Expansion of Dreissena into offshore waters of Lake Michigan and potential impacts on fish populations

Lake Michigan was invaded by zebra mussels (Dreissena polymorpha) in the late 1980s and then followed by quagga mussels (D. bugensis) around 1997. Through 2000, both species (herein Dreissena) were largely restricted to depths less than 50 m. Herein, we provide results of an annual lake-wide bottom trawl survey in Lake Michigan that reveal the relative biomass and depth distribution of Dreissena between 1999 and 2007 (although biomass estimates from a bottom trawl are biased low). Lake-wide mean biomass density (g/m²) and mean depth of collection revealed no trend between 1999 and 2003 (mean = 0.7 g/m² and 37 m, respectively). Between 2004 and 2007, however, mean lake-wide biomass density increased from 0.8 g/m² to 7.0 g/m², because of increased density at depths between 30 and 110 m, and mean depth of collection increased from 42 to 77 m. This pattern was confirmed by a generalized additive model. Coincident with the Dreissena expansion that occurred beginning in 2004, fish biomass density (generally planktivores) declined 71% between 2003 and 2007. Current understanding of fish population dynamics, however, indicates that Dreissena expansion is not the primary explanation for the decline of fish, and we provide a species-specific account for more likely underlying factors. Nonetheless, future sampling and research may reveal a better understanding of the potential negative interactions between Dreissena and fish in Lake Michigan and elsewhere.     

A Comparison of Lake Trout Spawning,Fry Emergence,and Habitat Use in Lakes Michigan,Huron, and Champlain

Restoration of self-sustaining populations of lake trout is underway in all of the Great Lakes and Lake Champlain, but restoration has only been achieved in Lake Superior and in Parry Sound, Lake Huron. We evaluated progress toward restoration by comparing spawning habitat availability, spawner abundance, egg and fry density, and egg survival in Parry Sound in Lake Huron, in Lake Michigan, and in Lake Champlain in 2000–2003. Divers surveyed and assessed abundance of spawners at 5 to 15 sites in each lake. Spawning adults were sampled using standardized gill nets, eggs were sampled using egg bags, and fry were sampled using emergent fry traps and egg bags left on spawning reefs overwinter. Spawning habitat was abundant in each lake. Adult lake trout abundance was low in Lake Michigan and Parry Sound, and very high at one site in Lake Champlain. Egg deposition was lowest in Lake Michigan (0.4–154.5 eggs•m⁻², median = 1.7), intermediate in Parry Sound (39–1,027 eggs•m⁻², median = 278), and highest in Lake Champlain (0.001–9,623 eggs•m⁻², median = 652). Fry collections in fry traps followed the same trend: no fry in Lake Michigan, 0.005–0.06 fry•trap⁻¹ day⁻¹ in Parry Sound, and 0.08–3.6 fry•trap⁻¹ in Lake Champlain. Egg survival to hatch in overwinter egg bags was similar in Lake Michigan (7.6%) and Parry Sound (2.3–8.9%) in 2001–02, and varied in Lake Champlain (0.4–1.1% in 2001–02, and 1.8–18.2 in 2002–03). Lake trout restoration appears unlikely in northern Lake Michigan at current adult densities, and failure of restoration in Lake Champlain suggests that there are sources of high mortality that occur after fry emergence.     

Fatty acid profiles of lake trout reveal the importance of lipid content for interpreting trophic relationships within and across lakes

