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This is to certify that the

thesis entitled
Seasonal Cycling of Seston and its Relationship
to PCB Concentrations in Grand Traverse Bay,
Lake Michigan

presented by
Eileen Marie McGervey McCusker

has been accepted towards fulfillment
of the requirements for

Master QLScience—degree in jeologica]. Sciences

(firm [kiln/Ax
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Major professor

Date 8/26/98

 

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

 

 

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FEB 1 9 2000

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1/98 chlRC/DathpfiS-p.“

SEASONAL VARIATION IN THE BIOGEOCHEMICAL CYCLING OF
SESTON AND ITS RELATIONSHIP TO PCB CONCENTRATIONS IN
GRAND TRAVERSE BAY, LAKE MICHIGAN
By

Eileen Marie McGervey McCusker

A THESIS

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

MASTER OF SCIENCE
Department of Geological Sciences

1 998

ABSTRACT

SEASONAL VARIATION IN THE BIOGEOCHEMICAL CYCLING OF SESTON
AND ITS RELATIONSHIP TO PCB CONCENTRATIONS IN GRAND
TRAVERSE BAY, LAKE MICHIGAN

BY

Eileen Marie McGervey McCusker

This study describes seasonal biogeochemical cycling Of seston in Grand
Traverse Bay, Lake Michigan and relates this to variation in total polychlorinated
biphenyl concentrations (ZPCB) in suspended solids. Seston was Characterized
by carbon and nitrogen elemental and isotopic abundances. Fluorescence,
temperature, light transmittance, and concentrations of dissolved inorganic
nitrogen were also determined. The vertical and seasonal trends in the 8'30

values Of seston exhibited a broad range (-30.7 tO -23.9%o). Seasonal 8‘5N

values of seston were highest in the spring and subsequently declined. The 8‘5N
values Of seston reflect a balance between fractionation during assimilation Of
NH4+ or NOg‘ and degradative processes. The seston EPCB and fluorescence
were both high in the spring and subsequently declined suggesting that variation
in PCB concentrations were associated with primary productivity. The strong
seasonal trends in the organic geochemical Characteristics Of seston and

concentrations Of PCBS emphasize the complex nature of particle cycling in the

bay.

DEDICATION

To my husband, Brent, whose encouragement and patience always keeps me

going and to my family who always believes in me.

iii

ACKNOWLEDGEMENTS

I would like to thank Peggy and Nathaniel Ostrom for both financial

support Of this research and their academic guidance.

iv

TABLE OF CONTENTS

LIST OF FIGURES ................................................................................. v
INTRODUCTION .................................................................................... 1
METHODS .............................................................................................. 4
RESULTS ............................................................................................... 9
DISCUSSION ......................................................................................... 20
Carbon Isotopes ......................................................................... 20
Nitrogen Isotopes ....................................................................... 25
Contaminants ............................................................................. 29
CONCLUSIONS ..................................................................................... 32
LIST OF REFERENCES ........................................................................ 33

LIST OF FIGURES

Figure 1. Map Of Grand Traverse Bay Michigan with stations GT 1 and GT 3 ............... 13

Figure 2. Fluorescence, concentrations of particulate organic nitrogen and particulate
organic carbon, temperature, seston 6‘30 and EN, transmittance, and concentrations Of
nitrate and ammonium for station GT 1 in 1997 .............................................................. 14

Figure 3. Fluorescence, concentrations of particulate organic nitrogen and particulate

organic carbon, temperature, seston 8‘30 and SN, transmittance, and concentrations of
nitrate and ammonium for station GT 3 in 1997 .............................................................. 16

Figure 4. Integrated water column fluorescence for stations GT 1 and GT 3 in
1997 ................................................................................................................................. 18

Figure 5. Concentration weighted average 8‘3C values of seston for GT 1 and GT 3 in
1 997 ................................................................................................................................. 1 8

Figure 6. Concentration weighted average 5‘5N values for seston for GT 1 and GT 3 in
1 997 ................................................................................................................................. 'I 9

Figure 7. Total PCB (ZPCB) concentration of seston for station GT 1 in
1997 ................................................................................................................................. 19

vi

INTRODUCTION

Seston is a dynamic reservoir of organic matter in lakes and oceans that can
change dramatically in composition on a seasonal and spatial basis. An
understanding of the origin and cycling of seston has important ecosystem
implications, as it is one of the pools of organic matter at the base Of pelagic food
webs and can facilitate the cycling of toxins in the environment. However,
detailed studies Of Changes in the geochemical nature Of seston on seasonal and
spatial scales are limited, particularly in the Great Lakes where seston has been
implicated in the role of contaminant transfers (Baker and Eisenreich, 1989;
Baker et al., 1991; Axelman et al., 1997). Consequently, I characterized
seasonal variation in the geochemical composition of seston in Grand Traverse
Bay and used these data to assist in an interpretation of temporal Changes in
surface water PCB concentrations.

