is
enriched in 18o by a kinetic isotope effect, in which the lighter 16o isotope is
preferentially consumed (Kiddon et al., 1993). Each of these processes uniquely affects
both the concentration and isotopic composition of 02. In general, 8180-02 values equal
to 0.7 %o reflect the predominant influences of atmospheric gas exchange, those less than
0.7 %o reveal the contribution of 02 from photosynthesis, and values greater than 0.7 %o
indicate the effect of 02 consumption by respiration (Bender and Grande, 1987). The
concentration and isotopic composition of the residual 02 pool will, therefore, reflect the
balance of these processes at any depth in the water column.

Based on the influences of photosynthesis, respiration and air/water gas exchange
on the concentration and isotopic composition of O2, equation 3 is expanded (Quay et al.,

1995)

dl‘g’moydt = Goa/z «gallon... ”“60. a.) — [02] ‘8"60} + (4)
P 18/16OW ap _ R 18/160 01’

18/16

where Z is depth, Ow is the measured isotopic composition of H20, cap is the
fi'actionation factor associated with photosynthesis (1.0000; Guy et al., 1993), 18”“0 is

the measured isotopic ratio of 02, orr is the fractionation factor associated with respiration
(0.9770; Luz et al., 2002), as is the fl’actionation factor associated with gas transfer
(0.9972; Knox et al., 1992), 18/160, is the isotopic ratio of atmospheric 02 (Kroopnick and

Craig, 1972), and as is the fractionation factor associated with gas dissolution (1.0073;

Benson and Krause, 1984). Fractionation factors are defined here as a ratio of the

reaction rates of the heavy, 180, to light, 1"0, isotope.

No measured value of the fractionation factor associated with respiration ((1,) for

Lake Superior has been published. The value of or,- may vary depending on the metabolic

diversity and planktonic species composition of the system (Kiddon et al., 1993; Luz et
al., 2002). Lake Superior’s plankton community is generally known to be dominated by
various phytoplankton species, however, potentially a large bacterial component may be
present due to the oligotrophic state of the lake (Munawar and Munawar, 1978; Scavia

and Laird, 1987; del Giorgio et al., 1997; Barbiero and Tuchman, 2001b). Previously

published or, values for bacterially dominated systems are approximately 0.982 (Quay et

al., 1995). Lacking an actual evaluation of the relative importance of bacteria and

phytoplankton to community respiration, a literature value of 0.977, the annual average

ctr for Lake Kinneret, was used for this study (Luz et al., 2002).

R:P ratios are calculated from the measured values of [02] and 5180-02 (Quay et

al., 1995):
R/P=(18/160w %- 18/16Og)/(l8/16O af_ 18/1608) (5)
“”1603 = a, (W160, cts — ([02]/[02]sat) 18”60} / {1 — ([021/[021sat)} (6)

R:P ratios greater than 1.0 signify a dominance of respiration, values less than 1.0
indicate a dominance of photosynthesis, and a value equal to 1.0 represent a balance of
respiration and photosynthesis (del Giorgio and Peters, 1993; 1994; Quay et al., 1995).

The air/water gas transfer rate (F02 in equation 2) is no longer included in
equations 5 and 6, since steady state of the system is assumed (Quay et al., 1995). At
steady state any changes in the concentration or isotopic composition of the residual O2
pool from the 02 gas exchange flux will be offset by the biological 02 fluxes of
photosynthesis and respiration, and an equilibrium of the total 02 flux is reached for the
system. The gas transfer rate becomes a function of the fluctuations of the concentration
and isotopic composition of 02 during photosynthesis and respiration and, therefore, is
not calculated directly from this method and not required to determine R:P ratios.

Results
HN transect (April through October 2000)

Complete mixing of the water column was evident in April, May and June,
resulting in only slight variations in temperature and fluorescence among stations and
with depth (Figures 1.2a, 1.3a). The initiation of a fluorescence peak was present in May
at stations HNOSO and HNO90, and all stations had a well-defined fluorescence peak in
June. Station HNOSO had the highest relative fluorescence value (Figure 1.3a).

Undersaturation of 02 was prevalent for the entire water column, for all stations, for the

April-June period (Figure 1.4a). Little variation was present in the 5180.0. in April and
May, and all values in the upper 50 m, at all stations, were greaterthan 0.7 96o,
representing a predominance of respiration over photosynthesis (Figure 1.5a).
Throughout the water column during spring, all stations were not heterotrophic with R:P
ratios greater than 1.0, and only a slight decrease in R:P ratios occurred from April to
June (Figure 1.6a).

The summer was a period of strong thermal stratification that formed a barrier to
gas exchange between the upper epilimnion and lower hypolimnion (Figure 1.2h). In
July, the shallowest epilimnion of this study was present. There was a progressive
increase in the depth of the epilimnion from July to August (Figure 1.2b). Maximum
fluorescence peaks below the thermocline were evident in July, indicating a deep
chlorophyll maximum (DCM) was prevalent at all stations. A reduction was apparent in
the magnitude of the fluorescence peaks from July to August at all stations, however a
DCM was still present (Figure 1.3b). Areas of slight supersaturation of 02 were evident
at all stations within the upper 30 m in July and August, but the depth interval in which
supersaturation was present differed among stations (Figure 1.4b). Minimum values of
6180-02 were found near areas of supersaturation for all stations (Figure 1.5b). R:P ratios
less than 1.0 were also evident within the upper 30 m of the water column, which
overlapped the areas of supersaturation and minimum 5180-02 (Figure 1.6b). In August,
an area of undersaturation of 02 was present at or below 45 m, for all stations. In this
region of the water column, some of the highest 6180-02 and R:P ratios for the entire

study were evident, signifying strong net heterotrophy (Figure 1.6b).

10

The water column was in transition between summer stratification and complete
fall turnover in September and October. Thermal stratification continued to weaken at all
stations during this period, and a decrease in surface water temperature was evident
(Figure 1.2c). A reduction in fluorescence was present at all three stations, and in
contrast to other months, fluorescence peaks in October were located within the
epilimnion (Figure 1.3c). The entire water column became undersaturated in 02 in
September, with the exception of station HNZlO at 45 m, which was supersaturated. In
October, the fiaction of 02 saturation values were equal to or less than 1.0, for the whole
water column at all stations (Figure 1.4c). At all depths, in September, 6180-02 values
were approximately 0.7 96o, except station I-INZlO at 35 and 40 m, where values were less
than 0.7 %o. In October at stations HNO90 and HN210, 8180-02 values were
approximately 0.7 %o, reflecting a predominance of gas exchange. At station HNOSO
8180-02 values varied from approximately 0.7 %o at S and 25 m, to greater than 0.7 %o at
20 and 30m (Figure 1.5c). The R:P ratios for all stations in September were greater than
1.0 signifying net heterotrophy, with the exception of station HNZIO at 40 m. Variations
in R:P ratios among stations were evident in October. All values at stations HNOSO and
HN210 were greater than 1.0, except at station HNOSO at 5 m. At station HNO90, R:P
ratios were less than 1.0 at 5 and 15 m, and were greater than 1.0 in the remaining upper
50 m (Figure 1.6c).

West-East transect (April and August 2001)

A well mixed water column lacking a distinct fluorescence peak was present for

all seven stations of the April 2001 West-East transect, reflecting conditions similar to the

April 2000 HN transect (Figure 1.7, 1.8). Once again, the entire water column was

11

undersaturated in 02 for all stations (Figure 1.9). All 6180-02 values were greater than
0.7 %o, signifying a predominance of respiration, similar to values observed in April
along the HN transect (Figure 1.10). The R:P ratios for the West-East transect and the
HN transect were also similar, with all individual R:P ratios greater than 1.0, representing
net heterotrophy for both transects during this period (Figure 1.11).

Thermal stratification was evident across the transect in August, however,
variations in the magnitude and depth of maximum 02 saturation and minimum 6180-02
values were apparent among stations fiom west (SU19) to east (SU01) (Figures 1.7,
1.12). All stations had a similar mixed layer depth of approximately 10 m. A DCM was
present at all stations, and the location of the maximum peak increased with depth from
west to east (Figure 1.8). Supersaturation of 02 was apparent at all stations, and was
located at or above the thermocline at the western and central stations (SU19 to SU10),
and extended below the thermocline at the eastern most stations (SU08 to SUOl). The
DCM was undersaturated in 02 at all stations except SUOl, the most eastern station
(Figures 1.8, 1.12). Values for the 6180-02 were less than 0.7 %o at all stations for the
majority of depths sampled, representing a predominance of photosynthesis (Figure
1.13). Net autotrophy was present across the transect at all stations. At stations SU19
through SU10 R:P ratios less than 1.0 were evident at or above the thermocline, and at
stations SU08 and SU04, R:P ratios less than 1.0 extended fiom surface to below the
thermocline, and throughout the DCM at station SUOl. Finally, R:P ratios greater than
1.0 were present at the DCM for all stations, except for station SUOl (Figure 1.14).

A distinct water column profile of both physical and biogeochemical parameters

was present at station SU22B (Figures 1.7, 1.12), in contrast to the other seven stations

12

that comprised the West-East transect. The epilimnion was deeper, located at 24 m, with
a surface water temperature ~ 18.0 °C, and a fluorescence peak within the epilimnion,
near the surface at 7m (Figure 1.7, 1.8). No DCM was present. Supersaturation in 02
was apparent at or above the thermocline, and the remaining upper 50 m was
undersaturated (Figure 1.7, 1.12). The 6180-02 values were less than 0.7 %o at and above
7m, approximately 0.7 %c at the thermocline to 41 m, and greater than 0.7 %c at
49 m (Figure 1.13). Minimum R:P ratios less than 1.0 were present at or above the
thermocline, signifying strong net autotrophy, and values greater than 1.0, representing
net heterotrophy, were evident below the thermocline (Figure 1.14).
Discussion

Two main factors predominantly influence variations in the balance of respiration
to photosynthesis within aquatic ecosystems: the flux of organic carbon entering the lake,
and the magnitude of primary production within the lake (W issmar et al., 1977; Odum
and Prentki, 1978; Scavia and Laird, 1987; del Giorgio and Peters, 1993; 1994).
Temporal and spatial variations in R:P ratios are not only a reflection of the balance of
respiration and photosynthesis (del Giorgio and Peters, 1993; 1994) but are also
influenced by physical mixing and stratification (Bennett, 1978a). Respiration and
photosynthesis also have a strong affect on O2, and therefore the concentration and
isotopic composition of 02 provides a basis to determine R:P ratios (Bender and Grande,
1987; Quay et al., 1995). This approach, however, has been rarely applied in a large
oligotrophic system on a spatial and temporal scale. This study uses [02] and 6‘80-02

within two multi-station transects in Lake Superior to quantify R:P ratios, and to

13

understand the factors controlling variations in the ratio within an oligotrophic system on
a seasonal and lake-wide basis.
Seasonal variations in R:Pratios

Within temperate lakes extensive mixing during the unstratified period in winter
and early spring results in lake-wide homogeneity of biogeochenrical and physical
characteristics. This trend is evidenced in Lake Superior by constant temperature and
fluorescence profiles with depth from April through June along the HN transect (Figure
1.2a, 1.3a). Furthermore, similarities in temperature, fluorescence and [02] between the
April 2000 IN and April 2001 West-East transects indicate that the homogeneity of
water column characteristics is a common feature of early spring and extends across the
lake (Figure 1.2a, 1.3a, 1.7, 1.8). Under these conditions of an extensively mixed water
column, the [O2] is expected to be at levels close to equilibrium with the atmosphere.
During April and May, however, uniform undersaturation in [02] and across lake R:P
ratios greater than one are present (Figure 1.4a, 1.6a, 1.9, 1.11), signifying excess
respiration over photosynthesis. Prolonged water column mixing may result in
resuspension of bottom sediments, potentially releasing organic carbon from past seasons
(Phillips, 1978). In addition, influx of allochthonous organic carbon fi'om spring
snowmelt and precipitation events adds to the input of organic carbon to the lake during
this time (Matheson and Munawar, 1978; Phillips, 1978; Robertson, 1997). The overall
result is an excess of respiration over photosynthesis and, thereby, extensive net
heterotrophy during April and May. This excess respiration, therefore, results in a period
of undersaturated [02] throughout the water column and, consequently, a net CO2 efflux

to the atmosphere.

