Changes in Fecundity and Egg Lipid Content of Lake Whitefish (Coregonus clupeaformis) in the Upper Laurentian Great Lakes between 1986–87 and 2003–05

Lake whitefish (Coregonus clupeaformis Mitchill), an important commercial species in the Laurentian Great Lakes, have experienced decreased growth and condition in regions of the upper Great Lakes over the past 20 years. Increases in lake whitefish density and decreases in the density of Diporeia spp., an energy rich and historically important part of the lake whitefish diet, have been implicated in the recent declines in lake whitefish growth and condition. The goal of this study was to describe lake whitefish fecundity, egg lipid content, and total ovary lipid content in selected regions of Lakes Huron, Michigan, and Superior in 1986–87 and 2003–05, two time periods with different lake whitefish and Diporeia densities. Under conditions of high lake whitefish density and low Diporeia density, female lake whitefish in the upper Laurentian Great Lakes generally produced fewer eggs. Egg lipid content was higher in 2003–05 than in 1986–87 at all sites, regardless of changes in lake whitefish or Diporeia densities. Total ovary lipid content and lake whitefish abundance were inversely related, while there was no significant relationship between total ovary lipid content and Diporeia density. The amount of energy that lake whitefish invested in egg production was more closely associated with lake whitefish abundance than with Diporeia density. This study provides evidence that recent changes in production dynamics of Great Lakes lake whitefish have not been driven solely by declines in Diporeia but have been significantly influenced by lake whitefish abundance.     

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.     

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.     

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.     

Lipid Concentration and Size Variation of Bythotrephes (Cladocera: Cercopagidae) from Lakes Erie,Huron, and Michigan

Lipid concentrations of Bythotrephes cederstroemi were compared among three Great Lakes, Erie, Huron, and Michigan, in an effort to investigate the phenotypic plasticity in size displayed among the lakes. Four developmental stages were measured in Lakes Erie and Huron and two stages were studied in Lake Michigan. With a gravimetric extraction method, the total lipid concentration range (μg lipid μg dry weight⁻¹, expressed as percent) for Bythotrephes was estimated to be 10–19%. Statistically significant differences were found in lipid concentrations of Bythotrephes among lakes and developmental stages. Lake Erie had significantly higher lipid concentration values than Lake Huron for stages 2 through 4, and had similar values to Lake Michigan for the analyzed stages 1 and 4. The first instar had indistinguishable lipid concentrations among Lakes Erie, Huron,and Michigan. Even though animals from Lake Erie were significantly smaller, the data suggest that they were not less well nourished. We hypothesize that selective mortality imposed by visual predators on larger Bythotrephes and the lack of deep water refuges in Lake Erie has encouraged the smaller size of Bythotrephes found there in comparison to those found in Lakes Huron and Michigan.     

Coexistence of the native benthic amphipod Diporeia spp. and exotic dreissenid mussels in the New York Finger Lakes

Populations of the benthic amphipod Diporeia spp. have sharply declined since the early 1990s in all North America's Great Lakes except Lake Superior. The onset and continued decline coincides with the invasion of these lakes by zebra (Dreissena polymorpha) and quagga (Dreissena rostriformis bugensis) mussels and the spread of quagga mussels to deep habitats. The six deepest Finger Lakes of central New York (Seneca, Cayuga, Skaneateles, Canandaigua, Keuka, and Owasco) have historically been Diporeia habitat and have had dreissenids for more than a decade. These lakes represent a wide range of trophic state, maximum depth, and dreissenid invasion history. We hypothesized that Diporeia abundance would be negatively impacted by dreissenid mussel expansion in the Finger Lakes. During 2006–2010, we sampled Diporeia and mussel populations in these six lakes. Diporeia was present in all six lakes, and was abundant (2000/m²) in Owasco Lake that has only zebra mussels and in Cayuga and Seneca Lakes that have had zebra and quagga mussels since 1994. Diporeia abundance was lowest (1000/m²) in Skaneateles, Canandaigua, and Keuka Lakes where quagga mussels have recently expanded. Productivity indicators explained much of the variability of Diporeia abundance. The persistence of Diporeia with quagga mussels in these lakes may be because of available alternative food resources. Fatty acid tracers indicate that Diporeia from Owasco Lake, the lake without quagga mussels, utilize diatoms, but Diporeia from Cayuga Lake that coexist with abundant quagga mussels also use food resources associated with terrestrial detritus that cannot be intercepted by dreissenids.     

