A high resolution study of spatial and temporal variability of natural and anthropogenic compounds in offshore Lake Superior sediments

An 8 km² area representative of deep offshore basins in Lake Superior was surveyed with multi-beam sonar and a high-frequency seismic-reflection system to create a high-resolution bathymetric map of the lake floor morphology, which is dominated by ring-shaped depressions attributed to the dewatering of glacial-lacustrine clays. Ten multi-cores were recovered from the survey area. The cores were scanned for magnetic susceptibility (MS), dated by ²¹⁰Pb and analyzed for water content, total organic carbon (TOC) and nitrogen (TON), biogenic silica (BSi), and total (THg) and methyl (MeHg) mercury. MS profiles varied considerably, inferring substantial centennial-scale differences in sedimentation history among the core sites. Concentration profiles of the analyzed constituents displayed differences of about ± 15% TOC, ± 40% BSi, ± 50% THg and ± 50% MeHg. Total mercury and methylmercury concentrations were typical of past measurements, and the mean THg accumulation rate (12 μg/m² year) was similar in magnitude to that of atmospheric Hg deposition. Sediment mass accumulation rates (MAR) ranged among the cores between average values of about 50 g/m² year in the ring centers to as high as 180 g/m² year between rings. Temporal variation in MAR within cores varied considerably on a decadal scale as well. Sediment redistribution by bottom currents over the complex morphology of the Lake Superior basin is not uniform in space and time, and indicates that a single core from any given area in the lake may not reflect the true history of environmental conditions that extend even a few hundred meters beyond the core site.     

Total gaseous mercury (TGM) concentration over Lake Superior and Lake Michigan

Mercury-contaminated fish are a serious problem in the Great Lakes basin, because mercury is a potent neurotoxin that poses a danger to both humans and wildlife. Lake Superior lake trout and walleye have the highest mercury concentrations of the five Great Lakes. Because the atmosphere is the major source of mercury to the Great Lakes, information on the over-water mercury concentration is essential to model the mercury biogeochemical cycle. For the first time in the peer-reviewed literature, this paper presents total gaseous mercury (TGM) measurements made over Lake Superior and Lake Michigan. The Lake Superior aircraft measurements were made at an altitude of 300 m, and the Lake Michigan aircraft measurements at a variable altitude of 30–300 m. The over-water Lake Superior TGM of 1.02 ± 0.34 ng/m³ is much lower than the TGM from nine stations in the Canadian Atmospheric Mercury Measurement Network (CAMNet) and six stations in the Atmospheric Mercury Network (AMNet). The land-based TGM concentrations average range from 1.25 to 1.75 ng/m³ which are in good agreement with current global average values of 1.3–1.6 ng/m³. The over-water Lake Michigan TGM is 1.65 ± 0.61 ng/m³. We also present Lake Superior over-water measurements of volatile organic compounds (VOC), ozone (O₃), nitrogen oxide (NO_y), and particulate matter. Elemental carbon (EC) is a tracer for mercury because mercury is released during the combustion of coal. EC is significantly correlated with TGM over both Lake Superior and Lake Michigan. TGM over Lake Michigan is also significantly correlated with organic carbon, sulfate, nitrate and ammonium.     

A mercury transport and fate model (LM2-Mercury) for mass budget assessment of mercury cycling in Lake Michigan

LM2-Mercury, a mercury mass balance model, was developed to simulate and evaluate the transport, fate, and biogeochemical transformations of mercury in Lake Michigan. The model simulates total suspended and resuspendable solids (TSRS), dissolved organic carbon (DOC), and total, elemental, divalent, and methylmercury as state variables. Simplified processes among the mercury state variables including net methylation, net reduction of divalent mercury, and reductive demethylation are incorporated in the model. Volatilization of elemental mercury as a kinetic (phase transfer) process and partitioning of total, divalent, and methylmercury as a set of instantaneous equilibrium processes were also simulated. The model was calibrated to data collected in 1994 and 1995 and corroborated by comparing model output generated from a long-term model hindcast to total mercury measured in high quality sediment profiles. Model hindcast predictions of total mercury in the water column were within estimates of total mercury calculated from observed lake trout bioaccumulation factors. Using the model, a mass budget assessment of mercury cycling in the lake was conducted. Atmospheric deposition, including wet and dry (particle) deposition and absorption of gaseous divalent mercury, was the dominant source of total mercury to the lake, followed by sediment resuspension, and then tributary loads. The major loss mechanism of total mercury from the water was associated with the settling of solids, followed by net volatilization. Methylmercury loading associated with wet deposition was the dominant source to the lake, followed by tributary loadings, and in situ net methylation.     