14

Net autotrophy is expected in June since observations of high diatom biomass and
decreases in [Si] suggest increasing levels of productivity in Lake Superior (El-Shaarawi
and Munawar, 197 8; Matheson and Munawar, 1978; Munawar and Munawar, 1978;
Weiler, 1978; Barbiero and Tuchman, 2001b). The observations of a slight increase in
fluorescence values, and a decrease in 6180-02 values, are consistent with an increase in
rates of photosynthesis and the formation of a late spring bloom (Figure 1.3a, 1.5a).
Undersaturation in [02] and R:P ratios greater than 1.0, however, seem inconsistent with
an increase in diatom biomass (Figure 1.4a, 1.6a). Autochthonous production at this
time, therefore, remains insufficient to override the excess respiration over
photosynthesis present in April and May. Overall, the result is persistent net
heterotrophy during June (Figure 1.6a).

In contrast to the uniformity of the spring water column, variations with depth in
temperature, light and nutrient concentrations due to seasonal stratification result in
spatial heterogeneity of summer R:P ratios. Regions with low 6180—02 values,
supersaturated [O2], and R:P ratios less than 1.0 are indicative of an overall metabolism
in which photosynthesis exceeds respiration. The depth of this net autotrOphy within the
water column was not consistent, however, between coastal and offshore stations or
across the lake (Figure 1.6b, 1.14). At coastal station HNOSO, R:P ratios less than 1.0 are
restricted to the upper epilimnion in July, in contrast to the two offshore stations of the
HN transect (Figure 1.6b). In addition, R:P ratios less than 1.0 are confined to the region
at or above the thermocline within the western and central portions of Lake Superior
(Figure 1.14). These results are consistent with previous studies that show temperature to

be a significant variable influencing phytoplankton production in Lake Superior,

15

especially during periods of thermal stratification (Nalewajko and Voltolina, 1986).
Temperature and R:P ratios were significantly correlated in the western and central
regions during summer (Figure 1.15), than in spring (It2 = 0.22, P = 0.001, or = .05, df=
18), indicating a phytoplankton community sensitive to changes in temperature
(Nalewajko and Voltolina, 1986). Lake Superior may be similar to many oligotrophic
fieshwater systems that maintain a large bacterial and phytoplankton community, whose
population and/or activity is influenced by temperature (Cotner and Biddanda, 2002; del
Giorgio et al., 1997; Currie, 1990; Scavia and Laird, 1987). Earlier coastal warming of
the water column in spring may, therefore, lead to accelerated chlorophyll formation and
summer phytoplankton production at inshore areas as compared to offshore regions
(Bennett, 1978a; Nalewajko and Voltolina, 1986). Community metabolism within the
lake may also be enhanced if a large bacterial population is present to respire dissolved
organic matter (DOM) and facilitate the transfer of some of this organic carbon to higher
trophic levels (Cotner and Biddanda, 2002; Azam, et al., 1983; Azam and Ammerman,
1984; Fuhrman, 1992).

Temperature, in contrast, is not as strong an influence on R:P ratios in the eastern
section of Lake Superior (Figure 1.15). Within this region and the offshore stations of
the HN transect, R:P ratios less than 1.0 are not restricted to the warmer epilimnion but
extend below the thermocline and throughout the DCM (Figure 1.2b, 1.6b, 1.7, 1.14).
Other factors, potentially light and/or nutrient concentrations, are the main drivers of
lower R:P ratios in the eastern section of the lake (Bennett, 1978b). The 1% light level
has been reported as deep as 30 m in the eastern section of Lake Superior and at 20 m off

the Keweenaw Peninsula, and therefore, above these depths light may not be a limiting

16

factor to phytoplankton production (Schertzer et al., 197 8). The observation of net
photosynthesis below the thermocline may also be the result of low-light adapted
phytoplankton that take advantage of higher nutrient concentrations within the
hypolimnion (Moll and Stoermer, 1982; Fahnenstiel and Glime, 1983; F ahnenstiel et al.,
1984; Barbiero and Tuchman, 2001a). The nutrient concentrations at this depth may be
enhanced within the eastern section of Lake Superior as a result of water mass exchange
between the lake and Whitefish Bay (Bennett, 1978b). Prior studies of the Straits of
Mackinac and the outlets of Lake Huron into Georgian Bay have reported a back flow of
water below the thermocline during stratified periods (Saylor and Sloss, 1976; Schertzer
et al., 1979). Phosphorus concentrations have been estimated to be 50 % higher entering
the St. Mary’s River from Whitefish Bay than leaving Lake Superior to the bay (Bennett,
1978b), implying half the St. Mary’s River phosphorus influx from Lake Superior and
Whitefish Bay is derived within the bay. High nutrient concentrations within Whitefish
Bay are suggested to be the result of the large land drainage area to water body area of
the bay, hence, the relatively small volume of water in the bay receives nutrient loadings
from a large portion of the surrounding drainage basin (Bennett, 197 8b). Nutrients from
Whitefish Bay, therefore, may enter Lake Superior during summer and become available
to phytoplankton within the hypolimnion, ultimately contributing to the summer period of
net autotrophy.

In addition to temperature, light and nutrients, another factor resulting in net
autotrophy in Lake Superior during the summer is a reduction in the input of
allochthonous organic carbon (del Giorgio and Peters, 1993; 1994) from tributary

influxes and sediment resuspension. The reduction in the fiequency and intensity of

17

precipitation events during the summer minimizes the magnitude of terrestrial run-off to
Lake Superior (Phillips, 1978). This, in turn, lowers the input of total dissolved solids
and organic matter fi'om the drainage basin to the lake, resulting in a decrease in the
availability of allochthonous organic carbon to fuel heterotrophic microbial respiration
(Phillips, 1978; del Giorgio and Peters, 1994; Robertson, 1997). In addition, the
development of a thermocline creates a barrier to vertical water movement from the
hypolimnion to the epilimnion, filrther impeding resuspension of sediments (Bennett,
1978a). Thus the supply of allochthonous organic carbon to the epilimnion may limit
respiration and, thereby, lower R:P ratios. The upper water column, consequently,
becomes extensively net autotrophic (Figure 1.6b, 1.14), augmented by generally higher
phytoplankton production in summer within the mixed layer as compared to spring (Fee,
et al, 1992). Net autotrophy within the summer, therefore, may result from a period of
decreased allochthonous organic carbon fluxes to the upper water column, reducing
respiration and not necessarily an increase in phytoplankton production.

Rates of phytoplankton productivity are typically greater in the fall than summer
in Lake Superior (Putnam and Olson, 1966; Fahnenstiel and Glime, 1983) as the fall
phytoplankton community utilizes nutrients redistributed as stratification deteriorates
(Munawar and Munawar, 197 8). Indeed, photosynthesis is dominant over respiration in
October, at station HNO90 (Figure 1.6c). Unexpectedly, respiration remains predominant
throughout the upper 50 m at stations HNOSO and HNZIO in October, and at all stations
in September, as signified by R:P ratios greater than 1.0 (Figure 1.6c). Autochthonous
organic carbon, from summer net autotrophy in the shallow epilimnion, may become an

organic carbon source at depths, since this organic carbon was originally produced at

18

surface during a period of intense restriction to vertical water movement (Bennett,

1978a). As this organic carbon is redistributed throughout the upper water column during

intense mixing in response to surface cooling and increased fall storm events, net

respiration prevails throughout the upper 50 m (Figure 1.6c). The spatial heterogeneity

of R:P ratios during the fall is, thereby, due to variations in the redistribution of nutrients

and/or organic carbon resulting in areas of photosynthesis or excess respiration.
Ecological implications - Duluth, Minnesota (Station SU22B)

One key factor that defines Lake Superior as an oligotrophic system is the
presence of a seasonal DCM (M011 and Stoerrner, 1982). In August, a DCM was present
below the thermocline at all stations, except at the western-most station (SU22B), near
Duluth, Minnesota, which had a chlorophyll maximum above the thermocline (Figure
1.7, 1.8). A chlorophyll maximum above the thermocline is consistent with a
mesotrophic or eutrophic system, defined by higher rates of phytoplankton productivity
than an oligotrophic environment (Moll and Stoermer, 1982). Indeed, some of the lowest
R:P ratios of this study are present within the chlorophyll maximum at station SU22B,
signifying a strong predominance of photosynthesis over respiration (Figure 1.8, 1.14).
Increased nutrient loading fi'om urban areas within the surrounding drainage basin is a
likely factor in shitting station SUZZB to a mesotrophic state. Previous studies have
reported relatively high phosphorus concentrations within the water column near Duluth
compared to other regions in Lake Superior, (Matheson and Munawar, 1978; Munawar
and Munawar, 1978; Weiler, 1978), potentially affecting trophic state by increasing
phytoplankton biomass. In addition, the increased nutrient loading around Duluth, may

affect trophic state in other areas of the lake, since the short mixing to long flushing times

19

of Lake Superior results in a rapid distribution of nutrients across the lake, and a slow
return to preconditions following high influx. The duration of this study was not
sufficient to resolve such long term alterations to lake-wide trophic state, however, the
sensitivity of the trophic state within the region of Lake Superior immediately
surrounding the area of increased nutrient loading was revealed.
Ecological implications — the microbial loop and climate change

Previous studies have indicated that the nricrobial loop is important to nutrient
cycling and energy transfer in oligotrophic environments (Fuhrman, 1992; Legendre and
Rassoulzadegart 1995; Biddanda et al., 2001; Cotner and Biddanda, 2002).
Bacterioplankton may be a large component of the heterotrophic biomass in oligotrophic
systems, and these organisms may dominate community respiration (del Giorgio et al.,
1997; Biddanda et al., 2001; Cotner and Biddanda, 2002). Within Lake Superior, greater
than 95 % of the less than l-um size fi'action are heterotrophic bacteria, which perform
82-91 % of planktonic respiration (Biddanda et al., 2001). Growth efficiencies of
bacterioplankton in Lake Superior are low, however, ranging from 4-13 % (Biddanda et
al., 2001). The heterotrophic community in Lake Superior, therefore, is comprised of a
large population of small organisms converting most of the organic carbon from primary
production back to C02. Only protozoans less than or equal to S-um consume small
bacterioplankton, thereby reducing the quantity of organic carbon available for larger
sized zooplankton (Ducklow et al, 1986; Legendre and Rassoulzadegan, 1995; Cotner
and Biddanda, 2002). In addition, previous studies estimate that greater than 50 % of the
phytoplankton biomass in oligotrophic systems is picoplankton, and hence a substantial

portion of nutrients released during protozoan grazing on bacterioplankton may be

20

assimilated by small sized phytoplankton (Ducklow et al., 1986; Legendre and
Rassoulzadegan, 1995; Bell and Kalff, 2001). Overall, an active microbial loop foodweb
may restrict a large portion of the organic carbon and nutrients to a “closed” microbial
community, diverting organic carbon fi'om higher trophic levels within the foodweb, and
thereby constraining fishery yields (Legendre and Rassoulzadegan, 1995; Kemp and
Smith, 2001; Cotner and Biddanda, 2002).

Climate change within the Great Lakes region has the potential to alter the current
foodweb dynamics of Lake Superior. Recent studies have determined that low rates of
primary production from picoplankton and high rates of respiration from
bacterioplankton result in overall net heterotrophy of oligotrophic lakes (del Giorgio et
al., 1997; Bell and Kalff, 2001). Indeed, based on R:P ratios, this study has shown Lake
Superior to be net heterotrophic, especially during the long period of spring turnover
(Figure 1.2a, 1.6a). Only during a short period of thermal stratification in July and
August was the lake net autotrophic (Figure 1.2b, 1.6b). Climate change, however, has
increased surface water temperatures and led to reduced ice cover within the Great Lakes
during winter. During ice-free periods mixing by winter storms may result in increased
nutrient release from sediment resuspension (Hanson et al., 1992; Mortsch and Quinn,
1996; Magnuson et al., 1997 ). Furthermore, an increase in surface water temperatures
may lead to an earlier occurrence of thermal stratification of the water column in spring
(Magnuson et al., 1997). An earlier onset of stratification may reduce the period of
strong net heterotrophy in spring and increase the duration of the period of summer net
autotrophy in Lake Superior. Finally, high nutrient concentrations from prolonged winter

mixing would favor phytoplankton as nutrient competitors over bacterioplankton.

