/ 07W! ’7yl6338 This is to certify that the thesis entitled SEASONAL VARIATION IN RATIOS OF COMMUNITY RESPIRATION TO GROSS PHOTOSYNTHESIS DETERMINED BY STABLE ISOTOPES AND CONCENTRATIONS OF DISSOLVED OXYGEN IN GRAND TRAVERSE BAY, LAKE MICHIGAN presented by AMANDA LEIGH FIELD has been accepted towards fulfillment of the requirements for the MS. degree in Environmental Geosciences Mil-dc Q. X 07L Major Prfiessor’s Signature 14pm! 30' 2007 Date MSU is an Affirmative Action/Equal Opportunity Institution LIBRARY MIChIQan State University PLACE IN RETURN Box to remove this checkout from your record. To AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 6/01 cJCIRCJDateDuepGS-pJS __ _ “.— SEASONAL VARIATION IN RATIOS OF COMMUNITY RESPIRATION TO GROSS PHOTOSYNTHESIS DETERMINED BY STABLE ISOTOPES AND CONCENTRATIONS OF DISSOLVED OXYGEN IN GRAND TRAVERSE BAY, LAKE MICHIGAN. By Amanda Leigh Field A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geological Sciences 2004 lulu? I'll-It: I IIIIIIIIIIIIII, ABSTRACT SEASONAL VARIATION IN RATIOS OF COMMUNITY RESPIRATION TO GROSS PHOTOSYNTHESIS DETERMINED BY STABLE ISOTOPES AND CONCENTRATIONS OF DISSOLVED OXYGEN IN GRAND TRAVERSE BAY, LAKE MICHIGAN. BY Amanda Leigh Field Dissolved oxygen concentrations (02) were used in conjunction with stable oxygen isotopes (5180-02) to determine the ratio of community respiration to gross photosynthesis (R:P ratios) in 2000 and 2001 in Grand Traverse Bay (GTB), Lake Michigan. Average R:P ratios in 2000 (1.2 :t 0.03 SE) and 2001 (1.1 i 0.04 SE) indicate that GTB is net heterotrophic on an annual basis and requires an aIlochthonous source or non-contemporaneous autochthonous source of organic carbon to support excess heterotrophic activity. On a seasonal basis, the system fluctuated between periods of net heterotrophy (R:P > 1) in early spring and late fall and net autotrophy (R:P < 1) in late spring and early summer. Periods of net autotrophy coincided with the onset of stratification. As stratification progressed, R:P ratios near unity were observed in the epilimnion indicating that heterotrophic activity in GT8 is strongly dependent upon autochthonous inputs of organic carbon during periods of stratification. We suggest that the temporal discontinuity between the introduction and utilization of organic carbon drives metabolism towards net heterotrophy. In memory of James E. Field: Pancakes and coffee are not the same without you. I wish you were still here with us. iii ACKNOWLEDGMENTS This work was sponsored by the Michigan Sea Grant College Program, project number RIES-18, under grant number 1997-99 NA76RGO133 from the Office of Sea Grant, National Oceanic & Atmospheric Administration (NOAA), US. Department of Commerce, and funds from the State of Michigan. The US. Government is authorized to produce and distribute reprints for governmental purposes notwithstanding any copyright notation appearing hereon. I thank the Grand Traverse Band of Ottawa and Chippewa Indians and the officers and crew of the Inland Seas for their dedicated assistance and good humor that made sampling both possible and enjoyable. This work is indebted to the efforts of my advisor, Dr. Nathaniel Ostrom, and my committee members, Dr. Peggy Ostrom and Dr. Jan Stevenson. I thank Dr. Hasand Gandhi and Brian Roberts for their assistance with sample analysis. A special thank you to Kim Frendo who aptly conducted the sampling in 2000 and took the time to demonstrate everything to the newcomer. I truly owe Dr. Mary Russ for her patient assistance and encouragement throughout my graduate career, not to mention her honesty and fabulous humor. Thank you to all those faculty and staff at Michigan State University who have provided encouragement and generous assistance: Dr. Cambray, Dr. Patino, Dr. Sibley, Mr. Todd Tarrant, Mr. Gabe Ording, and Mrs. Marsha Walsh. 1 especially thank my mother and father who put up with a tremendous influx of ‘stuff’ when I moved back and even a larger mess once I began writing. Your trust and courage in the face of adversity has given me more strength than anything or anyone else ever could — thank you. iv TABLE OF CONTENTS LIST OF TABLES ................................................................................................ vi LIST OF FIGURES -- -- vii INTRODUCTION _- - __ _ _ _ __ -----1 METHODS 6 FIELD AND LABORATORY METHODS ............................................................................... 6 DETERMINATION OF R:P RATIOS .................................................................................... 7 RESULTS 9 DISCUSSION......_ _ _______________ -- ______ - 13 R:P RATIOS ................................................................................................................... 17 CONCLUSIONS _ _ - - - - _________________ _ .25 TABLES 27 FIGURES - - , ..... -_-____-_.29 LITERATURE CITED _ - ............ - ...... 39 LIST OF TABLES Table 1: Summary of measurements collected at station GT3, 2000 ................ 27 Table 2: Summary of measurements collected at station GT3, 2001 ................ 28 vi LIST OF FIGURES Figure 1: Station GT3, Grand Traverse Bay, Lake Michigan (modified from http:l/www.glerl.noaa.gov). ................................................................. 29 Figure 2: Temperature ( ° C, bold line) and Fluorescence (RFU, thin line) as a function of depth (m) and sampling date in 2000. ....................... 30 Figure 3: Temperature ( ° C, bold line) and Fluorescence (RFU, thin line) as a function of depth (m) and sampling date in 2001. ....................... 31 Figure 4: Fraction of 02 saturation (023m) as a function of depth at station GT3 in Grand Traverse Bay in (a) April - June 2000, (b) July and October - November 2000, (c) April - June 2001, and (d) August and October 2001. .................................................................... 32 Figure 5: 5180-02 as a function of depth at station GT3 in Grand Traverse Bay in (3) April - June 2000, (b) July and October - November 2000, (G) April - June 2001, and (d) August and October 2001 .............................................................................................. 33 Figure 6: R:P ratios as a function of depth at station GT3 in Grand Traverse Bay in (a) April - June 2000, (b) July and October — November 2000, (0) April — June 2001, and (d) August and October 2001 .............................................................................................. 34 Figure 7: (a) Fraction of 02 saturation (02331). (b) 8180-02, and (c) R:P ratios as a function of depth (m) and sample date at station GT3 in Grand Traverse Bay in 2000 and 2001. The bold horizontal line represents (a) 100% 02 saturation, (b) air-water gas exchange (8180 = 0.7%), and (c) community respiration equal to gross photosynthesis (R:P = 1). ................................................................. 35 Figure 8. 