Lake Michigan Mass Balance

ABSTRACT: Air-water exchange of PCBs was determined in Lake Michigan on an event and seasonal basis in 1991-1993. Instantaneous fluxes of SPCB (sum of 77 congener peaks) based on air-water concentration gradients drawn from air and water samples collected simultaneously aboard ship demonstrated net volatization in September 1991. Air samples collected on the northeastern shore of Lake Michigan (Sleeping Bear Dunes State Park) between December 1991 and July 1993 showed no seasonal trend in vapor-phase SPCB concentrations and ranged from 30 to 400 pg/m³. These air concentrations were used to calculate seasonal water-air fluxes of SPCB that ranged from -18 ng m^-2 day^-1 (net deposition) to 60 ng m^-2 day^-1 (net volatization). The seasonal variation of vapor-phase and dissolved-phase PCBs in the impacted southern quarter of the lake are unknown, thereby hindering estimation of fluxes in this region. The estimated annual net SPCB flux is 12.3 mg m^-2 yr^-1, which corresponds to 520 kg for the northern three-quarters of Lake Michigan.

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• Hornbuckle, K.C.; Achman, D.R.; Eisenreich, S.J. 1993. Over-Water and Over-Land Polychlorinated Biphenyls in Green Bay, Lake Michigan. Environ. Sci. Technol. 27(1): 87-98.

ABSTRACT: Air samples were simultaneously collected over water and over nearby land in the Green Bay, Lake Michigan, region and analyzed for 85 polychlorinated biphenyls (PCBs). The concentration and distribution of PCB congeners collected from vapor over water, over land, and dissolved in the water support the hypothesis that volatization of PCBs from contaminated waters is a major source of PCBs to the local atmosphere. Concentrations of PCB congener (SPCBs) over the water are higher over southern Green Bay (670-2200 pg/m³) and lower over the northern bay (160-520 pg/m³) while SPCB concentrations over the land ranged from 70 to 760 pg/m³. The PCB concentration differences over land and over water were statistically significant for congener sums and for homolog groups in southern Green Bay. Regressions of air and water PCB distributions show high correlations (R² = 0.73-0.96) in southern Green Bay.

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• Hurley, J.P.; Cowell, S.E.; Shafer, M.M.; Hughes, P.E. 1998a. Partitioning and transport of total and methyl mercury in the Lower Fox River, Wisconsin.  Environ. Sci. Technol. 32(10): 1424-1432.

ABSTRACT: To investigate transport and partitioning processes of Hg_T in the Fox River, we coupled detailed time series data of total mercury (Hg_T) at the river mouth with transect sampling in the Lower Fox River. Unfiltered Hg_T concentrations in the Fox River during the study period (April 1994- October 1995) ranged from 1.8 to 182 ng L^-1 with a median of 24.8 ng L^-1, predominantly (93.6%) in the particulate phase. These levels were significantly elevated compared with other large tributaries to Lake Michigan (Hurley, J.P.; Shafer, M.M.; Cowell, S.E.; Overdier, J.T.; Hughes, P.E.; Armstrong, D.E. Environ. Sci Technol. 1996, 30, 2093-2098). Transect sampling revealed progressively increasing water column Hg_T concentrations and Hg_T particulate enrichment downstream, which were consistent with trends in sediment Hg_T levels in the river. Resuspended sediments are likely the predominant source of Hg from the Fox River into Green Bay. Despite elevated Hg_T concentrations, methyl mercury (MeHg) concentrations were relatively low, suggesting limited bioavailability of Hg_T associated with sediments.

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• Hurley, J.P.; Cowell, S.E.; Shafer, M.M.; Hughes, P.E. 1998b. Tributary loading of mercury to Lake Michigan: Importance of Seasonal events and phase partitioning.  The Science of the Total Environment. 213(1-3): 129-137.