Fatty acid profiles increasingly are being used to quantify foraging patterns of consumers, but the associated interpretation may vary with the tissue type and its lipid content. For salmonids, lipid deposits can be found in both dorsal and ventral (“belly flap”) areas of muscle tissues. However, it is uncertain whether belly flap and dorsal muscle fatty acid profiles are similar in natural populations of fish. We examined how fatty acid profiles of belly flap compared to those of dorsal muscle and the consequent impacts on dietary inferences. Fatty acid profiles were derived from lake trout (Salvelinus namaycush) caught in five North American lakes: Champlain, Flathead, Michigan, Ontario, and Swan. Fatty acid profiles were most similar between tissues when lipid content of muscle was > ~10%, the threshold below which similarities decreased and thus increasingly affected dietary inference. Some fatty acids commonly used as trophic indicators preferentially accrued in one tissue over the other depending on lipid content of the tissues. Regardless of tissue type, fatty acid profiles were specific to each lake indicating that food web structures were distinctive over a broad geographic range. Fatty acid profiles of tissues from lakes Michigan and Ontario were highly similar, so were those from Flathead and Swan lakes, whereas those from Lake Champlain were distinct, having comparatively high proportions of 18:1n-9. We conclude that lipid storage areas like belly flaps likely provide a more accurate signal than muscle when using fatty acids to investigate dietary patterns, particularly when muscle lipid levels are low.     

Horizontal and Vertical Distribution of PCBs in Southern Lake Michigan Sediments and the Effect of Waukegan Harbor as a Point Source

The spatial distribution of PCBs in southern Lake Michigan sediments was obtained from the analysis of 66 box cores and 8 grab samples. PCB concentrations in surficial sediments were closely related to sedimentation zone and to the oxidizable organic matter content of the sediments. Average surficial sediment concentrations ranged from 81 ng/g in depositional zones to 7.2 ng/g in non-depositional zones. The vertical distribution of PCBs was determined at several sites and was used to estimate the areal burden and flux of PCBs in each of the southern sedimentation zones. The total sediment PCB burden in the southern portion of the lake was estimated to be 5,900 kg. Southern Lake Michigan has received PCBs since approximately 1930 at an average flux of 7.1 μg/m² yr.The effect of Waukegan Harbor as a point source of PCBs to Lake Michigan was evaluated by comparing the PCB distribution and Aroclor composition of harbor sediments to those of sediments of decreasing distances from the harbor. Significant differences in total PCB concentration between non-depositional zones near the harbor and those in other areas of the basin indicate that Waukegan Harbor has influenced the PCB burden of Lake Michigan.     

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.     

Lake-wide Distribution of Dreissena in Lake Michigan, 1999

The Great Lakes Science Center has conducted lake-wide bottom trawl surveys of the fish community in Lake Michigan each fall since 1973. These systematic surveys are performed at depths of 9 to 110 m at each of seven index sites around Lake Michigan. Zebra mussel (Dreissena polymorpha) populations have expanded to all survey locations and at a level to sufficiently contribute to the bottom trawl catches. The quagga (Dreissena bugensis), recently reported in Lake Michigan, was likely in the catches though not recognized. Dreissena spp. biomass ranged from about 0.6 to 15 kg/ha at the various sites in 1999. Dreissenid mussels were found at depths of 9 to 82 m, with their peak biomass at 27 to 46 m. The colonization of these exotic mussels has ecological implications as well as potential ramifications on the ability to sample fish consistently and effectively with bottom trawls in Lake Michigan.     

Nitrogen dry deposition to Lake Superior and Lake Michigan

Nitrate (NO₃^?) levels in Lake Superior have increased from historic levels of about 5?μM to its current concentration of about 25?μM. The atmosphere makes a substantial contribution to the nitrogen budgets for Lake Superior and Lake Michigan. This study provides a more well-defined estimate of nitrogen dry deposition rates derived from the measurement of over-water concentrations, and in situ meteorological measurements, which were input into the Resistance Model. We obtained a nitrogen dry deposition rate of [(3.41?±?2.26)?×?10⁷?kg?N/yr; (5.90?±?3.91)?kg?N/ha/yr] over Lake Michigan, and [(1.54?±?1.06)?×?10⁷?kg?N/yr; (1.87?±?1.27)?kg?N/ha/yr] over Lake Superior. Nitric acid (HNO₃), which originates from the combustion of fossil fuels, contributes 84% of the total nitrogen dry deposition to Lake Michigan; and 66% to Lake Superior. Ammonia (NH₃), which originates from agricultural activities and gasoline combustion, is the second highest contributor of nitrogen dry deposition to both lakes: contributing 13% to Lake Michigan and 32% to Lake Superior. The nitrogen dry deposition is approximately 68% of the nitrogen wet deposition over Lake Superior, and approximately 80% of wet deposition over Lake Michigan. The over-water dry deposition velocity of HNO₃ and NH₃ were also evaluated. We obtained morning deposition velocities of 0.099?cm/s for NH₃ and 0.095?cm/s for HNO₃; and afternoon values of 0.137?cm/s for NH₃ and 0.132?cm/s for HNO₃. Another key finding is that the atmospheric concentrations of nitrogen compounds near Lake Michigan and Lake Superior have decreased since 2003.     