Seston is defined as the material collected on a filter with a pore size of 0.45
to 1.0 pm (Riley, 1970; Parsons, 1975) and primarily consists of small particles
which can have water column residence times on the order of several hundred
years (MCCave, 1975). Compositionally, seston can consist Of mineral grains,
phytoplankton cells and fragments, microzooplankton, amorphous inorganic and
organic matter, charcoal fragments, bacteria, small fecal pellets, and fibers. In
the Great Lakes, a seasonal progression Of particle sources has been
recognized with shoreline erosion predominating in winter, followed by biotic
production in early spring and fall, and resuspension of bottom sediments

prevailing in late fall following water column overturn (Eadie and Robbins, 1987;

Baker and Eisenreich, 1989). Although seston is Often assumed to consist
primarily of recently produced phytoplankton material, this is likely to be the case
only during periods of high primary production and at other times of the year
inorganic material or refractory organic matter may predominate. Consequently,
the geochemical composition of seston is a complex interaction of changes in
sources, in situ production, and microbial decay.

In this study, the geochemical composition Of seston was characterized by
deployments Of a CTD equipped with a fluorometer and transmissometer,
determinations Of the elemental and isotopic composition Of seston, and
measurements of the total concentration of PCB congeners (2PCB) from April to
September, 1997. Depth profiles of water column fluorescence and light
transmission provide insight into relative concentrations of chlorophyll a, a labile
constituent of phytoplankton (Furuya, 1990), and Changes in particle
concentrations, respectively. Carbon isotopes in aquatic systems can provide
insight into the sources of C02 utilized by phytoplankton and changes in levels of
primary productivity (Schelske and Hodell, 1991). In addition, stable carbon and
nitrogen isotopes have been used to trace organic material in ecosystems
(Minigawa and Wade, 1984; Peterson et al., 1985; Ostrom and Fry, 1993), to
identify sources Of sewage (Van Dover et al., 1992), to estimate trophic positions
and to quantify bioaccumulation of contaminants in food web studies (Cabana
and Rasmussen, 1994; Kidd et al., 1995; Kucklick et al., 1996). Organic
contaminants in Great Lakes fish have been a concern for several decades and

recent studies have proposed an efficient incorporation Of these compounds in

food webs via an association with organic rich particles (Baker and Eisenreich,
1989; Baker et al., 1991; Swackhamer and Skoglund, 1993; Kucklick and Baker,
1998). In the present study, the biogeochemical transformations of seston were
assessed using the water column characteristics of Grand Traverse Bay
concurrently with the carbon and nitrogen elemental and isotopic abundance Of
seston. An understanding of the biogeochemical transformations Of seston, will
provide insight into the complex transformations that can affect contaminant

concentrations on a temporal scale.

METHODS

Water column samples were collected from two stations within the
western arm Of Grand Traverse Bay, Lake Michigan, between April and
September 1997 on the vessels MN Northwestern and RN Shenehon. Grand
Traverse Bay is located in the northern part Of Lake Michigan and covers a
surface area of 681.6 km2 (Figure 1). This Bay was chosen as a study site
because it is an inland extension of Lake Michigan and many of its general
characteristics, such as, morphometry, land use, nutrient concentration, and
phosphorus limitation are similar to those of Lake Michigan (Auer, 1975). The
southern portion of the Bay is divided into an eastern and a western arm, and
most of the nutrient loading into Grand Traverse Bay enters through the
Boardman River in the southern end of the western arm of the Bay (Auer, 1975).
Our two sampling stations were located in the western arm, which has a
maximum depth of 122 m. Station GT1 is located 7 km from the southern shore
and has a depth of 98 m, and station GT 3 is 12 km from the southern shore with
a maximum depth of 112 m (Figure 1). Prior to sampling, the water column at
each station was characterized by deploying a SBE-25 conductivity -
temperature - depth profiler equipped with Sea Tech fluorometer and
transmissometer sensors (SeaBird, Electronics Inc.). Water column samples
were collected from several depths at each station using 5 L lever action Niskin
bottles (General Oceanics), 8 L Go-Flo (General Oceanics), or an 8 L Niskin- X
(General Oceanics). Depths were chosen such that samples were obtained

above, within, and below the chlorophyll maximum and in Close proximity to the

bottom.

For analysis of NH4+ and N05, an aliquot of water from each depth was
transferred into acid-washed 1L Nalgene bottles and frozen. Seston samples
were obtained by filtering 3 to 6 liters of water through a pre-combusted (500°C,
1 hour) and pre-weighed Whatman GF/F glass fiber filter. Seston samples were
frozen prior to isotopic and elemental analysis.

Ammonium concentrations were determined using an Orion ion specific
electrode (Garside et al., 1978). Nitrate concentrations were determined by
suppressor based anion chromatography (Shipgun and Zolotov, 1988) on a high
performance liquid Chromatograph (Rainin Instruments) with conductivity
detection (LDC Analytical). Separation Of anions was achieved on a Dionex
lonpac column (#AS4A-SC) with a suppressor using an eluent consisting of
2.4 mM NaHCOa. The limit of detection and precision for the analysis of N03'
and NH4+ by these methods is 0.1 uM (Ostrom et al., in press).

