Temporal Effects of St. Clair River Dredging on Lakes St. Clair and Erie Water Levels and Connecting Channel Flow

A Great Lakes hydrologic response model was used to study the temporal effects of St. Clair River dredging on Lakes St. Clair and Erie water levels and connecting channel flows. The dredging has had a significant effect on Great Lakes water levels since the mid-1980s. Uncompensated dredging permanently lowers the water levels of Lakes Michigan and Huron and causes a transitory rise in the water levels of Lakes St. Clair and Erie. Two hypothetical dredging projects, each equivalent to a 10 cm lowering of Lakes Michigan and Huron, were investigated. This lowering is approximately half the effect of the 7.6 and 8.2 meter dredging projects. In the first case the dredging was assumed to occur over a single year while in the second it was spread over a 2-year period. The dredging resulted in a maximum rise of 6 cm in the downstream levels of Lakes St. Clair and Erie. The corresponding increase in connecting channel flows was about 150 m³s^?1. The effects were found to decrease over a 10-year period with a half-life of approximately 3 years. The maximum effects on Lake Erie lagged Lake St. Clair by about 1 year.     

Investigation of viruses in Diporeia spp. from the Laurentian Great Lakes and Owasco Lake as potential stressors of declining populations

The benthic amphipod Diporeia is an ecologically and biogeochemically important constituent of deep freshwater lakes in North America. The proliferation of dreissenid mussels in the mid-1990s coincided with a sharp decrease in Diporeia populations in several Laurentian Great Lakes; however the ultimate cause and mechanisms of their decline are still unknown. Here we examined the composition of DNA viruses associated with Diporeia collected from populations of Lake Michigan that had declined and stable populations in Lake Superior and Owasco Lake (Finger Lake in central New York State). Viral metagenomic libraries from Owasco Lake and Lake Superior were comprised primarily of bacteriophages, which may infect bacteria within the amphipod microbiome. In contrast, the metagenomic library from Lake Michigan contained well-represented ssDNA circular viral genomes. The prevalence and viral load of one putative Type V ssDNA circular virus (LM29173) that recruited almost 30% of total viral sequence reads in the Lake Michigan library was analyzed by quantitative PCR. The prevalence of LM29173 was over two orders of magnitude greater in Lake Michigan compared to the other two lakes. Although further research is necessary to establish the pathology and epidemiological extent of viral-Diporeia interactions, our data suggest that viruses may be numerically significant constituents of the Diporeia microbiome, and if pathogenic some of these viruses may be a stressor of Great Lakes Diporeia populations. Our data further indicate that special attention should be given to the circovirus that was prevalent in the declining Michigan population but uncommon in the other two lakes.     

Characterization of Growth and Winter Survival of Age-0 Yellow Perch in Southeastern Lake Michigan

Reduced growth and increased winter mortality are possible mechanisms for poor recruitment of age-0 yellow perch (Perca flavescens) in southeastern Lake Michigan. To assess the role of growth variability on recruitment of age-0 yellow perch, we quantified length-by-date and abundance during 1996–2002. Analyses revealed significant differences in length among years and sites. To resolve spatial trends, we contrasted the 1998 cohort from southeastern Lake Michigan with conspecifics from populations located within and beyond the Lake Michigan watershed. These analyses identified significant differences in length; trends suggested that the capacity for growth in the cohort from southeastern Lake Michigan was similar to these other populations. To characterize winter survival for the 1998 age-0 cohort, otoliths from age-1 perch in 1999 were used to back-calculate length (BCL) from the previous autumn. The BCL distribution was significantly larger than the 1998 distribution, but analyses led to the inference that this age-0 cohort experienced little mortality during the winter of 1998–99. We used a regression model to quantify risk of winter starvation for the 1998 cohort, using 1981–2001 winter water temperatures from southeastern Lake Michigan. This analysis suggested that risk of starvation for the 1998 cohort across this period was low (1.0–4.4%). Back calculation and regression analyses allow us to reject the hypothesis that reduced growth led to a high rate of winter mortality for the 1998 cohort. If we extrapolate our findings to past cohorts, the results would be similar unless these fish were significantly smaller than the 1998 cohort. Variation in age-0 length, growth, or winter mortality failed to fully explain yellow perch recruitment patterns in southeastern Lake Michigan. We hypothesize that factors acting before the juvenile stage is sampled during summer are shaping recruitment; resolving this requires further, stage-specific studies.     