Abundance and Biomass of the Meiobenthos in Nearshore Lake Michigan with Comparisons to the Macrobenthos

The meiobenthos of nearshore southeastern Lake Michigan was quantified by taking cores from three depths (11,17, and 23 m) at monthly intervals from May to November 1976–79. Total meiobenthic abundance ranged from 69,700/m² to 1,300,000/m² and total biomass ranged from 0.03 to 0.87 g/m². Nematodes accounted for 80% of all individuals and 66% of the biomass. Most of the major groups peaked in late spring/summer, but some peaked in early spring and fall. With the exception of nematodes, mean annual densities varied from two-fold to twelve-fold at a given station. Harpacticoids, tardigrades, and ostracods tended to be more abundant at the deeper depths, while cyclopoids and cladocerans were less abundant. There was no consistent relationship between sampling depth and the abundance of nematodes and rotifers. Temporal and spatial variation in many of the groups appeared related to changes in the amount of bottom detritus. The overall macrobenthos:meiobenthos biomass ratio was 15:1 and ranged from 5:1 to 45:1 on an annual basis.     

Lake-wide,annual status of the Mysis diluviana population in Lake Michigan in 2015

Mysis diluviana is one of the most abundant zooplankton by biomass in the Laurentian Great Lakes of North America, a predator of other zooplankton and an important prey for fishes. Studies of long-term trends in Lake Michigan have shown 2005–2016 densities to be 50–80% lower than 1990s densities, but these observations have been based on annual monitoring that is either spatially or seasonally limited. We combined Lake Michigan Mysis data from three annual programs and the 2015 Cooperative Science and Monitoring Initiative to achieve broad spatial coverage during spring, summer, and fall of 2015 and broad depth coverage during spring 2016. Lake-wide, annual density and biomass were 82 (SE: 10) Mysis/m² and 200 (SE: 36) mg dry mass/m². Density and biomass estimates were highest offshore, generally higher in the north basin, and seasonally highest in summer. Annual lake-wide averages for depths >30 m were better captured by seasonally-extensive annual programs than spatially-extensive annual programs, although spring sampling may bias annual values low. Mysis cohorts grew 0.026 mm/d (age-0) and 0.007 to 0.027 mm/d (age-1). Annual mortality was 81–98%. Reproduction was fall-spring and seasonal lake-wide estimates ranged from 0.6 to 19.1% females brooding, 13–20 embryos/brood, and 3–46 embryos/m². Annual production (423 mg dry mass/m²/yr, SE: 31) was lower than all but one previous estimate from lakes Michigan, Huron, and Ontario. While Mysis tend to persist, low Mysis production may be a concern for prey fishes that feed on Mysis.     

Fatty acids reveal salmonine – prey relationships in Lake Michigan

Lake Michigan salmon and trout populations are important species for recreational fisheries and food web management, and are largely supported through stocking efforts, with varying degrees of natural recruitment. Ongoing fisheries management of these salmonine populations is dictated by relationships between predator and prey abundance as well as community structure within the lake. However, while prey fish biomass has declined, and species composition has changed in recent decades, knowledge of prey consumption by the salmonine community has lagged. Herein, we explore trophic relationships using fatty acids profiles, which offer insights into the foraging habits and energy pathways relied on over weeks to months prior to collection. Fatty acids of the prey base for salmonines in Lake Michigan indicate a gradient of foraging habits that range from pelagic (typified by alewife and rainbow smelt) versus benthic (i.e., slimy sculpin and round goby) resource use. Fatty acids implied that there was more variation in foraging habits among individual lake trout and brown trout compared to Chinook salmon, coho salmon and rainbow trout, which appeared to all rely almost exclusively on pelagic prey. Fatty acid profiles also indicated size-based shifts in foraging habits; for example, larger lake trout consuming a greater proportion of benthic prey than smaller individuals. Data herein suggest that Chinook and coho salmon, as well as rainbow trout, are more likely to experience competitive interactions during times of low pelagic prey-fish abundance in Lake Michigan, whereas brown and lake trout are able to utilize benthic resources to a greater degree.     

