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Geochemical and Isotope Dynamics of
Dissolved Inorganic Nitrogen in
Grand Traverse Bay, Lake Michigan

presented by 1

Amy N. Macrellis

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11100 chlRCJDmDmpBS-p.“

 

GEOCHEMICAL AND ISOTOPE DYNAMICS OF DISSOLVED INORGANIC
NITROGEN IN GRAND TRAVERSE BAY, LAKE MICHIGAN

By

Amy N. Macrellis

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Geological Sciences

1999

UMI Number: 1396639

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ABSTRACT

GEOCHEMICAL AND ISOTOPE DYNAMICS OF DISSOLVED INORGANIC
NITROGEN IN GRAND TRAVERSE BAY, LAKE MICHIGAN

By

Amy N. Macrellis

Spatial and temporal variations in the 515N of N03- in Grand Traverse Bay, Lake
Michigan were explored to better understand sources of inorganic N to the bay and the
effects of climate change on N cycling. The bay was thermally stratified approximately
one month earlier in 1998 than in 1997. The SISN of N03- showed minor seasonal
variation in 1997; however, in 1998, a marked decline in the SISN of N03- with time was
observed. Although decreasing N03. concentrations during the summer months indicated
that N03- uptake occurred during both seasons, isotopic fractionation consistent with
uptake was not observed. Phytoplankton likely assimilated a combination of 15N -
enriched NIL;+ and 15N--depleted N03- in 1997 and 1998, resulting in seston 8'5N values
that were almost always higher than those of N03: Average seston 8'5N values in 1998
were 7%o higher than values observed in 1997. The occurrence of 15N-enriched seston in
1998 was linked to increased NH4+ uptake relative to N03- assimilation by a smaller
phytoplankton biovolume. Seston organic N is an important source of N03- in Grand
Traverse Bay; thus, rapid internal regeneration of NH4+ and N03- from seston is a primary
control on the S'SN of inorganic N species. The 615N of seston appears to be sensitive to
changes in the timing of thermal stratification and may therefore bean indicator of

climate warming.

To my Family:

With your love, everything is possible.

iii

ACKNOWLEDGMENTS

This work would not be possible without the support of Michigan SeaGrant (U/MC grant
PO A31744 RIBS-16) and the US EPA (R825151-010). I would also like to thank the
crews of the RV Lake Guardian, RV Shenehon, RV Laurentian, RV W. G. Jackson, and
the late RV Northwestern for ship time and cheerful support. A great deal of gratitude
goes to my advisor, Nathaniel Ostrom, and my committee members, Dave Long and

Steve Hamilton, who answered endless questions and were always willing to explain

 

things more than once. All of the Geochem lab crew--Robin, Eileen, Colleen, Mary,
Tucker, and Andery--thanks for not getting too upset with all my experimentations and
messes. Many thanks to my family outside of family, especially Ted, Tifani, and Tom in
Vermont who provided the space I needed to finally write the first draft of this
manuscript. Mom and Dad, thank you for always being supportive of me, even when I
wasn't supportive of myself. Heather, dear sister, thank you for always being just a little

bit crazier than I am.

iv

TABLE OF CONTENTS

List of Tables ............................................................................................................ vi
List of Figures ........................................................................................................... vii
Introduction ............................................................................................................... 1
Methods ..................................................................................................................... 5
Field methods ................................................................................................. 5
Water column characteristics ........................................................................ 5
Nitrate and ammonium isotopic analysis ...................................................... 6
Results ....................................................................................................................... 9
1997 ............................................................................................................... 9
1998 ............................................................................................................... 12
Discussion ................................................................................................................. 22
Sediment resuspension and SRP dynamics ................................................... 22
N sources and cycling in Grand Traverse Bay .............................................. 24
P controls on N dynamics ............................................................................. 27
Synthesis: Conceptual model of factors affecting the SEN of
NO3' in Grand Traverse Bay .................................................................... 30
References ................................................................................................................. 36

LIST OF TABLES

Table 1: Dates of ice cover on Grand Traverse Bay, Lake Michigan from
the winter of 1995-96 to 1998-99 (data from http://www.natice.
noaa.gov/pub/Archive/Great_Lakes ........................................................... 33

vi

LIST OF FIGURES

Figure 1: Map of Grand Traverse Bay, Lake Michigan. .......................................... 8

Figure 2: Water column temperatures (°C), soluble reactive phosphorous
(SRP) concentrations (11M), chlorophyll fluorescence (RF U), and
NH4+ concentrations (11M) plotted as a function of depth (m) and
sampling date for station GT1 during the 1997 sampling season. ............. 15

Figure 3: Water column temperatures (°C), soluble reactive phosphorous
(SRP) concentrations (uM), chlorophyll fluorescence (RF U), and
NH4+ concentrations (uM) plotted as a function of depth (m) and
sampling date for station GT3 during the 1997 sampling season. ............. 16

Figure 4: Concentrations of NO3' (uM) and SUN values of NO3' (%o)
plotted as a function of depth (m) and sampling date for stations
GT1 and GT3 during the 1997 sampling season. ....................................... 17

Figure 5: Concentration weighted average SISN values (960) for N03’ and
seston plotted as a function of sampling date at stations GT1 and
GT3 during the 1997 sampling season. ...................................................... 18

Figure 6: Water column temperatures (°C), soluble reactive phosphorous
(SRP) concentrations (pM), chlorophyll fluorescence (RF U), and
NH4+ concentrations (11M) plotted as a function of depth (m) and
sampling date for station GT3 during the 1998 sampling season. ............. 19

Figure 7: Concentrations ofNO3' (11M) and 5‘5N values ofN03' (%o)
plotted as a function of depth (m) and sampling date for station
GT3 during the 1998 sampling season. ...................................................... 20

Figure 8: Concentration weighted average 515N values (%o) for N03' and
seston plotted as a function of sampling date at station GT3 during
the 1998 sampling season. ......................................................................... 21

Figure 9: Chlorophyll fluorescence values integrated over the depth of the
water column (RF U/mz) plotted as a function of sampling date for
stations GT1 and GT3 in 1997 (from McCusker er a1. 1999) and for
station GT3 in 1998. .................................................................................. 34