6180-02 (‘50, wrt air) as a function of the fraction of 02 saturation (023m) and sampling date for the upper 30 m at station GT3. The bold vertical line represents 100 % saturation. Air- water gas exchange (0.7 %o) is indicated by the bold horizontal line. The equilibrium locus represents the initial conditions of an aquatic system at equilibrium with the atmosphere and when biological activity is negligible. Quadrants I and III indicate 02 input and utilization, ll suggests 02 utilization, and IV represents 02 input. (a.) All data points for the upper 30 m are plotted and data representing R:P ratios greater than, less than, and approximately equal to one are encircled to more clearly display vii seasonal trends. (b.) Mean 8180-02 and OM values for the upper 30 m are plotted for each month. ..................................................... 36 Figure 9: Fraction of 02 saturation (02”,) as a function of 8180 - Oz (%0 , wrt air). Open squares are data for Lake Kinneret published by Luz et al. (2002), open triangles are data from Amazon floodplain lakes published by Quay et al. (1995), and filled circles are from the present study. The bold vertical line represents 100 % saturation. Air-water gas exchange (0.7 %o) is indicated by the bold horizontal line. Quadrants l and III indicate 02 input and utilization, ll suggests Oz utilization and IV represents 02 input. The majority of data for each study are encircled for ease of identification. .............................................................................................. 37 Figure 10: The progression of thermal stratification with depth over time. The bottom of the then'nocline (epi + meta) is indicated by a dotted line. Two vertical dashed line boxes indicate the start and end of thermostratitication (mid-May - Oct/Nov). Values depicted for 0233‘, 5180-02, and R:P ratios were averaged for the upper water column and the hypolimnion during the stratified period. .................. 38 viii INTRODUCTION The balance between community respiration and gross primary production (R:P ratio) in an aquatic ecosystem is influenced by the fixation of organic carbon from in situ primary production (autochthonous production) and influx of external carbon transported into a system from surrounding aquatic and terrestrial environments (aIlochthonous production). In a system driven by autochthonous production, community respiration is equal to or less than gross primary production (R s P) and the system is net autotrophic. Conversely, community respiration in excess of gross primary production (R > P) results if additional sources of organic carbon to autochthonous production are available to respiring organisms. Such systems are net heterotrophic. The degree to which net heterotrophy is expressed in an aquatic system, and thus the degree to which allocthonous production influences a system, has been shown to vary with lake productivity (Odum and Prentki 1978, del Giorgio and Peters 1994, del Giorgio et al. 1997, Cole et al. 2000). In the majority of measured oligotrophic temperate lakes, community respiration exceeds gross primary production (del Giorgio and Peters 1994, Coveney and Wetzel 1995, del Giorgio et al. 1997, Cole et al. 2000). Such observations led to the conclusion that aIlochthonous inputs of organic carbon play a significant role in the metabolism of unproductive systems (del Giorgio and Peters 1994, Coveney and Wetzel 1995, del Giorgio et al. 1997). Recently, however, this interpretation was challenged when gross primary production was observed to exceed community respiration in oligotrophic Laurentian Shield lakes (Carignan et al. 2000). Subsequent research has indicated that elevated DOC concentrations result in net heterotrophy and lower concentrations result in net autotrophy (Praire et al. 2002, Hanson et al. 2003). Such observations imply that DOC concentration, and ultimately that watershed characteristics controlling DOC influx (del Giorgio and Peters 1993, Praire et al. 2000), control the balance between net aIlochthonous and autochthonous organic carbon in lake ecosystems. Current assessments of the balance between R and P in lacustrine environments are generally restricted to the epilimnetic and metalimnetic waters during periods of therrnostratification (del Giorgio 1994, Carignan et al. 2000, Praire et al. 2002, Hanson et al. 2003). Thermally stratified layers restrict the vertical movement of nutrients and organic matter and significantly affect the metabolic balance of a system. For example, physically forced sediment resuspension events during isothermal conditions stimulated heterotrophic activity in Southeastern Lake Michigan (Cotner et al. 2000). This is in contrast to indications that nutrient regeneration, not external sources of organic carbon, supports metabolism in the upper water column during late summer and fall stratification in Lake Michigan (Van Mooy et al. 2001). Meanwhile, metabolism in the hypolimnion during the stratified period is strongly influenced by sediment resuspension events (Eadie et al. 1984, Scavia and Laird 1987, Cotner et al. 2000, Schneider et al. 2002) and sedimenting particles from the epilimnion (Schneider et al. 2002). With measurements restricted to stratified periods and to epilimnetic/metalimnetic depths, a gap exists in our current understanding of the seasonal and depth trends in the balance between R and P. The occurrence of net autotrophy and heterotrophy was investigated as a function of time and depth in Grand Traverse Bay (GTB), Lake Michigan, based upon the variations in R:P ratios over time. Ratios of R to P were determined from in situ measurements of oxygen concentrations [02] and stable oxygen Isotopes (5‘8002) utilizing a model developed for Amazon Basin floodplain lakes and rivers (Quay et al. 1995). Values of 818002 are expressed with respect to air (wrt air) where 5‘80 = 0.0%., wrt air is equal to 23.5%» V-SMOW (international standard for O) in per mil (%o) notation. (1) 6‘°0= «‘“officiampulwoflioxsm- 1) * 1000 The model takes advantage of the quantifiable effects that photosynthesis, respiration, and gas exchange have on [02] and 8'80-02. Effects of photosynthesis, respiration, and gas exchange on 6130-02 values are a result of kinetic fractionation processes whereby small differences in the mass of an isotope (‘60 or 1"0) influence the rate at which a reaction occurs (Kroopnick 1975, Kiddon et al. 1993, Quay et al. 1995). During respiration, "30150 is utilized at a faster rate than 16O-"30 and the residual 02 is enriched in 1"0 relative to the initial substrate (Lane and Dole 1956, Bender and Grande 1987, Kiddon et al. 1993, Quay et al. 1993, and Quay et al. 1995). During the splitting of water molecules in photosynthesis, H2180 and H2160 are utilized at approximately the same rate and the 02 released has a 6180 value that is equal to that of the water (Stevens et al. 1975, Guy et al. 1993). In GTB, the average isotopic signature of the water is -28.8%o wrt air and a predominance of photosynthesis over respiration results in 6180-02 values approaching -28.8%o wrt air. Fractionation during the exchange of 02 between the atmosphere and surface waters results in a slight enrichment in 18O of surface waters by 0.7% (Kiddon et al. 1993, Benson and Krause 1984). As a result, 5130-02 