Convergence of trophic state and the lower food web in Lakes Huron,Michigan and Superior

Signs of increasing oligotrophication have been apparent in the open waters of both Lake Huron and Lake Michigan in recent years. Spring total phosphorus (TP) and the relative percentage of particulate phosphorus have declined in both lakes; spring TP concentrations in Lake Huron are now slightly lower than those in Lake Superior, while those in Lake Michigan are higher by only about 1 μg P/L. Furthermore, spring soluble silica concentrations have increased significantly in both lakes, consistent with decreases in productivity. Transparencies in Lakes Huron and Michigan have increased, and in most regions are currently roughly equivalent to those seen in Lake Superior. Seasonality of chlorophyll, as estimated by SeaWiFS satellite imagery, has been dramatically reduced in Lake Huron and Lake Michigan, with the spring bloom largely absent from both lakes and instead a seasonal maximum occurring in autumn, as is the case in Lake Superior. As of 2006, the loss of cladocerans and the increased importance of calanoids, in particular Limnocalanus, have resulted in crustacean zooplankton communities in Lake Huron and Lake Michigan closely resembling that in Lake Superior in size and structure. Decreases in Diporeia in offshore waters have resulted in abundances of non-dreissenid benthos communities in these lakes that approach those of Lake Superior. These changes have resulted in a distinct convergence of the trophic state and lower food web in the three lakes, with Lake Huron more oligotrophic than Lake Superior by some measures.     

Cattail Invasion of Sedge/Grass Meadows in Lake Ontario: Photointerpretation Analysis of Sixteen Wetlands over Five Decades

Photointerpretation studies were conducted to evaluate vegetation changes in wetlands of Lake Ontario and the upper St. Lawrence River associated with regulation of water levels since about 1960. The studies used photographs from 16 sites (four each from drowned river mouth, barrier beach, open embayment, and protected embayment wetlands) and spanned a period from the 1950s to 2001 at roughly decadal intervals. Meadow marsh was the most prominent vegetation type in most wetlands in the late 1950s when water levels had declined following high lake levels in the early 1950s. Meadow marsh increased at some sites in the mid-1960s in response to low lake levels and decreased at all sites in the late 1970s following a period of high lake levels. Typha increased at nearly all sites, except waveexposed open embayments, in the 1970s. Meadow marsh continued to decrease and Typha to increase at most sites during sustained higher lake levels through the 1980s, 1990s, and into 2001. Most vegetation changes could be correlated with lake-level changes and with life-history strategies and physiological tolerances to water depth of prominent taxa. Analyses of GIS coverages demonstrated that much of the Typha invasion was landward into meadow marsh, largely by Typha × glauca. Lesser expansion toward open water included both T. × glauca and T. angustifolia. Although many models focus on the seed bank as a key component of vegetative change in wetlands, our results suggest that canopy-dominating, moisture-requiring Typha was able to invade meadow marsh at higher elevations because sustained higher lake levels allowed it to survive and overtake sedges and grasses that can tolerate periods of drier soil conditions.     

Stimulation of Lake Michigan Plankton Metabolism by Sediment Resuspension and River Runoff

Previous work during a major sediment resuspension event (March 1988) in southern Lake Michigan demonstrated that nutrients and carbon derived from resuspended sediment stimulated intense winter heterotrophic production while simultaneously decreasing light availability and autotrophic biomass. However, the role of riverine inputs on plankton metabolism remained unclear. Here we present results from a simulated enrichment experiment (March 2000) designed to examine the influence of resuspended sediments and riverine inputs on Lake Michigan plankton dynamics. Lake water amended with realistic levels of river water, coastal resuspended sediment and river water + sediment all showed enhanced heterotrophic bacterial production and plankton respiration rates, relative to the lake water control. Bacterial production increased by approximately 4× in river water treatments and by a factor of 2.5× for the sediment only treatment compared to lake water controls. Rates of net primary production were stimulated by river water (8.5×) and resuspended sediment (3×), but most by a combination of river water + sediments (11×). Community respiration showed a similar response with rates approximately 8x higher in river water amendment treatments and 3.5× higher in the sediment treatment. Extrapolating experimentally determined production rates to the southern Lake Michigan basin indicated that heterotrophic and autotrophic production in this nearshore region may be enhanced by as much as 3× and 5.2× due to these source inputs. Indeed, field measurements throughout southern Lake Michigan from 1998–2000 support these experimental results. Experimental and field observations suggest that both seasonal riverine inputs and episodic resuspended sediments influence the regional scale ecosystem metabolism and biogeochemistry in Lake Michigan.     