Contaminant concentrations in bald eagles nesting on Lake Superior,the upper Mississippi River,and the St. Croix River

We measured concentrations of DDE, total PCBs, and mercury in bald eagle (Haliaeetus leucocephalus) nestlings at three locations in the upper Midwest: Lake Superior, the upper Mississippi River, and the St. Croix River, 2006–2008. We also analyzed trends in concentrations of these contaminants for eagles on the southern shore of Lake Superior, from 1989 to 2008, using the current and previously published data. Concentrations of DDE in nestling blood plasma samples were greatest on Lake Superior (geometric mean: 16.2 μg/kg, n = 29), whereas concentrations of total PCBs were highest in Mississippi River samples (88.6 μg/kg, n = 51). Mercury concentrations were highest along the upper St. Croix River (6.81 μg/g wet weight in feathers, n = 19). For Lake Superior, DDE concentrations declined significantly in nestling blood plasma samples from 1989 to 2008, an average of 3.0% annually. Similarly, total PCBs in Lake Superior eaglets decreased 4.0% annually from 1989 to 2008, and mercury concentrations in nestling feathers from Lake Superior nests also decreased significantly from 1991 to 2008, 2.4% per year. With the possible exception of mercury on the upper St. Croix River, mean concentrations in 2006–2008 of all three compounds were below levels associated with significant impairment of reproduction for all sites, and reproductive rates at all three sites averaged > 1.2 young per occupied territory, which is greater than the rate indicative of a healthy population.     

Watershed influences on the transport, fate and bioavailability of mercury in Lake Superior: Field measurements and modelling approaches

The issuance of fish consumption advisories in US states bordering Lake Superior has heightened the need for understanding the biogeochemical cycling and transformations of mercury in this great lake. Major routes of mercury (Hg) transport to lakes include atmospheric deposition (wet and dry), direct discharges and riverine (watershed) inputs. The specific objectives of this ongoing study are to: (i) determine the speciation and bioavailability of Hg transported to Lake Superior (ii) determine the importance of watershed‐specific characteristics that control physical and chemical forms of Hg; (iii) identify key mechanisms controlling Hg bioavailability and speciation in near‐shore zones relative to open lake regions; and (iv) provide process‐level information to compliment concurrent development of Hg fate and transport models of the Lake Superior ecosystem. Three tributaries of Lake Superior were chosen for detailed field study based on previous results and particular watershed characteristics. Mixing zones represent a potentially important zone of Hg entry into the food chain, and were sampled for biota, water and sediment in transects between the tributary mouth and the lake end member. Sampling of open‐water sites was conducted during Environmental Protection Agency‐sponsored cruises on the research vessel Lake Guardian. Results from our work on Lake Superior tributaries demonstrate that dissolved organic carbon and methylmercury (MeHg) export was greatest from watersheds containing wetlands. In Lake Superior, concentrations of Hg species were small at pelagic stations (total mercury (HgT) averaged 0.49 ng/L, MeHg averaged 6.4 pg/L). In contrast, MeHg concentrations in tributaries ranged from 100 to 250 pg/L. Watershed sites dominated by wetlands exhibited the greatest MeHg concentrations, occasionally in excess of 12 ng/L. Samples of bulk zooplankton ranged from 20 to 130 ng/g dry weight HgT and from 15 to 45 ng/g dry weight MeHg, and demonstrated typical bioacculmulation of mercury as well as distinct seasonality in concentrations.     

Modeling historical trends in Lake Superior total nitrogen concentrations

Nitrate concentrations in Lake Superior increased fivefold between 1900 and 1980, and have remained nearly constant since that time. Such rapid changes in concentration in a lake with a long hydraulic residence time (~ 190 years) are surprising. We developed a model to better understand the causes of the historical changes and to predict future changes in nitrate concentrations. Historical loadings were reconstructed based on average national NO_x emissions estimates, recent (past ~ 30 years) atmospheric N deposition data, recent tributary concentration data, and basin-wide runoff estimates. Increases in atmospheric N deposition alone were insufficient to have resulted in the observed trends. However, model runs combining increased atmospheric deposition with increased tributary N loading and/or decreased burial + denitrification mid-century reproduced the observed accumulation of N. Because internal N fluxes are an order of magnitude greater than external fluxes, relatively small changes in the lake's internal N cycle may produce relatively large changes in total N concentrations. Land-use changes in the watershed, particularly increases in logging activity, may have altered riverine N inputs. Regardless of the historical mechanisms leading to the rise in nitrate concentrations, it appears as though the system is currently at or is approaching peak N content.     