Sediment Trap Studies in Lake Superior: Insights into Resuspension,Cross-margin Transport,and Carbon Cycling

As part of the Keweenaw Interdisciplinary Transport Experiment in Lake Superior (KITES) project, sediment traps were deployed at multiple locations along the northern coast of the Keweenaw Peninsula in Lake Superior. Traps were deployed at multiple depths (25 m below water surface to 5 m above sediments) at varying distances from shore (0.5–21 km) during the months May-October over a 3-year period. Material captured in the sediment traps was analyzed for total carbon (C), total nitrogen (N), total phosphorus (P), total copper (Cu), ²¹⁰Pb, and stable isotope ratios of C and N. Copper in sediment trap material, a tracer for mine residues, indicated transport of material in both directions along the peninsula. Cross-margin transport of material occurred at all trap locations along the coast and in all seasons. The most significant finding was that sediment traps suspended just below the thermocline collected large amounts of resuspended sediments even when the trap was moored in 120–220 m of water (9–21 km from shore). Estimates of the contribution to settling fluxes of organic carbon from resuspended sediments ranged from 10–30% in offshore traps; estimates based on ²¹⁰Pb agreed well with estimates based on a carbon mixing model. Element ratios suggest that resuspended material in offshore traps originated in near-shore regions. Despite the strong influence of resuspension on the composition of sedimenting material, stable isotope ratios were controlled by processes occurring in the water column. Seasonal variations in isotope ratios may reflect seasonal shifts between predominance by autotrophic and heterotrophic processes in the water column.     

Impact of Zebra Mussels (Dreissena polymorpha) on Fingernail Clams (Sphaeriidae) in Extreme Southern Lake Michigan

The demography of the zebra mussel (Dreissena polymorpha) and its impacts on native fingernail clams (Sphaeriidae) in Lake Michigan near Michigan City, Indiana were studied from 1992 to 1997 at 5, 10, and 15 m depths. Zebra mussel densities ranged from 0 to 6,209/m², and were greatest at deeper stations. Size-frequency distributions suggest that initial colonization of the Michigan City area occurred in 1991, with adults typically living for 2 to 3 years. Abundance of adult populations may be limited by the sandy habitat typical of the areas studied and the occasional high-energy wave action in the shallow (5 m) near-shore zone of this area. Densities of fingernail clam ranged from 0 to 1,312/m² and were also greatest at deeper stations. Approximately half of the fingernail clams shells between 1 and 2 mm in size were used as substrate for attachment by juvenile and adult zebra mussels, while over 90% of the clams > 2 mm showed attachment by zebra mussels. Overall median densities of Sphaeriidae decreased from 832/m² to 13/m² at the 15 m depth and from 234/m² to 0/m² at the 10 m depth during the study. It appears that zebra mussel colonization caused a dramatic reduction of Sphaeriidae density by 1997 that may eventually result in their loss from the area.     

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.     

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.     

Downflux of Sediment,Organic Matter,and Phosphorus in the Niagara River Area of Lake Ontario

Vertical arrays of sediment traps were installed near the mouth of the Niagara River and at nearshore and offshore sites. Surveys of temperature, turbidity, and conductivity were used to establish that the traps nearest the river mouth were under the river outflow plume. A single offshore array in 1980 indicated downfluxes of 1-2 g · m^?2 · d^?1 of dry matter in the mid-water column. These fluxes were similar to those of small lakes but much higher fluxes were found near the bottom of Lake Ontario. More extensive work in 1981 confirmed the presence of a near-bottom nepheloid layer and the effect it has on increasing downflux extimates. Sediment traps near shore and at the river mouth caught substantially more material at all depths than did traps at the offshore stations. Downfluxes of phosphorus and sediment were consistent with independent estimates of loading and retention.     