Figure 10: Conceptual model of the factors affecting the BISN of NO3° in
Grand Traverse Bay. .................................................................................. 3 5

vii

INTRODUCTION

Variations in climate can have a dramatic influence on the Great Lakes ecosystem
by causing increases or decreases in precipitation (Montroy 1997), reduction in lake
levels, altered frequency and timing of lake turnover, reduced ice cover (Hanson et a1.
1992), increased likelihood of anoxia, and changes in water chemistry and biota (Mortsch
and Quinn 1996). For example, during an ice-free winter, strong winds may result in the
resuspension of P that would otherwise be trapped in sediments (N icholls 1998). Long-
terrn changes in water column stratification and nutrient sources, such as stream runoff ‘
and precipitation, may ultimately result in the perturbation of primary production and of
interactions between trophic levels in the Great Lakes ecosystem. To better understand
sources of inorganic N, the influences of dissolved inorganic nitrogen ODIN) on BlsN at

the base of a foodweb, and the effects of climate change on N cycling, we explored

spatial and temporal variations in the isotopic composition of NIL.+ and N03- in Grand
Traverse Bay, Lake Michigan. Strong El Nifio years such as 1982-83, 1987, and 1992
have been associated with climatic changes in the Great Lakes (N icholls 1998).
Understanding the response of this ecosystem to such short-term climatic changes can
enable a more accurate prediction of the response of aquatic systems to future climatic
events.

N isotope ratios have been used to assess the origins of N03- and NH.“ in
terrestrial and aquatic environments (Liu and Kaplan 1989, Cifuentes et a1. 1989, Macko
and Ostrom 1994, Ostrom et a1. 1997, Ostrom et al. 1998b). Sources of inorganic N‘to

aquatic environments primarily include atmospheric deposition, fertilizers, sewage inputs,

N-fixation, and in situ rernineralization and nitrification. Inputs of N from these sources
may be distinguished if the sources are characterized by distinct 8‘5 N values. For
example, low 6'5 N values for N03- and phytoplankton in coastal ecosystems and in Lake
Superior have demonstrated the importance of atmospheric deposition as a source of N to
primary production (Fogel and Paerl 1993, Paerl et a1. 1993, Paerl and Fogel 1994,
Ostrom et al. 1998b). Yoshioka er al. (1988) observed that 515N values of NO; in Lake
Kizaki and River Naka-Nogu were close to 0960 while the S'SN of NO3' in eutrophic Lake

Suwa was 5.0 to 5.6%. High SISN values for N03- in Lake Suwa relative to values in

Lake Kizaki and River Naka-ngu were strongly indicative of a large contribution of

NO3' from sewage. In the Truckee River, however, N inputs to phytoplankton via N-
fixation could not be distinguished from fertilizer inputs because both of these N sources
tend to have low and similar 5'5N values (Estep and Vigg 1985). Consequently, N03-
source differentiation based on 8'5 N is dependent on the existence of sources with unique
isotopic compositions and may be confounded by inputs from multiple sources.

Stable isotopic studies of biogeochemical reactions in natural environments are
complicated by the fact that isotopic variation may be the result of numerous reactions
and processes (Ostrom et al. 1998b). Spatial and temporal variation in the isotopic
composition of DIN may occur because DIN is subject to isotopic fractionation during
uptake by phytoplankton (Cifuentes et al. 1989, Waser et al. 1998). N uptake by
phytoplankton is the rate-controlling step in an ecosystem when NH.+ or N03- is the
limiting nutrient. Little or no fractionation occurs during uptake under N limitation and

the 8‘5 N of phytoplankton reflects that of its nutrient source (Wada and Hattori I978,

Wada 1980). If concentrations of N03- or NH; are not limiting, however, then the 815 N
of both the phytoplankton and the residual N may be altered. For example, assimilatory
uptake of NH4+ or NO3' by phytoplankton or bacteria has been shown to leave the
residual N enriched in ”N by as much as 7%o (Mariotti et a1. 1984, Horrigan et al. 1990,
Hoch er al. 1996). The direction of an isotope shift can also be used to reveal the
predominant biological and microbial processes affecting DIN. For example, high and
low BUN values for N03- can be used to recognize regions of denitrification and
nitrification, respectively (Cline and Kaplan 1975, Mariotti et al. 1981, Cifuentes et al.
1989, Liu and Kaplan 1989, Horrigan et al. 1990, Hedin et a1. 1998).

The study of isotopic variation at the base of foodwebs is essential toward an
understanding of N sources supporting foodwebs and trophic relationships. For example,
Van Dover et al. (1992) used stable N isotopes to demonstrate that sewage was a
nutritional support for a deep-sea foodweb. Isotopic studies of trophic relationships often
make assumptions about the number of trophic levels present in a food web based on
differences in 515 N between primary producers and predatory fish (e.g., Cabana and
Rasmussen 1996). The ultimate control of foodweb 515N, however, is not the isotopic
composition of the primary producers but that of the inorganic N assimilated by primary
producers. Thus, variation of the 8'5 N of consumers in different aquatic systems may be
a result of different 615 N values for DIN, and comparisons between these foodwebs
without an understanding of the corresponding inorganic N isotope values may be

misleading. Furthermore, the BISN of suspended organic material (seston) is likely to

change on a seasonal basis and depends at least in part upon the relative use of NO3' and
NH; by phytoplankton (McCusker et al. 1999).

The relative use of different inorganic N species by primary producers over time
may be affected by changes in climate such as El Nifio events. For example, longer
stratified periods and decreased epilimnetic nutrient availability may cause Lake
Michigan phytoplankton populations to shift from the present diatom- and
phytoflagellate-dominated communities (Fahnenstiel and Scavia 1987) to communities
composed primarily of blue-green algae (Chang and Rossman 1988) or other
phytoplankton species which are better adapted to low nutrient concentrations.
Alterations at the base of a food web, as well as changes in the number of trophic levels
present in that food web, may occur as a result of climate change and can be accurately
documented using stable N isotopes. The present study explores spatial and temporal
variations in the S‘SN of NIL;+ and N03' in Grand Traverse Bay, Lake Michigan, in order
to better understand the relationship between DIN and nutrients at the base of the

foodweb and to explore the effects of climate change on N cycling.