Pelagic Occurrence of Benthic Animals Near Shore in Lake Michigan

Collections with towed nets showed that presumably sedentary benthic animals occurred continually in the water above bottom in small numbers over depths of 6 m and 9 m in southeastern Lake Michigan. Densities were usually higher at night and in the months from June through September. Pelagic individuals at both depths consisted mainly of two chironomids, Saetheria tylus and Chironomus sp., and two naidids, Stylaria lacustris and Nais pardalis, but a total of 71 species were collected over a seven-month period. About 0.1% of the benthic animals were present above bottom at any time during a night in July 1974. The dominant pelagic species were the same as the benthic dominants at 6 m, but were much less important in the benthos at 9 m. We propose that this behavior, analogous to drifting of stream invertebrates, serves to maintain benthic populations in the frequently disturbed sediments of shallow (6 m deep or less) areas in Lake Michigan, and probably many other large lakes.     

A 90-year record of lake whitefish Coregonus clupeaformis abundances in Michigan waters of the upper Laurentian Great Lakes

Fishery managers throughout the upper Laurentian Great Lakes are currently faced with a two-decade decline in abundance and harvest of lake whitefish Coregonus clupeaformis stocks. We used multivariate auto-regressive state-space (MARSS) models to develop long-term (90-year; 1929–2018) time-series of lake whitefish relative abundance based on commercial catch-per-effort (CPE) data for 13 statistical districts in State of Michigan waters, including 1836 and 1842 Treaty ceded waters, of Lakes Superior, Michigan, and Huron. CPE time-series were used to estimate historical baseline conditions, which were compared to more recent conditions, specifically with reference to select regulatory, environmental, and ecological conditions in each lake and fishing intensity. Population growth rates suggested that lake whitefish stocks responded: (1) negatively to high levels of harvest and expansion of sea lamprey Petromyzon marinus populations during the early-1900s; (2) positively to commercial fishery regulation and sea lamprey control during the late-1950s and early-1960s; and (3) negatively to establishment of dreissenid mussels Dreissena spp. in Lakes Michigan and Huron by 2005 and the recent period of low productivity in all three lakes since the mid- to late-2000s. When placed in a historical context, the most recent (2011–2018) lake whitefish abundances are low, intermediate, and high in 31 %, 46 %, and 23 % of all districts examined. Although environmental and ecological conditions likely drove recent declines, correlation analysis suggested that higher levels of fishing intensity were associated with greater district-specific declines in abundance during the last two decades (1999–2016), a period characterized by lower overall productivity and limited recruitment in most lake whitefish stocks.     

Contaminant Residues in Tissues of Adult and Prefledged Herring Gulls from the Great Lakes in Relation to Diet in the Early 1990s

In the early 1990s, herring gulls (Larus argentatus) were collected in 15 breeding colonies throughout the Great Lakes basin and in two reference colonies on Lake Winnipeg and the Bay of Fundy. Organochlorine and metal concentrations, and stable isotope ratios (¹⁵N/¹⁴N and ¹³C/¹²C) were measured in their tissues, and we qualitatively assessed their diet. Breast muscle δ¹⁵N suggested that adults fed on planctivorous or insectivorous fish at six colonies, on piscivorous fish at four, and at a lower trophic level at the remaining three. The concentrations of Co, Ni, Al, Cr, Sn, Fe, and Pb in kidneys of adults suggested anthropogenic enrichment in the Great Lakes basin. Concentrations of contaminants were highest most often in tissues of gulls from Lake Ontario and northern Lake Michigan colonies. Concentrations of Pb in adults from Hamilton Harbour and the Detroit River, and of Se in adults from the southern Lake Huron colony, were similar to published toxicity thresholds. Tissue levels of Cd have increased, while those of Pb have decreased markedly since 1983. DDE, dieldrin, mirex, and ΣPCB concentrations in livers collected from nine of these colonies revealed declines of 16 to 87% at most locations since the early 1980s. 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents were highest in adults from the offshore colonies in western Lake Erie and northern Lake Michigan, where gulls feed on piscivorous fish, and were driven by non-ortho PCBs. There was evidence of an unusually high bioavailablity of organochlorines, especially dieldrin, near the northern Lake Michigan colony during the period of chick growth, and of an ongoing loading of mercury to eastern Lake Ontario. Tissues of adult gulls from colonies on Lakes Ontario, Erie, and Michigan best reflect local conditions whereas those from Lake Superior and northern Lake Huron reflect contaminants accumulated from time spent on the lower lakes.     