ABSTRACT: As a component of a lakewide mass balance study for Lake Michigan, we measured total mercury (Hg_T) concentrations and fluxes in 11 selected tributaries.  Unfiltered Hg_T concentrations ranged from 0.56 ng l^-1 at the Pere Marquette River to 182 ng l^-1 at the Fox River. Highest mean Hg_T concentrations were observed in the Fox R., Indiana Harbor Ship Canal, Grand R. and the Kalamazoo R. Mean particulate matter Hg_T content ranged from about 0.1 to 1.5 mg g^-1, with highest levels from the industrialized basins of the Indiana Harbor and Fox River. Highest tributary loading rates (g day^-1) were observed from the Fox, Grand, Kalamazoo and St. Joseph Rivers. Increased loading rates during spring melt and summer/fall storm events in these tributaries were generally associated with particulate loading from either sediment resuspension or erosional processes. In contrast, filtered Hg_T represented 80% of the Hg_T flux in the Manistique R., whose watershed is comprised almost entirely of wetlands and forest. 

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• Hurley, J.P.; Shafer, M.M.; Cowell, S.E.; Overdier, J.T.; Hughes, P.E.; Armstrong, D.E. 1996. Trace Metal Assessment of Lake Michigan Tributaries Using Low Level Techniques. Environ. Sci. Technol. 30(6): 2093-2098.

INTRO: Assessment of trace metal levels in freshwaters and marine waters requires strict adherence to trace metal protocols (1-8). Recent advances in analytical methodologies for Hg (cold vapor atomic fluorescence spectrophotometry; 9-11) and other trace metals (inductively coupled plasma-mass spectrometry or ICP-MS; 12-14) have enabled redefinition of trace metal levels in lakes, rivers, and oceans. However, applications of these analytical protocols are typically limited to specific questions on trace metal cycling. To acquire data for environmental management decisions, a task which usually involves development of large data sets and extensive environmental sampling, it is essential that estimates of trace metal levels and phase partitioning are made by accurate and precise methods. From a geochemical viewpoint, accurate “clean” assessment of trace metal levels and partitioning behavior is necessary to understand processes controlling transport and fate in divergent river systems.

In 1993, the U.S. EPA, in cooperation with the Wisconsin Department of Natural Resources, the Michigan Department of Natural Resources, and the U.S. Geological Survey, began an assessment of tributary inputs of trace metals and selected organic compounds to Lake Michigan through the Lake Michigan Tributary Monitoring Project (LMTMP; 15). The work was also coordinated with the Lake Michigan Mass Balance Project (16). The tributary project was the first such effort to evaluate annual loadings of trace metals to Lake Michigan while utilizing low-level trace metals techniques. Strict adherence to clean field and laboratory protocols, adapted specifically for this study, were required to attain accurate trace metal assessment of 11 Lake Michigan tributaries. We present initial quality assurance results and data from 1994 to compare trace metal concentrations and partitioning during three selected flow regimes fro these selected tributaries. Field sampling continued through October 1995, and the results of these analyses will be included in subsequent publications.

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• Madenjian, C.P.; DeSorcie, T.J.; Stedman, R.M.; Brown, Jr., E.H.; Eck, G.W.; Schmidt, L.J.; Hesselberg, R.J.; Chernyak, S.M.; Passino-Reader, D.R. 1999. Spatial Patterns in PCB Concentrations of Lake Michigan Lake Trout. J. Great Lakes Res. 25(1): 149-159.

  ABSTRACT: Most of the PCB body burden in lake trout (Salvelinus namaycush) of the Great Lakes is from their food. PCB concentrations were determined in lake trout from three different locations in Lake Michigan during 1994-1995, and lake trout diets were analyzed at all three locations. The PCB concentrations were also determined in alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), bloater (Coregonus hoyi), slimy sculpin (Cottus cognatus), and deepwater sculpin (Myoxocephalus thompsoni), five species of prey fish eaten by lake trout in Lake Michigan, at three nearshore sites in the lake. Despite the lack of significant differences in PCB concentrations of alewife, rainbow smelt, bloater, slimy sculpin, and deepwater sculpin from the southeastern nearshore site near Saugatuck (Michigan) compared with the corresponding PCB concentrations from the northwestern nearshore site near Sturgeon Bay (Wisconsin), PCB concentrations in lake trout at Saugatuck were significantly higher than those at Sturgeon Bay. The difference in the lake trout PCB concentrations between Saugatuck and Sturgeon Bay could be explained by diet differences. The diet of lake trout at Saugatuck was more concentrated in PCBs than the diet of Sturgeon Bay lake trout, and therefore lake trout at Saugatuck were more contaminated in PCBs than Sturgeon Bay lake trout. These findings were useful in interpreting the long-term monitoring series for contaminants in lake trout at both Saugatuck and the Wisconsin side of the lake.