METHODS

Field methods

Water samples and conductivity-temperature-depth profiles were taken at two
stations, GT1 (44° 50.00 N, 85° 37.00 W; 98 m depth) and GT3 (44° 59.00 N, 85° 34.80
W; 112 m depth), in Grand Traverse Bay, Lake Michigan (Figure 1). Eight cruises were
conducted between April and September 1997 and seven cruises were performed between
March and September 1998. A Seabird SEE-25 conductivity-temperature-depth profiler
equipped with a fluorometer and transmissometer (Seatech) was deployed initially at each
station to assess the physical and biogeochemical characteristics of the water column.
Water samples were collected using 8 L or 5 L Go-Flo or lever-action Niskin bottles
(General Oceanics), respectively, at 5-6 depths, including points above, within, and below
the chlorOphyll maximum. Each sample was filtered through a precombusted (500 °C, 2
hours) 0.45 pm glass fiber filter (Whatman GF/F) and transferred to acid-washed Nalgene
bottles. Samples were stored on ice, frozen within 12 hours of collection, and stored

frozen (-20°C) until analysis.

Water column characteristics

Concentrations of N03- in water samples were determined by suppressor-based
anion chromatography (Shipgun and Zolotov 1988) using a Rainin HPLC with
conductivity detection (LDC Analytical). A Dionex IonPac (AS4A-SC) column and a
2.4 mM NazCO3/NaHCO3 eluent were used to separate anions. Concentrations of N114+

were measured using an Orion (model 95-12) ion specific electrode (Garside et al. 1978,

Ostrom et al. 1998b). The accuracy and limits of detection for both techniques were
approximately 0.1 uM (Ostrom er al. 1998a). Soluble reactive P (SRP) analysis was
conducted on filtered samples, and concentrations were determined colorimetrically as
molybdate reactive P (Murphy and Riley 1962). The accuracy and limit of detection of

SRP analysis was approximately 0.01 uM.

Isotopic analysis of N0; and NH4+

Purification of NH; and N03- from water samples for stable isotopic analysis was
conducted using a steam distillation procedure (Bremner and Keeney, 1966; Velinsky et
al. , 1989; Ostrom, 1992; Ostrom et al., 1998a). An initial distillation was performed to
recover and purify NH; for isotopic analysis. Sample volumes were Optimized to
recover between 1 and 20 umol N. Sample pH was adjusted to > 10 by the addition of
2.0 mL 5 N NaOH (previously distilled to remove NI-I;) to convert NH; to volatile NH3
gas. The collection rate was adjusted to 11.0 mL/minute, and the condensate from the
distillation was passed through an Erlenmeyer trap flask containing 25 mL of 0.084 N
HCl. If the sample contained less than 0.5 umol NH;, the trap flask solution was
discarded because this small amount of NH; could not be accurately analyzed for SUN.
Immediately following the distillation for NH;, 0.3 g of finely ground Devarda’s alloy
(50% Cu, 45% A1, 5% Zn) was added to the sample in order to reduce N03: to NH;. The
Erlenmeyer trap flask was replaced with a clean flask containing the same volume and
concentration of HG] as discussed above, and the sample was distilled a second time as

previously described. The NH; in the condensate was bound onto 0.1 g of a zeolite

molecular sieve (Union Carbide Ionsiv W-85), which exchanges H" for NH; (V elinsky
et al. 1989). Optimal binding conditions involve adjusting the sample pH to 5.0, adding
0.1 g of the sieve, and stirring gently for 30 minutes (Ostrom et al. 1998a). The zeolite
was then filtered onto a precombusted glass fiber filter (Whatman GF-F) and dried
overnight at 40°C. The binding procedure was repeated twice to insure complete
recovery of NH;, and the two filters were combined as one sample and prepared for
combustion. In preparation for combustion, the samples were transferred to
precombusted quartz tubes and mixed with ca. 3 g each of precombusted (850°C, 4 h)
copper and copper oxide and sealed under vacuum. Samples were combusted using a
modified Dumas procedure (Macko 1981), after which they were cryogenically purified
and analyzed for isotopic abundance using a Micromass Prism mass spectrometer.
Distillation of deionized water yielded a background contribution of 0.4 umol N with a

815N of +8.0 i 1.3%o derived primarily from the Devarda’s alloy; consequently, all 8'5N

values of N03- were corrected for this background contribution using a mass balance
equation (Ostrom 1992). Background levels of NH; from the distillation of deionized
water without the addition of Devarda’s alloy were negligible. Maintenance of isotopic
integrity through the distillation process was demonstrated by analysis of an international
NO3' standard, IAEA N-3, which has a reported value of 4.72%o (Bohlke and Coplen
1995). Analysis of three replicates of the IAEA N-3 standard in our laboratory yielded a

515N value of 4.1 i 0.1%. Precision of replicate samples was S 0.5%.

85°15’

45°15’

45°00’

 

 

 

 

Lake Michigan (http://www.gler|.noaa.gov)

Figure 1: Map of Grand Traverse Bay.

RESULTS

1997

Temperature profiles from stations GT1 (Figure 2a) and GT3 (Figure 3a) during
the 1997 season were indicative of a cold, protracted spring. Stratification in Grand
Traverse Bay was not apparent until early June. The thermocline was well-developed at a
depth of 15 m in July and the bay remained stratified through September. The entire
water column remained oxic throughout both the 1997 and 1998 sampling seasons.

Concentrations of SRP ranged between 0.07 to 0.22 pM at station GT1 (Figure
2b) and from 0.08 to 0.20 uM at station GT3 (Figure 3b). Spring SRP concentrations
were generally higher than concentrations measured during the summer months. A sharp
decline in SRP from 0.16 uM to 0.08 uM was observed following stratification in early
J une at both stations, after which SRP concentrations remained low (ca. 0.08 pM) for the
duration of the sampling season.