21

Nutrient cycling would thereby be limited in bacterioplankton during spring and the
ability of these consumers to utilize the early spring influx of allochthonous organic
carbon may be reduced (Bentzen et al, 1992; Cotner and Wetzel, 1992; Coveney and
Wetzel, 1995; Cotner and Biddanda, 2002). The overall result would likely be larger
sized phytoplankton and zooplankton, which in turn would be directly available to higher
trophic levels (Legendre and Rassoulzadegan, 1995). Climate change, therefore, may
shift Lake Superior from a foodweb dominated by a microbial community to a shorter
foodweb, funneling more organic carbon to higher trophic levels and the potential to
increase fish populations. This shift in the foodweb may not occur, however, if a longer
period of thermal stratification leads to an earlier depletion of nutrients within the
epilimnion (Magnuson, et al., 1997). In this case, the period of net autotrophy may not
be extended but merely shifted from summer to early spring, leaving insufficient time for
larger sized phytoplankton to develop. Nutrient depletion would ultimately favor
bacterioplankton over phytoplankton during nutrient uptake (Bentzen et al., 1992; Cotner
and Wetzel, 1992; Coveney and Wetzel, 1995; Cotner and Biddanda, 2002), and hence,
reduce phytoplankton production. Indeed, recent studies within Lake Michigan suggest
that climate change will follow this second scenario and ultimately lower current rates of
offshore primary production (Brooks and Zastrow, 2002). In the future, therefore,
climate change is likely to enhance the active microbial community in Lake Superior and
further divert organic carbon from higher trophic levels, and ultimately fisheries.

Relationship between 02 gas exchange and R:P ratios in an oligotrophic lake

An influence of 02 gas exchange on [02] was expected within Lake Superior,

however, biological fluxes were shown to equally affect 6180-02 and the fraction of O2

22

saturation despite low rates of primary productivity and planktonic biomass in
oligotrophic environments. Net autotrophy was evident during July, August and October,
and was characterized by supersaturation ofO2 and 6180-02 values less than 0.7%.
signifying a predominance of photosynthesis (Figure 1.16b, 1.16c). A significant
correlation was found between R:P ratios and 02 gas exchange (Figure 1.17) as
photosynthesis from summer and fall phytoplankton blooms increased [02] to
supersaturated levels within the upper water column, and created an O2 efflux to the
atmosphere from the lake. In contrast, net heterotrophy was present in April and May as
indicated by an undersaturation of O2 and 6180-02 values greater than 0.7 %o representing
a predominance of respiration (Figure 1.16a). R:P ratios and 02 gas exchange are
significantly correlated during this period (Figure 1.17). April and May is a period of
extensive water column mixing and low phytoplankton biomass resulting in inputs of
atmospheric O2 and low rates of photosynthesis, respectively. The undersaturation of 02,
therefore, is from respiration in excess of photosynthesis, which in turn drives the influx
of 02 from the atmosphere to the lake. Finally, June and September are periods in which
R:P ratios reveal equal rates of respiration and photosynthesis (Figure 1.16a, 1.16c). At
this time a significant correlation was not found between R:P ratios and 02 gas exchange
(Figure 1.17). These periods represent a transition between early spring net heterotrophy
and summer net autotrophy and between summer net autotrophy and late fall net
heterotrophy, creating a temporary balance between respiration and photosynthesis
(Figure 1.16a, 1.16b, 1.16c). Consequently, these are also periods of transition in
atmospheric O2 flux, since biological fluxes are driving the physical flux. During these

periods, therefore, a small change in respiration or photosynthesis at one location in the

23

lake leads to a corresponding small change in the atmospheric 02 flux. Since the water
column is in transition during this time, however, the changes in respiration and
photosynthesis that are affecting the variation in the atmospheric O2 flux at one location
in the lake may not be reflected in other parts of the lake. Overall, the spatial variations
in respiration, photosynthesis and 02 gas exchange characteristic of June and September
lead to a large scale spatial heterogeneity in R:P ratios and 02 gas exchange and a poor
correlation between the two variables during this time. Significant correlations between
R:P ratios and 02 gas exchange in oligotrophic systems, therefore, are strongly dependent
on the biological processes of photosynthesis and respiration even within environments
of low primary production. If this relationship between R:P ratios and 02 gas exchange
proves robust in more detailed studies and in other environments, R:P ratios have the
potential to be utilized as an indicator of 02 gas exchange and ultimately could be used to
determine the CO2 flux between a lake and the atmosphere.
Conclusions

Lake Superior, similar to many oligotrophic systems, was found to be
predominately net heterotrophic, although the balance of respiration to photosynthesis
within the lake shifted on a seasonal basis. The intensity of net heterotrophy was
strongest in spring when organic carbon, fireling excess respiration, is more readily
available presumably due to resuspension of sediments or high tributary influx.
Respiration prevailed even though cold water temperatures were evident and extensive
water column mixing indicated that 02 influx from the atmosphere must have been
substantial. Net autotrophy was extensive across the lake in summer, with temperature,

light, nutrient concentrations, and the reduction of allochthonous organic carbon inputs

24

during thermal stratification controlling the intensity and location of these regions. Lake
Superior, therefore, was not consistently net heterotrophic on a temporal scale, but the
balance between respiration and photosynthesis shified on a seasonal basis and a short
period of lake-wide net autotrophy was observed during summer thermal stratification.

With the exception of the station near Duluth, Minnesota, Lake Superior was
spatially homogeneous with respect to R:P ratios. A relatively short nrixing time of a
few years relative to a long flushing time of 177 years (Matheson and Munawar, 1978;
Bennett, 1978b) allows for lake-wide homogeneity of nutrients and organic carbon,
resulting in a similar pattern in R:P ratios across the lake. The predominance of net
autotrophy near Duluth indicates Lake Superior’s potential to develop mesotrophic
conditions as a consequence of high nutrient loadings from urbanized areas. A
movement toward mesotrophy emphasizes the extreme sensitivity of a predonrinantly net
heterotrophic lake to potential filture increases in organic carbon and nutrient loadings,
and the ability of 6180-02 to monitor these variations.

Results from this study are consistent with a foodweb heavily dominated by an
active microbial community, however, climate change may redefine the overall foodweb
dynamics of Lake Superior. Other studies have shown that a majority of small sized
consumers, grazers and primary producers maintains a large portion of organic carbon
and nutrient cycling within a “closed” microbial loop, diverting resources fi'om higher
trophic levels, including fish populations. Climate change has increased surface water
temperatures within the Great Lakes and may lead to earlier onset and extended periods
of thermal stratification. Earlier stratification may create a prolonged period of net

autotrophy in the lake that could result in a shorter foodweb and a potential increase in

25

fishery yield. More likely, an extended period of stratification will result in earlier
nutrient depletion in the epilimnion. Our expectation, therefore, is that climate change
will enhance metabolism within the microbial community and potentially compromise
fishery yields.

Despite the oligotrophic nature and low levels of primary production, biological
factors proved significant in controlling the physical flux of O2 in Lake Superior. The
role of 02 gas exchange within the lake is demonstrated by a clustering of all data around
the atmospheric O2 flux axis (Figure 1.16a, 1.16b, 1.16c). Respiration and
photosynthesis drive [02] in Lake Superior and thereby create atmospheric O2 influx
during periods of higher respiration to photosynthesis and atmospheric O2 efflux during
periods of lower respiration to photosynthesis. Consequently, a significant relationship
between R:P ratios and 02 gas exchange was found during periods of net autotrophy and
net heterotrophy. This correlation may enable the use of R:P ratios as an indicator of 02
gas exchange and an alternative measurement of C02 flux between the atmosphere and

freshwater lakes.

26

SAMELINQfiIIES

 

 

 

HN TRANSECT
Maximum Distance
Station Latitude (N) Longitude (W) Depth (m) from Shore (km)
HNOSO 47° 17.2' 88° 36.9' 110 5
HNO70 47° 18.1' 88° 37.8' 120 7
HN090 47° 19.0' 88° 38.8' 110 9
HN210 47° 24.3' 88° 44.2' 160 21
WESTiAST TRANSE_CI
Maximum Distance from
Station Latitude (N) Longitude flV) Depth 4111) Western Shore (km)
SUZZB 46° 47.6' 91° 45.0' 55 15
SU19 47° 22.2' 90° 51 .2' 192 80
SU16 47° 37.3' 89° 27.7' 184 183
SU12 47° 51 .3' 88° 02.5' 243 289
SU10 47° 30.8' 89° 32.7' 159 329
SU08 47° 36.3' 86° 49.0' 307 382
SU04 47° 15.5' 86° 20.8' 149 422
SU01 46° 59.5' 85° 09.9' 96 515

Table 1.1: Latitude and longitude coordinates, maximum depth and distance
to shore for all stations along the the HN and West-East transects. Note: Mid-
station HNO70 was sampled in June instead of HN090.

27

 

 

 

 

 

HN210 _
HN090.
HNOSO'
0
§U12 suoa a:
sure surs- - _ suor
suro su04 _
suzze
0 25 50
_
MILES

 

 

 

Figure 1.1: Locations of the stations comprising the West-East
and HN (insert) transects in Lake Superior.

28

Temperature (°C) April 28, 2000

 

 

 

0 15 20 25
10
E 20
f} 30
o 40
HNOSO
5° HN090
so ------- HN210

Temperature (°C) May 11, 2000
0 5 1 0 1 5 20 25

o I I I I I I

 

 

10
E 20
g 30
8 4o
50 ——HN050
60 —HN090
------ HN210
Temperature (°C)1 June 22, 2000
O 20 25
o I j
10 -
E 20 -
g. 30 -
8 4o-
—HN050
5° ' —HN070
60 . ------ HN210

 

Figure 1.28: Temperature as a function of depth for
all stations along the HN transect in April through June
2000. Note: Mid-station HNO70 was sampled in

June instead of HNO90.

29

Temperature (°C) July 30, 2000
0 5 1 0 1 5 20 25

 

 

 

 

 

 

------ HN210

 

Temperature (°C) August 25, 2000

 

 

 

 

5 30 d

8

o 40 -
50 - -—l-lN050
60 ------ HN210

 

 

 

Figure 1.2b: Temperature as a function of depth for all stations
along the HN transect in July and August 2000.

30

Temperature (°C) September 27, 2000

O 5 1O 15 20 25
o I

I I 4

 

 

 

HNOSO
.1
6° ------ HN210

 

 

 

 

Temperature (°C) October 22, 2000
0 5 1 0 1 5 20 25

 

 

 

 

0 l I L I
10 ‘
:E: 20 -
5 30 '
O.
0
D 40 d
50 ‘ HNOSO
60 . —HN090
------ HN210

 

 

 

Figure 1.2c: Temperature as a function of depth for all stations
along the HN transect in September and October 2000.

31

Fluorescence (RFU) April 28, 2000
0.03 0.06 0.08 0.11 0.14

 

 

 

o L , I I I
104
E 2° ‘
g 30 -
0
0 401
50 - ——HN050
J ——HN090
6° ------ HN210

 

 

 

Fluorescence (RF U) May 11, 2000
0.03 0.06 0.08 0.11 0.14

 

 

 

 

o a a I
10 j
g I
5 30 -
O
50 ‘ _HNW
60 . —HN090
------ HN210

 

 

 

Fluorescence (RFU) Jule 22, 2000
0.03 0.08 0.08 0.11 0.14

 

 

 

 

10 -
i 30 - ‘1
I)
0 40-1
5° ‘ —HN050
60 , -—HN070
------ HN210

 

 

 

Figure 1.3a: Chlorophyll fluorescence as a function
of depth for all stations along the HN transect in April
through June 2000. Note: Mid-station HNO70 was
sampled in June instead of HN090.

32

Fluorescence (RFU) July 30, 2000
0.03 0.06 0.08 0.11 0.14

 

 

 

 

 

 

 

 

 

 

 

0 I j
10 d
E 20 d
5 30 - 33-"
O
0 40 1
50 cl _HN050
—HN090
6° ‘ ------ HN210
Fluorescence (RF U) August 25, 2000
0.03 0.06 0.08 0.11 0.14
‘i
10 r
a 30- h
m ..
o 40 a
50 1 —HN050
60 . —HN080
------ HN210

 

 

 

Figure 1.3b: Chlorophyll fluorescence as a function of depth for
all stations along the HN transect in July and August 2000.

33

Fluorescence (RFU) September 27, 2000
0.03 0.06 0.08 0.11 0.14

 

 

 

 

0 i I I 4I

10 -

A “i,

E, 20 4 /

5 30 .