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• Madenjian, C.P.; Desorcie, T.J; Stedman, R.M. 1998a. Maturity Schedules of Lake Trout in Lake Michigan. J. Great Lakes Res. 24(2): 404-410.

ABSTRACT: We determined maturity schedules of male and female lake trout (Salvelinus namaycush) in Lake Michigan from nearshore populations and from an offshore population on Sheboygan Reef, which is located in midlake. Gill nets and bottom trawls were used to catch lake trout in fall 1994 and 1995 from two nearshore sites and Sheboygan Reef. Each lake trout was judged immature or mature, based on visual examination of gonads. Probit analysis, coupled with relative potency testing, revealed that age-at-maturity and length-at-maturity were similar at the two nearshore sites, but that lake trout from the nearshore sites matured at a significantly earlier age than lake trout from Sheboygan Reef. However, length at maturity for the nearshore populations was nearly identical to that for the offshore population, suggesting that rate of lake trout maturation in Lake Michigan was governed by growth rather than age. Half of the lake trout males reached maturity at a total length of 580 mm, whereas half of the females were mature at a length of about 640 mm. Over half of nearshore males were mature by age 5, and over half the nearshore females matured by age 6. Due to a slower growth rate, maturity was delayed by 2 years on Sheboygan Reef compared with the nearshore populations. Documentation of this delay in maturation may be useful in deciding stocking allocations for lake trout rehabilitation in Lake Michigan.

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• Madenjian, C.P.; Desorcie, T.J.; Stedman, R.M. 1998b. Ontogenic and Spatial Patterns in Diet and Growth of Lake Trout in Lake Michigan. Trans. Amer. Fisher. Soc. 127: 236-252.

ABSTRACT: Lake trout Salvelinus namaycush in nearshore waters of Lake Michigan grow faster than lake trout residing offshore on Sheboygan Reef, which is in midlake. We examined the stomachs of lake trout, spanning ages 1 through 16, caught in both nearshore and offshore environments of Lake Michigan during 1994 and 1995 to determine whether diet differences may be responsible for the difference in growth rate. A comparison of the diets, coupled with bioenergetics modeling, indicated that juvenile lake trout on Sheboygan Reef experienced slow growth due to low food availability rather than to cold water temperatures. The availability of appropriate-size prey appeared to regulate lake trout growth. Small prey fish were probably not readily available to small (200- to 399-mm total length) lake trout on Sheboygan Reef, a substantial portion of whose diet consisted of invertebrates; in contrast, nearshore juveniles had a nearly 100% fish diet. Growth rate on the reef remained slow through intermediate lake trout sizes (400-599mm total length), presumably due to low availability of rainbow smelt Osmerus mordax on the reef. Once lake trout achieved total lengths of approximately 600mm, they grew slightly faster on Sheboygan Reef than near shore, indicating that large (>170-mm total length) prey fish were readily available to lake trout in the reef area. On a wet-weight basis, alewife Alosa pseudoharengus dominated the diet of large ($600 mm total length) lake trout from both the nearshore and offshore regions of the lake, although bloater Coregonus hoyi composed over 30% of the diet on Sheboygan Reef and in southeastern nearshore Lake Michigan. Size of alewife prey increased with lake trout size. The bloater population currently represents the bulk of the biomass of the adult prey fish community, so out diet analysis suggests that large lake trout are continuing to select alewives.