Chlorophyll fluorescence ranged from 0.1 to 1.1 relative fluorescence units (RFU)
at station GT1 (Figure 2c) and from 0.1 to 1.5 RF U at station GT3 (Figure 3c) throughout
the sampling season. Extensive mixing of the water column was reflected by relatively
high chlorophyll fluorescence readings (0.6 - 0.7 RFU) throughout the water colmnn
during April, May, and early June 1997. After stratification in early June, chlorophyll
fluorescence was generally lower and a maximum tended to occur just below the
thermocline.

Concentrations of NH; ranged from 0 - 1.3 M at station GT1 (Figure 2d) and

0 - 1.1 uM at station GT3 (Figure 3d) throughout the season; however, the lowest NIL+

concentrations occurred throughout the water column in the early spring and in
September. Maximum concentrations of 1.0 to 1.3 uM at both stations occurred in the
lower water column in late July through early August. Gradually increasing NH:
concentrations (from 0.5 to ca. 1.1 M) in the lower water column were observed
following a sharp decline in SRP concentrations at both stations.

Concentrations of N03- ranged from 12.6 to 18.7 uM at station GT1 and from
12.8 to 17.4 uM at station GT3 during the 1997 sampling season (Figure 4a and 4b,
respectively). The small variation in N03- concentration throughout the water column in
May and early June at both stations (15.8 i 0.8 uM at GT1 and 15.6 i 0.7 uM at GT3)
was indicative of extensive water column mixing. Declining N03- concentrations in the
upper water column at Station GT1 (15.0 to 12.6 uM) and at Station GT3 (15.5 to
12.8 uM) between July and September were likely a result of N03. uptake by
phytoplankton. Concentrations of N03- in the lower water column remained relatively
constant (ca. 16 11M) at both stations throughout the sampling season.

The isotopic composition of N03- ranged from +0.4%o to +78% at Station GT1
over the course of the 1997 sampling season (Figure 4c). The small degree of variation in
SlsN-NOg’ (less than 1%o) observed at both stations in May 1997 was consistent with
extensive water column mixing. At Station GT1 in mid-June and early July, the 5'5N of
N03- tended to be more 15N-enriched throughout the water column (+4.0 to +78%)
relative to the rest of the sampling season. A general decline in SISN values of N03' (to
+3.5 to +05%) was observed throughout most of the water column in mid-July through

September. The 6'5N of NO3' at station GT3 ranged from +04% to +6.1%o; however,

10

there were no easily discemable seasonal trends in 6'5N-N03' at GT3 during the 1997
season (Figure 4d).

The 6'5N of NH.+ was higher than that of N03- whenever sufficient NH; was
available to facilitate the measurement of its isotopic composition. The 6'5N of NH;
ranged from +86%» to +17.2%o at GT1 (n = 19) and +10.3%o to +17.5%o at GT3 (n = 11).
Sufficient data to clearly resolve temporal or seasonal trends in the isotopic composition
of NH; were not available owing to the small concentrations of NH; present in Grand
Traverse Bay.

Seasonal variations in the isotopic composition of N03- throughout the water

column were described using the following relationship, which weights the SISN of N03-

at each sampling depth by its respective concentration (Ostrom et al. 1997):

25‘5Nw = a 1 (1)

where SUN“, is equal to the N isotopic composition of N03. or seston (in %o) weighted by
concentration at a station on a specific date, C; corresponds to the concentration of N03-
or seston organic N for an individual sample (1.1M), 5, is equal to the S'SN of an individual

N03- or seston sample (%o), and n is the number of N03- or seston samples collected at a
station on a particular date.

The concentration-weighted 8'5N of N03. at station GT1 increased from +28% to
+53%: between May and mid-June, then gradually decreased to +14% in September
(Figure 5a). There was no easily discernible seasonal trend in the weighted 515N of N03-

at station GT3 during the 1997 sampling season, and values ranged from +2.1%o to

11

+4.3%o (Figure 5b). In contrast, the concentration-weighted SUN of seston at station GT1
(Figure 5c) decreased gradually from +5.9%o in early May to +1 .9%o in August, then
increased to +64% in mid-September. The weighted 615N of seston at station GT3
(Figure 5d) also decreased from +6.1 to +3.6%o, then increased dramatically to +92% in
September. While there was a significant relationship between the seasonal trends of the
weighted isotopic composition of NO3' and that of seston at station GT1 (or = 0.05, p =

0.46, n = 7) in 1997, such a relationship was not present at station GT3 (or = 0.05, n = 6).

I 998

In 1998, temperature profiles were indicative of an early, warm spring (Figure 6a).
The bay showed signs of stratification as early as May 7, and was well-stratified from
mid-May through September. During most of the sampling season, the thermocline was
apparent near 15 m, but in mid-September a deeper (ca. 25 m) and more gradual
thermocline was observed.

SRP concentrations were slightly lower (0.07 to 0.16 11M) at station GT3 in 1998
than in 1997 (Figure 6b), although a similar seasonal trend of rapidly decreasing SRP
concentrations (0.10 to 0.07 uM) concurrent with stratification was observed. In 1998, P
decline was apparent in early May; however, a similar rapid decline in SRP
concentrations did not occur until late June of 1997.

Lower chlorophyll fluorescence values (0.10 - 0.85 RFU) were generally observed
at station GT3 during the 1998 sampling season (Figure 6c), although the overall trend in
chlorophyll fluorescence was similar to that observed in 1997. Water column mixing was

apparent in March as shown by consistent values with depth (ca. 0.5 RF U). During May

12

and June, the highest chlorOphyll fluorescence levels (0.65 - 0.85 RFU) occurred in a
broad peak just beneath the thermocline. In August and September 1998, however, a

chlorophyll fluorescence maximum occurred above the thermocline.

Concentrations of NH; were slightly higher (0 - 1.5 uM) at station GT3 during
the 1998 sampling season (Figure 6d). As in 1997, increased NH; concentrations (1.0 to
1.4 uM) were generally observed in deeper waters following stratification, and the lowest
concentrations were observed throughout the water column in March and in September.