In preparation for analysis, seston samples were dried at 40°C for
approximately 12 hours and monofilament fibers and zooplankton were removed
from the dry sample. The filters were weighed, acidified (10% HCI) to remove
carbonate, and dried again at 40°C. The surface of the filter that contained the
seston was placed in a precombusted (500°C, 1 hour) quartz tube with excess
precombusted CuO and Cu (approximately 3 g of each). The tubes were
evacuated, sealed, and combusted at 850°C. The combustion products were
separated cryogenically on a vacuum line and the isotopic composition of the

purified carbon dioxide and nitrogen gas was determined on a PRISM

(Micromass) mass spectrometer. Nitrogen and carbon isotope ratios are

expressed in per mil notation (%o):

s'E = [(RammJRmm) - 11* 1000

where, l is the heavy isotope of element E and R is the abundance ratio of the
heavy to light isotope. The internationally recognized standards for 515N and
8130 are atmospheric nitrogen gas and V-Peedee Belemnite, respectively. The
precision for this technique is 0.1%o (Macko et al., 1987).

The concentration of organic carbon in seston samples was estimated
using a calibrated baratron capacitance manometer (MKS Instruments) during
gas separation. Organic nitrogen concentrations in the seston samples were
determined based on a calibration Of the ion beam (mass 28) produced from
syringe injection of purified N2 gas within a calibrated volume of the mass
spectrometer.

Seasonal variation in the isotopic composition Of seston for the entire
water column was described with concentration weighted averages as detailed in
Ostrom et al., (1997). Chlorophyll fluorescence was also integrated as relative
fluorescence units (RFU) per square meter for the entire water column.

For contaminant and lipid analysis, monthly epilimnetic water samples
were collected by pumping water directly through pre-combusted (550° C,

24 hrs), 293 mm diameter glass fiber filters (GFFS, Schleicher and Schuell,

0.7 mm) from depth (5 to 35 m) using a submersible pump (March Mfg, model

SCMD) to collect the operationally defined particulate phase. Filter samples
typically ranged from 400 to 740 L. Lipid analysis was done on 2 to 4 L of water
from the Niskin or Go-Flo bottles. Water was filtered on 47 mm diameter GFF
filters and lipids analyzed by the method Of Bligh and Dyer (1959).

Suspended solids were extracted and analyzed for PCB congeners using
methods similar to those previously published by KC and Baker (1995) and
Kucklick et al. (1996). The samples were extracted with a 50:50 mixture Of
acetone and hexane for 24 hours. Prior to extraction, a PCB surrogate
consisting of 3,5 dichlorobiphenyl, (PCB 14); 2.3.5.6 tetrachlorbiphenyl (PCB 65);
and 2.3.4.4'. 5.6 hexachlorobiphenyl (PCB 166) were added to each sample.
Following extraction. back-extraction using water and hexane removed acetone.
The sample was reduced in volume by rotary evaporation and fractionated using
Florisil (Norstrom et al., 1988). The purified extract was reduced in volume to
1-2 mL and 2.4.6 trichlorobiphenyl (PCB 30) and 2,2'. 3,4.4’. 5,6,6”
octachlorobiphenyl (PCB 204) were added as internal standards. The samples
were further reduced in volume to < 100 pL under a gentle stream of purified
nitrogen prior to analysis. All samples were analyzed for polychlorinated
biphenyl congeners by gas chromatography with a 63Ni electron-capture
detection (GC-ECD) using a Hewlett-Packard 5890 series ll GC equipped with a
0.32 mm x 60 m DB-5 capillary column (J&W Scientific). The carrier gas was H2
at a flow rate of 35 cm/s; the injector and detector temperatures were 250°C and
320°C, respectively. PCBS were quantified by the method Of Mullin (1985), that

uses a calibration mixture of Aroclors 1232:1248z1260 in a ratio Of 25:18:18. In

the present study. 69 PCB congeners or congener groups were quantified by
dividing the Chromatogram in half based on retention time and quantifying the
first half against PCB 30 and the second half against PCB 204. The zPCB for
seston was calculated by summing all of the congeners quantified in each

sample.

RESULTS

Changes in thermal structure and primary production were clearly evident
in temperature and fluorescence profiles throughout the season in Grand
Traverse Bay. Generally. the uniform temperature. fluorescence. and
transmittance depth profiles in April and May reflected vertical mixing throughout
the water column at GT 1 and GT 3 (Figures 2 and 3). The cold and protracted
spring of 1997 was indicated by a lack of density stratification at either station
and maximum temperatures less than 5°C until early June. After this time,
density stratification became more distinct and surface temperatures gradually
rose from 4° C to approximately 22° C in August. The water column remained
thermally stratified through mid September.