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• Madenjian, C.P.; Elliot, R.F.; Schmidt, L.J.; Desorcie, T.J.; Hesselberg, R.J.; Quintal, R.T.; Begnoche, L.J.; Bouchard, P.M.; Holey, M.E. 1998. Net Trophic Transfer Efficiency of PCBs to Lake Michigan Coho Salmon from Their Prey. Environ. Sci. Technol. 32(20): 3063-3067.

ABSTRACT: Most of the polychlorinated biphenyl (PCB) body burden accumulated by coho salmon (Oncorhynchus kisutch) from the Laurentian Great Lakes is from their food. We used diet information, PCB determinations in both coho salmon and their prey, and bioenergetics modeling to estimate the efficiency with which Lake Michigan coho salmon retain PCBs from their food. Our estimate was the most reliable estimate to date because (a) the coho salmon and prey fish sampled during our study were sampled in spring, summer, and fall from various locations throughout the lake, and (b) detailed measurements were made on the PCB concentrations of both coho salmon and prey fish over wide ranges in fish size, and (c) coho salmon diet was analyzed in detail from April through November over a wide range of salmon size from numerous locations throughout the lake. We estimated that coho salmon from Lake Michigan retain 50% of the PCBs that are contained within their food. 

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• Madenjian, C.P.; Hesselberg, R.J.; Desorcie, T.J.; Schmidt, L.J.; Stedman, R.M.; Quintal, R.T.; Begnoche, L.J.; Passino-Reader, D. 1998. Estimate of Net Trophic Transfer Efficiency of PCBs to Lake Michigan Lake Trout from Their Prey. Environ. Sci. Technol. 32(7): 886-891.

ABSTRACT: Most of the polychlorinated biphenyl (PCB) body burden accumulated by lake trout (Salvelinus namaycush) from the Laurentian Great Lakes is from their food. We used diet information, PCB determinations in both lake trout and their prey, and bioenergetics modeling to estimate the efficiency with which Lake Michigan lake trout retain PCBs from their food. Our estimates were the most reliable estimates to date because (a) the lake trout and prey fish sampled during our study were all from the same vicinity of the lake, (b) detailed measurements were made on the PCB concentrations of both lake trout and prey fish over wide ranges in fish size, and (c) lake trout diet was analyzed in detail over a wide range of lake trout size. Our estimates of net trophic transfer efficiency of PCBs to lake trout from their prey ranged from 0.73 to 0.89 for lake trout between the ages of 5 and 10 years old. There was no evidence of an upward or downward trend in our estimates of net trophic transfer efficiency for lake trout between the ages of 5 and 10 years old, and therefore the efficiency appeared to be constant over the duration of the lake trout’s adult life in the lake. On the basis of our estimates, lake trout retained 80% of the PCBs that are contained within their food.

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• Madenjian, C.P.; O'Connor, D.V.; Nortrup, D.A. 2000. A New Approach Toward Evaluation of Fish Bioenergetics Models. Can. J. Fish. Aquat. Sci. 57: 1025-1032.

  ABSTRACT: A new approach was used to evaluate the Wisconsin bioenergetics model for lake trout (Salvelinus namaycush). Lake trout in laboratory tanks were fed alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus Mordax), prey typical of lake trout in Lake Michigan. Food consumption and growth by lake trout during the experiment were measured. Polychlorinated biphenyl (PCB) concentrations of the alewife and rainbow smelt, as well as of the lake trout at the beginning and end of the experiment, were determined. From these data, we calculated that lake trout retained 81% of the PCBs contained within their food. In an earlier study, application of the Wisconsin lake trout bioenergetics model to growth and diet data for lake trout in Lake Michigan, in conjunction with PCB data for lake trout and prey fish from Lake Michigan, yielded an estimate of PCB assimilation efficiency from food of 81%. This close agreement in the estimates of efficiency with which lake trout retain PCBs from their food indicated that the bioenergetics model was furnishing accurate predictions of food consumption by lake trout in Lake Michigan.

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• Madenjian, C.P.; Schmidt, L.J.; Chernyak, S. M.; Elliott, R.F.; DeSorcie, T.J.; Quintal, R.T.; Begnoche, L.J.; Hesselberg, R.J. 1999. Variation in Net Trophic Transfer Efficiencies among 21 PCB Congeners. Environ. Sci. Technol. 33(21): 3768-3773.