Concentrations of N03- ranged from 10.9 to 17.7 M at station GT3 throughout
the 1998 sampling season (Figure 7a). A decreasing trend in N03- concentration
throughout the upper water column during the summer months was observed in 1998,
although the magnitude of the decline (15.5 11M to 11.0 11M) was greater than the decline
observed in 1997. Concentrations of N03- in the lower water column increased from 16.2
uM in March to 17.7 11M in mid-September of 1998.

The 5'5N of NO3' (Figure 7b) ranged from -1 .3%o to +7.6%o at Station GT3 over
the course of the 1998 sampling season. During March 1998, the 615N of N03. was
relatively high, and declined gradually throughout the water column from May through
August, reaching a low of -l .3%o at 25 m on August 7. Subsequently, the SISN of N03.
began to increase and attained values near +4.5%o in September.

Concentration-weighted average 8‘5N values for N03. (Figure 8a) were strikingly
more variable seasonally at station GT3 in 1998 than in 1997. The weighted 815N of
NO3' exhibited a decreasing trend from +5.1%o in May 1998 to +0.2%o in August,

followed by a slight increase to +38% in mid-September. The concentration-weighted

13

average SUN of seston (Figure 8b) was much higher overall in 1998 than in 1997, with a

range of 8.8%o to 13.7%. A strong relationship exists between the weighted isotopic
composition of seston and that of NO3' in 1998 (or = 0.05, r2 = 0.87, n = 7), whereas in

1997 there was no significant relationship between the weighted 815N of N03- and that of

seston at station GT3. The isotopic composition of NH; (range of 7.0 to 20.9%, n '= 20)

remained higher than that of NO3' at all times in 1998; however, the data were not

complete enough to allow the delineation of seasonal trends.

14

 

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21

DISCUSSION

Sediment resuspension and SRP dynamics

The highest SRP concentrations were observed during the unstratified period in
both 1997 and 1998 (Figures 2b, 3b, 6b). This relatively large pool of available P is
likely the result of sediment resuspension during winter mixing. Resuspension can be
regarded as a mechanism which accelerates the flux of solutes between sediment and lake
water, both in the sense of increasing flux of organic P from sediment to water and in
increasing the introduction of 02 and other oxidizing elements from water to sediment
(Sondergaard et al. 1992). In the open waters of Lake Michigan, water column trends of
SRP and total P were generally similar, although TP was always higher than SRP (Eadie
et al. 1984). Higher SRP and TP concentrations in the benthic nepheloid layer relative to
the rest of the water column firrther support the idea of a source of SRP originating from
the resuspension of sediments. Sediment resuspension tends to be greatest in Lake
Michigan during the winter months and therefore may control total P and SRP
concentrations in early spring (Chambers and Eadie 1981). The large amount of winter
resuspension is a mechanism that provides a greater amount of contact between recent
sediments and the water column and thus loads the lake with P (Eadie et al. 1984).
Sediment resuspension during the winter months may explain the relatively high SRP
concentrations observed in Grand Traverse Bay prior to stratification in both 1997 and
1998. The analysis of sediment trap material from the bay, however, is not yet complete.

The importance of winter water column mixing to spring P concentrations in the

water column was demonstrated by the observation that when ice cover inhibits winter

22

 

mixing of Lake Michigan, spring total P concentrations are significantly less than in years
without ice cover (Rodgers and Salisbury 1981). In Grand Traverse Bay during the
spring of 1998, however, lower SRP concentrations were recorded following a winter
with no observable ice cover than following the winter of 1996-1997 when significant ice
cover was observed (Table 1). Lower spring P concentrations in the water column prior
to stratification in 1998 may be the result of the occurrence of fewer major storm events
contributing to extensive mixing during the winter of 1997-98 and early spring
stratification in 1998.

Sedimentation plays a major role in the decline of P concentrations during
stratified periods and is influenced by plankton community structure (Guy et a1. 1994).
Diatoms, a major component of the spring phytoplankton community in Lake Michigan,
are characterized by relatively high sedimentation losses during early stratification. This
sinking loss is important in removing diatoms and associated nutrients from the
epilimnion (Fahnenstiel and Scavia 1987). Evidence of rapid nutrient removal from the
water column was observed in Grand Traverse Bay. During 1997, SRP concentrations
were generally higher during the unstratified period, and a decline in SRP concentrations
was not apparent until 2-3 weeks following stratification. In 1998, however, SRP
concentrations declined throughout the water column even before stratification was
strongly developed, suggesting that much less P was available to support the
phytoplankton population.

Sediment resuspension may control SRP concentrations in the water column
during unstratified periods, but resuspension in deeper waters during stratification cannot

contribute to primary production in the epilimnion (Conley et al. 1988). During stratified

23

periods, additional P may be supplied to the water column via external sources including
surface runoff, stream inputs, atmospheric deposition, and sediment fluxes. In Lake
Michigan, however, the annual P utilization for primary production is about 100 times
greater than the estimated SRP sediment flux and 57 times greater than the combined
external loadings of total P (Conley et al. 1988). Thus, internally regenerated P is
necessary to sustain the rates of primary production commonly observed in Lake
Michigan during stratified periods, and the internal regeneration of P must occur on very
short time scales in the water column (Conley et al. 1988).

The timing and strength of stratification had a dramatic effect on P (SRP)
availability to primary producers in Grand Traverse Bay. During 1997, when the bay
experienced an exceptionally cold and protracted spring, the water column remained
unstratified until early June. The lack of stratification during this long period may have
resulted in greater sediment resuspension and higher SRP concentrations in the water
column. In 1998, however, a warm spring and early stratification may have inhibited

resuspension and therefore less P was available to support the phytoplankton population.