<1 l

o ':

D 40 d “I
50 r ' HNOSO

d —HN090

60 ------ HN210

 

 

 

Fluorescence (RFU) October 22, 2000
0.03 0.06 0.08 0.11 0.14

 

 

 

 

 

 

0 I I I j
10 r
E 20 -
:5 30 r
O.
O
O 40 .1
50+ HNOSO
60 . HN090
------ HN210

 

 

 

Figure 1.3c: Chlorophyll fluorescence as a fiinction of depth for
all stations along the HN transect in September and October 2000.

34

Fraction of O2 Saturation April 28, 2000

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

0

 

J; I I I I I I

 

 

—o—HN050
+1-01on
---A- - -HN210

 

 

 

Fraction of O2 Saturation May 11, 2000

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

0
101
Em‘
€301
8404
so.

 

60-

I I J I I L I

 

 

+HN050
—a—HN090
-- -A- - -HN210

 

 

 

Fraction 0102 Saturation June 22,2000
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

0
10-

€20-

vao‘

§

0404
50+
604

 

Figure 1.4a: The fraction of 02 saturation as a function
of depth for all stations along the HN transect in Apirl,
May and June 2000. Note: Mid-station HNO70 was

I I I I I I j

 

 

—o—HN050
-a—l-mo70
- - a- - -HN210

 

 

 

sampled in June instead of HNO90.

35

Fraction of 02 Saturation July 30, 2000
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

 

 

 

 

o I I I I I I I
10-1
A J
E, 20
5 30-
O.
0)
c3 40-
50‘ A +HN050
+HN080
60 ---A--HN210

 

 

 

Fraction of O2 Saturation August 25, 2000
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

 

 

 

 

0 I I I I I I I
10‘
E 20‘
5 301
O.
0
o 40-
50‘ -o—HN050
60‘ +HN090
---A--HN210

 

 

 

Figure 1.4b: The fraction of 02 saturation as a filnction of depth for all
stations along the HN transect in July and August 2000.

36

Fraction of O2 Saturation September 27, 2000
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

 

 

 

 

0 I I I I I I I
101
E 20‘
5 30'
Q.
0
0 40- "A
50* -e—HN050
60- +HN080
---A--HN210

 

 

 

Fraction of O2 Saturation October 22, 2000
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

 

 

 

 

0 L I I I I I I
10 r
E 20 -
C a
'5. 30
O
0 40« A
50 ‘ —-0-—HN050
—E—HN090
J
60 -~A--HN210

 

 

 

Figure 1.4c: The fraction of 02 saturation as a firnction of depth for all
stations along the HN transect in September and October 2000.

37

Woo2 (11.) April 28, 2000

 

 

 

 

 

 

 

 

 

 

 

 

 

—2.9 -2.0 -1.1 -0.2 0.7 1.6 2.5
o I I I I I I
10-
:20-
g 30«
0
O 40-
50' —e—HN050
60- -a—HNoeo
---A--'HN21O
8180-02 (11,.) May11,2000
-2.9 —2.0 -1.1 -0.2 0.7 1.6 2.5
o I I I I I J
101
:20-
g 30 -
O
o 40-
50' —-e—HN050
60‘ +HNOQ)
---A--»HN210
6130—02 (11.) June 22, 2000
-2.9 —2.0 -1.1 02 0.7 1.6 2.5
o I I I I I I
10-
:20-
30-
i
c 40- ‘
50‘ .15 —e-—HN050
60- —e—HN070
---A---HN21O

Figure 1.511: The isotopic composition of 02 as a
function of depth for all stations along the HN transect
in April, May and June 2000. Note: Mid-station
HNO70 was sampled in June instead of HNO90.

38

omo-o2 (11.) July 30, 2000
-2.9 -2.0 -1.1 -0.2 0.7 1.6 2.5

 

 

 

 

o I I I I I J
10‘
E 20-
5 30'
O.
0
c: 401
501 —-e-HN050 "A
1 +HN090
60 ---A--HN210

 

 

 

6180-02 (11...) August 25, 2000
-2.9 -2.0 -1.1 -0.2 0.7 1.6 2.5

0 I L j I I J

 

 

60 ‘ +HN090
- - a- - HN210

 

 

 

 

Figure 1.5b: The isotopic composition of 02 as a firnction of depth for
all stations along the HN transect in July and August 2000.

39

5‘30-02 (95.) September 27, 2000
-2.9 -2.0 -1.1 oz 0.7 1.6 2.5

 

o I I L I I J
10-

E20-

fiSO-

o ,A'

0 40- A

 

 

50 1 —o—HN050
- - ~A - - HN210

 

 

 

 

6180-02 (95.) October 22, 2000
-2.9 -2.o -1.1 oz 0.7 1.6 2.5

O I I I L I A

 

 

50 - —0-HN050
+HN090
- - it - - HN210

 

 

 

 

Figure 1.5c: The isotopic composition of 02 as a function of depth for
all stations along the HN transect in September and October 2000.

40

R:P ratio April 28, 2000
0.5 1.0 1.5 2.0 2.5 3.0

 

 

 

 

o I I I I I
10-
320‘
£130"
3 40-
50‘ —o—HN050
60‘ +HN090
---A--HN210

 

 

 

R:P ratio May 11, 2000
0.5 1.0 1.5 2.0 2.5 3.0

 

 

 

 

o I I I I I
101
Ex"
5 304
8‘
o 40‘
50‘ —O—HN050
60' +Hm
"-A" HN210

 

 

 

R:P ratio June 22, 2000
0.5 1.0 1.5 2.0 2.5 3.0

 

 

 

o I I I I I
101
E”
5 30-
8
0 40¢
604 +HN070
--~A--HN210

 

 

 

Figure 1.6a: Ratios of respiration to photosynthesis
(R:P ratios) as a function of depth at all stations along

the HN transect in April through June 2000. Note: Mid-
station HNO70 was sampled in June instead of HNO90.

41

R:P ratio July 30, 2000
0.5 1.0 1.5 2.0 2.5 3.0

 

 

 

 

o I L I I I
10‘

A .l

g 20

5 sol

O.

0

0 40d
504 'A -0—HN050

—a—HN090

60‘ ---A--HN21O

 

 

 

R:P ratio August 25, 2000
0.5 1.0 1.5 2.0 2.5 3.0

0 I I J I I

 

 

5° ‘ —e—HN050
- - A - - HN210

 

 

 

 

Figure 1.6b: Ratios of respiration to photosynthesis (R:P ratios) as a

function of depth at all stations along the HN transect in July and August
2000.

42

0.5

10«
AzoJ
£30.
40‘

(m

Dept

 

60d

 

 

R:P ratio September 27, 2000
1.0 1.5 2.0 2.5 3.0

I I I I j

 

—'9—HN050
+HN090
- - fl - - HN210

 

 

 

R:P ratio October 22, 2000
1.0 1.5 2.0 2.5 3.0

I I j

 

-9— HN050
-E- HN090

---A--HN210

 

 

 

Figure 1.6e: Ratios of respiration to photosynthesis (R:P ratios) as a
function of depth at all stations along the I-[N transect in September and

October 2000.

43

Temperature (°C) Western Section 2001
0 5 10 15 20 25

 

 

 

 

 

 

 

 

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

o I I
J
10 - /
E 20 at .
i 3° ‘
3 4° ‘ SU16Apr
50 ‘ —SU19Apr
—SU16Aua
60 - SU19Aug
------- SU228 Aug
Temperature (°C) Central Section
2001
0 5 10 15 20 25
o I I z”... I I
10 1 ”2
E 2°i
i 3°i
8 404
—--su12Apr
60 - -------------- SU10Aug
SU12Aug
Temperature (°C) Eastern Section 2001
0 5 10 15 20 25
O I I I I I
10 -
’2‘ 2° ‘
i 3° ‘
8 40 d SUO1 Apr
—su04Apr
50 1 —summ
60 . _SUO1 M
SU04Aug
suoeAug

 

 

 

 

Figure 1.7 : Temperature as a function of depth for all
stations along the West-Bast transect in April and
August 2001.

Fluorescence (RF U) Western Section 2001
0.0 0.8 1.6 2.4 3.2 4.0

 

 

   
 

 

 

o I II
10
g 20
g sol
8 40‘ smeApr
—SU19Apr
50 . —SU16Aug
—su19Aug
60 ‘ ------- suzzemg

 

 

 

Fluorescence (RFU) Central Section 2001

0.0 0.8 1.6 2.4 3.2 4.0
0

   
  

 

 

 

 

 

10 q u '1 ..............
‘.\
A 20 . J
g _ ,,-r
i 3° ‘
3 4o-
50 . [ SU10Apr
SU12Apf
60 . ....... sumAug
SU12Aug

 

 

 

 

Fluorescence (RFU) Eastern Section
2001

0.0 0.8 1.6 2.4 3.2 4.0

 

 

 

 

 

 

O I I II
10
g 20
‘5 30
3
0 4° sum Apr
50 SU04 Apr
60 ——-su01 Aug
sum Aug
suoa Aug

 

 

 

Figure 1.8: Chlorophyll fluorescence as a ftmction of
depth for all stations along the West-East transect in
April and August 2001.

45

Fraction of Oz Saturation Western Section
April 2001

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

0 I I I I I I I I

Depth(m)
8 8 8 8 8 a

—e—su19
—e—sute
Fraction of 02 Saturation Central Section

April 2001
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

I I I I I I I I

Depth(m)
8 ‘e" 8 8 B 3 e

+SU12
+SU1O

Fraction of 02 Saturation Eastern Section

 

 

April 2001
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15
0 I I I é I I I I
10
g 20
5 30
O.
8 4o
60 +SU04
---G---SU01

 

 

 

Figure 1.9: The fraction of 02 saturation as a function

of depth for all stations along the West-East transect in
April 2001.

46

313002 (in) Western Section April 2001
-2.9 -2.0 -1.1 oz 0.7 1.6 2.5

o I I I I I I

 

60 - —0-SU19
—O—SU16
t3"’o-o2 (at...) Central Section April 2001

-2.9 -2.0 -1.1 -0.2 0.7 1.6 2.5

o I I I I I I I

Depth (m)
0) 01 # on N —-
O O O O O o

+SU12
+SU10
5130-02 (*0) Eastern Section April 2001

-2.9 -2.0 -1.1 -0.2 0.7 1.6 2.5

 

 

 

0 L I I I I I I
10
g 20
5 30
S
o 40
50 —er-suoe
60 +SUO4
non-sum

 

 

 

Figure 1.10: The isotopic composition of 02 as a

function of depth for all stations along the West-East
transect in April 2001.

47

Depth (m)

Depth (m)

Depth (m)

R:P ratio Western Section April 2001

 

 

 

 

 

 

 

 

0.5 1.0 1.5 2.0 2.5 3.0
0 . . 1 . .
10 -
20 -
30 -
4o -
50 -
—e—su16
R:P ratio Central Section April 2001
0.5 1.0 1.5 2.0 2.5 3.0
0 . . 1 . .
10 4
20 -
30 -
40 -
50 s
R:P ratio Eastern Section April 2001
0.5 1.0 1.5 2.0 2.5 3.0
0 . 1 . . .
10 -
20 -
30 -
40 .
50 - —a—suoe
60 ‘ —B—SU04
no --suor

 

 

 

Figure 1.11: The R:P ratios as a fimction of depth for
all stations along the West-East transect in April 2001.

48

Fraction of 02 Saturation Western Section

 

 

 

August 2001
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15
o I
10-
E I
*2 3° ‘
0
O 401
50- qty—$0223
—e—su19
50‘ non-sure

 

 

 

 

Fraction of 02 Saturation Central Section
August 2001

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15

o I I I I- I I I

10‘
320‘
§3°‘
840.

50-
604

 

+SU12
*SU1O

 

 

 

 

Fraction of 02 Saturation Eastern Section
August 2001
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15
o I

    

I I I L I I
n

 

50 ‘ +sum
GOJ +SU04
- - o- - .3001

 

 

 

 

Figure 1.12: The fraction of 02 saturation as a fimction

of depth for all stations along the West-East transect in
August 2001.

49

5% (at...) Western Section August 2001
-2.9 -2.0 -1.1 -o.2 0.7 1.6 2.5

 

 

 

50 - +SU228
—o—su19
6° ‘ -- e - -su1e

 

 

 

6180 (%o) Central Section August 2001
-2.9 -2.o -1.1 -o.2 0.7 1.6 2.5

o I I I I L I

 

 

60 g —a—Su12
—a—su1o

 

 

 

5% (%o) Eastern Section August 2001

-2.9 -2.0 -1.1 -0.2 0.7 1.6 2.5
0

10‘
€20“

 

 

 

5° ‘ —e—suoe
60 . —a—su04
- - o- - -suo1

 

 

 

 

Figure 1.13: The isotopic composition of 02 as a

function of depth for all stations along the West-East
transect in August 2001.