  ABSTRACT: We tested the hypothesis that the efficiency with which fish retain polychlorinated biphenyl (PCB) congeners from their food strongly depends on K_ow and degree of chlorination of the congener. We used diet information, determinations of concentrations of individual PCB congeners in both coho salmon (Oncorhynchus kisutch) and their prey, and bioenergetics modeling to estimate the efficiencies with which Lake Michigan coho salmon retain various PCB congeners from their food. The retention efficiency for the tetrachloro congeners averaged 38%, whereas retention efficiencies for higher chlorinated congeners ranged from 43 to 56%. Not including tetrachloro congeners, we found neither decreasing nor increasing trends in the efficiencies with which the coho salmon retained the PCB congeners from their food were either increasing K_ow or increasing degree of chlorination of the PCB congeners. We concluded that (a) for PCB congeners with 5-8 chlorine atoms/molecule, K_ow and degree of chlorination had little influence on the efficiency with which coho salmon retained tetrachloro PCB congeners in their food appeared to be slightly lower than that for higher chlorinated PCB congeners.

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  Mason, R.P.; Sullivan, K.A. 1997. Mercury in Lake Michigan. Environ. Sci. Technol. 31(3): 942-947.

INTRO: Mercury contamination of aquatic systems is an important worldwide health concern (1,2). Recent research has demonstrated that many freshwater lakes in North America, Europe, and Asia contain fish with elevated mercury (Hg) concentrations, i.e., concentrations that exceed state, federal, or international health guidelines (3-12). In the United States in the last decade, an ever-increasing number of states have issues health advisories for freshwater fish consumption, based primarily on the elevated Hg concentrations in piscivorous fish (11). In conjunction, the U.S. Environmental Protection Agency (EPA) has lowered the reference dose for Hg in fish, based on EPA’s Integrated Risk Information System, and many states in the United States-for example, Minnesota (12)-have also lowered the acceptable criteria for Hg in fish. These regulatory actions have proven to be contentious (11) and have led to a re-evaluation of the sources of Hg to U.S. waters and the factors controlling Hg accumulation in fish, as there are significant gaps in our knowledge of the sources of Hg to aquatic systems and the role of atmospheric deposition and anthropogenic emissions in providing “bioavailable “ Hg to aquatic systems (11-15).

            A number of studies, focused primarily of smaller lakes, have sought to correlate elevated concentrations of mercury in fish with environmental parameters (e.g., refs 7-10 and 15-17) and have demonstrated the importance of anthropogenic inputs to the atmosphere in contributing to mercury contamination of both nearby and remote watersheds (13, 18). There is, however, little published data for Hg in Lake Michigan and the other Great Lakes. A previous study of mercury and other trace metals in the Great Lakes (19) reported average total mercury concentrations ranging from 10 pM (2 ng/L) for Lake Superior to 225 pM for Lake Michigan (data collected in 1980-1983). Our data, collected in 1994 and 1995 from offshore waters (Figure 1) during the EPA-sponsored Lake Michigan Mass Balance Study (LMMB; 20), averaged 1.6 pM total Hg and are 2 orders of magnitude lower than these earlier data for similar sites in Lake Michigan. The concentration is more comparable to that of the open ocean (e.g., ref  21). Historical contamination of samples likely accounts for the differences between our values and previous data, as has been found in Lake Michigan for some of the other trace metals, most obviously for lead (22). It has occurred even recently during the analysis of open ocean waters for trace metals (23).

            Other recent measurements in Lake Michigan (24) found values for mercury ranging from 5 to 50 pM at a station within 6 km of Chicago. These concentrations are elevated as compared to our measurements, but this site likely receives enhanced inputs, both fluvial and from the atmosphere, from Chicago. One of our sites, approximately 20km offshore, did not however show any enhancement in concentration as compared to sites more remote from urban influence (20). Gill and Bruland (25) found values of 4.5 and 18 pM for samples collected from the shores of Lake Ontario and Lake Erie, respectively, somewhat elevated relative to the open Lake Michigan water. A limited survey of Lake Superior found total Hg concentrations around 5 pM. (26).