Prior to stratification, April through early June, the majority of the water
column was Characterized by Chlorophyll fluorescence greater than or equal to
0.5 relative fluorescence units (RFU) at both stations (Figures 2 and 3). With the
onset of stratification in mid June at GT 1, and in July at GT 3. a maximum in
RFU was Observed in the mid water column (15 to 40 m) through September
(Figures 2 and 3). These mid water column maximums occurred below the
thennocline at both stations. Another salient feature of the fluorescence data
are distinct peaks that are prevalent in the mid water column at 40 m in July at
GT 1 and 20 m in September at both stations (Figures 2 and 3). These peaks
are associated with a decrease in light transmittance. Light transmittance
reached a minimum near the bottom between June and July at both stations,

suggesting the presence Of a benthic nepheloid layer (Figures 2 and 3), a

localized region Of high total suspended matter that is a common feature Of the
Great Lakes during stratified periods (Bell et al., 1980; Eadie et al., 1983;
Sandilands and Mudroch. 1983). A slight increase in integrated chlorophyll
fluorescence in September may indicate a fall bloom at both stations (Figure 4).
Integrated water column fluorescence was high between May and June at both
stations (between 68.1 and 73.3 RFU/m2 at GT1 and between 66.3 and
69.9 RFU/m2 at GT3). and suggested that productivity was highest in the early
Spring (Figure 4).

Concentrations of particulate organic carbon (POC) ranged from 3.7 to
10.0 M at GT 1. and 3.0 to 8.7 uM at GT 3, while concentrations Of particulate
organic nitrogen (PON) ranged from 0.2 to 1.3 uM at GT 1. and 0.2 to 1.0 M at
GT 3 (Figures 2 and 3). Average C/N values for seston were 10.8 2 3.0 and
10.2 2 3.4 for GT 1 and GT 3, respectively.

Concentrations of POC and PON in April and May at both stations
exhibited little variation with depth at either station, consistent with a well-mixed
water column, as indicated by uniform fluorescence and transmittance depth
profiles (Figures 2 and 3). A peak in POC and PON occurred on June 4 at GT 1
at 20 m that was not Observed at GT 3 (Figure 2). Increases in fluorescence
coincident with high concentrations of POC and/or PON between 20 and 60 m
occurred between mid June and August at GT 1 and GT 3. The depth at which
this trend was Observed varied among months. From mid June through
September. an increase in particle concentration in the lower water column

coincident with decreases in light transmittance further suggests the

10

development Of a benthic nepheloid layer.

Dissolved inorganic nitrogen concentrations varied throughout the season
at both stations. Concentrations of N03' ranged from 12.6 to 18.7 ”M at GT 1
and from 17.4 to 12.8 M at GT 3. The concentration of NH4+ ranged from the
detection limit at both stations to 2.4 ”M at GT 1 and 1.1 uM at GT 3 and was
always less than the concentrations of N02. From April tO June at GT 1. and
May to June at GT 3, N03' concentrations were uniform with depth but
decreased with time (Figures 2 and 3). With the exception of April and May.
NH4+ concentrations throughout the water column were consistently above 0.5
”M at either station. The highest NHI+ concentration of the season was seen at
GT 1 on June 4 concurrent with peaks in POC and 51°C at a water depth Of 20 m
(Figure 2).

Carbon isotope values for seston ranged from 23.9 tO -30.6%o at GT 1
and -24.9 to -30.7%o at GT 3. There was little variation in 5‘30 values with depth
in April and May and values were low at both stations (Figures 2 and 3). Between
mid June and August. seston 5‘3 C values were higher at the surface (5 to 10 m)
relative to the hypolimnion (between 15 and 60 m) at both stations (Figures 2
and 3). The low 5‘3 C values in the mid water column at this time and on
September 16 at GT 1 were associated with peaks in chlorophyll fluorescence.
Lower depths (below 60 m) showed an increase in seston 5‘3 C values at GT 1
and GT 3 from mid June through September that was Often coincident with a
decrease in light transmittance.

The water column concentration weighted average 5‘3 C values for seston

11

were low in April and May (-29.9%o in April at GT 1, and -29.7°/oo and -28.9°/oo in
May at GT 1 and GT 3, respectively) (Figure 5). The 6‘3 C value at GT 1
increased by approximately 1.5%: in early June, and decreased to -29.3°/oo in
July, while the 5'3 C concentration weighted average concentrations at GT 3
declined steadily from May to July. After July the concentration weighted 8‘3 C
values increased steadily at both stations and reached a maximum in September
(-25.3%o and -27.0%o at GT1 and GT 3 respectively).

Nitrogen isotope values for seston in Grand Traverse Bay ranged from 2.5
tO 8.7%o at GT 1. and 1.7 to 11.6%0 at GT 3. In April and May. high seston 515M
values (> 5%o) were observed at GT 1 above 20 m. At GT 3 in May. a maximum
in the 51°N value of seston of 97% occurred at 30 m (Figure 3). In June. July,
and August, there was a tendency for minimum 81°N values to occur in the mid
water column (20 to 60 m) at both stations. The water column concentration
weighted average 51°N values for seston at GT 1 and GT 3 exhibited a decline
from approximately 6%o in April to approximately 2%o in August (Figure 6). From
August to mid September, the concentration weighted average for GT 3
remained relatively constant. while it exhibited an increase to 3.3%o at GT 1.