Horizontal and Vertical Distributions of Mercury in 1983 Lake Superior Sediments with Estimates of Storage and Mass Flux

Persistent elevated mercury concentrations in some species of Lake Superior fish and the general lack of information on mercury storage in and fluxes to the lake's sediments prompted the analysis of samples that were collected in 1983. Results of the analyses support the conclusion that Lake Superior sediments have mercury concentrations above background levels at all sites sampled. For those cores which penetrated the sediment deeply enough (the majority of the cores), background mercury concentrations ranged between 0.016 and 0.048 mg/kg. Mercury concentrations in surficial sediments ranged between 0.027 and 0.96 mg/kg. The maximum mercury concentration found in sub-surface sediments (2 to 20 cm deep) was 6.5 mg/kg. The surficial 20 cm of sediment contained 342 metric tons of mercury of which 51% or 174 metric tons was anthropogenic. The surface 2 cm of sediment contained 29 metric tons of mercury of which 76% or 22 metric tons was anthropogenic. Estimated total mercury fluxes to surficial sediments ranged between 0.1 and 10 ng/cm²/y with a mean of 3.2 ng/cm²/y. Background total mercury fluxes to the lake ranged between 0.20 and 0.72 ng/cm²/y with a mean of 0.48 ng/cm²/y. Estimated fluxes of anthropogenic mercury to surficial sediments ranged between –0.42 and 10 ng/cm²/y with a mean of 2.7 ng/cm²/y. The fluxes reported here are only the second known reporting of mercury fluxes to Lake Superior sediments. The inventory of mercury in the sediments is the first reported.     

Lead in Lake Michigan and Green Bay surficial sediments

Sediment samples were collected in 1987–1990 from Green Bay and in 1994–1996 from Lake Michigan. Surficial sediments (0–1 cm) from both locations were analyzed for lead for the purpose of describing the horizontal variation of lead in 1994–1996 Lake Michigan and 1987–1990 Green Bay sediments, estimating lead fluxes to surficial sediments, and comparing results to earlier studies. With Lake Michigan concentrations ranging from below the method detection limit to 180 μg/g, the surficial sediments had mean and median lead concentrations of 70 μg/g and 64 μg/g, respectively. Lead concentrations in Green Bay surficial sediments were similar to those in Lake Michigan and ranged between the method detection limit and 160 μg/g. For the bay, mean and median concentrations were 58 and 59 μg/g, respectively. Surficial lead concentrations were highest in the Southern, Waukegan, and Grand Haven basins of Lake Michigan and in the central region of Green Bay in the vicinity of Chambers Island. For Lake Michigan and Green Bay, dated sediment cores illustrate the decline in lead concentrations during the last 30 and 10 years, respectively. Lead fluxes ranged between < 0.049 and 7.2 μg/cm²/yr for Green Bay and between 0.47 and 20 μg/cm²/yr for Lake Michigan. Lead fluxes to Lake Michigan were lower than those reported for 1972. These are the most comprehensive fluxes of lead to Lake Michigan and Green Bay surficial sediments reported to date.     