N Sources and Processes in Grand Traverse Bay

External sources of DIN to Grand Traverse Bay may include atmospheric
deposition, stream inflows, groundwater, and exchange with the open waters of Lake
Michigan. Inputs of NH4+ and N03- to surface waters via atmospheric deposition in 1997
were calculated using NADP / NTN monthly precipitation-weighted mean concentrations

from the Wellston, MI station (http://nadp.sws.uiuc.edu/nadpdata). Atmospheric

deposition of NH4+ and N03- combined accounted for less than 1% of the total mass of

24

DIN in the bay during the 1997 sampling season; thus, precipitation was not considered to

be a significant external DIN source in this model. Inputs of N03- from the Boardman
River, the major riverine source to the west arm of Grand Traverse Bay, accounted for
only 0.005% to 0.02% of the total mass of DIN in the bay from May to October of 1998
(S. Woodhams, personal communication). Contributions of DIN to the bay from
groundwater were also likely to be minimal relative to the bay’s volume (D. Boutt,
personal communication). Water exchange between Grand Traverse Bay and Lake
Michigan is limited by the presence of a shallow sill at the north end of the bay (Lauff
1957), and currents in the bay are weak relative to those in the open waters of the lake (G.
Miller and T. Miller, GLERL, unpublished data). Consequently, internal regeneration of
NH; and N03- is more likely to control the SlSN of DIN species than are external sources
of DIN to the bay.

N fixation is a potentially significant source of 15N-depleted N. The
mineralization and nitrification of Nz-fixing algae may yield N03' with low 8‘5 N.

Therefore, N fixation is a mechanism that may cause seasonal variation in the SISN of

N03: N fixation may occur at N03- concentrations greater than 10 M but is
significantly inhibited by NH4+ concentrations as low as 0.5 1.1M (Takahashi and Saijo
198 8). In Grand Traverse Bay, NH4+ concentrations below 0.5 [AM did not occur at any
time in the epilimnion during August and early September of 1997 or 1998, when the

lowest SISN values for N03- were observed. Furthermore, in oligotrophic Lakes Superior,

Huron, and Michigan, biological N fixation accounts for only 0.02% of total N inputs to

25

these systems (Mague and Burris 1973). Thus, N fixation is not likely to be a source of
15N-depleted N03- to Grand Traverse Bay.

Sediments may serve as either a net source or a net sink for NO3', depending upon
the conditions that exist in a specific sedimentary environment. Denitrification in Lake
Michigan sediments was found to be an unlikely sink for N03- dissolved in hypolimnetic
water overlying the sediments; however, this process was a sink for N mineralized in the
sediments (Gardner et a1. 1987). Recent work in estuarine sediments suggests that even if
sediments are a sink for hypolimnetic NO3', isotopic evidence for sediment denitrification
would not be observed in the overlying waters (Brandes and Devol 1997). Consequently,
sediments in Grand Traverse Bay are not likely to be a significant source of N03' to
overlying waters, nor is sediment denitrification likely to influence the 515N of
hypolimnetic N03:

The isotopic composition of N03' is often controlled by uptake in aquatic systems
(Saino and Hattori 1980, Mariotti et al. 1984, Cifuentes et al. 1988, Montoya et al. 1990,
Altabet et al. 1991). During assimilation of N03- or N114+ by phytoplankton, decreasing
DIN concentrations are usually observed concurrent with an increase in the S'SN of the
residual DIN. While decreasing NO3' concentrations were observed in the upper water
column of Grand Traverse Bay following stratification in 1997 and in 1998, concurrently
increasing 8'5N values for N03' were generally not observed. Thus, N03. uptake by
phytoplankton is not a dominant factor influencing the isotopic composition of N03' in
this system. Similarly, Cifuentes et al. (1988) did not observe fractionation effects during

uptake when NH4+ concentrations were below 20 M in the Delaware Estuary, suggesting

26

that isotopic fractionation during uptake may not be observed below a critical
concentration.
N inputs from external sources, N fixation, or sediments are not likely to affect the

6'5N of N03. in Grand Traverse Bay. Although evidence of N03- uptake by

phytoplankton was observed, assimilation does not appear to strongly affect the 515 N of
N03. in this system. Consequently, internal recycling of N via remineralization of

organic matter and nitrification is likely a primary control on the B‘SN of N03. in Grand

Traverse Bay.

P controls on N dynamics

In 1997 and 1998, NIL;+ concentrations gradually increased with time following
the sharp decline in SRP concentrations. Higher NH; concentrations in the lower water
column during July and early August in both years were also associated with changes in
the pattern of chlorophyll fluorescence throughout the water column. Before
stratification, NIL+ concentrations were generally low and chlorophyll fluorescence
values were relatively high throughout the water column. A relatively large
phytoplankton population was being mixed throughout the water column during the
unstratified period, and this population likely assimilated a large proportion of the
available NHX. After stratification, water column mixing was greatly reduced. Changes
in the availability of NH: to phytoplankton in the water column occurred as a result of
reduced mixing following stratification. The amount of available NIL+ in aquatic
systems is controlled by a balance of losses due to nitrification and uptake and inputs-due

to internal regeneration (Axler et al. 1982, Laird et al. 1988). The observation of low

27

NIL:+ concentrations in the upper water column of Grand Traverse Bay indicates that
following stratification, NH4+ uptake was more rapid than the remineralization of organic
N. Higher NH: concentrations in the lower water column suggested that there was more
regeneration of NI-I4+ than uptake. The apparent relationship between [NHX] and [SRP]
is likely a function of phytoplankton growth limitation by P and a relaxation of NI-I4+
uptake relative to remineralization once P becomes limiting, particularly in deeper waters
following stratification.

In many freshwater aquatic systems, NH4+ is the preferred DIN species for uptake
by phytoplankton (Takahashi and Saijo 1981, Axler et al. 1982, McCarthy et a1. 1982, Gu
and Alexander 1993, Takahashi et al. 1995). Because most phytoplankton prefer NH4+,
N03. uptake may be significantly inhibited at NH4+ concentrations as low as 0.5 uM
(Takahashi and Saijo 1981, Priscu and Priscu 1984). In oligotrophic Flathead Lake,
however, low rates of N03. uptake occurred during most of the year at NH4+
concentrations of up to 5 uM (Dodds et al. 1991). Consequently, because NH4+
concentrations in Grand Traverse Bay were generally less than 2 uM, it is likely that
phytoplankton assimilated both NI-I4+ and N03- in varying proportions throughout 1997
and 1998.