Seasonal trends in the zPCB Of surface seston (5 to 10 m) were evident
at GT 1. The PCB concentrations ranged from a maximum of 44.2 ng/g dry
weight in April to a minimum Of 9.3 ng/g dry weight in September (Figure 7). The
concentration Of PCBS in the seston of the surface waters was highest in the
spring, and showed a steady decline throughout the season. This trend was

concurrent with a general decline in integrated fluorescence with time.

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23 Na 8 N: 93 an S 2:.

 

a a q 4 4 N d d _
4N
1n
1v
Ln

8.0....
Eb... lo

 

 

 

00/0 NNQ P91L|5IGM

19

DISCUSSION

The biogeochemical cycling of suspended particles in Grand Traverse
Bay is characterized by variation in seasonal and depth profiles of fluorescence,
concentrations of POC and PON, 813C, and 515N and seasonal 2PCB. Temporal
and spatial changes in these geochemical characteristics reflect the complex and
dynamic nature of seston. Fluorescence, concentrations of NH4" and N03] and
the 5‘30 and 515N values of seston characterize temporal and spatial changes in
productivity, nutrient uptake, and microbial activity. Temporal trends in these
data can be related to variation in seasonal 2PCB of seston in the water column

of the Bay.

Carbon Isotopes

Variation in the 513C values of seston can be controlled by many factors,
including variation in the relative contribution of different types of seston, degree
of isotopic fractionation during carbon fixation, variation in phytoplankton species
composition, pH, temperature, isotopic composition of dissolved inorganic
carbon (DIC), degree of lipid production, and changes in water masses (Wong
and Sacket, 1978; Fontugne and Duplessy, 1978; Farquhar et al., 1982; Guy et
al., 1986; Flau et al., 1989; Rau et al., 1991; Nakatsuka et al., 1992; Fogel and
Cifuentes,1993). With regard to variation in the composition of seston, the two
primary sources of organic matter to lacustrine environments are autochthonous
(algal) and allochthonous (terrestrial) production, which may differ in isotopic

composition. Therefore, variation in the relative contribution of each of these

20

reservoirs influences the isotopic composition of seston. Given that the

proportion of terrestrial and aquatic inputs may change spatially and seasonally,
variation in the 5130 of seston can be expected. Within Lake Michigan and the
other Great Lakes more than 90% of the organic matter is of an aquatic origin

(Andren and Strand, 1981; Meyers and Eadie, 1993; Meyers and lshiwatari,

1993). Average C/N values for seston in Grand Traverse Bay (10813.0 and
10.2:3.4 for GT 1 and GT 3, respectively) are similar to those found in other
Great Lake environments and are indicative of an algal origin (Muller, 1977;
Prahl et al., 1980; Meyers et al., 1984). Therefore, terrestrial inputs into Grand
Traverse Bay are expected to be quite low.

Among the processes that can affect the 6130 composition of seston,
strong evidence suggests that variance in the growth rate of phytoplankton and
intracellular and extracellular C02 concentrations predominate (Fogel and
Cifuentes, 1993;Laws et al., 1995). In the process of photosynthesis,
discrimination against 130 occurs during diffusion of C02 into the phytoplankton
cell and, to a greater extent during fixation by the enzyme RuBP carboxylase
(O’Leary, 1981). When DIC concentrations are high, the isotope effect
associated with RuBP carboxylase is more fully expressed, resulting in larger
isotope fractionation between the phytoplankton and BIG (Fogel and Cifuentes,
1993). In aqueous systems, the rate of the enzymatic reaction often exceeds the
rate of 002 transport into the cell, allowing diffusion to become the rate limiting
process (O’Leary, 1981; Raven et al., 1987; Cifuentes et al., 1988). When

diffusion of C02 into the cell is limiting, the 6‘3C value for the photosynthetic

21

tissue is primarily determined by the smaller isotope effect associated with this
process (Fogel and Cifuentes, 1993). Some phytoplankton species have
concentrating mechanisms that can actively transport bicarbonate, which is
enriched in 13C relative to dissolved 002, into the cell during periods of low DIC
concentration (Lucas, 1983; Fogel and Cifuentes, 1993). When the processes of
diffusion and active transport of bicarbonate dominate, the result is a heightened
degree of 130 assimilation and an increase in the 5130 values of phytoplankton
(Fogel and Cifuentes, 1993).

Seston collected during April and May in Grand Traverse Bay was
characterized by low 8130 values (less than -29.0%o) and showed little variation
with depth. Although integrated water column fluorescence suggests that
productivity was high at this time relative to later in the summer, the low 5130
values suggest that CO: was not limiting (Figures 2 and 3). Diatoms have been
observed to predominate in the water column of Lake Michigan in the early
spring (Gala and Giesy, 1991) and accumulate large amounts of lipid (Jacobson
et al., 1970). Given that the 5130 of lipids is lower than that of the material from
which they are derived (Abelson and Hoering, 1961 ), low 6130 values of seston in
the spring may be a consequence of a high lipid content in diatoms at that time.