Great Lakes Flood Thresholds and Impacts

Using the location, data, and water levels from flood events along the Canadian shore of the Great Lakes, flood damage thresholds were determined to identify and compare water levels at which static and storm-induced high water impact shoreline interests on several shore reaches of Lakes Erie, Huron, Ontario, and St. Clair. Spatial differences identified may be related to several factors, including: 1) nearshore bathymetries; 2) extent of residential development along low-lying shorelines; 3) degree of riparian adjustment to flooding; and 4) location relative to dominant wind or storm directions. Correlation analyses found that flood damage levels are more closely correlated to fluctuations in static levels on Lakes Ontario, Huron, and St. Clair, while flood damage levels are more closely correlated to maximum instantaneous water levels on Lake Erie. Correlation analyses of individual gauge data identified locations possibly more susceptible to storm surges. A conservative approach to determining flood damage thresholds is suggested, being based on a standard deviation below the mean of maximum instantaneous flood levels for a given gauge. The standard deviation threshold, while lower than current “critical levels” used in management, is more representative of the majority of flood damage levels than thresholds based on lowest maximum instantaneous lake levels. However, caution is urged in applying any critical level solely based on water level gauge information as Great Lakes flooding is a highly site-specific phenomenon influenced by meteorologic factors.     

NOTE: First record of natural reproduction by Atlantic salmon (Salmo salar) in the St. Marys River,Michigan

Atlantic salmon (Salmo salar) are native to Lake Ontario; but their populations severely declined by the late 1800s due to human influences. During the early to mid-1900s, Atlantic salmon were stocked throughout the Great Lakes in effort to reestablish them into Lake Ontario and introduce the species into the upper Great Lakes. However, these efforts experienced minimal success. In 1987, Lake Superior State University and the Michigan Department of Natural Resources began stocking Atlantic salmon in the St. Marys River, Michigan, which has resulted in a successful, self-supporting hatchery operation and stable recreational Atlantic salmon fishery. Possibly due to a combination of competition with other salmonid species for spawning habitat, prey selection causing detrimental effects on early life stages and high rates of early mortality syndrome, Atlantic salmon appeared to be severely limited in their ability to naturally reproduce within the upper Great Lakes. In 2012, the first unequivocal documentation of naturally reproduced Atlantic salmon in the St. Marys River was recorded, downstream from the compensation works and parallel to the Soo Locks in Sault Ste. Marie, Michigan.     

Great Lakes chloride trends: Long-term mass balance and loading analysis

Surveillance data collected over the past 150 years are compiled and analyzed to identify chloride trends in the Laurentian Great Lakes. These data indicate that chloride levels started rising in the mid-19th century and began accelerating in the early twentieth century. Lake Superior's and Lake Michigan's concentrations have continued to increase steadily and currently stand at their maximum recorded levels. In contrast, lakes Huron, Erie and Ontario reached peak levels between 1965 and 1975, but then began to decline. However, recent data indicate that the chloride concentrations in these lakes are now increasing again. Because loading data are not readily available, a mass-balance model is employed to estimate the chloride inputs required to account for the concentration trends. This inverse analysis yields computed load reductions that are consistent with reported industrial load reductions during the last three decades of the 20th century. Hence, it appears that the improvements were for the most part attributable to industrial controls. The model is also used to predict that if loads are held fixed at 2006 levels, concentrations in all lakes will continue to increase with the most dramatic rise occurring in Lake Michigan which will ultimately approach the level of Lake Erie.     