Perfluoroalkyl acids in Lake Superior water: Trends and sources

Perfluoroalkyl acids (PFAs) are a family of highly persistent compounds which are present in the environment as a result of degradation of polyfluorinated precursors, from use as processing aids for production of fluoropolymers, and use in fire fighting foams. The purpose of this study was to investigate prevailing concentrations and possible sources of PFAs in Lake Superior, as well as in Siskiwit Lake on Isle Royale. Between 2001 and 2005, replicate water samples were taken from lake surface waters, and from depth profiles, as well as from major tributaries including municipal waste water treatment plants (WWTPs) at three major population centers. Perfluorooctanoate (PFOA) was the predominant PFA in Lake Superior, with concentrations ranging from 0.07 to 1.2 ng/L in surface waters. PFOA concentrations were generally 1.5 to 2-fold greater than perfluorooctanesulfonate (PFOS) levels. WWTPs were found to contribute up to 20 fold higher concentrations of PFOA (22 ng/L) relative to the intake water from Lake Superior, while most tributaries contained lower concentrations of perfluorocarboxylates (PFCA) and perfluoroalkylsulfonates (PFSs) (< 0.1 ng/L). Overall tributaries and precipitation were estimated to be the major sources of PFCAs and PFSs to Lake Superior. Tributaries were estimated to be the largest source contributing 59% of PFOA and 57% of PFOS inputs to the lake. Profiles conducted over the deepest points in the lake showed that PFAs were found throughout the water column, however, there was no distinctive trend with depth.     

Lake Superior zooplankton biomass: Alternate estimates from a probability-based net survey and spatially extensive LOPC surveys

We conducted a probability-based net tow sampling of Lake Superior in 2006 and compared the zooplankton biomass estimate with an estimate from laser optical plankton counter (LOPC) surveys. The net survey consisted of 52 sites stratified across three depth zones (0–30, 30–150, > 150 m). The LOPC tow surveys were extensive and spatially covered much of Lake Superior (> 1300 km of towing). The LOPC was field calibrated to Lake Superior zooplankton samples collected across the years of 2004 to 2006. The volume-weighted lake-wide zooplankton biomass determined by traditional net tows to 100-m sample depth was 20.1 (± 7.8 SD n = 52) mg dry-weight m^− 3. The estimates varied by depth zones within the lake, where nearshore (0–30 m) estimates were highest and highly variable. Net sites for the LOPC field calibration were removed to allow for LOPC validation with independent nets; the resulting net-based estimate 20.0 (± 9.3 SD n = 38) mg dry-weight m^− 3 and LOPC lake-wide estimate 19.1 (± 3.3 SD) mg dry-weight m^− 3 agreed well. Consistency across survey methods for lake-wide estimates suggested that LOPC survey data provides a comparable assessment tool to traditional nets for collecting zooplankton biomass data. We briefly compare our results with some observed historical patterns. Onshore–offshore trends in zooplankton biomass concentrations were similar to the last major lake-wide survey in 1973. The LOPC provided high resolution data on zooplankton biomass distribution. Using simultaneously collected in situ sensor data, the LOPC zooplankton biomass distributions over horizontal and vertical space can be modeled as a function of temperature and fluorescence.     

Genetic assessment of straying rates of wild and hatchery reared lake sturgeon (Acipenser fulvescens) in Lake Superior tributaries

Natal philopatry in lake sturgeon (Acipenser fulvescens) has been hypothesized to be an important factor that has lead to genetically distinct Great Lakes populations. Due to declining abundance, population extirpation, and restricted distribution, hatchery supplementation is being used to augment natural recruitment and to reestablish populations. If hatchery-reared lake sturgeon are more likely to stray than naturally produced individuals, as documented in other well-studied species, outbreeding could potentially jeopardize beneficial site-specific phenotypic and genotypic adaptations. From 1983 to 1994, lake sturgeon propagated using eggs taken from Lake Winnebago adults (Lake Michigan basin) were released in the St. Louis River estuary in western Lake Superior. Our objective was to determine whether these introduced individuals have strayed into annual spawning runs in the Sturgeon River, Michigan. Additionally, we estimated a natural migration rate between the Sturgeon River and Bad River, Wisconsin populations. Presumed primiparous lake sturgeon sampled during Sturgeon River spawning runs from 2003 to 2008 were genotyped at 12 microsatellite loci. Genotypic baselines established for the Sturgeon River (n = 101), Bad River (n = 40), and Lake Winnebago river system (n = 73) revealed a relatively high level of genetic divergence among populations (mean F_ST = 0.103; mean R_ST = 0.124). Likelihood-based assignment tests indicated no straying of stocked Lake Winnebago strain lake sturgeon from the St. Louis River into the Sturgeon River spawning population. One presumed primiparous Sturgeon River individual likely originated from the Bad River population. Four first-generation migrants were detected in the Sturgeon River baseline, indicating an estimated 3.5% natural migration rate for the system.     