Variation in 6130 with depth was evident beginning in June prior to
stratification (Figures 2 and 3). On June 4 at GT 1, there was an increase in the
5130 of seston from -28.4%o at the surface to -26.2%o at 20 m (Figure 3). This
trend is concomitant with an increase in the concentration of POC, PON, and

NH4+ at the same depth. Integrated water column fluorescence was still

22

relatively high in June. The high particulate elemental concentrations and
fluorescence data suggest that high 8130 values at 20 m may be related to a
reduction in isotopic discrimination resulting from decreases in the aqueous C02
reservoir during photosynthesis.

From mid June to August at both stations and on September 16 at GT1,
minimum 5130 values were present in the mid water column (15 to 60 m), usually
coincident with the chlorophyll maximum and high P00 or PON concentrations
(Figures 2 and 3). This trend could be associated with the accumulation of
isotopically depleted lipids or uptake of respiratory 002 by phytoplankton. Lipid
data, particularly in August at GT 1 and GT 3, indicate increases in lipid
concentration coincident with decreases in 5130 values. Lipid concentrations at
GT 1 during August increased from 15.4 (lg/L at 5 m to a maximum of 33.5 (lg/L
at 35 m concomitant with a decline in 8130 values between the surface and 35 m.
We suggest that assimilation of respiratory 002 is also likely because the peak
of primary production, as indicated by the chlorophyll fluorescence profiles,
appears below the therrnocline where active respiration occurs. This suggestion
is supported by preliminary data indicating that the 5130 values of DIC in the
hypolimnion of Grand Traverse Bay are low in July and August (average =
-1.1 i: 0.3%0 in July and average = -2.4 :l: 1.0%o in August), and previous
observations that respiratory 002 is 130 depleted (Jacobson et al., 1970;
Peterson and Fry, 1989). In addition, there is evidence of intense microbial
respiration at the base of the chlorophyll maximum (Ostrom et al., 1997).

In addition to the mid water column minima in the 5‘30 values of seston,

23

depth profiles of 5130 are characterized by high values in the water column below
60 m in late summer. Decreases in light transmittance at these depths suggest
the presence of a benthic nepheloid layer. The low 5130 values for seston below
60 m could result from isotopic discrimination during microbial degradation.
Active bacterial cycling is a common feature of the benthic nepheloid layer (Hicks
and Owen, 1991).

An overview of seasonal variation in the 5130 composition of seston
is facilitated by an assessment of the concentration weighted average data
(Ostrom et al., 1997). These data are also useful in that they serve to
emphasize the primary factors that control thecarbon isotopic composition of the
entire reservoir of seston in the water column. The concentration weighted
average 5130 of seston in early spring (April and May) exhibited low values
(< -28.5%o) at both stations indicating production without 002 limitation
(Figure 5). The increase in the concentration weighted average (1 .5%o) between
May 7 and June 4 at GT 1 occurred at a point when P00 was high, and 002
may have been limiting (Figure 2 and 5). The weighted 6130 values attained a
low value in July at both GT 1 and GT 3 (-29.3%o and -30.7%o respectively).
Peak fluorescence at this time was below the therrnocline. Utilization of a large
pool of 130 depleted respiratory 002 in the hypolimnion is the most likely cause
of this substantial decrease in the carbon isotope ratios in July. Weighted
average 5130 values for seston reached a maximum in September at both
stations. Fluorescence profiles indicate the presence of a fall phytoplankton

bloom at this time. Thus, decreases in the size of the 002 pool may have

24

resulted in less isotopic discrimination and an increase in 5130 value of seston.
Both down water column 5130 values and seasonal weighted 5130 averages
suggest that, in Grand Traverse Bay, lipid production and assimilation of
respiratory 002 are the predominant controls on the carbon isotopic composition

of seston in Grand Traverse Bay.

Nitrogen Isotopes

Variation in the 515N values of seston can be attributed to many factors,
including changes in (1) the relative contribution of different nutrient sources,
(2) types of particulate matter, (3) the degree of isotopic fractionation during
nutrient uptake and/or diagenetic transformation (e.g. ammonification). In Grand
Traverse Bay, where autochthonous material predominates the seston,
fractionation during nutrient uptake and subsequent fractionation during

biogeochemical cycling are the primary controls the 515N of seston. Variation in

the 515N value of algal derived seston has traditionally been interpreted to be a
consequence of the isotope effect associated with nutrient uptake and
assimilation (Saino and Hattori, 1980,1987; Altabet and McCarthy, 1986; Altabet
1988; Nakatsuka et al., 1992; Ostrom et al., 1997). Low 515N values for seston
have been attributed to isotopic fractionation during nutrient uptake (Saino and
Hattori, 1980,1987). In this case, inorganic nitrogen that is enriched in 14N is
preferentially utilized when nitrogen is not limiting to primary production (Altabet
and Deuser, 1991; Altabet and McCarthy, 1985, Saino and Hattori, 1980,1987).