values greater than air (0.7%) indicate that the rate of respiration is greater than that of photosynthesis while values less than air indicate a predominance of photosynthesis relative to respiration (Bender and Grande 1987, Quay et al. 1995). The primary objective of the current study was to investigate the balance between R and P (R:P ratios) as a function of season and depth in Grand Traverse Bay (GTB). Station GT3 is located in the upper west arm of Grand Traverse Bay (GTB), a lobed inlet Of Lake Michigan (Figure 1). Northern Lake Michigan is characterized as oligotrophic to mesotrophic (Stoermer et al. 1972, Auer et al. 1976, and Tarapchack and Stoerrner 1976). Surface waters from northern Lake Michigan flow into the west arm of GTB (Lauff 1957) and water quality is correspondingly similar (Auer et al. 1976). The glacially formed Bay is large (total area is equal to 681.6 kmz) and deep (mean depth for the bay is 55 m; Lauff 1957). It is expected that deep, oligotrophic systems such as GTB will be strongly impacted by aIlochthonous material from the surrounding watershed due to the relatively low contribution of organic matter from established macrophyte communities that impact shallow systems (del Giorgio and Peters 1993). However, low riverine and groundwater inputs (Smith 1973, Schneider et al. 2002) and average CIN ratios previously measured at station GT3 imply that terrestrial inputs of organic carbon into GTB are low (McCusker et al. 1999). As a result, we expected that metabolism would be predominately supported by in- situ primary production and would be reflected on an annual basis by R:P ratios near unity. METHODS Field and Laboratory Methods Station GT3 (Figure 1, 44°59.00 N, 85°34.80 W, Z = 112 m) was sampled six times between April and November in 2000 and five times between April and October in 2001 (Table 1 and 2). Sampling was conducted between 800 and 1100 hours to reduce error resulting from diurnal variations in photosynthesis and respiration. A SeaBird Electroan SBE25 CTD profiler equipped with a fluorometer (Seatech) was deployed to characterize temperature and to approximate the chlorophyll a (Chl a) abundance of the water column. In 2000, three to five depths were chosen for sampling dissolved 02. Sampling depths were based upon the fluorescence depth profile of the water column. Five fixed depths (5, 15, 25, 50, and 100 m) were chosen in 2001. Water (3 - 5 L) was collected from these depths using standard and lever-action Niskin bottles (General Oceania). A modified \Mnkler method was employed to determine dissolved 02 concentrations (Carpenter 1965, Emerson et al. 1999). The collection procedure for 6180-02 followed that of Emerson et al. (1991 and 1999). Approximately 100 mL of sample water was slowly introduced into pre-evacuated 200 mL glass vessels fitted with high vacuum stopcocks that contained dried mercuric chloride (1 mL HgCIz, saturated solution) to eliminate biological activity. Prior to and after introducing the sample, the arm on the vessel was flushed with 002 gas to prevent air contamination. Following equilibration in a constant temperature bath (8 hours, 28°C), water was removed by vacuum until approximately 1 mL remained. Determination of 6180-02 followed the gas chromatograph—isotope ratios mass spectrometry (GC-IRMS) technique of Roberts et al. (2000). Briefly, the sample was introduced into an evacuated inlet system consisting of LiOH (to remove C02 and H20) and carried by He on a GC column. As the sample passed through the GC column, the molecular sieve trapped any remaining 002 and H20. The resulting sample was then allowed to flow into the mass spectrometer. Values of 8130-02 were determined on a GV Instrument Prism mass spectrometer with an analytical precision of i 03‘!» (Roberts et al. 2000). Determination of R:P ratios The ratio of the community respiration rate (R) to gross primary production (P) for a system at steady state (assumed) was calculated from the measured values of 5‘30-02 and [Oz] (Quay et al. 1995): (2) R'P = (182130wap _ 1821609)/(182160ar_ 1821609) and (3) 18”60., = agi‘t‘ioaa. - (oz/oas)‘°=‘°011[1«oz/02.)] where 02 is the concentration of dissolved oxygen (measured), 02,, is the concentration of dissolved oxygen at atmospheric saturation (Benson and Krause 1984, Garcia and Gordon 1992), 18”60,. is equal to the isotopic composition of 02 in the atmosphere (0% wrt air, Kroopnick and Craig 1972), 18”"0 is the isotopic composition of 02 in solution (measured), "“60W is the isotopic composition of water (measured, -28.8%o wrt air), 18”‘50,, is the isotopic composition of the gas invasion flux (calculated), 0,, is the ratio of the 180-‘50 to 160-‘60 gas transfer velocities (0.9972, Knox et al. 1992), a, Is the ratio of the solubilities of ‘80-‘60 to ‘60-‘60 gases in water (1.0007, Kroopnick and Craig, 1972), orp is the ratio of the photosynthetic reaction rates of H2‘80 and H2160 (1.000 :I: 0.003; Stevens et al. 1975, Guy et al. 1993), and Or is the ratio of 180160 to 160—‘60 reaction rates during respiration (estimated 0.9770; Luz et al. 2002). Samples for the determination of 6‘80-H20 were analyzed by Mountain Mass Spectrometry (Evergreen, Colorado) via a MultiPrep and reduction furnace system. The net respiratory-fractionation factor, 0,, reflects the combined isotope effects associated with the Mehler, photorespiration, cytochrome oxidase, and alternative oxidase respiration pathways for an aquatic community (Kroopnick and Craig 1972, Kroopnick 1975, Guy et al. 1989). Previously measured values range between 0.9780 for ocean surface waters (Kroopnick 1975, Quay et al. 1993) to 0.9820 for Amazon rivers dominated by bacterial respiration (Quay et al. 1995). Recently, Luz et al. (2002) determined the respiratory fractionation factors as a function of season in the epilimnion of a phytoplankton-dominated lacustrine environment, Lake Kinneret. As the community in GTB resembles the community present in Lake Kinneret, we used the average epilimnetic respiratory fractionation factor reported by Luz et al. (2002) of 0.9770 in our calculations of R:P ratios. RESULTS The expected seasonal progression in thermal structure for a temperate lake was evident at station GT3 in GTB by isothermal conditions in spring, themtostratification in the summer, breakdown of stratification in fall, and a return to isothermal conditions in late fall (Figures 2 - 3). Sampling in May and June of 2001 occurred several weeks later than in May and June of 2000 and temperatures were 1 - 2 °C higher in the upper 20 m in the latter portion of each month in 2001. The water column was isothermal in May 2000 and weakly stratified in May 2001 (Figures 2 - 3). Surface water temperatures continued to rise throughout the summer stratification period with the maximum epilimnetic temperatures observed in July of 2000 (20°C, Figure 2) and August of 2001 (~22°C, Figure 3). By October, epilimnetic temperatures had decreased signaling the breakdown of therrnostratification. However, a cooler (~ 14°C) and well-developed mixed layer extending to 25 m in depth was evident