50

R:P ratio Western Section August 2001

0.5 1.0 1.5 2.0 2.5 3.0

o 0... I I I I

 

 

—&—SU223
—O—SU19
60 ---0---SU16

 

 

 

 

R:P ratio Central Section August 2001

0.5 1.0 1.5 2.0 2.5 3.0
O I I I a a

 

60 A +3012
—&—SU10
R:P ratio Eastern Section August 2001
0.5 1.0 1.5 2.0 2.5 3.0

I I I I

 

+SU08
+SU04
- - O- - - SU01

 

 

 

 

 

Figure 1.14: The R:P ratios as a function of depth for
all stations along the West-East transect in August 2001.

51

Western and Central Section
Stations SU10. SU12, SU16, SU19, SUZZB

 

 

 

 

 

 

August 2001
2.0 - y=-0.044x+1.4099
R2 = 0.81
1.5 « P =1.87e’7
.Q
9 .
0. 1.0
a:
0.5 d
0.0 . . r u r
0 5 10 15 20 25
Temperature (°C)
Eastern Section
Stations SU01, SU04, SU08
August 2001
2'0. y=-0.0198x+1.0926
R2 = 0.15
1.5 1
o P = 0.235
‘43 O
h 1.0 ‘ _‘ o
E o o 04-
0.5 4 °
0-0 I r i r 1
0 5 10 15 20 25
Temperature (°C)

Figure 1.15: R:P ratios as a function of temperature for the western and
central sections (df = 17, a = .05) and the eastern section (df = 9, a = .05)
in August 2001.

52

Figure 1.1611: The fraction of 02 saturation as a function of the isotopic composition of
02 during April, May and June 2000, within the mixed layer along the HN transect. The
locus of the fraction of 02 saturation equal to 1.00 and the isotopic composition equal to
0.7 %o represents a system at atmospheric 02 saturation. Data points within quadrant H
represents R:P ratios greater than 1.0 or net heterotrophy, within quadrant HI R:P ratios
equal to 1.0 or a balance of respiration to photosynthesis, and within quadrant IV R:P
ratios less than 1.0 or net autotrophy.

53

April 28, 2000

 

 

 

 

 

 

 

 

 

 

 

 

2.5- m 1.
,.. 1.6- fig
go, 0.7
g -024
3° 4.11
20 III. IV.
0.80 0.85 0.90 0.95 1.00 1.05 1.10
Fraction of 02 Saturation
May11,2000
2'5. n. 1.
A 1.6a
s fioc
N 0.7
-0.2«
'°° -1.1-
20 m‘ N
0.80 0.85 0.90 0.95 1.00 1.05 1.10
Fraction of 02 Saturation
June 22,2000
2.5-
II. I
g 1.6‘
s. 0.7
8
$30 -0.2‘ 0 go
-1.1 4 %
-2o “1' N“
0.80 0.85 0.90 0.95 1.00 1.05 1.10
Fraction of 02 Saturation
Figure 1.16a

54

Figure 1.16b: The fraction of 02 saturation as a function of the isotopic composition of
02 during July and August 2000, within the mixed layer along the HN transect. The
locus of the fraction of 02 saturation equal to 1.00 and the isotopic composition equal to
0.7 %o represents a system at atmospheric 02 saturation. Data points within quadth II
represents R:P ratios greater than 1.0 or net heterotrophy, within quadrant III R:P ratios
equal to 1.0 or a balance of respiration to photosynthesis, and within quadrant IV R:P
ratios less than 1.0 or net autotrophy.

55

 

July 30, 2000

2'5 q n. I.
A 1.6‘
v... 0.7 O
”('3 -0.2 -t * O

“3 -1.1-l

 

III. IV.

-2.0 r r r # r

0.80 0.85 0.90 0.95 1.00 1.05 1.10
Fraction of 02 Saturation

 

 

 

August 25, 2000

2.5- I.
A 1.6d
‘3 0.7
5') -0.2-l
7° -1.1-

-2.0 r r r r
0.80 0.85 0.90 0.95 1.00 1.05 1.10

Fraction of 02 Saturation

 

III. IV.
j

 

 

 

Figure 1.16b

56

Figure 1.16c: The fiaction of 02 saturation as a fiinction of the isotopic composition of
02 during September and October 2000, within the mixed layer along the HN transect.
The locus of the traction of Oz saturation equal to 1.00 and the isotopic composition
equal to 0.7 %o represents a system at atmospheric 02 saturation. Data points within
quadrant H represents R:P ratios greater than 1.0 or net heterotrophy, within quadrant III
R:P ratios equal to 1.0 or a balance of respiration to photosynthesis, and within quadrant
IV R:P ratios less than 1.0 or net autotrophy.

57

September 27, 2000

 

 

 

 

 

 

 

 

 

 

2.0-i 11' l.
2‘; 1.0-
65' Q
«(’5 0.0"‘ 060
'30 -1.0-l
-20 m‘ “L
0.80 0.85 0.90 0.95 1.00 1.05 1.10
Fraction of 02 Saturation
October22,2000
2.5-
II. I.
3 1.64
"g. 0.7 8
0
£3 -0.2«
'°° 4.11
‘20 m. I t r t Iv. I
0.80 0.85 0.90 0.95 1.00 1.05 1.10
Fraction of 02 Saturation
Figure 1.16c

58

Figure 1.17 : 02 gas exchange as a function of R:P ratios during periods of net
autotrophy (df = 2, or = .05), periods of respiration equal to photosynthesis (df = 7, or =
.05), and periods of net heterotrophy (df = 7, ct = .05) for all depths within the mixed
layer for all stations along the HN transect in April through October 2000. Note: For the
delineation of each period see Figures 1.16a-c.

59

 

Periods of Net Autotrophy

 

 

2.0 s
y a 14.325x + 0.8957
R2 . 0.99
g 1'5 i P - 0.003
g, 1.0 .
m M
0.5 i
000 I r I I r U I i
-0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06
02 Gas Exchange (moi/m2 day)

Periods of Respiation equal to Photosynthesis

y= 1.w7x+ 1.1014

 

 

 

2.0 a R2 8 0.33
1.5 ‘ P = 0.107
_O
‘3 c_ 3 W
‘- 1.0 -l
9:
I!
0.5 -l
0.0 I T I U I I I i
-0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06
02 Gas Exchange (mollmz day)
Periods of Net Heterotrophy
2.0 -

1.5J (V9
O

 

 

'3 .

9: y = 11.594)( + 1.0038

m it2 = 0.67
0.5 . P . 0.013
0.0 I I I r I I f *I

-0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06
02 Gas Exchange (movm2 day)
Figure 1.17

60

Chapter 2

TEMPORAL, SPATIAL AND STORM RELATED CHANGES IN R:P RATIOS IN
THE NORTH PACIFIC OCEAN

Background

Phytoplankton photosynthesis within oceanic environments is estimated to
contribute approximately one half of total global primary production (Falkowski, 1994;
Duarte and Agusti, 1998; Williams, 1998; Karl, 1999). New phytoplankton production
within the ocean mixed layer provides, via the biological pump, a source of organic
carbon to the water column below the pycnocline, which is then available to fuel
heterotrophic respiration at depth (Volk and Hoffert, 1985; F iedler et al., 1991). In
addition, photosynthesis within the ocean surface waters results in atmospheric C02
drawdown and the ocean, therefore, acts as a C02 sink (V olk and Hoffert, 1985;
Sarmiento and Siegenthaler, 1992). Respiration within the mixed layer has the potential
to reduce the flux of organic carbon from surface waters to regions below the pycnocline
(Geider, 1992). Furthermore, if a predominance of respiration over photosynthesis exists
within surface waters, the ocean would be a C02 source to the atmosphere (Smith and
Mackenzie, 1987; Duarte and Agusti, 1998; Williams, 1998). Previous studies have
disagreed on the global ratio of respiration to photosynthesis (R:P ratio) within the upper
ocean, claiming both overall net heterotrophy (R:P ratio greater than 1) and overall net
autotrophy (R:P ratio less than 1) (Smith and Mackenzie, 1987; Williams, 1998). Duarte
and Agusti (1998) further suggest that there exists a spatial variation in R:P ratios, such
that oligotrophic environments are net heterotrophic, and eutrophic environments are net

autotrophic and that there is an overall balance between respiration and photosynthesis

61

within the global ocean. The resolution of this debate is required in order to determine
the status of the world’s ocean as a source or sink of CO2, especially during the current
period of increasing atmospheric CO2 levels (Lashof and Ahuja, 1990; Falkowski, 1994).

In order to address this question, this study uses the concentration and isotopic
composition of O2 to calculate R:P ratios and gross 02 production (Bender and Grande,
1987; Quay et al., 1995; Luz et al., 1999; 2000) along a transect from the open ocean
oligotrophic waters of the North Pacific subtropical gyre (NPSG) to the coastal upwelling
zone of the eastern tropical North Pacific (ETNP). The spatial extent of this transect,
across regions of varying nutrient concentrations, [02], and degrees of phytoplankton
productivity, allows for evaluation and comparison of R:P ratios within contrasting ocean
environments. This study, therefore, is an extensive evaluation of the balance between
respiration and photosynthesis across a broad spatial scale within the central and eastern
North Pacific ocean.

The ETNP is an active area of tropical cyclone formation, with the second highest
number of named tropical cyclones compared to all other oceanic regions (Vincent and
Fink, 2001). Such episodic disturbances are believed to enhance autotrophic production,
as increased wind speeds during storms intensify water column mixing and redistribute
nutrients and phytoplankton biomass to surface waters from below the pycnocline
(Falkowski, 1994; Karl, 1999). Due to the intensity of the tropical cyclones (Vincent and
Fink, 2001), few in my studies of the effects of storm events on phytoplankton
production have been attempted within the Pacific ocean (Ditullio and Laws, 1991).
During this study, however, two tr0pical cyclones formed in the ETNP, offering a rare

opportunity to quantify variations in nutrient concentrations, [02] and the isotopic

62

composition of 02 before, during and after a storm event. A second objective, therefore,
was to evaluate the effects of an episodic disturbance on the balance of respiration to
photosynthesis, and evaluate the impact of storm events on phytoplankton productivity
within the ocean mixed layer.
Methods

Sampling during the Eastern Pacific Redox Experiment (EPREX) cruise was
performed aboard the RV Roger Revelle, fi'om May 24 to June 28, 2000. Six stations
were sampled along a transect consisting of station ALOHA, located 100 km north of the
island of Oahu within the NPSG, a station on the fiinge of the NPSG, $2, and four
stations within the ETNP, located along 16° N from 136° W to 98° W, S3 through 86
(Figure 2.1). Four additional cruises to station ALOHA, three aboard the RV Moana-
Wave during, October 17-21, 1998 (HOT98), January 11-15, 1999 (HOTlOl), and April
12-16, 1999 (HOT104), and the Aka Aka Ea cruise aboard the RV Ka’imikai-O-Kanaloa
during July 20-28, 1999 (AKA) were conducted as part of the Hawaii Ocean Time Series
study. At each station water was collected at discrete depths throughout the water
column for analysis of nutrients, [O2], and 5180—02, using a rosette containing 24 Niskin
sampling bottles. In addition, water was collected during the EPREX cruise at station
ALOHA, S3, S4, SS (prior to the storm) and S6 (after the storm) for 5'70-02 analysis. A
SeaBird Electronics CTD profiler was deployed to determine temperature, salinity and
fluorescence as a firnction of depth within the water column. A modified Winkler
method was used to measure [02] (Grasshoffet al., 1933). The analysis of the amortzo

was performed at the University of Hawaii, Honolulu, following the method of Epstein

63

and Mayeda (1953). Water samples from the EPREX cruise were analyzed for [N03'] by
the University of Washington Technical Services, Seattle, Washington.