Under nutrient depleted conditions, the degree of isotopic discrimination

25

. . . . 1 .
decreases, resulting In an Increase In 5 5N values for seston. If the Isotope
. . . . . . . . 15 . .
effect assocnated wuth nutnent assumllatlon IS the major control on 5 N vanatlon

of seston, then the 515N value of seston should be less than or equal that of the
inorganic nitrogen source (Ostrom et al., 1997). In many oligotrophic systems,
such as Grand Traverse Bay, where N03' is abundant it is considered an

- important source of nitrogen for phytoplankton growth (Dugdale and Goering,
1967; Eppley and Peterson, 1979). In Grand Traverse Bay, 515N values for N03'
were characterized by an average of 2.6 :l: 0.9%o for 33 samples. Although
phytoplankton in Lake Michigan are not nitrogen limited, seston in Grand
Traverse Bay was enriched in 15N relative to N03' values. Similar 15N
enrichments in seston relative to N03“ have been found in Lake Michigan, Lake
Superior, and Conception Bay, Newfoundland (Ostrom at al., 1997; and Ostrom
et al., in press). The observation that the 515N values of seston are higher than
those of N03' suggests that phytoplankton may be utilizing 15N enriched NH4+, or
that other processes in addition to the isotope effect associated with assimilation
of N03' are controlling the 615N value of seston (Ostrom et al., 1997).

Previous studies suggest that phytoplankton prefer NH4+ to N03"
(McCarthy, 1980), and in the Pacific Ocean concentrations of NHias low as
0.3 M were observed to cause inhibition of N03’ uptake (Wheeler and
Kokkinakis, 1990). In Grand Traverse Bay, NH4+ concentrations were typically
higher than 0.3 m, suggesting that NH4+ may be an important source of

inorganic nitrogen to phytoplankton despite N03' being the most abundant

26

nutrient. Preliminary measurements of the isotopic composition of NH4+ in Grand
Traverse Bay indicate that NH4” can be extremely enriched in 15N (average =
12.3 :t 3.0%o, n = 16) and this is consistent with high values for seston,
especially in the early spring. Alternatively, high 615M values for the suspended
particulate matter in the spring may be related to isotopic fractionation during
biogeochemical cycling of this particulate pool. The long residence time of
seston increases the potential for its transformation in the water column
(McCave, 1975). Processes other than nutrient assimilation that can cause an
increase in the 515M values of seston include microbial cycling, microzooplankton
grazing, peptide bond hydrolysis, and the formation of dissolved organic nitrogen
(DON) (Saino and Hattori, 1987; Altabet, 1988; Checkley and Miller, 1989; Silfer
et al., 1991,1992; Ostrom and Macko, 1992; Hoch et al., 1996; Feuerstein et al.,
1997).

Although the high 515N values for seston in the spring may indicate
utilization of NH4+, decreases of ca. 3 ”M in N03’ concentrations in the upper
water column from April through August at both stations (Figures 2 and 3)
suggest that phytoplankton may have also been utilizing 15N depleted NOa'.
Relative to earlier in the season, lower 515N values for seston between 20 and
60 m during June, July and August at GT 1, and in June and August at GT 3
suggest utilization of isotopically light N03' or discrimination against 15N during
uptake (Figures 2 and 3). The temporal decline in the 515M weighted average of

seston and decrease in N03' concentration with time is indicative of increased

utilization of N03' as the season progresses.

27

The vertical water column trends for the nitrogen isotopic composition of
seston reflect a balance between discrimination against 15N during assimilation
of N03“ and NH4”, and 15N enrichment resulting from the uptake of enriched
NH4+, and microbial degradation. Despite large variation in 515N depth profiles
throughout the season, there are specific cases where subsets of these
processes appear to predominate. When 515N values for seston are high, the
uptake of 15N enriched NH4+ appears to be an influential process. For example,
high 515N values (> 5%o) in the surface 20 m of the water column at GT 1
combined with a decrease in NH4+ concentrations from April to May are
consistent with uptake of NH4+ by phytoplankton (Figure 2). A sharp increase
from 4.0 to 9.7%o at 30 m depth in May at GT 3 is concurrent with a decrease in
the NH4+ concentrations at the same depth and also suggests uptake of 15N
enriched NH4" (Figure 3). In contrast, the high 515N value (9.4%0) in the
hypolimnion (60 m) on July 2 at GT 3 is indicative of loss of 14N during
degradation. High 515N values for seston are also observed within the benthic

nepheloid layer (Figures 2 and 3). This layer is believed to originate from near
shore erosion and resuspension (Eadie et al., 1983). The material comprising
the nepheloid layer in these profiles is not likely to be recently produced material
since there is no corresponding increase in chlorophyll fluorescence. In addition,
it is enriched in 130 and 15N at both stations relative to seston in the upper water

column, and this observation is consistent with more heavily degraded material.