in October of 2000 whereas a warmer (~16°C) and shallow mixed layer extending to 5 m in depth was present in October 2001 (Figure 2 - 3). The variability in the breakdown of stratification between years was likely the result of the intensity and timing of storm events. Chlorophyll a (chl a) abundance in 2000 and 2001 ranged from 0.2 to 2.0 relative fluorescence units (RFU). A deep chlorophyll layer (DCL) was present throughout the stratified period in 2000 and a weak DCL was evident in late summer and fall 2001. A DCL was not present in early spring (April and May, Figure 2 - 3) and epilimnetic decreases in fluorescence toward the surface were likely due to interference from sunlight with the fluorometer. The DCL was most prominent in June of 2000 and consequently deteriorated throughout the summer and fall. In June 2001, as a broad fluorescence layer existed from 30 m to 80 m (Figure 2 - 3). Dual florescence peaks were evident in August of 2001, when an epilimnetic florescence maxima coincided with a equally strong DCL (Figure 2 - 3). By October of both years, the DCL was no longer present, and a strong epilimnetic peak was evident between 5 - 25 m in depth (Figure 2 - 3). The water column florescence profile in November was relatively uniform reflecting mixing. Values of 023m ranged from 0.82 - 1.05 in 2000 and 0.76 -1.05 in 2001 (Figure 4). Early spring was consistently undersaturated in 02 and indicated that 02 utilization by respiring organisms exceeded 02 input by primary production and/or atmospheric introduction (Figures 4a and 40). With the onset of therrnostratification in June, the upper 30m became supersaturated with respect to 02 reflecting photosynthetic and/or atmospheric input in excess of 02 utilization by heterotrophic activity (Figure 4a and 4c). Excluding near saturation conditions at 25 m in July of 2000, the water column was increasingly undersaturated from mid-summer to fall reflecting a predominance of respiration during the stratified period (Figure 4b and 4d). Thus, despite a brief period when supersaturation was observed in the eplimnion, the water column at GT3 was predominately undersaturated reflecting a predominance of respiration over photosynthesis and/or atmospheric exchange. Observed values of 8180-02 ranged between -1.5 and 1.7 and are driven by seasonal variations in the relative importance of photosynthesis, respiration, 10 and gas exchange (Figure 5). Spring 8130-02 values were near 0.7% (Figure 5a and 5c), thereby reflecting atmospheric input of 02 (Kiddon et al 1993). After the onset of thennostratification in late spring, isotope values for 02 in the upper 25 to 50 m were predominately less than 0.7% (Figure 5a - d) thus indicating the predominance of photosynthesis over respiration (Stevens et al. 1975, Bender and Grande 1987). In general, the lowest monthly 6180-02 values were observed as a single minimum at 15 m (Figure 53 - d) that rarely coincided with the observed fluorescence maxima (Figure 2 - 3). The exception was August of 2001 when low 8‘80-02 values at 5 m and 25 m, reflecting a predominance of photosynthesis over respiration, were observed within the epilimnetic (~0 - 15 m) and hypolimnetic (> 20 m) fluorescence maxima (Figure 2 - 3, Table 2). By October 2000 and 2001, low 6180-02 values were restricted to the upper 25-30m (Figure 5b and 5d). Deeper waters (> 30 m) in October 2000 and 2001 and all depths in November 2000 were characterized by 6180-02 values greater than 0.7%» (Figure 5b and 5d), indicating a prevalence of respiration over photosynthesis (Lane and Dole 1956, Kroopnick 1975, Bender and Grande 1987, Quay et al. 1995). At 100 m, 6180-02 values were generally greater than 0.7%, indicating that respiration consistently exceeded photosynthesis in the deep hypolimnion (Figure 5a - d). Ratios of community respiration to gross primary production (R:P) ranged from 0.8 to 1.4 in 2000 and 0.6 to 1.3 in 2001 (Figure 6). While R:P ratios at depths greater than 80 m were consistently above 1.0 and varied little between seasons and years (1.3 — 1.4 in 2000 and 1.1 — 1.3 in 2001), values varied 11 seasonally in the upper 50 m (1.2 - 1.4 in 2000 and 1.0 — 1.3 in 2001, Figure 6). Observed R:P ratios during the isothermal period of early spring were greater than 1 thereby indicating that community respiration exceeded gross primary production. From late spring (May 2001) and early summer (June 2000 and 2001), R:P ratios less than 1 indicated that gross primary production was greater than community respiration in the upper 20—30 m (Figure 6a and 6c). Values were near unity (R:P = 1) in summer and increased in the fall when the highest R:P ratios were observed (Figure 6b and 6d). The overall predominance of community respiration over gross photosynthesis in both 2000 and 2001 provided a strong indication that station GT3 is net heterotrophic. 12 DISCUSSION The balance between community respiration (R) and gross primary production (P) in an aquatic system can be understood by the measurements of 02 concentrations and stable isotopes. Values less than 02 saturation in temperate oligotrophic to mesotrophic lakes reflect respiration in excess of atmospheric and photosynthetic 02 introductions (Quay et al. 1995, Praire et al. 2002). In Grand Traverse Bay (station GT3), the water column was predominantly undersaturated, as indicated by 0233, values less than 1, throughout the sampling periods in 2000 and 2001 (Figure 7a). A brief period of supersaturation in the upper 25 m in May 2001 and June 2000 and 2001 indicated that the introduction of 02 into the water column exceeded that utilized by heterotrophic activity. Solely based upon values of 023,“, it is difficult to discern if supersaturation is the result of primary production or increased atmospheric introduction of 02. Despite this uncertainty, the predominance of undersaturated conditions is an indication that station GT3 was predominately net heterotrophic. Previously, 6180-02 values have been used to qualitatively evaluate the relative importance of atmospheric exchange, P, and R on water column 02 (Bender and Grande 1987, Quay et al. 1995). Oxygen isotope values reveal which process is most influential based upon the following criteria: (1) atmospheric exchange is suggested by 6180-02 values equal to 0.7 (Benson and Krause 1984, Kiddon et al. 1993), (2) R by values greater than 0.7 (Kroopnick 1975, Bender and Grande 1987, Quay et al. 1995), and (3) P by values less than 13 0.7 (Stevens et al. 1975, Guy et al. 1993). Variations in 6180-02 with depth and season at station GT3 therefore reflect changes in the relative importance of P, R, and gas exchange over time. During the isothermal periods (April and November 2000, 2001, and May 2000), water column 6180-02 values were near, but greater than 0.7% indicating that the predominant metabolic process was 02 utilization by the respiring community (Figure 7b). Values did not deviate far from 0.7%, however, and suggested a significant influx of atmosphericelly derived 02 to the water column occurred in early spring and late fall. When GTB was