Variations in the N isotopic composition of seston are primarily understood to
reflect isotope effects associated with nutrient uptake by phytoplankton (Saino and
Hattori 1980, Mariotti et al. 1984, Montoya 1990, Nakatsuka et al. 1992). When uptake

by phytoplankton is a dominant control on the SN of seston, low seston 515N values are

observed concurrent with a sharp decrease in N03- concentrations (Saino and Hattori

28

1980). High S'SN values for seston relative to NOg', however, may be a consequence of
several processes, including assimilation of 15N-enriched NHX, losses of 15N-depleted
material from seston, microzooplankton grazing, and microbial degradation. Seston 5'5 N
values in Grand Traverse Bay were almost always higher than those of N03- in both 1997
and 1998, thus providing isotopic evidence for the suggestion that phytoplankton
assimilated a combination of lsN-enriched NI-I4+ and lSN-depleted N03- throughout both
sampling seasons.

In central Lake Michigan, the vertical distribution of phytoplankton biovolume
was quite similar to the vertical distribution of chlorophyll (Brahce 1980). Thus,
chlorophyll fluorescence may be used in some cases as a rough approximation of
phytoplankton biovolume. Integrated chlorophyll fluorescence values at both stations
GT1 and GT3 in 1997 were higher than values recorded in 1998 (Figure 9). Relatively
low seston 8'5N values in Grand Traverse Bay throughout most of the 1997 sampling
season were likely the result of the assimilation of both 15N-enriched NH4+ and 15N -
depleted N03- by a relatively large phytoplankton biovolume. The decreasing trend in
concentration-weighted SISN values of seston observed in 1997 reflected increasing
assimilation of 15N-depleted N03- as the sampling season progressed. In 1998, however,
there was less P available to phytoplankton (as a result of early stratification).
Phytoplankton biovolume, as reflected by chlorophyll fluorescence, was smaller overall
because of reduced P availability (Figure 9). Higher 5'5N values for seston in 1998
relative to 1997 may be a result of less overall demand for inorganic N by phytoplankton.

Higher proportions of lsN-enriched NH4+ uptake by phytoplankton in 1998 relative to

29

1997 may have contributed to the marked increase in the SISN of suspended organic
material between the two years.

The 815N of seston in Grand Traverse Bay appears to be largely controlled by a
balance between uptake of 15N-enriched NH: and lsN-depleted NOg'. Furthermore,
isotopic fractionation during uptake of NH4+ and N03- in Grand Traverse Bay is minimal,
although fractionation due to degradative processes affecting seston cannot be ruled out.
Changes in the timing of stratification between 1997 and 1998 caused a subtle shift in P
availability, and this markedly affected the timing of phytoplankton reliance on NH:
versus N03- as reflected in the SUN of seston in Grand Traverse Bay. The chain of events
described in the bay is an illustration of how SISN at the base of a food web may be
affected by longer stratified periods as a result of climate warming. Here, a longer
stratified period in 1998 altered P dynamics and affected the 8'5N of both N03- and
seston. Recent increases in sediment 8'5N in eastern Lake Ontario (Hodell and Schelske
1998) may also be linked to climate warming. In Grand Traverse Bay, we observed
higher 615N values for seston during a warm year with a relatively long stratified period.
If the high seston S'SN values we observed are mirrored in sinking POM data and
consequently in recent sediments, then the increasing 815N of sediment may be the
indirect result of changing phytoplankton nutrient preferences as a consequence of

climate warming.

30

Synthesis: Conceptual model of the factors affecting the 6'5N of N03. in Grand
Traverse Bay

A conceptual model was devised to explain the apparent relationship between the
SN of seston and that of NO3' which relates the isotopic composition of N03' to its
sources via nitrification (Figure 10). Seston and sinking POM are the ultimate sources of
N03' in the water column via mineralization and subsequent nitrification (represented by
arrows marked a in Figure 10). Although N03- inputs from sediment were likely

unimportant in this system (Gardner et al. 1987), rates of N mineralization and

denitrification were not measured. Therefore, a source of NO3' originating in the
sediments of the bay (represented by arrows marked b in Figure 10) cannot be completely

discounted. Isotopic fractionation during uptake did not appreciably affect the BISN of

N03- in Grand Traverse Bay, so fractionation due to uptake (indicated by arrows marked
c in Figure 10) is considered to be negligible. External N sources and possible inputs
from N fixation were not likely to be significant, as discussed previously, and were not
included in the model. Thus, the major internal sources of N03' to Grand Traverse Bay
may be limited only to seston and sinking POM via remineralization and nitrification. A
preliminary average 515N value of 10.3%o for sinking POM (N. Ostrom, unpublished
data) agrees well with recent sinking POM isotopic data from Lake Ontario (Hodell and
Schelske 1998). The relative contributions of seston and sinking POM to the N03' pool
and isotopic fractionation that may be associated with the processes of remineralization

and nitrification (indicated by a in Figure 10) have yet to be determined in the bay.

31

Organic N from seston and potentially from sinking POM, however, contributes to and

influences the isotopic signature of N03' in Grand Traverse Bay.

During most of the 1997 sampling season, the 815N of seston differed from that of
N03' by 3960 or less. No significant seasonal trend in the SISN of N03- was observed (or =
0.05, n = 6) and a relationship between the isotopic composition of seston and that of
NO3’ could not be easily described. The weak relationship between the SISN of N03- and
that of seston is indicated by the homogenous color of the N03- reservoir at the top of
Figure 10. However, the 5151\1 of seston at station GT3 was much higher overall in 1998
(avg. = 12.2%o,ln = 32) than in 1997 (avg. = 5.6%, n = 32), likely because of increased
15N-enriched NH4+ uptake by phytoplankton in 1998. When organic N from this 15N-
enriched seston was remineralized and nitrified, the high isotopic composition of seston
was transferred into the N03' pool. As a result, a striking seasonal trend in the 815N of
NO3' was observed in 1998, coupled with a significant relationship between the isotOpic

compositions of seston and N03. (or = 0.05, r2 = 0.87, n = 7). The strong relationship
between the 5”N of N03- and that of seston in 1998 is indicated by the strong gradation

from dark gray (high SISN) to light gray (low 8'5 N) in the N03- reservoir at the bottom of
Figure 10. Seston acts as a naturally lsN-enriched tracer in 1998 and allows the

qualitative observation of the contribution of seston organic N to the dissolved N03'
reservoir in Grand Traverse Bay. In this system, the S'SN of N03’ is dependent upon the

isotopic signatures of the reservoirs of organic N contributing to NO3’ via
remineralization and nitrification. Seston is a significant source of regenerated N03. in

the bay and therefore, replenishment of N03- in the bay is largely dependent upon internal