Model forecasts of atrazine in Lake Michigan in response to various sensitivity and potential management scenarios

For more than forty years, the herbicide atrazine has been used on corn crops in the Lake Michigan basin to control weeds. It is usually applied to farm fields in the spring before or after the corn crop emerges. A version of the WASP4 mass balance model, LM2-Atrazine, was used to assess the impact of the historical and future usage of this chemical on lake water concentrations. Long-term model forecasts were performed under various sensitivity and potential management scenarios. The model was calibrated to available lake data and results indicate that atrazine, under average conditions, is decaying very slowly in the lake (0.009/year). This kinetic decay translates into a half-life estimate of 77 years. If the average condition scenario were assumed to remain constant into the future and reflective of conditions in January 1, 2005, it is expected that the lake (excluding Green Bay) would eventually reach a volume-weighted average atrazine concentration of approximately 67 ng/L in the year 2157 (current model prediction is 48 ng/L for year 2011). These forecasted lake-wide concentrations are below known water quality criteria for the protection of aquatic ecosystems.     

Total and methyl mercury accumulation in 1994–1995 Lake Michigan lake trout and forage fish

As part of the Lake Michigan Mass Balance Project, total and methyl mercury were determined for lake trout (Salvelinus namaycush) and five forage fish species collected from Lake Michigan near Saugatuck, Michigan, and Port Washington, Sheboygan Reef, and Sturgeon Bay, Wisconsin, between 1994 and 1995. With a mean concentration of 179 ng/g wet wt., whole lake trout total mercury (Hg_T) concentrations ranged between 27.6 and 348 ng/g wet wt. For combined sites, 1–4 yrs, 5–6 yrs, 7–11 yrs, and 12–15 yrs lake trout mean Hg_T concentrations were 73.7, 130, 212, and 280 ng/g, respectively. Forage fish species alewife (Alosa pseudoharengus), bloater (Coregonus hoyi), slimy sculpin (Cottus cognatus), deepwater sculpin (Myoxocephalus thompsoni), and rainbow smelt (Osmerus mordax) had mean Hg_T concentrations of 63.8, 55.3, 36.7, 51.4, and 35.2 ng/g wet wt., respectively. With the exception of alewife, bloater, and slimy sculpin, all fish species contained approximately 100% methyl mercury (MeHg). Field bioaccumulation factors (BAF) were consistent with a Lake Michigan food chain that is more efficient at transferring MeHg to higher trophic levels than some inland lakes. This and other studies of lake trout from Lake Michigan document decreasing Hg_T concentrations in lake trout from 1971 to 1985 and constant or increasing concentrations between 1985 and 2000. These observations were supported by a similar trend in Lake Michigan Hg sediment fluxes. To our knowledge, this is the most intense two year study of mercury in fish for any Great Lake or other large fresh water system and is one of the most complete studies of mercury cycling in the Lake Michigan food chain.     

Acoustic estimates of abundance and distribution of spawning lake trout on Sheboygan Reef in Lake Michigan

Efforts to restore self-sustaining lake trout (Salvelinus namaycush) populations in the Laurentian Great Lakes have had widespread success in Lake Superior; but in other Great Lakes, populations of lake trout are maintained by stocking. Recruitment bottlenecks may be present at a number of stages of the reproduction process. To study eggs and fry, it is necessary to identify spawning locations, which is difficult in deep water. Acoustic sampling can be used to rapidly locate aggregations of fish (like spawning lake trout), describe their distribution, and estimate their abundance. To assess these capabilities for application to lake trout, we conducted an acoustic survey covering 22 km² at Sheboygan Reef, a deep reef ( < 40 m summit) in southern Lake Michigan during fall 2005. Data collected with remotely operated vehicles (ROV) confirmed that fish were large lake trout, that lake trout were 1–2 m above bottom, and that spawning took place over specific habitat. Lake trout density exhibited a high degree of spatial structure (autocorrelation) up to a range of ∼ 190 m, and highest lake trout and egg densities occurred over rough substrates (rubble and cobble) at the shallowest depths sampled (36–42 m). Mean lake trout density in the area surveyed (∼ 2190 ha) was 5.8 fish/ha and the area surveyed contained an estimated 9500–16,000 large lake trout. Spatial aggregation in lake trout densities, similarity of depths and substrates at which high lake trout and egg densities occurred, and relatively low uncertainty in the lake trout density estimate indicate that acoustic sampling can be a useful complement to other sampling tools used in lake trout restoration research.     