. . . 15
Seasonal trends In concentration weighted average 5 N values for seston

28

are similar at both stations. The most salient feature of these data is that 515M
weighted averages peak in May, and decrease steadily through August

(Figure 6). The high values in the spring are consistent with the utilization of
NH4+ with an elevated 515M or loss of 14N during degradation, while the decrease
with time can be caused by preferential assimilation of 14N during nutrient uptake
or increase in the uptake of 15N depleted NOa'. Declines in N03' concentration
support an increased utilization over the season. Both the temporal and spatial

trends in this study suggest that 515N values for seston in Grand Traverse Bay

are controlled by a balance between depletions in 15N associated with

fractionation during uptake of NH4+ and N03' and depletions caused by

degradative processes in the water column.

Contaminants

PCBs have persisted in the Great Lakes for several decades, despite
regulations reducing their use and manufacture. These hydrophobic compounds
tend to degrade slowly and bioaccumulate (Thomann and Connolly, 1984;
Swackhamer and Skoglund, 1993), and can be transported great distances in
the atmosphere before they enter surface waters via wet or dry deposition
(Eisenreich et al., 1981; Swackhamer and Armstrong, 1986; McVeety and Hites,
1988). Following atmospheric deposition PCBs are known to partition into
organic rich particles, thus subsequent cycling of PCBs is closely related to the
production, physical transport, and loss of particles within the water column

(Baker et al., 1991). Although sedimentation and burial has been cited as a

29

mechanism of contaminant removal of PCBs (Eadie and Robbins, 1987), only
small amounts of these contaminants were found to be incorporated into surficial
sediment in Lake Superior (Baker et al., 1991;Jeremiason et al., 1994). This
suggests that PCBs are being recycled in the water column (Baker et al., 1985;
Jeremiason et al., 1988; Stow et al., 1995) or transported into the food chain via
zooplankton grazing (Swackamer and Skoglund, 1993; Stange and
Swackhamer, 1994).

Phytoplankton play a significant role in the incorporation of contaminants
into the aquatic food web (Swackamer and Skoglund, 1993; Stange and
Swackhamer, 1994; Skoglund et al., 1996). Our data show that zPCB of surface
seston decline throughout the season from a maximum in the spring (44.20 ng/g
dry weight) and appear to be correlated with integrated water column
fluorescence (R2 = 0.71) (Figures 4 and 7). This trend is indicative of high
concentrations of labile phytoplankton material in the spring that decreases as
the season progresses. If these hydrophobic compounds partition into the lipid
fraction or organic carbon of phytoplankton, then the observed trend in 2PCB
may result from a decrease in labile phytoplankton material. Specifically, our
data suggests that PCBs preferentially associate with the labile fraction of the
phytoplankton reservoir.

In addition to new production, high 2POB in the spring may be a result of
the resuspension of sediment or the benthic nepheloid layer. Although 0/N
ratios indicate that seston is comprised mainly of algal material, a small

contribution of refractory resuspended material may be significant if it is enriched

3O

in PCBs. Particulate matter from the benthic nepheloid layer has been implicated
as an important source of contaminants to the food web (Baker and Eisenreich,
1989; Baker et al., 1985,1991). The benthic nepheloid layer, as seen by
decreases in transmittance, developed following stratification in mid June at both
stations (Figures 2 and 3). In April, prior to stratification and the formation of the
nepheloid layer, zPCB in the surface waters (5 to 10 m) at GT 1 were at the
highest of the season. Later, when the benthic nepheloid layer begins to form,
2PCB at the surface decrease. The high concentration of PCBs in the surface
waters of GT 1 in the spring may result from mixing of nepheloid material rich in
organic contaminants throughout the water column. As the water temperature
rises, and stratification sets in, resuspended benthic nepheloid layer material
may settle out from the surface waters, resulting in a decrease in the
concentrations of PCBs in the surface seston. However, the 5130 values of
spring surface seston (-29.7°/oo for both April and May.) differ from that of the
benthic nepheloid layer (average = -25.9 :t 1.7%o, n = 9 for August to
September) and sediment from several locations in the western arm of Grand
Traverse Bay (average = -24.8 :t 1.1%, n = 8), suggesting that resuspension is
not the major contributor to the seston. In this case, a large contribution of PCBs
from resuspension would necessitate that resuspended material is enriched in
PCBs. Quantifying the relative contribution of PCBs from phytoplankton and
resuspended material is problematic owing to the difficulty in isolating

phytoplankton from other suspended particles.

31

CONCLUSIONS

Carbon and nitrogen isotopes, coupled with other water column data such
as temperature and fluorescence, can be used to provide a better understanding
of the dynamics of the cycling and the transformations of suspended particulate
matter in a lake system. Biogeochemical analysis of suspended particle
dynamics is critical for characterizing the base of the pelagic food web and
understanding the cycling of contaminants within food webs. The carbon
isotopic composition of seston in Grand Traverse Bay was largely affected by
that of the source of CO2, and the lipid content of the phytoplankton. The
nitrogen isotopic composition of seston, however, appeared to reflect a balance
between fractionation during uptake of 15N enriched NH4+ or 15N depleted N03”,
and other degradative processes that cause 5‘5N enrichments. High zPCB in
surface seston appears to be associated with increases in the labile fraction of

phytoplankton in the spring.

32

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