            Studies in the Great Lakes have generally focused on fish and the higher trophic levels of the food chain (e.g., ref 27). The National Contaminant Biomonitoring Program (NCBP) has measured total mercury concentrations in pooled samples of whole fish at a number of sites in the Great Lakes between 1976 and 1984 (28), and there has been no significant decrease in fish concentration over the study period. The concentrations of mercury in bloater and perch (0.04-0.07 ppm wet weight) for the three Lake Michigan sites are lower than those found in lake trout (0.2-0.22 ppm), likely reflecting the food preferences of the different species. Fish Hg concentration data for Lake Ontario collected between 1977 and 1988 (29) are similar to those found by the NCBP.

            We report here the results obtained during the LMMB study. In addition to our charge under the project of measuring total and particulate Hg in the water column, we were able to collect and analyze invertebrate and fish samples for their total and methylmercury (MMHg) concentration. Water samples were also analyzed for MMHg. Here we present the overall data and discuss the bioaccumulation of Hg in Lake Michigan organisms. Our water column data will be discussed in further detail elsewhere (20).

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• Miller, S.M. 1999. Spatial and Temporal Variability of Organic and Nutrient Compunds in Atmospheric Media Collected During the Lake Michigan Mass Balance Study. M.S. thesis. University of New York at Buffalo, Buffalo, New York. 181 pp.

  ABSTRACT: As part of the Lake Michigan Mass Balance (LMMB) study, others obtained atmospheric samples (gas, particulates, and precipitation) at sites on the shore around the lake and at sites on the lake from April 1994 to October 1995. Samples were later analyzed by others for a suite of organic (PCBs, trans-Nonachlor, and Atrazine) and nutrient (Total Phosphorus, Total Kjeldahl Nitrogen, and Nitrate) compounds. The interpretation of these data is best summarized in terms of ten major conclusions. All measures of quality (i.e., recovery of known mass, analytical reproducibility, number and quality of blanks, and replicates) exceed the standards set for the study. With respect to gas exchange, SPCBs are near chemical equilibrium, except near Chicago where the net flux is into the lake and statistically greater than zero (~334 mg/yr m²). Much more PCB mass (~2 to 54 mg/yr m² gross volatization and 65 to 334 mg/yr m² gross deposition) crosses the air-water interface through gas exchange than through particulate deposition (~0.010 to 0.075 mg/yr m²) or precipitation deposition (~0.002 to 0.090 mg/yr m²). Much more Atrazine mass (~12 to 190 mg/yr m²) crosses the air-water interface through precipitation deposition than through gas exchange (~7 mg/yr m² net deposition) or particulate deposition (~1 to 12 mg/yr m²). Since gas Atrazine concentrations were near or below the method detection limit and the detection limit was used gas flux calculation, 7 mg/yr m² is the maximum deposition flux. Actual flux for gas Atrazine is possibly lower. While the concentration of Total Phosphorus in precipitation has decreased from 56 mg/L in 1976 to 6.36 mg/L in 1994-95, loading estimates have remained nearly the same (2.5x10⁵ kg/yr in 1976 versus 2.9x10⁵ kg/yr for 1994-95). This lower than expected loading for 1976 is due to low average annual precipitation amount and the method used.

  Flux calculations for gas exchange and particulate deposition yield statistically similar results for samples collected at shoreline sites and aboard the R/V Lake Guardian, meaning that there is no ship effect when the yardarm sampler is used. There may be a ship effect when the air sampler is located on the bow, an artifact that is obvious during the LMMB 1994-95 field sampling effort; however, there no evidence of it prior to or after that time. Estimation of annual whole-lake fluxes requires an interpolation of data collected around and over the lake. Averaging the the flux calculations from each of the sampling sites does not introduce large errors except for gas PCBs near Chicago. Use of data collected only from the Sleeping Bear Dunes sampling site, the site used by the Integrated Atmospheric Deposition Network since 1990, to describe chemical fluxes for the entire lake will tend to underestimate fluxes. This is due, in part to observed concentrations that are routinely lower at this site than at any of the other seven shoreline and R/V Lake Guardian sites included in the LMMB.