stratified, values of 6180-02 less than 0.7% were observed in the upper 25 m from late May to October and in the upper 50 m in June and July (Figure 7b). Therefore, the primary metabolic process in the upper water column during the stratified period was P. At 100 m, 5‘80-02 values ranged from 0.8 — 1.5%.. indicating that the primary process influencing 02 at depth was R (Figure 7b). Consequently, 8180-02 values reveal that primary production provided a significant input of 02 into the upper water column during the stratified period and that R was the predominant process in the hypolimnion and during isothermal periods. Furthermore, 8'80-02 values ’ indicated that the system was net autotrophic from late spring/early summer to fall and net heterotrophic in early spring and late fall. This conclusion, however, is in contrast to the interpretation of 0288, data where undersaturation indicated that R predominates during the stratified months. Although 023,, and 6180-02 each provide independent indications of the balance between R and P, there are instances when these two measures yield conflicting results. 14 To better understand the relationship between 0283. and 6180-02 in the upper water column, these two parameters were plotted against one another (Figure 8). The intersection of the vertical saturation line (023m = 1.0) and the horizontal 0.7% line, hereafter referred to as the equilibrium locus, represents a point where the flux of 02 is primarily due to atmospheric exchange and the biological fluxes are negligible (Figure 8). The equilibrium locus theoretically represents the initial conditions of a system at equilibrium with the atmosphere. Initial system conditions were assumed to originate at the equilibrium locus in order to provide a point of comparison with measured values. To better understand the relative importance of G, P and R, the figure is broken into four quadrants indicating that 02 is input and/or consumed (Figure 8). Interpretations of the relative importance of R and P based upon 02,,“ and 6‘80-02 are contradictory in quadrants l and Ill. Interestingly, data that were plotted within quadrant III were measured when the water column was thermally stratified. Measurements made at this time indicated that the water column was undersaturated yet isotope values indicated a predominance of photosynthetically produced 02. This discrepancy likely represents a system in which the equilibrium locus is not the initial condition; but rather offset by an influence from biological activity. Therefore, the apparent contradiction between 0238, and 6180-02 is likely the result of a biologically influenced shift away from air-water equilibrium conditions and demonstrates that a combination of 0233. and 6‘80-02 is needed to understand variations in the balance between R and P. 15 In order to frame the trends observed at station GT3 into a larger context, the distribution of 6130-02 and 02$,” for all depths and times at station GT3 was compared with values reported for the floodplain lakes of the Amazon Basin (Quay et al. 1995) and Lake Kinneret (Luz et al. 2002; Figure 9). The Amazon lakes and Lake Kinneret display contrasting metabolic balances and organic carbon inputs. Amazon Basin floodplain lakes are considered to be strongly net heterotrophic due to high bacterial biomass and high aIlochthonous inputs of organic carbon (Quay et al. 1995 and references within). Lake Kinneret, however, is characterized by high primary production and high autochthonous inputs (Dubowski et al. 2002). In comparison to the Amazon Basin lakes and Lake Kinneret, the data from station GT3 are narrowly distributed around the equilibrium locus (Figure 9). Therefore, biological activity (P and R) must be low in GTB relative to that in the Amazon lakes or Lake Kinneret. The proximity of GT3 data to the equilibrium locus indicates that the abundance of 02 within GTB is closer to atmospheric equilibrium. Data points from the Amazon Basin lakes and Lake Kinneret data are skewed to the left and right, respectively, of the equilibrium locus. The distribution of GT3 data indicate that GTB is closer to atmospheric equilibrium at all times and that atmospheric exchange consistently dominates 02 fluxes in a deep, oligotrophic-mesotrophic, temperate lake to a greater extent than in systems characterized by high biological activity like the Amazon Basin floodplain lakes and Lake Kinneret. 16 R:P ratios At station GT3, R:P ratios observed for all depths and times (mean = 1.2 i 0.03 SE) were significantly greater than one (student t-test, p < 0.001) and indicated that the system on an annual basis was net heterotrophic (Figure 8). Net heterotrophy was expected as respiration rates exceed primary production in the majority of systems characterized as oligotrophic to mesotrophic (Quay 1995, del Giorgio et al. 1994 and 1997, Cole et al. 2000). As net heterotrophy was observed, the contribution of organic materials by the autotrophic community cannot be the sole support to heterotrophic activity. Thus, an additional source to autochtonous organic carbon production is required to support the observed excess respiration. Within net heterotrophic lakes, additional sources to new in situ primary production are largely presumed to be of terrestrial origin introduced into a system by groundwater or rivers (del Giorgio and Peters 1994, Coveney and Wetzel 1995, del Giorgio et al. 1997, Cole et al. 2000). However, aIlochthonous inputs of organic carbon are considered low in Lake Michigan (Scavia et al. 1986) and GTB (Smith 1973, McCusker et al. 1999, and Schneider et al. 2002) and additional sources of organic carbon must be considered in order to explain the observance of net heterotrophy. Another possibility of organic carbon introduction exists in Lake Michigan in the form of autochthonously produced material that accumulates within the system and is metabolized at a later point in time (Scavia et al. 1986, Scavia and Laird 1987, Cotner and Biddanda 2002, and Biddanda and Cotner 2002). In Lake Michigan, organic carbon is hypothesized l7 to accumulate in the upper water column and hypolimnion during periods of low bacterial respiration (Scavia and Laird 1987, Cotner et al. 2000), rapid sedimentation (Biddanda and Cotner 2002), and high phytoplankton productivity in the spring (Scavia et al. 1986, Cotner et al. 2000). Three possible internal sources of organic carbon to the upper water column are the deep chlorophyll layer (DCL), benthic nephloid layer (BNL), and sediments. The DCL is a common feature in Lake Michigan during the stratified summer months (Figure 2 and 3; Brooks and Torke 1977, Mortonson 1977, Moll and Stoerrner 1982, Barbiero and Tuchman 2001b, Fahnenstiel and Scavia 1987) and may provide up to 70% of net primary production (Moll and Stoerrner 1982). Material introduced to the upper water column from the DCL includes autochthonously produced organic carbon that may have been retained within the system for days or months. Another common feature when Lake Michigan is thermally stratified is the development of a BNL; a concentrated layer