Resurgence of Cisco (Coregonus artedi) in Lake Michigan

In recent decades, many factors that were linked with the decline of Great Lakes cisco (Coregonus artedi) populations have subsided. The goal of this study was to investigate where cisco exist in Lake Michigan and evaluate evidence for recovery including when, where, and to what extent it is occurring. We evaluated datasets from several independent monitoring efforts that did and did not target cisco. We also evaluated trends in commercial and recreational catches of cisco. Across these datasets, there was strong evidence of a sustained recovery of cisco stocks that began in Lake Michigan in the mid-2000s. Fall gill net surveys and commercial fisheries provided reasonable indications of a population recovery in the northeast by 2011. Further south, Ludington Pump Storage barrier net monitoring also recorded increasing numbers of cisco starting in 2011. Recreational harvest estimates were valuable in evaluating spatial distributions but were less valuable as an early signal of abundance shifts. Measures of the recreational harvest of cisco most notably increased in 2014. The highest catch rates and harvest occurred in Grand Traverse Bay and northern Lake Michigan as evidenced by recreational, commercial, and fall netting surveys. Observations of cisco are expanding and have increased in intensity along the eastern shore of Lake Michigan south to Muskegon in both fishery dependent and independent surveys. The similarity in trends from all data sources indicate that cisco abundance has increased, and their range within the basin continues to expand.     

Trends in Mysis diluviana abundance in the Great Lakes, 2006–2016

With the large Diporeia declines in lakes Michigan, Huron, and Ontario, there is concern that a similar decline of Mysis diluviana related to oligotrophication and increased fish predation may occur. Mysis density and biomass were assessed from 2006 to 2016 using samples collected by the Great Lakes National Program Office's biomonitoring program in April and August in all five Great Lakes. Summer densities and biomasses were generally greater than spring values and both increased with bottom depth. There were no significant time trends during these 10–11 years in lakes Ontario, Michigan, or Huron, but there was a significant increase in Lake Superior. Density and biomass were highest in lakes Ontario and Superior, somewhat lower in Lake Michigan, and substantially lower in Lake Huron. A few Mysis were collected in eastern Lake Erie, indicating a small population in the deep basin of that lake. On average, mysids contributed 12–18% (spring-summer, Michigan), 18–14% (spring-summer, Superior), 30–13% (spring-summer, Ontario), and 3% (Huron) of the total open-water crustacean biomass. Size distributions consisted of two peaks, indicating a 2-year life cycle in all four of the deep lakes. Mysis were larger in Lake Ontario than in lakes Michigan, Superior, and Huron. Comparisons with available historic data indicated that mysid densities were higher in the 1960s–1990s (5 times higher in Huron, 2 times higher in Ontario, and around 40% higher in Michigan and Superior) than in 2006–2016.     

Diet of lake trout and burbot in Northern Lake Michigan during spring: Evidence of ecological interaction

We used analyses of burbot (Lota lota) and lake trout (Salvelinus namaycush) diets taken during spring gill-net surveys in northern Lake Michigan in 2006–2008 to investigate the potential for competition and predator–prey interactions between these two species. We also compared our results to historical data from 1932. During 2006–2008, lake trout diet consisted mainly of alewives (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax), whereas burbot utilized a much wider prey base including round goby (Neogobius melanostomus), rainbow smelt, alewives, and sculpins. Using the Schoener's diet overlap index, we found a higher potential for interspecific competition in 1932 than in 2006–2008, though diet overlap was not significant in either time period. No evidence of cannibalism by lake trout or lake trout predation on burbot was found in either time period. In 2006–2008, however, lake trout composed 5.4% (by weight) of burbot diet. To determine whether this predation could be having an impact on lake trout rehabilitation efforts in northern Lake Michigan, we developed a bioenergetic-based consumption estimate for burbot on Boulder Reef (a representative reef within the Northern Refuge) and found that burbot alone can consume a considerable proportion of the yearling lake trout stocked annually, depending on burbot density. Overall, we conclude that predation, rather than competition, is the more important ecological interaction between burbot and lake trout, and burbot predation may be contributing to the failed lake trout rehabilitation efforts in Lake Michigan.     

Some Fungi and Water Molds in Waters of Lake Michigan with Emphasis on Those Associated with the Benthic Amphipod Diporeia spp.