Biological Redistribution of Lake Sediments by Tubificid Oligochaetes: Branchiura sowerbyi and Limnodrilus hoffmeisteri/Tubifex tubifex

Mechanisms and rates of sediment mixing by the largest oligochaete in Lake Erie, Branchiura sowerbyi, have been quantitatively investigated using a multiple ¹³⁷Cs tracer layer microcosm technique and compared with a mixture of the dominant tubificids, Limnodrilus hoffmeisteri + Tubifex tubifex. These worms feed head down in the sediment (up to 20 cm for B. sowerbyi and up to 10 cm for L. hoffmeisteri/T. tubifex) on organic-rich particles and deposit fecal pellets at the sediment-water interface (conveyor-belt feeding). Obliteration of tracer layers by these worms was attributed to mixing by both diffusive- and feeding-style (advective) processes. The downward velocities were 2.87 to 3.66 cm/d/100,000 indiv/m² for the cells with B. sowerbyi (∼10 cm body length, 13 mg body mass) and 0.33 to 0.49 cm/d/100,000 indiv/m² for the cells with L. hoffmeisteri / T. tubifex (∼5 cm body length, 1 mg body mass). These downward velocities correspond to sediment fluxes across the sediment-water interface of 66.4 to 86.4 g dry sediment/indiv/m²/yr in cells with B. sowerbyi and 5.91 to 9.09 g dry sediment/indiv/m²/yr in the cells with L. hoffmeisteri / T. tubifex). The differences between species was due to differences in biomass, with recycling rates of 5.11 to 6.65 and 5.91 to 9.09 g dry sediment/mg biomass/m²/yr for B. sowerbyi and L. hoffmeisteri/T. tubifex, respectively. Similarly, biomass normalized downward velocities were 806 to 1,028 cm/yr/kg biomass/m² and 1,205 to 1,789 cm/yr/kg biomass/m² for B. sowerbyi and L. hoffmeisteri/T. tubifex, respectively. Both B. sowerbyi and L. hoffmeisteri / T. tubifex feed selectively on organic-rich fine-grained particles and showed an increase in particle selectivity with an increase in population density. The particle selectivity factor values ranged from 1.0 to 2.5. Food competition at a higher population density might force these organisms to selectively feed on a smaller size range of sediments. The maximum feeding rate for B. sowerbyi (4,000 to 8,000 indiv/m²) ranged from 9.10 to 13.9 per yr at depths between 11.7 and 13.6 cm while the maximum biodiffusion coefficient, D_b, ranged from 0.78 to 1.02 cm²/yr at depths between 1.6 and 2.3 cm. The maximum feeding rate for L. hoffmeisteri / T. tubifex (20,000 to 40,000 indiv/m²) was 8.13 to 13.1 per yr at depths between 5.21 and 5.27 cm and D_b ranged from 0.20 to 0.72 cm²/yr at depths between 0.87 and 2.0 cm.     

Southern Lake Michigan Sediments: Changes in Accumulation Rate,Mineralogy, and Organic Content

Data combined from three one-meter gravity cores raised from a single site in southeastern Lake Michigan permit reconstruction of the last 3,550 years of sedimentation at that location. Sediment has been accumulating at about 50 g m^?2 y^?l through most of the upper 50 cm of the core and nearly 900 g m^?2 y^?1 below that depth. Several of the physical and chemical properties of the sediments change when the sedimentation rates change; more rapidly accumulating sediments have a finer grain size, more carbonate minerals and more inorganic carbon, less organic carbon, and, among the organic fractions, relatively more humin and less fulvic acid. All these rate and abundance changes apparently occurred about 3,300 years ago and seem to be associated with the lowering of lake levels since the Nipissing high stand which ended about 4,000 years ago.     