of small particles 5 - 30 m above the sediments (Eadie et al. 1984, Hicks and Owens 1991, McCusker et al. 1999, Schneider et al. 2002). The BNL is composed of particulate matter from the water column (Schneider et al. 2002) and from sediment resuspension (Eadie et al. 1984). Thus, the BNL potentially represents a source of organic carbon that is retained within the system for days to years. Sediment resuspension events occur throughout the year in GTB (Schneider et al. 2002) although the largest sediment resuspension events are observed in the winter months when lake ice is not present (Eadie et al. 1984, Cotner et al. 2000, Schneider et al. 2002). Although sedimentary organic carbon is often refractory, such organic 18 carbon can be altered by solar electromagnetic radiation to biologically available forms (Biddanda and Cotner 2003). Material within the sediments may have been deposited recently to decades ago. For purposes of this discussion, the sources will be grouped under the term non-contemporaneous organic carbon as they represent material that has been retained for some time within the system and respired at a later point in time. The observed range of R:P ratios, 0.6 to 1.4 (T able 1 and 2), at station GT3 demonstrates that this system alternates between periods of net autotrophy and net heterotrophy. Whereas net autotrophy implies that autochthonously produced organic material is produced in excess and may be retained within the system, net heterotrophy implies that an external or non-contemporaneous organic carbon substrate is utilized within the water column. An imbalance between R and P signifies that heterotrophic activity and autotrophic production are temporally uncoupled. As previously suggested, the metabolism of non- contemporaneous organic carbon provides one mechanism that may result in periods of uncoupling between R and P (Scavia et al. 1986, Cole et al. 2000). The relationship between carbon inputs and the balance between R and P is not unidirectional, however. Temporal uncoupling between R and P may result in the retention of organic matter within a system that is then utilized at a later time (Scavia et al. 1986, Scavia and Laird 1987, Cole et al. 2000, Biddanda and Cotner 2002). Whether R and P are uncoupled or coupled at one point in time is quantitatively expressed by R:P ratios. As a result, the R:P ratio indicates when accumulation of organic carbon within the system occurs (due to excess 19 phytoplankton production) and when additional sources to autotrophic production of organic carbon are required to satisfy excess heterotrophic consumption of carbon. At station GT3, the alternation between net autotrophy and net heterotrophy implies that this system shifts between periods of accumulation and periods of organic carbon introduction and utilization. Throughout this study, R exceeded P in the hypolimnion (Figure 6 - 7). While R is expected to be elevated at depths below 30 - 50 m, P is expected to be greatly reduced or absent due to low light conditions. Consequently, it was expected that the R:P values would greatly exceed unity. This was not the case (Figure 6, 7, and 10). The presence of 02 in the hypolimnion is likely the result of mixing and diffusion of atmospheric and/or photosynthetic 02 from the upper water column to the lower water column. However, respiration in the hypolimnion also requircs a source of organic carbon. While sedimentation of photosynthetically reduced carbon may support hypolimnetic respiration during periods of net autotrophy in the epilimnion, net heterotrophic conditions in the epilimnion indicate that additional inputs to sedimenting products of photosynthesis are required to support R at other times. Sediment resuspension is likely an important source of autochthonously and allochthonously produced organic carbon and nutrients (Cotner et al. 2000) to the water column. Although the largest sediment resuspension events are observed in the winter months when lake ice is not present (Eadie et al. 1984, Cotner et al. 2000, Schneider et al. 2002), significant resuspension events resulting from seiche activity also occur periodically in GTB during stratified periods (Schneider et al. 2002). Such 20 events in southeastern Lake Michigan result in increased bacterial productivity, decreased phytoplankton productivity, and ultimately in an uncoupling between autotrophic and heterotrophic activity (Cotner et al. 2000). Therefore, diffusion of 02 from the epilimnion and resuspension of sedimentary material into the hypolimnion likely explain the strong net heterotrophy observed in the hypolimnion throughout both years observed in this study. Net heterotrophy was observed at station GT3 when the water column was unstratified (November and April - May, Figures 2 - 3 and 6 - 7). The occurrence of net heterotrophy in early spring and fall indicates that aIlochthonous and non-contemporaneous inputs to the water column exceed autotrophic inputs. Vertically uniform oxygen isotope values (6180-02 near 0.7%) and isothermal conditions in early spring and fall indicate total water column turnover. Mixing conditions imply that sediment resuspension may be an important source for heterotrophic activity in early spring and late fall. Sediment resuspension events occur more often during unstratified periods (Schneider et al. 2002) and would support the introduction of non-contemporaneous organic carbon into the upper water column. Therefore, our observation of net heterotrophy during isothermal periods in early spring and late fall indicates that autotrophic and heterotrophic activity is uncoupled and that excess respiration may be stimulated by sediment resuspension events. Net autotrophy was Observed in the upper water column at station GT3 during the onset of therrnostratification in May of 2001 and June in both years (Figure 2 - 3 and 6 - 7). Late spring and early summer have been reported to be 21 periods of high P in GTB (Stoermer et al. 1972). However, the observation of strong net autotrophy (R:P ratios as low as 0.6; Table 2) was not expected during this period. Rates of primary production in the west arm of GTB indicate that primary production typically peaks in late summer/fall and spring values are only slightly elevated above winter rates (Auer et al. 1976). Low R:P ratios (< 1) observed at station GT3 imply that either P rates are elevated or R rates are reduced. A reduction in R rates in spring/summer relative to early spring is unlikely as rising water temperatures stimulate bacterial activity and an increase in R is expected to occur (Pomeroy and Deibel 1986, vaia and Laird 1987). However, the expected magnitude of R may reduced as diatoms dominate in spring (Auer et al. 1972, Barbiero and Tuchman 2001a) and are able to competitively depress the heterotrophic consumption of DOC when Si is available (Havskum et al. 2003). Therefore, the low R:P ratios observed in the transition from late spring to early summer are likely the result of an increase in both R and P; however, P is likely increasing more rapidly than R. In July of 2000 and August of 2001, community respiration and gross photosynthesis were approximately in balance in the upper 25 m at station GT3. After the period of net autotrophy in late spring/earty summer, the water column was undersaturated with respect to 02 by neariy 10% for the remainder of the summer. Although 6‘80—02 values revealed an increasing significance of photosynthetic input into the upper water column in the late summer (Figure 6), R:P ratios approaching unity indicate that the increased autotrophic input was accompanied by elevated heterotrophic utilization (Figure 8 and 10). 