Landscape Evolution and Deglaciation of the Upper Peninsula,Michigan: An Examination of Chronology and Stratigraphy in Kettle Lake Cores

We propose a radiometric chronology bracket for the last glacial advance/retreat, called the Marquette readvance, in the Upper Peninsula of Michigan (Upper Peninsula) using organic material from kettle lakes and previously published age determinations on wood buried by glaciofluvial sediments. The lakes are located both inside and outside the ice-contact margin of the Marquette readvance. Wood buried in glaciofluvial sediments from the Marquette readvance was previously dated at 10,025 ± 100 ¹⁴C yr BP (Hughes and Merry 1978, Lowell et al. 1999, and Pregitzer et al. 2000). Ackerman Lake, a kettle lake located inside the ice-contact margin, yielded a basal radiocarbon date of 9,495 ± 70 ¹⁴C yr BP representing the time of organic accumulation after ice retreat. These dates above and below the glacial deposit bracket the age of the Marquette readvance/retreat to 360–700 ¹⁴C yr, or the midpoint of 530 ¹⁴C yr. Ackerman Lake yielded multiple radiocarbon dates, including an average date of 8,811 ± 11 ¹⁴C yr BP (9,736–9,913 cal yr BP) at a change in stratigraphy from red clay to gray silt. This transition along the northern Upper Peninsula is interpreted to represent ice sheet retreat into Lake Superior and the reworking of older glacial sediments by ∼8,500 ¹⁴C yr BP. Organic material from the kettle lake sediments spanning multiple geomorphic locations (both inside and outside of the ice-contact margin) and previous radiocarbon dates from the entire Upper Peninsula yielded dates concentrating around 9,500 ¹⁴C yr BP. We attribute this synchronous organic accumulation in the Upper Peninsula to be a result of climatic signature corresponding with the Preboreal Oscillation, so the duration of the Marquette glacial cover may have been less then implied by the Ackerman Lake basal age.     

The Lake Michigan contaminant transport and fate model,LM2-toxic: Development,overview, and application

The Lake Michigan contaminant transport and fate model LM2-Toxic was developed to gain a better understanding of PCB cycling dynamics and to predict environmental exposure concentrations of 54 polychlorinated biphenyl (PCB) congeners in Lake Michigan water and sediment from 1994 to 2055 as a function of a variety of forcing functions including constant conditions, continued recovery forecasts, and load reduction scenarios. LM2-Toxic couples the organic carbon sorbent and chemical dynamics conceptualized for a natural water system. Based on 1994–1995 model results, a mass budget analysis showed that air–water exchange was the most important mass transfer process. Volatilization was the largest PCB loss and gas absorption was the largest PCB input to Lake Michigan. Model-predicted environmental exposure concentrations suggest that the water quality criterion for protection of wildlife (0.074 ng/L) and human health (0.026 ng/L) will be attained in approximately 2018 and 2045, respectively, based on a slow recovery scenario. For this scenario, atmospheric components, including vapor phase concentration and wet and dry particulate loadings, were assumed to decline with a 20 year half-life, and tributary loadings were assumed to decline with a 13 year half-life.     

Evidence of a remnant self-sustaining strain of lake trout in the Lake Michigan basin

Managers have long embraced the need to maintain diversity as a requisite condition for population and community sustainability. In the case of Great Lakes lake trout, diversity has been severely compromised. The identification of new gamete sources may be beneficial to lake trout reintroduction efforts, particularly in situations where native stocks have been completely extirpated such as in Lake Michigan. Lake trout from Elk Lake, Michigan, are genetically distinct from domestic hatchery strains and historical forms of lake trout from Lake Michigan. Importantly, Elk Lake fish were genetically distinct from Marquette strain lake trout which were previously stocked into Elk Lake. Elk Lake fish were most similar to Lake Michigan basin-derived Lewis Lake (LLW) and Green Lake (GLW) hatchery strains and to historical Lake Michigan populations from the Charlevoix, Michigan area. While all individuals exhibited characteristics of lean form lake trout, the body shape of lake trout from Elk Lake, stocked lean fish from Lake Michigan and Lake Superior wild lean strains from near Isle Royale differed. Elk Lake fish were more fusiform, elongate, and streamlined with a narrower caudal peduncle compared to hatchery lean strains and wild lean forms from the Isle Royale region of Lake Superior. The lake trout population in Elk Lake is a remnant of a now extirpated native Lake Michigan population that was established either by natural colonization or stocking from historical Lake Michigan populations. Elk Lake lake trout is as genetically diverse as other strains used in Great Lakes reintroduction efforts and likely represent a viable gamete source representing genetic diversity lost from Lake Michigan.     