Collection of water samples for determination of the isotopic composition of O2
and R:P ratios followed the protocol of Emerson et al. (1991,1999). Samples were
collected in pre-evacuated 200 mL glass vessels fitted with high vacuum stopcocks. Prior
to use, 1 mL of saturated HgCl2 was added to each vessel and dried, in order to eliminate
biological activity following collection. Immediately before and after samples were
collected, the vessel inlet was flushed with CO2 to displace air. Headspace gases were
equilibrated at a constant temperature water bath (~ 24 °C) for at least 4 hours, under
continuous rotation. Afier equilibration, water was removed by vacuum until 1 mL
remained in the vessel. Due to limited sampling vessels and storage space, the majority
of samples collected during the EPREX cruise were cryogenically transferred within a
high vacuum system onto molecular sieve 5 A 1/ 16” x 1/4” Alltech pellets and sealed in
10” pyrex tubes for later onshore analysis. Samples not transferred remained in sampling
vessels and the inlets were flushed periodically with CO2 to prevent air contamination
from potential leakage across the stopcock seals.

Upon returning to the lab, samples stored on molecular sieve were cryogenically
transferred, under vacuum, back to sampling vessels via liquid He, and the determination
memo-oz was accomplished using gas chromatography interfaced to a stable isotope
ratio mass spectrometer (Roberts et al., 2000). The sample vessel was connected to an
inlet system on the gas chromatograph that consists of, in series, an ascarite trap to
remove water and CO2, a 3 mL gas sampling loop between two Valco sampling valves

(one 6 port and one 4 port), a vacuum isolation valve, and a vacuum pump. Initially, the

inlet system was completely evacuated before closing the isolation valve separating the
inlet system from the vacuum pump. The stopcock on the vessel was opened and sample
gas was allowed to equilibrate for 10 seconds within the inlet system. Upon rotation of
the Valco valves, sample gas was carried by He flow onto a 5 m by 1/8” OD molecular
sieve S A GC Alltech column, and N2 and 02 were separated in time. Any residual water
or C02 entering the GC column was efficiently trapped onto the molecular sieve column
and removed later by heating. The effluent of the gas chromatograph was routed to the
mass spectrometer and sample isotopic ratios were determined by comparison to a
reference pulse of previously characterized pure 02 tank standard. Stable isotope ratios
for O are expressed in per mil (%o) notation:
618O:[(Rsample/Rstandlird)‘1]I"1000 (1)
where R is the ratio of 18O to 160. All 6180—02 values are expressed with respect to air,
which is enriched in 18o by 23.5 96.. relative to VSMOW, resulting in a also value of
VSMOW, with respect to air, of - 23.5 96o. The analysis of 6170-02 was performed at the
Institute of Earth Sciences at the Hebrew University of Jerusalem, Jerusalem, Israel,
following the method in Luz et al., (1999; 2000).
Determination of R:P ratios

Photosynthesis, respiration and gas exchange at the air/water interface control the
concentration and isotopic composition of 02 (Bender and Grande, 1987; Quay et al.,
1995). These three processes are represented by the following equation:
d[02]/dt = P - R + Pg: (2)
where d[02]/dt is the change in the concentration of 02 over time, P is the rate of

photosynthesis, R is the rate of respiration, and F 02 is the air/water gas exchange rate.

65

Air/water gas exchange is generally the primary process controlling the isotopic
composition and concentration of 02 within the upper water column. The 8180-02 in the
atmosphere is defined as O %o with respect to air. A small fractionation effect during
dissolution results in 18O enrichment of 02 in surface waters by approximately 0.7 960
(Knox et al., 1992). The 02 produced during photosynthesis is derived from water
(Stevens et al., 1975; Guy et al., 1993), and the also-H20 at station ALOHA was
determined to have an average isotopic value of -23.1 :t 0.5 %o (n = 33) with respect to
air. Photosynthesis, therefore, not only increases [02], but also results in a decrease in
5'80-02. In contrast, during respiration, 02 is consumed and the residual 02 pool is
enriched in 18o by a kinetic isotope effect, in which the lighter 16o isotope, is
preferentially consumed (Kiddon et al., 1993). Each of these processes uniquely affects
both the concentration and isotopic composition of 02. In general, 6180-02 values equal
to 0.7 %o reflect the predominance of atmospheric gas exchange, those less than 0.7 %o
reveal the contribution of 02 from photosynthesis, and values greater than 0.7 %o indicate
the effect of 0; consumption by respiration (Bender and Grande, 1987). The
concentration and isotopic composition of the residual 02 pool will, therefore, reflect the
balance of these processes at any depth in the water column.

Based on the influences of photosynthesis, respiration and air/water gas exchange
on the concentration and isotopic composition of 02, equation 2 is expanded (Quay et al.,
1995):
d(18l16

Oydt=Go7/Zag{([02]sat18/160a cal—[oz] W160} + (3)
P 18/160w up _ R 18/160 Gr

66

where G02 is the gas transfer coefficient, Z is depth, [Oflsat is the saturation concentration
of 02 (Weiss, 1970), [02] is the concentration of 02 in the water column, 18”60“, is the
measured isotopic composition of water, up is the fiactionation factor associated with
photosynthesis (1.0000; Guy et al., 1993), 18”60 is the measured isotopic ratio of 02, a, is
the fractionation factor associated with respiration (0.9800; Kiddon et al., 1993), org is the
fractionation factor associated with gas transfer (0.9972; Knox et al., 1992), 18”60,, is the

isotopic ratio of atmospheric 02 (Kroopnick and Craig, 1972), and as is the fractionation

factor associated with gas dissolution (1.0073; Benson and Krause, 1984). Fractionation
factors are defined here as a ratio of the reaction rates of the heavy, '80, to the light, 160,
isotope.

Previously published otr values for marine systems range from 0.9780 for the

subarctic Pacific ocean (Quay et al., 1993), to an average value of 0.9800 for specific

marine plankton and bacteria (Kroopnick, 1975; Kiddon et al., 1993). The slightly higher
averaged value was used in this study because an on, value reflecting the subarctic Pacific

ocean may not be valid in the warm waters of the subtropical/tropical Pacific ocean.

R:P ratios are calculated from the measured values of [02] and 6180-02 (Quay et

al., 1995):
R/P = (l8/l6ow up _ 18/1603) / (18/160 (11" 18/1603) (4)
18"‘50g = a, {W160a as — ([021/[021sa0 ”"60; / {1 — (toil/lozlao} (5)

67

Ratios greater than 1.0, signify a dominance of respiration, values less than 1.0 indicate a
dominance of photosynthesis, and a value equal to 1.0 represent a balance of respiration
and photosynthesis (del Giorgio and Peters, 1993; 1994; Quay et al., 1995).

The air/water gas transfer rate (F02 in equation 2) is no longer included in
equations 4 and 5, since steady state of the system is assumed (Quay et al., 1995). At
steady state any changes in the concentration or isotopic composition of the residual 02
pool from the 02 gas exchange flux will be offset by the biological 02 fluxes of
photosynthesis and respiration, and an equilibrium of the total 02 flux is reached for the
system. The gas transfer rate becomes a filnction of the fluctuations of the concentration
and isotopic composition of 02 during photosynthesis and respiration and, therefore, is
not calculated directly from this method and is not required to determine R:P ratios.

Determination of grass 02 production

A constant relationship between 170 and 180 exists for nearly all materials on
Earth, such that the 5‘70 ofa sample is approximately halfthat ofthe also (5‘70 = 0.521
* 5'80) (Luz et al., 1999). Biological processes, such as respiration and photosynthesis,
adhere to this linear relationship, and these processes are known as mass dependent
fi'actionation reactions (Luz et al., 1999; 2000). Photochemical reactions within the
stratosphere and exchange of 02 with O3 and C02, however, result in a slight enrichment
in 17O of tropospheric 02 (0.3 96o) (Thiemens, 1992; Luz et al., 1999). This anomalous
tropospheric 02 mixes into aquatic ecosystems through air/water gas exchange at the
surface waters (Luz et al., 1999). Within the water column, photosynthetically produced
02 lacks this anomalous enrichment in 17o, and produces 02 with a A'7o value equal to

that of the surrounding water:

68

NC = 6‘70 - 0.521(5‘30) (o)
where 5180 and 5170 are the measured isotopic compositions of 02 (Luz et al., 1999).

The greater the magnitude of photosynthesis, therefore, the closer the A170 value of 02
becomes to the maximum A170 of water, which has been determined to average to 249
per meg [per meg = %o * 1000] for all ocean water (Luz et al., 2000). At steady state, any
02 efl'luxed to the atmosphere from the ocean will be replaced by 02 produced during

photosynthesis, therefore, by accounting for air/water gas exchange, an estimate of gross

02 production may be obtained (Luz et al., 2000):
GP = KCo (Adiss " Aeq)/(Amax - Adiss) (7)
where GP is gross 02 production, K is the gas transfer coefficient, C0 is the saturation

concentration of 02, Adjss is the measured A170 value of 02, Aeq is the A170 of

stratospheric 02 in air/water equilibrium (16 per meg; Luz et al., 2000) and, Am is the

A170 of ocean water (249 per meg; Luz et al., 2000). Gas transfer coefficients (K) were
calculated for this study fi'om the empirical relationship of Clark et al. (1995), with
average daily wind speeds determined from data recorded in the deck logs of the RV
Roger Revelle for the dates sampled.
Results
Transect - station ALOHA to S6 (May 25 to June 22)

Variations in sigma 0, fluorescence and [N03] were present fiom west to east
along the transect. A decrease in the density of surface water (5 m) was evident from
station ALOHA (23.3) to S6 (21.5) (Figure 2.2). Values of sigma 9 greater than or equal

to 26, were observed at 300 m at station ALOHA, at 225 m at $2, at 175 m at $3, at 150

69

m at S4, at 100 m at 85 and at 80 m at S6 (Figure 2.2). This observation is evidence of
the upwelling of dense deep water in the eastern portion of the transect, and shallowing of
the mixed layer from west to east. A single fluorescence peak was evident at 125 m at
station ALOHA and 82, and at 100 m at S3 and S4. Two fluorescence peaks were
observed at S5 at 50 and 100 m, and at S6 at 40 and 80 m (Figure 2.3). The deeper peaks
at SS and S6 were present at the surface of the pycnocline. Surface [NOg'] were less than
0.50 M for all stations, however, the nitricline was shallower from west to east (Figure
2.4). At station ALOHA through S4, [NOg'] were highest at or below the pycnocline, in
contrast to S5 and 86 where [N 03'] above the pycnocline were greater than or equal to
concentrations below the pycnocline.

Distinct water column profiles with depth in [02], the fi’action of 02 saturation,
SRO-On and R:P ratios were apparent along the transect. Within the upper 300 m, [02] at
station ALOHA fluctuated only slightly around 200 umol/L (Figure 2.3). A reduction in
[02] with depth along the transect was apparent at S2 and S3. Minimum [02] were
observed at 300 m of 68 umol/L at 82 and 48 umol/L at S3. At S4, S5 and S6 [02] at
surface were approximately 210 umol/L, however, anoxic zones within the upper 300 m
were present at all these station, and anoxia occurred at shallower depths fi'om west to
east (Figure 2.5). The fiaction of 02 saturation within the upper mixed layer at station
ALOHA, S3, S4 and S6 were approximately 1.0, and in equilibrium with the atmosphere.
Supersaturation of 02 was evident at S2 and S5 at surface (Figure 2.6). A predominance
of atmospheric gas exchange at surface was prevalent at station ALOHA, S3 and S4 as
indicated by 6180-02 values near 0.7 %o (Figure 2.7). In contrast, 8180-02 values less

than 0.7 96o, indicative of a predominance of photosynthesis, were evident at S2, S5 and

70

S6, and the lowest 6180-02 value of —2.0 %o was present at S6. 6‘80-02 values greater
than 0.7 %o, signifying a predominance of respiration, were observed at 250 m at station
ALOHA, between 175 and 250 m at $2, between 100 and 150 m at S3, and at 75 m at SS.
R:P ratios varied from 0.8 to 2.0 along the transect. Within the upper 100 m, from station
ALOHA to S4, respiration was approximately equal to photosynthesis (Figure 2.8). At
only five depths in the upper 100 m were R:P ratios slightly less than 1.0, representing a
predominance of photosynthesis over respiration; at $2 at S and 50 m, at S3 at 50 m and
at S5 at 5 and 25 m. At all remaining depths, at all stations, a predominance of
respiration over photosynthesis was evident by R:P ratios greater than 1.0. The highest
R:P ratio in this study of 2.0 was present at 250 m at station ALOHA.
Storm events - S5 and S6

Strong storm events occurred during the cruise, at SS from June 14-16, and at S6
from June 19-22. The water column was sampled prior to the storm and after the storm at
SS and S6 and during the storm at S6. Intense water column mixing was apparent fiom
decreased variations in fluorescence, [NO3'], [02], and the fi'action of 02 saturation
measured before, during and after storm events (Figure 2.9, 2.10, 2.11, 2.12). A
deepening of the pycnocline was evident afier the storms, at both stations. In contrast to
the sharp, narrow fluorescence peaks prior to the storms at SS and S6, peaks after the
storms were broader and reduced in magnitude (Figure 2.9, 2.10). At S5 and S6,
maximum [N03] increased in the mixed layer and decreased below the pycnocline alter
the storm events as compared to prior to the storms (Figure 2.9, 2.10). Overall a

deepening of the nitricline in response to the storms was evident at SS, from 30 to 40 m,

71

and at S6, from 25 to 45 m. Between 25 and 60 m at S6, both [02] and the fraction of 02
saturation were observed to increase as the result of storm (Figure 2.12).