22 Consequently, a strong coupling between autotrophic and heterotrophic communities was implied as the autotrophic input of 02 did not return the system to saturation. The occurrence of R:P ratios approaching unity (Figure 6) during a period when organic matter substrate limitation (Gardner et al. 1989) and nutrient depletion are expected (Fahnenstiel and Scavia 1987, Conley et al. 1998), indicates that heterotrophic activity must be increasingly dependent upon photosynthetic metabolites in the upper water column as stratification persists. Our results are consistent with previous studies conducted in southeastern Lake Michigan where similar patterns in bacterial cell production and net primary production provides evidence that autochthonous production could potentially meet the bacterial organic carbon demand during summer stratification (Scavia et al. 1986). As the R:P ratios presented in this study represent the balance between community respiration and gross primary production, bacterial metabolic demands are included within the R:P ratio. Our results demonstrate that R:P ratios approach unity and indicate that epilimnetic heterotrophic consumption of organic carbon is coupled to autotrophic production in late summer. Net heterotrophy was strongly indicated in the upper 25 m in October of both years as R:P ratios were equal to 1.2 (Figure 6 - 7, Table 1 and 2). As the water column was still stratified, sources of organic carbon to the epilimnion from the hypolimnion are restricted. Autotrophic and heterotrophic activity in the upper water column in the fall is likely supported by epilimnetic nutrient recycling (Van Mooy et al. 2001) and the metabolic utilization of organic matter from the DCL as the mixing layer increases in depth (Mortonson 1977, Brooks and Torke 1977, 23 Scavia et al. 1986, Scavia and Laird 1987, Van Mooy et al. 2001). Therefore, the Observation of net heterotrophy in the epilimnion in October at station GT3 indicates that autotrophic and heterotrophic activity are uncoupled in the fall and likely triggered by mixing of materials from the DCL. 24 CONCLUSIONS The predominance of net heterotrophy at station GT3 implies that additional sources of organic carbon are influential to net metabolism within unproductive systems like GTB. However, relative to other systems, such as Chesapeake Bay (where R:P ratios range from 0.211 to 1.05; Smith and Kemp 2001) and the Amazon River (where R:P ratios ranged from 1.5 to 4; Quay et al. 1995), the range in R:P ratios observed in GTB (0.6 - 1.4) is narrow and never deviates far from unity. The narrow range in R:P values observed at station GT3 indicates that gross primary production and community respiration are temporally coupled. Despite this, R:P ratios near unity were only observed in the upper water column during summer stratification. Periods of imbalance between R and P were frequent and occurred during isothermal periods, the onset of stratification, destratification, and in the hypolimnion (Figure 2 - 3 and 8). While community structure dynamics may influence R (Havskum et al. 2003) resulting in net autotrophy in late spring and early summer, metabolism of non- contemporaneous and aIlochthonous organic carbon results in a shift away from balance towards net heterotrophy. The Occurrence of net heterotrophy during the unstratified periods and throughout the year in the hypolimnion, indicates that non-contemporaneous and aIlochthonous organic carbon are metabolized. In fact, the persistence of net heterotrophy during the unstratified periods and in the hypolimnion may largely be responsible for the predominance of net heterotrophy observed in GT3. When the average values for 023... 6180-02, and R:P ratios are calculated separately for the upper water column (epilimnion + metalimnion) 25 and the hypolimnion during the stratified periods, the epilimnetic R:P ratios converge towards unity (Figure 10). Community respiration is therefore predominately supported by autochthonous production in the upper water column during stratification. The coincidence of periods of imbalance with disruptions to thermostratification and balance during thermostratification implicates the strong influence exerted on metabolism by the restriction of nutrient and organic matter distribution during thermostratification. In summary, the deviation of R:P ratios from unity that are Observed at station GT3 are likely the result of the temporal and spatial discontinuity between non-contemporaneous and autochthonous inputs thereby resulting in an imbalance between the autotrophic and heterotrophic communities. Similar trends in the temporal and spatial balance between R:P are evident in Lake Superior (Russ et al. 2004) and suggest that the presence of deep waters and isothermal conditions likely contribute to the predominance of net heterotrophy in large temperate lacustrine ecosystems. The occurrence of net autotrophy during periods of thermostratification (Russ et al. 2004) supports the observation in this study that autochthonous products largely support metabolism in unproductive systems and additional sources of organic carbon result in a shift towards net heterotrophy. As a result, large unproductive temperate lakes may alternate between periods of net autotrophy and net heterotrophy over the course of a single year. 26 Table 1: Summary of measurements collected at station GT3, 2000. Sample Date Depth Temperature [0;] Fraction O2 5150 R:P ratio 2000 m (° C) (M) Saturation (You , wrt air) 24-Apr 5 3.35 393.24 0.94 0.9 1.2 24—Apr 15 3.35 431.47 1.04 *10 *1.9 24-Apr 25 3.35 405.78 0.97 1.3 1.3 24-Apr 50 3.35 379.14 0.91 0.9 1.3 24-Apr 80 3.33 388.23 0.93 1.1 1.3 2-May 5 4.52 397.94 0.99 0.6 1.1 2-May 15 4.12 394.50 0.97 0.7 1.2 2-May 25 4.09 396.38 0.97 1.1 1.2 9-Jun 5 12.09 339.97 1.01 -0.4 1.0 9-Jun 10 10.59 384.10 1.05 .1.0 0.8 9-Jun 30 6.35 373.50 0.97 0.4 1.1 9-Jun 50 5.06 374.44 0.94 0.4 1.2 25qu 5 19.20 279.19 0.97 —0.5 1.1 25—Jul 15 15.30 313.34 1.00 -1.1 1.0 25qu 25 11.07 333.71 0.97 -0.3 1.1 25-Jul 50 5.87 362.85 0.93 0.0 1.1 25-Jul 100 4.80 354.70 0.89 1.5 1.4 3-Oct 5 14.51 294.85 0.93 0.2 1.2 3-Oct 15 14.48 294.54 0.93 0.1 1.2 3—Oct 25 14.35 297.99 0.93 0.2 1.2 23-Oct 50 7.58 319.81 0.86 1.7 1.4 3-Oct 100 5.98 336.84 0.87 1.5 1.4 29-Nov 5 8.05 344.67 0.93 1.0 1.3 29-Nov 15 8.05 312.71 0.85 0.8 1.3 29—Nov 25 7.97 314.91 0.85 1.0 1.3 29-Nov 50 7.60 305.51 0.82 0.9 1.3 29—Nov 100 7.50 312.71 0.84 1.3 1.3 27 *Associated data omitted from analysis due to error in measurement of 6180-02. Table 