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• Miller, S.M.; Sweet, C.W.; DePinto, J.V.; Hornbuckle, K.C. 2000. Atrazine and Nutrients in Precipitation:  Results from the Lake Michigan Mass Balance Study. Environ. Sci. Technol. 34(1): 55-61.

ABSTRACT: Atmospheric deposition of anthropogenic air pollutants contributes to degradation of water quality in the Great Lakes and other water bodies and is indicative of atmospheric pollution. In this paper, we discuss deposition of three air pollutants: atrazine; total phosphorus; and nitrogen (total Kjeldahl nitrogen and nitrate) to Lake Michigan. Throughout 18 months in 1994-1995, over 600 atmospheric samples (gas, particulate, and precipitation combined) were collected and analyzed for persistent organic pollutants and nutrients. Here the measurements and modeled deposition estimates are presented for atrazine, phosphorus, and nitrogen. Results indicate that concentrations of atrazine in precipitation have remained constant over 5 years (0.10-0.40 mg L^-1), consistent with the nearly constant sales rate of the herbicide over that time period. Actual loading of atrazine to the lake was less in 1994-1995 (1.04 x 10³ kg yr^-1) than in 1990-1991 (2.6 x 10³ kg yr^-1 ). This difference in loading is due to lower overall precipitation in 1994-1995. Phosphorus concentrations in precipitation, on the other hand, have decreased from an average of 57 mg as P L^-1 in 1976 to an average of 6.36 mg as P L^-1 for 1994-1995. Nitrate deposition has decreased by a small, but not statistically significant, amount since the late 1970s.

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• Rossmann, R. 2002. Lake Michigan 1994-1995 Surficial Sediment Mercury. J. Great Lakes Res. 28(1): 65-76.

ABSTRACT: Between 1994 and 1996, sediment samples were collected from 118 stations in Lake Michigan.  Samples were collected using both a box corer and a PONAR grab sampler.  Samples were collected for the purpose of describing the current spatial variation of mercury in surficial sediment and for comparing current concentrations and fluxes to historic data.  This work includes the first description of the spatial variation of mercury fluxes to the lake's sediment.  It also provides a more recent description of the spatial distribution of mercury concentrations within the lake's sediments.  For the 118 sites sampled and analyzed, the surficial sediment mercury concentration averaged 78 ng/g and ranged between 2 and 260 ng/g.  Concentrations were highest in depositional basins and exhibited a distribution that conformed to the lake's bathymetry.  The spatial distribution of mercury fluxes did not conform to the bathymetry.  Instead maximum fluxes were shifted toward the southeastern shore.  Fluxes to and concentrations in surficial sediment of each depositional basin were not significantly different from one another.  Fluxes averaged 7.2 ng/cm²/y and ranged between 0.85 and 32 ng/cm²/y.  Regional atmospheric fluxes account for roughly 50 percent of the total mercury flux to surficial sediment. 

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• Rygwelski, K.; Richardson, W.; Endicott, D. 1999. A Screening-Level Model Evaluation of Atrazine in the Lake Michigan Basin. J. Great Lakes Res. 25(1): 94-106.

ABSTRACT: Atrazine, a widely used herbicide in the agricultural regions of the Lake Michigan basin, was selected as a priority toxic chemical for study in the United States Environmental Protection Agency (U.S. EPA)-sponsored Lake Michigan Mass Balance Project. A surface-water, screening-level model for atrazine in the Lake Michigan basin was developed to obtain an initial insight into its transport behavior and ultimate fate. Estimates of tributary loadings and atmospheric loadings for model computations were made for the period-of-usage of the chemical beginning in 1964. Most of these loading estimates were based on total annual usage rates in the United States. Data from the literature were used to estimate these historical loadings. Approximately 30% of the total load of atrazine entering the lake is associated with precipitation, and the remainder is from tributary loads. An unsteady state, Water Quality Simulation Program (WASP) model based on the principle of conservation of mass, was used to predict concentrations of atrazine in Lake Michigan and Green Bay, Wisconsin. Model predictions agree with recent field measured atrazine concentrations in the lake when atrazine assumed to be conservative in the lake. The persistence of atrazine predicted for Lake Michigan contrasts sharply with the relatively short half-lives of the chemical measured on agricultural fields as reported in the literature. It was estimated that if loadings of atrazine were to continue into the future at a rate equivalent to that of 1993, the lake would reach steady-state concentrations of 160 ng/L in approximately 300 years.