32

processes instead of external N sources. The fact that the SISN of NO3', the largest
available pool of N in this system, exhibits seasonal variation in 1998 is evidence that
internal NO3' regeneration must be rapid in Grand Traverse Bay.

In conclusion, changes in the timing of stratification resulted in less P availability
to phytoplankton in Grand Traverse Bay. Although phytoplankton likely assimilated a
combination of 15N-enriched NH4+ and 15N-depleted N03- throughout both 1997 and
1998, isotopic fractionation normally associated with DIN uptake was generally not
observed in the N03- pool during either sampling season. Changes in SRP availability
during the 1998 season affected the timing of phytoplankton reliance on NH4+ as opposed
to NO3', and this shift was reflected in higher 815N values for seston. The alterations that
occurred in Grand Traverse Bay between 1997 and 1998 serve as an illustration of how
S'SN at the base of a foodweb may be affected by longer stratified periods as a result of

climate warming. At this time, inputs from processes such as N fixation are not

observed, and the replenishment of NO3' in Grand Traverse Bay is dependent upon rapid
internal regeneration via mineralization and nitrification of organic N from seston and
potentially from sinking POM. Long-term changes in the timing of stratification, nutrient
chemistry, or external source inputs as a result of climate change, however, may
permanently shift phytoplankton populations in favor of those species adept at utilizing
low concentrations of nutrients such as NH: and P. Thus, long-term changes in water
chemistry as a result of longer stratified periods may significantly affect nutrients at the

base of the food web, and may ultimately change the trophic structure of Grand Traverse

Bay.

33

 

 

 

 

 

 

 

 

 

 

 

 

Location - Date first ice Date last ice
recorded recorded
1 995-1996 East Arm 02/02/96 04/29/96
West Arm 02/02/96 04/29/96
1 996-1997 East Arm 01/14/97 04/15/97
West Arm 01/1 7/97 04/1 53/97
1997-1998 No ice recorded on composite images in East or West Arm
l I
1998-1999 No ice recorded on composite images in East or West Arm

 

 

Table 1: Dates of ice cover on Grand Traverse Bay, Lake Michigan from the
winter of 1996-96 to 1998-99 (data from httpzllwww.natice.noaa.govlpub/archive/
Great_Lakes).

34

 

.wmg c. m...0 cozmum .0». .625 Saar ..m .m 56.2.02). E05 mam? 2.. 9.0 new Eb 95.2me .2 Emu DEE—Ema .0.

2.0.8.2 6 mm note... ANESHEV 525.8 .262, o... .0 562. o... 56 62662:. mo:.m> mocoomocozc _.>2.ao.o.2.o. ”a 659“.

9.4. .2. tr :2. .

.35.

l I

E<

 

 

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nmmw .mHO 6
209. ......O .0.

 

 

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Toe 3
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too n
I
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35

1997 0‘? Uptake

a , . 7 Nitrate
2.53..

 

   

 

 

 

Seston . Sinking
5.6 °I.,, POM
10.3 ‘1!”
(XE Sediment

 

 

or u 1
1998 69 ptake ,_ 6

up Nitrate "
3 3%.,

   

 

   

 

 

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12.2 °I°o 4 “ “ POM
@ 10.3 °I,,
Sediment

(I

Figure 10: Conceptual model of factors controlling the 815N of N03 in Grand
Traverse Bay. The 815N of NO3', represented by the large shaded box, is
affected by inputs from various internal sources (represented by arrows). Inputs
from internal sources that appeared to affect the 815N of N03 are indicated by
shaded arrows with solid borders (a), while internal sources or processes whose
contributions were not appreciable are shown by unshaded arrows (b and c).
Isotopic fractionation associated with the processes of remineralization,
nitrification, and uptake has not yet been determined, or is negligible; the
potential for fractionation to occur during these processes is indicated by a. The
scale to the right of the figure denotes the 815N of all N reservoirs discussed In
the model. In 1997, no strong seasonal trends In the 815N of N03 were
observed (note lack of gradation in 1997 N03 reservoir). In 1998, however, a
highly significant relationship was observed between the 815N of N03 and that of
seston (note strong gradation In 1998 N03 reservoir). The occurrence of15N-
enriched seston in 1998 allowed the qualitative observation of the contribution of
seston organic N to the N03' reservoir via remineralization and nitrification.

36

 

 

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AXLER, R. P., GERSBERG, R. M., AND C. R. GOLDMAN. Inorganic nitrogen assimilation
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BRAHCE, M. Z. 1980. The vertical distribution of phytoplankton and primary production _
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BRANDES, J. A., AND A. H. DEVOL. 1997. Isotopic fractionation of oxygen and nitrogen
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BREMNER, J. M., AND D. R. KEENEY. 1966. Determination and isotope-ratio analysis of 1

 

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CHAMBERS, R. L. AND B. J. EADIE. 1981. Nepheloid and suspended particulate matter in
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CONLEY, D. J ., QUIGLEY, M. A., AND C. L. SCHELSKE. 1988. Silica and phosphorous
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ESTEP, M.L.F. AND S. VIGG. 1985. Stable carbon and nitrogen isotope tracers of trophic
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FAHNENSTIEL, G. L. AND D. SCAVIA. 1987. Dynamics of Lake Michigan phytoplankton:
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FOGEL, M.L. AND H. W. PAERL. 1993. Isotopic tracers of nitrogen from atmospheric
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