Delivery of nutrients and seston from the Muskegon River Watershed to near shore Lake Michigan

Drowned river mouth lakes are major features of coastal Great Lakes habitats and may influence nutrient and organic matter contributions from watersheds to near shore coastal zones. In May through October 2003, we measured loads of nutrients, surficial sediment, and seston to track the delivery of riverine-derived materials from the lower Muskegon River Watershed (MRW) into the near shore area of southeast Lake Michigan. Nutrient flux data indicated that seasonal loads of 1800 metric tons (MT) of particulate organic carbon, 3400 MT of dissolved organic carbon, and 24 MT of total phosphorus were discharged from the lower Muskegon River, with approximately 33% of TP load and 53% of the POC load intercepted within the drowned river mouth terminus, Muskegon Lake. Carbon: phosphorus molar ratios of seston in Muskegon River (C:P = 187) and Muskegon Lake (C:P = 176) were lower than in Lake Michigan (C:P = 334), indicating phosphorus limitation of phytoplankton in near shore Lake Michigan. Isotopic signatures of seston collected in Muskegon Lake were depleted in δ¹³C (− 30.8 ± 1.6‰) relative to the isotope signatures of seston from Lake Michigan (− 26.2 ± 1.3‰) or the mouth of the Muskegon River (− 28.1 ± 0.5‰), likely due to the presence of biogenic methane in Muskegon Lake. Seston δ¹⁵N increased on a strong east-to-west gradient within Muskegon Lake, indicating significant microbial processing of nutrients. The extent of nutrient uptake in Muskegon Lake altered the chemical and isotopic characterization of seston flowing into Lake Michigan from Muskegon River.     

Atmospheric Concentrations of Toxaphene on the Coast of Lake Superior

Thirty-seven air samples were collected from September 1996 through December 1997 at Eagle Harbor, Michigan to determine the atmospheric concentration of toxaphene near Lake Superior. The concentrations ranged between 0 and 63 pg/m³, with most less than 20 pg/m³, which agreed well with concurrent measurements over the lake and near Lake Michigan made by other research groups. These concentrations are significantly lower than those measured in 1988 and 1989 in Egbert, a small city in southern Ontario. The phase transition energy was calculated to be 47 kJ/mol, half the value calculated for the Egbert site; this disparity has been seen in comparisons of lakeshore and over-land values for other compounds. If temperature effects are removed, the average toxaphene concentration for the Eagle Harbor samples was 6.4 ± 2.2 pg/m³, which was lower than the 16 pg/m³ average at Egbert, Ontario. This difference in concentration may (or may not) be due to differences in sampling dates or locations or in measurement techniques.     

A total phosphorus budget for the Lake of the Woods and the Rainy River catchment

The overall goal of this study was to quantify the major and minor sources and losses of total phosphorus (TP) to the Lake of the Woods (LOW), summarized as a nutrient budget. This research was initiated in response to degradation in lake water quality, including elevated TP concentrations and increased cyanobacterial blooms, which has resulted in LOW's classification as an “Impaired Waterbody” in Minnesota. The whole-lake LOW TP budget shows that tributary inflow is largely dominated by a single source, the Rainy River, draining 79% of the LOW catchment by area. Currently, there is only a small TP contribution from shoreline residential developments (6 t; ~ 1%) at a whole-lake scale, relative to the large TP loads from atmospheric deposition (95 ± 55 t; 13%) and the Rainy River (568 ± 186 t; 75%). Overall, the annual TP load to LOW was ~ 754 t with ~ 54% TP retained within the lake. The nutrient budget for the Rainy River catchment revealed that contributions from point sources along the river constitute the largest anthropogenic TP source to the Rainy River and eventually to LOW. Historical load calculations along the Rainy River show that this load has been significantly reduced since the 1970s, and presently just over 100 t of P enters LOW from anthropogenic point sources. These TP budgets provide insights into the major sources of TP influencing the overall LOW water quality and with future refinement may provide a greater understanding of linkages between TP loading and spatial and temporal water quality changes in the LOW.