Before the storm at SS, 6180-02 values in the upper 50 at indicated a
predominance of photosynthesis, however, below 50 m 5180-02 values greater than 0.7
%o were present, signifying a strong predominance of respiration (Figure 2.11). At S6,
prior to the storm, 6‘80-02 values less than 0.7 960 were observed at all depths,
representing a predominance of photosynthesis, and a minimum value of -2.0 %o was
found at 25 m (Figure 2.12). A predominance of respiration over photosynthesis was
observed at and below 40 m at both stations, as indicated by R:P ratios greater than 1.0
before the storm events (Figure 2.13). During the storm at S6 a predominance of
atmospheric gas exchange was present at 5 and 15 m, as indicated by 6180-02 values
approximately equal to 0.7 960. 6180-02 values less than 0.7 %o were observed at 30 and
40 m during the storm, however, 8180-02 values of 3.3 %o and 5.6 %o indicated a strong
predominance of respiration at 50 and 60 m, respectively (Figure 2.12). At S6 R:P ratios
throughout the upper 60 m greater than 1.0 were evident during the storm (Figure 2.13).
A decrease in the 6180-02 values was evident at 70 m, from 4.5 %o prior to the storm and
1.5 %o after the storm at SS (Figure 2.11). After the storm at S6 5180-02 values,
signifying a predominance of photosynthesis, were observed throughout the upper 50 m,
and the lowest value of -2.5 %o at 50 m (Figure 2.12). At SS, a similar trend in R:P ratios
after the storm to those observed before the storm was evident (Figure 2.13). At S6 after
the storm, however, R:P ratios equal to 1.0 were observed, representing a balance
between respiration and photosynthesis. As expected, atmospheric gas exchange was

apparent at the beginning of the storm, however, a predominance of respiration over

72

photosynthesis during the storm was also observed and was followed by a predominance
of photosynthesis throughout the water column after the storm.
Discussion
mm»: the NPSG to the ETNP (station ALOHA to S6)

Variations in the balance of respiration to photosynthesis at station ALOHA
within the core of the NPSG, based on our data and that collected 10 years prior (S.
Emerson and P. Quay, University of Washington, unpublished data, http://usjgofs.whoi.
edu/jg/serv/jgofs/hot/ancillary__measurements/gas_ratio.htrnl), allows for a unique
analysis of R:P ratios on a seasonal basis. In 1990, the lowest R:P ratios were present in
June and July (1.1) and maximum R:P ratios of 1.3 occurred in February, March, May
and December (Table 2.1), and both minimum and maximum R:P ratios signified net
heterotrophy. The dominance of respiration over photosynthesis, however, was 20 %
lower in the summer than in the spring and winter (Table 2.1). Low R:P ratios in summer
are expected within the NPSG, since this is a period of an annual primary production
maximum despite also being a period of strong stratification (Karl et al., 1995; Karl,
1999). Stratification is a barrier to mixing and, thereby, the influx of nutrients, organic
carbon, and plankton biomass from below the pycnocline to surface is reduced, which
potentially decreases both photosynthesis and respiration within the mixed layer. Blooms
of the cyanobacteria T richodesmium, however, are reported during periods of strong
stratification. N2 fixation by these organisms contributes approximately half of total new
production at station ALOHA (Karl et al., 1995; Karl et al., 1997). Summer within the
NPSG, therefore, is a period of reduced respiration and increased photosynthesis,

ultimately resulting in lower average R:P ratios as compared to other seasons. This

73

temporal trend is not evident during 1998/2000, and R:P ratios are approximately 1.0 for
all months sampled (Table 2.1). The negligible seasonal variation in R:P ratios may be a
consequence of the lack of sufficient sampling during the 1998/2000 season. A second
possibility for the dissimilarity between 1990 and 1998/2000 periods is a shifi in the
balance of respiration and photosynthesis caused by decadal climate fluctuations.

In contrast to a weak El Nifio event of 1990, La Nina conditions persisted
throughout the 1998/2000 period within the tropical Pacific Ocean (http://www.cpc.
noaa.gov/products/analysis_monitoring/ensostufl‘/enso.years/html). Average R:P ratios at
station ALOHA were higher in 1990 (1.2) and indicated a predominance of heterotrophic
conditions that contrasts with the balance of R:P ratios during 1998/2000 (Table 2.1).
Karl et al. (1995) noted that high primary production and low organic carbon export from
the euphotic zone characterizes station ALOHA during El Nifio events. This observation
may suggest high consumption of organic carbon within the surface waters during El
Nifio conditions. Indeed, the net heterotrophy in 1990 is indicative of a predominance of
respiration over photosynthesis within the upper 100 m (Table 2.1). In comparison, non-
El Nir‘io conditions are represented by a relatively high export production from the
euphotic zone at station ALOHA (Karl et al., 1995). The balance between respiration
and photosynthesis during the La Nina of 1998/2000, therefore, may reflect the reduction
in the consumption of organic carbon within the euphotic zone as compared to during the
El Niflo event. Despite this apparent difference in the balance of respiration to
photosynthesis between 1990 and the later samplings the majority of the R:P ratios are
equal to or greater than 1.1 and this observation defines station ALOHA as a region of a

predominance of respiration over photosynthesis (Table 2.1). This net heterotrophy

74

 

 

requires a supply of organic carbon external to the core of the NPSG, and the primary
production in environments at the fringe of the NPSG may provide an organic carbon
source that is transported to station ALOHA via horizontal advection.

An external organic carbon source is difficult to reconcile for the NPSG, a region
geographically isolated from a direct near-shore terrestrial influx of organic carbon
(Smith and Mackenzie, 1987; Kirchman, 1997). One possible source of external organic
carbon is horizontal transport from the more productive equatorial fiinge of the NPSG
(Abell et al., 2000; Emerson et al., 2001). Karl (1999) defines the NPSG as the area from
approximately 15° N to 35° N latitude and 135° E to 135° W longitude, thereby, placing
$2 on the southeastern edge of this vast oceanic environment (Figure 2.1). Similar depth
profiles of sigma 9, [N03] and [On] for the upper 100 m at station ALOHA and 82
support the characterization of these two stations as part of the same water mass (Figure
2.2, 2.4, 2. 5). In contrast to station ALOHA, however, net autotrophy was prevalent
throughout the upper 100 m at 82 (Figure 2.8, Table 2.1). Excess organic carbon
produced at S2 may be transported to the euphotic zone of station ALOHA by horizontal
transport from the edge toward the center of the gyre, due to the effects of downwelling
generated from the overall clockwise circulation of the NPSG (Six and Maier-Reimer,
1996; Karl, 1999; Emerson et al., 2001). This downwelling condition of the NPSG that
previously defined this region as a homogeneous environment may actually be a
necessary mechanism interconnecting carbon cycling between areas, since the primary
production within the fringe environment sustains the excess respiration at the core.

In contrast to the downwelling conditions that predominate within the

oligotrophic NPSG, the ETNP is a eutrophic environment supported by upwelling of

75

nutrient rich waters to the surface (Ohman et al., 1982; Barber and Chavez, 1991; F iedler
et al., 1991; Karl, 1999), and thereby net autotrophy might be expected within this region.
For all stations sampled along the ETNP transect, however, R:P ratios equal to or greater
than 1.0 were observed (Table 2.1). A previous study also noted that within the ETNP,
the majority of the organic carbon produced within the euphotic zone was consumed
before being exported below the pycnocline (King et al., 1978). We observed regions of
net autotrophy within the mixed layer but net heterotrophy above the pycnocline more
than balanced any primary production, resulting in average R:P ratios greater than 1.0
within the upper 100 m (Figure 2.8, Table 2.1). Export of excess organic carbon via
horizontal advection fiom east to west (Ohman et al., 1982) from areas of net autotrophy
at one station may fiiel the net heterotrophy at an adjacent station along the transect from
S6 to S3. Despite reported high rates of primary production with the ETNP (F iedler et
al., 1991; Murray et al., 1994), these high rates of photosynthesis appear to be coupled
with equal or higher rates of respiration. The ultimate result is not the anticipated
extensive net autotrophy but an environment either with respiration and photosynthesis in
balance or slightly net heterotrophic.

Pre-stonn conditions along the transect (station ALOHA to S6)

Primary production prior to the storm event, based on l7O-02 data, increases fiom
west to east along the transect from the oligotrophic NPSG to the eutrophic ETNP
(Figure 2.14, Table 2.2). Based on a literature review of the ratio of community
respiration to gross primary production in various aquatic environments, Duarte and
Agusti (1998) determined the minimum threshold of gross 02 production required for net

autotrophy in an oligotrophic or open ocean environment to be 0.035 g O: m“3 day".

76

Both the oligotrophic core of the NPSG at station ALOHA and the open ocean fringe of
the ETNP at S3, therefore, had sufficient gross 02 production for the establishment of net
autotrophic conditions within the upper 150 m (Table 2.2). Only at the depth of 50 m at
SB was net autotrophy observed, and all remaining depths at both stations were net
heterotrophic (Figure 2.8). This precarious equilibrium between respiration and
photosynthesis may be shifted toward net autotrophy either from a decrease in external
organic carbon inputs resulting from a reduction in horizontal transport to stations
ALOHA and S3, or an influx of nutrients from below the pycnocline during episodic
mixing events. Due to the oligotrophic characteristic of these two stations and, therefore,
a low threshold of net autotrOphy, the balance of respiration to photosynthesis at each
station is sensitive to slight variations in the flux of nutrients or organic carbon. In
contrast to these oligotrophic environments, in eutrophic coastal regions, with high
influxes of terrestrial organic carbon filelling respiration, a base value of 1.62 g 02 m'3
day'1 is necessary for net autotrophy (Duarte and Agusti, 1998). Indeed, SS, located
within the ETNP near the Mexican coast, had sufficient gross 02 production prior to the
storm event for net autotrophy, and R:P ratios less than 1.0 were present within the upper
25 m (Figure 2.8, Table 2.2). Ultimately the variations in R:P ratios along the entire
transect from station ALOHA to S6 revealed an overall balance between respiration and
photosynthesis within the upper 100 m during the EPREX cruise (Table 2.2). The areas
of net heterotrophy and CO2 efflux from the ocean to the atmosphere, therefore, offset the
regions of net autotrophy and CO2 influx from the atmosphere to the ocean. This balance
in R:P ratios and CO2 flux, however, is sensitive to shifts in the magnitude of primary

production, which in turn, may be effected by perturbations to the physical environment.

77

Storm events — S5 (June 14-16) and S6 (June 19-22)

Strong episodic mixing events redistribute both phytoplankton biomass and
nutrients to surface waters and may, thereby, increase rates of primary production within
the mixed layer (King, 1986; Hayward, 1987; Falkowski, 1994; Karl, 1999), potentially
causing a short-term lowering of the R:P ratio and net autotrophic conditions. The initial
impact of storm events at both S5 and S6 was intense mixing of the water column, as
indicated by a deepening of the pycnocline, homogeneity in fluorescence, [N 03’] and
[02] profiles with depth, and a shift in the isotopic composition of 02 toward values
indicative of gas exchange (Figure 2.9, 2.10, 2.11, 2.12). During the storm, R:P ratios
greater than 1.0 between 40 and 60 m at S6 and a corresponding 6180—02 value greater
than 0.7 %o at these depths were followed by after the storm increases in the fraction of
02 saturation, and low also-02 values throughout the upper 100 m (Figure 2.11, 2.12,
2.13). These observations were suggestive of a period of predominance of respiration
over photosynthesis following the initial storm-induced mixing, which in turn proceeded
to a period of net photosynthesis alter the storm. A period of net heterotrophy may
precede a period of photosynthesis as organic carbon is redistributed with nutrients to the
mixed layer from below the pycnocline (F alkowski, 1994). Heterotrophic bacteria
present in the mixed layer may consume this newly accessible organic carbon source and
initially outcompete phytoplankton, especially during a period of reduced light levels
associated with cloud cover. Eventually after the storm, as light returns to pre-storm
levels, the increase in the influx of nutrients fiom below the pycnocline results in an

increase in primary production (Ohman et al., 1982; King, 1986).