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ABSTRACT: Levels of arsenic, cadmium, chromium, copper, lead, and zinc in filterable and particulate phases are measured, using validated trace metal clean techniques, in eleven Lake Michigan tributaries, as a component of the Lake Michigan Tributary Monitoring Program. The eleven rivers drain watersheds which represent a wide range of surficial geologies and land use, and one focus of our research will concern the relationships between metal signatures and land use-land cover. Preliminary data indicate that for many systems trace metals levels and partition coefficients vary within relatively narrow characteristic ranges, and that correlations with river discharge are poor. Principal trace metal loading events over the 94 calendar year occurred during summer and fall storms, not during the spring melt, with the Grand R., Kalamazoo R., and St. Joseph R. contributing the large majority of the flux to the lake proper. Order-of-magnitude differences in metal levels are seen between the study rivers, with the low end represented by the Manistique, Muskegon, and Pere Marquette rivers, and high end by the Fox, Kalamazoo, and Milwaukee rivers. Significant variations in KD are also evident between study sites, however, partitioning relationships between the metals are generally consistent, following the pattern Pb> Zn > Cd > Cu > Cr > As.

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• Sullivan, K.A.; Mason, R.P. 1998. The concentration and distribution of mercury in Lake Michigan. The Science of The Total Environment. 213(1-3): 213-228.

ABSTRACT: Total and particulate mercury (Hg) concentrations were determined for the water column of Lake Michigan as part of the EPA Lake Michigan Mass Balance Project. Concentrations for total Hg averaged 1.60±0.25 pM while the particulate was 0.60±0.18 pM (20¯50% of the total). Overall, fluctuations in total Hg both spatially and vertically showed no consistent trends, while particulate Hg may have been affected by calcium carbonate precipitation in late summer and sediment resuspension late in the season. Dissolved methylmercury (MMHg) concentrations ranged from the detection limit (25 fM) to 210 fM and the particulate fraction was from 5 fM (DL) to 45 fM (2% of total Hg). Dissolved gaseous Hg (DGHg) concentrations during one cruise averaged 140±85 fM. Incubation experiments suggest that biotic processes are primarily responsible for elemental Hg production (2.6-6.5% per day) in Lake Michigan. Overall, the cycling and speciation of Hg in Lake Michigan is more akin to that of the open ocean, than smaller lakes in the mid-western USA. 

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• Swackhamer, D.L.; Schottler, S.; Pearson, R.F. 1999. Air-Water Exchange and Mass Balance of Toxaphene in the Great Lakes. Environ. Sci. Technol. 33(21): 3864-3872. 

ABSTRACT: This study determined the importance of air-water exchange of toxaphene in the Great Lakes by comparing this flux to other inputs and outputs in a mass balance model. Our overall goal was to test the hypothesis that the current water concentrations of Lake Superior are due to physical limnological differences between it and the lower Great Lakes and secondarily to evaluate whether nonatmospheric inputs of toxaphene have had an impact on current toxaphene burdens in lakes Superior and Michigan. A series of water samples from the Great Lakes were analyzed for toxaphene. A static mass budget is presented for lakes Superior, Michigan, and Ontario. A dynamic model of toxaphene behavior in lakes Superior and Michigan from 1950 to 1995 is then presented. The results of this model support our hypothesis that the colder temperatures and lower sedimentations rates in Lake Superior are responsible for its high water concentrations of toxaphene to Lake Superior. However, the model supports the conclusion that there were nonatmospheric sources of toxaphene to Lake Michigan. 

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