To determine types of fungi in the water and associated with the benthic amphipod Diporeia spp., samples were collected at various depths in Lake Michigan in an area where the Diporeia population was in a severe state of decline. No fungi were found associated with living, freshly-dead, or dried Diporeia cultured separately from Lake Michigan water. When dead Diporeia and other organic substrates (snake skin and hemp seeds) were used to grow fungi in Lake Michigan water, a rich and diverse fungal and water mold community was revealed. A total of 31 species were found, with the most common genera being Achlya, Aphanomyces, Myzocytium, and Pythium. In general, species were homogeneously distributed in the water; that is, few differences were found in species richness between nearshore (10–15 m) and offshore (60–80 m) waters, and between near-surface (1 m) and near-bottom waters (1 m off bottom). Sampling occurred during the unstratified period (April and October) to maximize the number of species collected, which may have contributed to the uniform spatial pattern observed. While conclusions must be placed in context with our methods of detection, we found no evidence that a fungal infestation was associated with Diporeia in this region of the lake.     

Mean Circulation in the Great Lakes

In this paper new maps are presented of mean circulation in the Great Lakes, employing long-term current observations from about 100 Great Lakes moorings during the 1960s to 1980s. Knowledge of the mean circulation in the Great Lakes is important for ecological and management issues because it provides an indication of transport pathways of nutrients and contaminants on longer time scales. Based on the availability of data, summer circulation patterns in all of the Great Lakes, winter circulation patterns in all of the Great Lakes except Lake Superior, and annual circulation patterns in Lakes Erie, Michigan, and Ontario were derived. Winter currents are generally stronger than summer currents, and, therefore, annual circulation closely resembles winter circulation. Circulation patterns tend to be cyclonic (counterclockwise) in the larger lakes (Lake Huron, Lake Michigan, and Lake Superior) with increased cyclonic circulation in winter. In the smaller lakes (Lake Erie and Lake Ontario), winter circulation is characterized by a two-gyre circulation pattern. Summer circulation in the smaller lakes is different; predominantly cyclonic in Lake Ontario and anticyclonic in Lake Erie.     

Ecological Interactions Between Bythotrephes cederstroemi and Leptodora kindtii and the Implications for Species Replacement in Lake Michigan

The zooplankton predator, Leptodora kindtii, declined in abundance at an offshore reference station in Lake Michigan in the mid-1980s following the invasion of another zooplankton predator, Bythotrephes cederstroemi. Both predators feed largely on daphnid prey and it was observed that densities of three Daphnia species declined in abundance at the reference station following the Bythotrephes invasion. Circumstantial evidence would suggest that the native predator, Leptodora, was competitively suppressed by Bythotrephes. However, results of laboratory experiments presented here show that Bythotrephes will readily attack and eat Leptodora when the predators are maintained under concentrated densities, even if alternative prey are available for the Bythotrephes. There was no evidence in these experiments that Leptodora attacked or ate Bythotrephes. These results imply that predation by Bythotrephes on Leptodora may alternatively account for the collapse of Leptodora in offshore Lake Michigan. In this note I discuss evidence in support of competition and predation as alternative hypotheses to explain the pattern of species replacement, Bythotrephes for Leptodora, observed in Lake Michigan. The existing data are not definitive and tempt further inquiry.     

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.     

Patterns of Lake Balkhash water level changes and their climatic correlates during 1992–2010 period

The variation in Lake Balkhash water levels during the period from 1992 to 2010 and their relationship with climate dynamics were investigated in this study, using satellite altimetry data and meteorological records from climate stations located in the lake catchment basin. The altimetry‐derived water level demonstrated a general water level increase, reaching a mean value of 8.1 cm year^?1 in July 2005, with a maximum value of 342.52 m. The increased Lake Balkhash water level was accompanied by an overall upward trend in precipitation and temperature in the catchment basin during the study period. A strong increase in the winter and spring temperature was the main contributor to the general upward temperature trend, whereas a significant change of summer and autumn precipitation was the major contributor to the annual precipitation trend. Neither precipitation nor temperature increased uniformly across the entire lake drainage basin. The study results identified the most pronounced climate change occurring in the mountainous part (>2000 m above sea level) of the basin, in the upper reaches of the Ili river, which is the main water inflow to the lake. Statistical analysis indicated the Lake Balkhash water level is strongly correlated with both precipitation and temperature. The correlations were investigated for three altitudinal strata (<1000 m, 1000–2000 m, >2000 m) corresponding to the lower, middle and upper reaches of the Ili river. The best correlations were obtained for the upper reaches of the Ili river, indicating a changing snow cover and glacier equilibrium are the main factors controlling the water level trends in Lake Balkhash.