78

Despite the apparent increase in primary production following the storm events,
however, net autotrophy was not present at SS and S6 after the storm (Table 2.1).
Average R:P ratios indicated slight net heterotrophy at SS, and at S6, shifted toward a
balance between respiration and photosynthesis. This balance of respiration to
photosynthesis may be a transition period between net heterotrophy during the storm and
net autotrophy, but sampling after the storm may have ceased before sufficient time had
elapsed for net autotrophy to be established. Furthermore, an increase in photosynthesis
from physical redistribution of nutrients does not necessarily equate to net autotrophy, if
respiration also increases from a redistribution of organic carbon from below the
pycnocline. Indeed, the large increase in primary production recorded by Ditullio and
Laws (1991) following a storm event required a nutrient source external to the ocean, and
was attributed to atmospheric N03' and Fe deposition and not to redistribution of
nutrients from vertical mixing. Ultimately, the storm events in the ETNP resulted not in
a shift to net autotrophy due to an increase in primary production as expected, but in a
greater balance in R:P ratios due to a temporal decoupling of respiration and
photosynthesis. The presence of equal respiration to photosynthesis suggests a minor
impact on the net flux of CO2 between the atmosphere and the ocean following these
episodic disturbances, and not a substantial drawdown of atmospheric CO2, which may

have occurred had expected net autotrophy prevailed after the storms.

79

AVERAGE R:P RATIOS FOR THE UPPER 100 m

 

 

 

 

CRUISE STATION DATE R:P RATIO CRUISE STATION DATE R:P RATIO
HOT13 ALOHA 01/90 1 .2 HOT98 ALOHA 10/98 1.0
HOT14 ALOHA 02/90 1.3 HOT101 ALOHA 01/99 1.1
HOT15 ALOHA MISC 1.3 HOT104 ALOHA 04/99 1.0
HOT16 ALOHA 04/90 1 .2 AKA ALOHA 07/99 1 .2
HOT17 ALOHA 05/90 1 .3 EPREX ALOHA 05/00 1 .0
HOT18 ALOHA 06/90 1.1
HOT19 ALOHA 07/90 1.1
HOT22 ALOHA 12190 1 .3

AVERAGE: 1 .2 AVERAGE: 1 .1
STDEV: 0.09 STDEV: 0.09

CRUISE STATION DATE R:P RATIO CRUISJEl STATION TIME SAMPLED R:P RATIO
EPREX ALOHA (500 1.0 EPREX S5 PRIOR TO STORM 1.1
EPREX 62 W00 0.9 EPREX $5 AFTER STORM 1.1
EPREX $3 06100 1.1
EPREX 84 06/00 1.0 EPREX $6 PRIOR TO STORM 1.1
EPREX $5 06100 1.1 EPREX S6 DURING STORM 1.3
EPREX 86 m 1.1 EPREX 86 AFTER STORM 1.0

AVERAGE: 1.0
STDEV: 0.08

Table 2.1: Average R:P ratios for the upper 100 m of the water column for past and
current cruises, and before, during and alter the storm events at stations SS and S6.

80

GROSS 02 PRODUCTION

 

 

 

GP(02) GP(02)
(9 m“3 day") (9 fine day“)
De th m station ALOHA De th m S3
100 0.052 5 0.21 1
125 0.063 100 0.205
150 0.056 125 0.155
150 0.209
GP(02) GP(02)
(9 m“ day") (9 mg day")
Depth (m) §=5 Depth (m) 86
25 2.839 5 0.602
15 0.160
30 0.266
40 0.723
50 38.555

 

Table 2.2: The calculated gross 02 production for stations ALOHA
S3, SS and S6. Station S6 was sampled afler the storm event.

81

 

Utted States a WCB

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1m 1m 140W 130w 120w 110W 1” W

 

 

 

Figure 2.1: The location of all stations comprising the EPREX transect.
Note: The shaded area approximates the NPSG.

Approximate scale: 1:37000000

82

 

'l
50‘ ‘5
100-
150‘
2001
250«
300i

Depth (m)

 

—stntion ALOHA
_ _32

 

 

 

 

 

 

504
1007
150«
2001

Depth (m)

 

2507
- -s4

 

 

 

 

 

300-

 

150-
2004

Depth (m)

 

250‘

 

 

 

 

 

300-

Figure 2.2: Sigma 6 as a function of depth for all
stations along the EPREX transect from May 24 to
June 28, 2000.

83

Fluorescence (RF U)
0 0.2 0.4 0.6 0.8

0 JD l l I

50-1
1001
150i

Depth (m)

200 -

250 q —-—etntion ALOHA
— —82

 

 

 

 

 

Fluorescence (RFU)
0 0.2 0.4 0.6 0.8

0d 1 l l I

 

-—S4

 

N
0'1
0
l
-—‘

 

 

 

Fluorescence (RFU)
0 0.2 0.4 0.6 0.8

 

 

250 1
300 -

 

 

 

 

Figure 2.3: Fluorescence as a function of depth for all
stations along the EPREX transect from May 24 to
June 28, 2000.

84

[NOa'] (11M)

 

 

50

 

100

 

 

150

Depth (m)

200
250

 

300

 

 

 

 

 

 

 

 

 

 

 

 

 

35
E
5
53*
[N05] (PM)
0 5 10 15 20 25 30 35
o I I i I I I I
L... _ __
50 d _ h "' ~~
’E‘ 100- 1‘
5 150- t’
a. \
O
Q 200- /
—35 /
250 - /
— —se
300 d

 

Figure 2.4: [N03] as a function of depth for all
stations along the EPREX transect from May 24 to
June 28, 2000.

85

[02] (PM)

0 40 80 120 160 200 240
0 L I I

 

 

50 '1 +Itation ALOHA
+82

 

 

100-

 

150‘

Depth (m)

200‘
250‘

 

 

300. :r'

[021(PM)
o 40 80 120 160 200 240

0 L L I

 

50%

(m

 

c 150 ”‘6'

Dept
N
8

 

 

 

 

   
 

[021(PM)
0 4o 80 120 160 200 240

I I J

      

 

fl-e-'--

 

 

 

 

 

Figure 2.5: [02] as a fimction of depth for all stations
along the EPREX transect from May 24 to ere 28,
2000.

86

Fraction of 02 Saturation

(10 (12 (14 (16 (18
0 I I I I

10

 

 

50 ‘ +etation ALOHA

—D—82

 

 

1004

 

150-

Depth (m)

200-
250‘

 

300-

Fraction of 02 Saturation

(10 (12 (14 (16 (18
0 I I I I

10

 

50-
100-
150.

Depth (m)

200d

12

III

 

 

250-l

 

 

+83
_x_s4

 

300d

Fraction of 02 Saturation

10

 

 

:5: 100a
5150-
Q
8 2004
250.

 

 

300-

 

 

Figure 2.6: The fraction of 02 saturation as a function of
depth for all stations along the EPREX transect fi'om

May 24 to June 28, 2000.

87

 

 

 

100-
150d

Depth (m)

200+

 

 

2501

 

 

 

 

300-

429
0

50‘
@1004
£5 1504
D.

82004
2504

 

300-l

 

-1J

I

-11

0. L I
0’2]: E?
50-1

518002 060
(17 :25 413 61

I L I

x—

 

—X—S4

 

 

 

6180-02 (he)

(17 225 443 61

 

 

 

 

Figure 2.7 : The isotopic composition of 02 as a
function of depth for all stations along the EPREX
transect fiom May 24 to June 28, 2000.

88

R:Pratio
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

I I I I

 

 

-D—S2

 

 

 

 

R:P ratio
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

 

 

 

 

 

 

R:P ratio
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

 

 

 

 

 

01 a + n n n n
50 ‘K
E 100-
5 1504
O.
8 200+
250- +35
—O—86
300.

 

Figure 2.8: Ratios of respiration to photosynthesis
(R:P ratios) as a function of depth at all stations along the
EPREX transect from May 24 to June 28, 2000.

89

20-
40‘
60‘
30.1
100‘
120.
140-

Depth (m)

 

0

201
40.
60«I
30..
100-
120i
140-

Depth (m)

 

 

 

 

————Pflxkuhmn$5
--Amssmnn85

 

 

Fluorescence (RF U)

02 04 I06 08

I I I

 

-———-R*thumn85
'--Amusmnn85

 

 

 

I NOa' I (HM)

 

 

 

 

 

Figure 2.9: Sigma 0, fluorescence and [N03] as a
flmction of depth prior to and after the storm at SS
from June 14 to 16, 2000.

90

Depth (m)

Depth (m)

Depth (m)

20

 

20%
404
604
80-1
100‘
120-

 

140-

on
O
I

100‘
120‘

 

140.

 

 

-———memmumn86
--n-Dmuusmnn86
-—Afterstorm86

 

 

 

Fluorescence (RF U)

02 (14 06

 

 

 

 

 

I N03'] (PM)

 

20-
404
60d
30.
100‘
120-
140-

 

 

 

menummnsa
---memsmnn86
---Amusmnn86

 

 

 

 

Figure 2.10: Sigma 9, fluorescence and [N03] as a

ftmction of depth prior to, during and alter the storm
at S6 from June 19 to 22, 2000.

91

Depth (m)

Depth (m)

Depth (m)

[02] (PM)

0 40 80 120 160 200 240

 

20 a
40 «
60 -
30

100

 

 

120 .

 

 

 

 

140

Fraction of O2 Saturation
0.0 0.2 0.4 0.6 0.8 1.0 1.2

o I L I I

20.
404
60-1
804
100i

 

 

i—iF—Pflxtummnss
-ir-Anushnnss

1204

 

 

 

 

1404

6“0-02 06o)

429 -1J 07' 215 413 61
o I I I I

 

804
1004

 

i—iF—metummnss

120‘
-aF-Anusmmn85

 

 

 

 

1404

Figure 2.11: [02], the fraction of 02 saturation and the
isotopic composition of 02 as a function of depth
priortoandafierthe storm at SS from June l4to 16,

2000.

92

[02] (PM)

 

Depth (m)

 

 

 

 

 

Fraction of 02 saturation

0.0 0.2 0.4 0.6 0.8 1.0 1.2

 

 

 

 

 

 

 

 

 

IE:
5
Q
8
100‘
+Prlortosbrm86
120‘ -B-Durinoshrm88
+Nbrsbrm$8
140-
18
8 O-02 (%o)
-2.9 -1.1 0.7 2.5 4.3 6.1
0‘ In I I I
204 ,m
,I
A 40. s .....
T-‘“fl3—-~
g 60. ‘~.D
5
53* °°‘
1004
+Priortnstonn$6
1201 -B-Durlngstonn$6
140 +Aflerstorm$6

 

 

 

Figure 2.12: [02], the fraction of 02 saturation and the
isotopic composition of 02 as a function of depth prior
to, duringandaflerthe stormat S6fromJune 19t022,
2000.

93

 

R:P ratio
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

 

 

 

 

 

 

 

 

 

 

 

 

0‘ I I I I I I I I
20-
A 40‘
g 60+
5
a 8°‘
100-
120 1 +Pnor to storm SS
+Afterstorm$5
140-
R:Pratio
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
o I I I I I I
204 m
A 40‘ ~-“-
““13-‘
S 60. ~E,
£-
§ 8°‘
100.
+Priortostorm86
120. -E—Duringstonn86
+AfteretormS6
140-

 

Figure 2.13: Ratios of respiration to photosynthesis (R'P ratios) as a flmction of
depth for storm events prior to and alter the storm at SS from June 14 to 16, 2000
and prior to, during and after the storm at S6 from Jtme 19 to 22, 2000.

94

A170 (per meg)

 

 

 

 

 

 

0 50 100 150 200 250 300
0 . i i i i i i
304:-
A 60¢
g l'
5
8' mdb
0 °, -o-surtlonALOHA
g -o—s3
1201b . 0 $5
1507 0

Figure 2.14: The A170 values of 02 for stations ALOHA, S3, SS and S6.

95

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