Trends in Diporeia populations across the Laurentian Great Lakes, 1997-2009

Benthic communities in the Laurentian Great Lakes have been in a state of flux since the arrival of dreissenid mussels, with the most dramatic changes occurring in population densities of the amphipod Diporeia. In response, the US EPA initiated an annual benthic macroinvertebrate monitoring program on all five Great Lakes in 1997. Although historically the dominant benthic invertebrate in all the lakes, no Diporeia have been found in Lake Erie during the first 13 years of our study, confirming that Diporeia is now effectively absent from that lake. Populations have almost entirely disappeared from our shallow (< 90 m) sites in lakes Ontario, Huron, and Michigan. In Lake Ontario, three of our four deep (> 90 m) sites still supported Diporeia populations in 2009, with densities at those sites ranging between 96 and 198/m². In Lake Michigan, populations were still found at six of our seven deep sites in 2009, with densities ranging from 57 to 1409/m². Densities of Diporeia in 2009 at the four deep sites in Lake Huron were somewhat lower than those in Lake Michigan, ranging from 191 to 720/m². Interannual changes in population size in Lake Huron and Lake Michigan have shown a degree of synchrony across most sites, with periods of rapid decline (1997-2000, 2003-2004) alternating with periods of little change or even increase (2001-2002, 2005-2009). There has been no evidence of directional trends at any sites in Lake Superior, although substantial interannual variability was seen.     

Thiamine and Fatty Acid Content of Lake Michigan Chinook Salmon

Nutritional status of Lake Michigan Chinook salmon (Oncorhynchus tshawytscha) is inadequately documented. An investigation was conducted to determine muscle and liver thiamine content and whole body fatty acid composition in small, medium and large Chinook salmon. Muscle and liver thiamine concentrations were highest in small salmon, and tended to decrease with increasing fish size. Muscle thiamine was higher in fall than spring in large salmon. The high percentage of Chinook salmon (24–32% in fall and 58–71% in spring) with muscle thiamine concentration below 500 pmol/g, which has been associated with loss of equilibrium and death in other Great Lake salmonines, suggest that Chinook appear to rely less on thiamine than other Great Lakes species for which such low concentrations would be associated with thiamine deficiency (Brown et al. 2005b). A positive correlation was observed between liver total thiamine and percent liver lipids (r = 0.53, P < 0.0001, n = 119). In medium and large salmon, liver lipids were observed to be low in fish with less than 4,000 pmol/g liver total thiamine. In individuals with greater than 4,000 pmol/g liver thiamine, liver lipid increased with thiamine concentration. Individual fatty acids declined between fall and spring. Essential omega-3 fatty acids appear to be conserved as lipid content declined. Arachidonic acid (C20:4n6), an essential omega-6 fatty acid was not different between fall and spring, although the sum of omega-6 (Sw6) fatty acids declined over winter. Elevated concentrations of saturated fatty acids (sum) were observed in whole body tissue lipid. In summary, thiamine, a dietary essential vitamin, and individual fatty acids were found to vary in Lake Michigan Chinook salmon by fish size and season of the year.     

European Valve Snail Valvata piscinalis (Müller) in the Laurentian Great Lakes Basin

Previously reported from the lower Great Lakes basin and St. Lawrence and Hudson rivers, the nonindigenous gastropod Valvata piscinalis was found for the first time in Superior Bay (Minnesota) of Lake Superior, Lake Michigan (Wisconsin), and Oneida Lake (New York) of the Lake Ontario basin. This snail was not abundant in Lakes Superior and Michigan, whereas in eutrophic Oneida Lake it reached a maximum density of 1,690 individuals/m² (mean density = 216 individuals/m²). Human-mediated disturbances could facilitate the range extension of this snail by providing dispersal opportunities (e.g., canals, shipping traffic) or increasing nutrients (e.g., eutrophication). A native of the Palaearctic region, V. piscinalis has colonized sites across the Great Lakes basin, suggesting that it will likely become common in disturbed Great Lakes areas.