2: Summary of measurements collected at station GT3, 2001. Sample Date Depth Temperature [0,] Fraction 0, 5150 R:P ratio (2000 (mi (°Cl (uni saturation (%o,wrt air) 19-Apr 5 2.12 397.32 0.92 1.1 1.3 19-Apl' 15 2.14 407.97 0.95 1.0 1.2 19-Apr 25 2.15 410.79 0.96 1.0 1.2 19-Apl' 50 2.16 408.91 0.95 1.3 1.3 19-Apr 100 2.18 408.91 0.95 1.1 1.3 23-May 5 6.61 396.38 1.04 0.2 0.6 23-May 15 5.60 398.26 1.01 0.1 0.9 23-May 25 4.88 397.94 1.00 0.0 1.0 23-May 50 4.23 391.05 0.96 0.3 1.1 23-May 100 3.83 403.58 0.98 0.8 1.1 21-Jun 5 12.88 344.05 1.04 -0.3 0.7 21-Jun 15 9.90 360.34 1.02 -0.9 0.9 21-Jun 25 9.08 377.58 1.05 —0.9 0.8 21-Jun 50 5.66 385.41 0.98 -0.5 1.0 21-Jun 100 4.04 377.58 0.92 0.8 1.2 22-Aug 5 21.00 271.04 0.97 -1.5 1.0 22-Aug 15 19.63 260.07 0.91 0.1 1.2 22-Aug 25 8.10 352.51 0.96 -0.8 1.1 22-Aug 50 5.53 362.85 0.92 0.7 1 .2 22-Aug 100 4.89 376.32 0.94 1.1 1.3 2-Oct 5 16.01 297.67 0.97 *-2.5 *1.0 2-0Ct 15 12.84 288.27 0.87 -0.2 1.2 2-0ct 25 10.29 301.12 0.86 0.2 1.2 2-Oct 50 8.30 280.75 0.76 0.7 1.3 2-Oct 100 5.75 325.87 0.83 1.2 1.3 28 *Associated data omitted from analysis due to error in measurement of 8180-02. 85°30 85‘? 15' Grand Traverse 1 EBtIy! GT3 45° 00'- ! r 44°45~ 6 I j Figure 1: Station GT3, Grand Traverse Bay, Lake Michigan (modified from http:l/www.glerl.noaa.gov). g . km 3 r 29 24 Apr" 2000 Temperature (’C) 2 May 2000 Temperature (‘0) 9 June 2000 Temperature (°C) 0 510152025 0 5 10152025 0 510152025 G 20" Depth (m) S 100- l l l l vch vvvvvv 0.0 V I I I I V T V I ' 1.0 2.0 J J 1 J T T W 1 | i vvvvvvvvvv 3 October 2000 l I t 0.0 T IJLAJAAI v11f11—vtfi 1.0 2.0 vc‘vfi vvvvvv 29 November ____.IL. nlnnngt I c v r VTfrTTT 1.0 2.0 Fluorescence (RFU) Fluorescence (RF U) F'UONNOMO (RF U) Figure 2: Temperature (°C, bold line) and Fluorescence (RFU, thin line) as a function of depth (m) and sampling date in 2000. 30 19 Apr" 2W1 23 May 2W1 21 June 2W1 Temperature (’0) Temperature (’C) Temperature (’6) 0510152025 0510152025 0510152025 o L 1 1 t #1 1 J 1 1 I _L l V I I I U U I I I 20* f -l l st 5. a 40+ " . " a 60- .1 t q 80- I 4 l - l 100~ i - ~- —+—+—I—+—+—+—+—I—I—+—I ~—+—I—+—I—+—+—+—+—t—I—l 4 a: t t t t t: t 0.0 1.0 2.0 22 August 2001 2 October 2N1 Fluorescence (RFU) o 11 : L l l .. 2°“ l g : 9 ”i E 8 so- 80' 100‘ '* l 4 ”44‘ :44 H I—+-H—1—I-H-—l—l—+-I 0.0 10 2.0 .0 1.0 2.0 Fluorescence (RFU) Fluorescence (RFU) Figure 3: Temperature (‘0, bold line) and Fluorescence (RFU, thin line) as a function of depth (m) and sampling date in 2001. 31 5808+ 353.8101 W V u FOON =on=..OEE—.w 8.62.8101 a 8.80.71 ,. 8578151 - n F. _. OJ. ad ad Nd guano cocN =uu=..oEE:w .38 6850 sec .583. a. sec .38 as... - __a< a. .88 88.62 - 6850 use 22. a. .88 as... - __a< a. 5 6.. cease: 9.80 :_ 2.0 cozfim 5 Scan .o .8325. a mm 880. 82238 NO .o 5:08“. .v 9:9“. 3.57811 . 8.. 5.8.2.81 - 8.. d - 8 m r 3 (mt - on o . . . o 38 .cEEamasam $5311 4 8. 8.85.8101 . 8.. 38.811 - on w d - 8 m - 8 M - an e . . . . a ..... o... a... a... s... 9so ocow .oEEsm.a:_.am 32 3.50811 3.3.2.8101 1' .l vln I I T ..ch =0...=..0EE..m 8.82-81? 8.80..” 1T oo._:...m~lol fl T T I I _.III A 1 A N F e P. Ea :3 . ...... NO . 02m ocow =0...=..0EE:m .38 6850 see .882 6. sec .38 6:2. - .8... a. .88 88962 - 6860 new 22. a. .88 6:8 - __a< a. 5 ..mm 0905; 0:90 5 2.0 no.8.» .m 5000 .0 20.85... a mu N0.03m .m 2:9“. 3.5.7311 a 8.. $628801 . 3.. 3.88.1 .. 8 m d - 8 m - 3M . on 0 _ . . O 38 3883905....» 8.57311 4 8.. 88281-1 . 03 8.63.811 - 8 w d .. 8 m - s. m c/ - 8 a . a . c .. o ... Sc Es . a... . NO - 033 Saw .oEEam.m:_.am NI 33 ..8~ 6850 can .833. .3 can .38 as... - __a< a. .88 88952 - 6860 sec ...... a. .88 25.. - .52 .3 c. 8m 0805.... .580 c. 8.0 no.5.» .m 500.. .0 3.85. a mm «0.5. an”. .0 050.“. 5808+ . ..o..5.rwu+ 5 cu? 592.8101 4 53281-1 - 8. - 3.22.2191 . on - a 00 T - 3 4 - 8 a o a Son =un=..oEE=m EON ..oEEamBEam 8.628101 - 8.37311 . 83 8158+ r 81>OSIN+ r 89 8.378151 - 382.811 F 8 - - co - - 3. - f - ca 0 a a II III- 7 1 O m... 0;. 0.0 m... a... ad on! mum 0:2 as... 008 ..uutoEEam coon ..oEEamBEEm (111) who (1») W00 34 0.7 2.0 1.0 - 0.0 1 -1.0 - 8"0 - 02 (1t... , wrtalr) -2.0 1.5 1.3 1 1.0 R:P rstlo 0.8 ~ 0.5 I f T T T Mar-00 Jun—00 Oct-00 Jan-01 Apr—01 Jul-01 Nov-01 —c—5m --c--15m —O—25m --c--50m —I:I—100m Figure 7. (a) Fraction of 02 saturation (02”,), (b) 5‘80-02, and (c) R:P ratios as a function of depth (m) and sampling date at station GT3 in Grand Traverse Bay in 2000 and 2001. The bold horizontal line represents (a) 100% 02 saturation, (b) air-water gas exchange (8180 = 0.7 ‘1»), and (0) community respiration equal to gross photosynthesis (R:P = 1). 35 as... 0.8 .....s 28....» 0.8. .8~ 8c .8... 8.8.. ...... 238.... .58. 88 5. .....oE .58 .0. 00.5... 0.0 E on .00.... 0... .o. 0028 580 0:0 N0.02m :00... ..0. .00:0.. 0:800» 0.00.0 2.00.0 0.0:. o. 00.2.80 0.0 0:0 0. 0:00 ...0.0E.xo.nn0 0:0 60... 000. .:0:. 0.00.0 09.0. mum 055000.00. 0.00 0:0 0002.. 0.0 E on .08... 0... .0. 0.50.. 0.00 __< ..0. .52. NO 05000.00. 2 0:0 6008...... NO 0.000050 .. .:o..0~._.... 0:0 .8... N0 98...... ... 38 . 98.830 830.8: a. 0.25.. .885... 8...... ...... 9688...... 9.. ...... E58586 .3 50.9.0 9.0.60 :0 ..o 0:050:00 .0...:. 0... 05000.00. 0:00. E:..n...:00 0.. .. .0:._ .0.:o~..o.. 0.0.. 0... .... 00.00.05 0. .2... ...o. 00:08.8 00m 0.02.8.4. .:o..05.00 e\e 8. 05000.00. 0.... .00...0> 0.0.. 0.. .. .20 500.0 .0 E cm .00.... 0... .o. 0.00 05.9.50 0:0 .580. 8005.00 NO .0 5.80.. 0... .o 8.0:... 0 00 ...0 ...s .3... N0.08m .0 050.". 8.52... 3.50s 8.500 3...... E >. . . E u. x 9 - 0 . .15... - .- o... 4 Iflflivx’ .vey. x c X» a . M ... . . p, . _ \ s x . ,. .. I 7- .. 0 fix r L 03001— II gx. . r? m 5.5.800 . . w. .8... . = . . a 36 5"0 -o,(%. ,wrtalr) Figure 9. Fraction of 02 saturation (020:1) as a function of 8180 - 02 an, wrt air). Open squares are data for Lake Kinneret published by Luz et al. (2002), open triangles are data from Amazon floodplain lakes published by Quay et al. (1995), and filled circles are from the present study. The bold vertical line represents 100 % saturation. Air-water gas exchange (0.7 %o) is indicated by the bold horizontal line. Quadrants I and III indicate 02 input and utilization, 11 suggests 02 utilizationand IV represents 02 input The majority of data for each study are encircled for ease of identification. 37 {—— 2001 fl Apr May Jun Aug Oct Apr May Jun Jul Oct Nov 0 i \ 0ng I -.__ \ long-m i‘ ff ' ‘ Ii. \ 20 J» ” 8"o-ozs-o.y ’ _ \81'o-oza-o.3 5,. xiv-1.1 I E ' \ R:P51.0\ 40 J’.’ ‘\ I " \x/ \ E : \\‘I \‘ i 60 ~: , . 8 L.” %'0.91 . . . - Owl o.“ 80 ””130 . 1. '_ 1. 3’“ “ 5 0-ozso.9 6 o-oz-os 100 -; M R:P-1.3 ~_ 39.11 V .4 [5 like; 120 Figure 10. The progression of thermal stratification with depth over time. The bottom of the therrnocline (epi + meta) is indicated by a dotted line. Two vertical dashed lines boxes indicate the start and end of thermostratification (mid-May - Oct/Nov). Values depicted for 0230. 6180-02, and R:P ratios were averaged for the upper water column and the hypolimnion during the stratified period. 38 LITERATURE CITED Auer, M., Canale, R., and Freedman, P. 1976. 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