Contemporary diets of Lake Superior lake whitefish off the Keweenaw Peninsula and changes in condition from the 1980s to 2010s

Over the last two decades, declines in lake whitefish (Coregonus clupeaformis) recruitment and growth in many areas of the Laurentian Great Lakes have raised concerns about the status of this important species. Although Lake Superior populations have been less affected than those in other Great Lakes, these populations still face multiple threats. We characterized lake whitefish diets collected off the Keweenaw Peninsula between 2015 and 2017 and compared results to previous Lake Superior studies. We additionally estimated length-weight relationships to determine whether lake whitefish body condition (i.e., expected weight-at-length) had changed since the 1980s. Diet diversity was low, although individual specialization was moderate to high. Fish transitioned from consuming Diporeia in the spring to Mysis and fish eggs during fall and winter; sphaeriids composed 20–30% of diets across all seasons. Compared to findings for other Lake Superior regions, lake whitefish diets comprised lower percentages of high energy items (e.g., Diporeia, Mysis) and higher percentages of low energy items (e.g., sphaeriids). Expected weights in the 2000s and 2010s were lower in the 400- and 500-mm length groups but similar in larger lengths groups compared to the 1980s; condition was highest across all lengths in the 1990s. The observed decline in condition since the 1990s in the 400- and 500-mm length groups, in combination with possibly greater consumption of less energetically profitable items, suggests that lake whitefish <600 mm or preferred prey resources in this lake region may be experiencing stressors leading to condition declines, although what these stressors are remain unknown.     

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.     

A cross-lake comparison of crustacean zooplankton communities in the Laurentian Great Lakes, 1997–2016

Spring and summer open-water crustacean zooplankton communities were examined across all five Laurentian Great Lakes from 1997 to 2016. Spring communities were dominated by calanoid (lakes Superior, Huron and Michigan) or cyclopoid (lakes Erie and Ontario) copepods. Volumetric biomass of summer communities increased along an assumed trophic gradient (Superior, Huron, Michigan, Ontario; eastern, central and western Erie), as did dominance by cyclopoids and cladocerans. Over the time series of the study, summer communities in lakes Michigan, Huron and Ontario shifted towards greater dominance by calanoids and greater similarity with Lake Superior. Trajectories of changes were different; however, reductions in cladocerans accounted for most of the change in lakes Michigan and Huron while reductions in cyclopoids and increases in Leptodiaptomus sicilis were behind the changes in Lake Ontario. Shifts in the predatory cladoceran community in Lake Ontario from Cercopagis pengoi to occasional dominance by Bythotrephes longimanus, a species much more vulnerable to planktivory, as well as the appearance of Daphnia mendotae in a daphnid community previously consisting almost exclusively of the smaller Daphnia retrocurva, suggest impacts of reduced vertebrate predation. In contrast, strong correlations between cladocerans and chlorophyll in lakes Michigan and Huron point to the possible importance of bottom-up forces in those lakes. Large interannual shifts in cladoceran community structure in the central and eastern basins of Lake Erie suggest intense but variable vertebrate predation pressure. The zooplankton communities of lakes Huron, Michigan and Ontario may be approaching a historic community structure represented by Lake Superior.     

Feeding Ecology of Lake Whitefish in Lake Huron

We determined diet composition, feeding strategy, prey size, and effects of prey type on food weight and energy in stomachs for lake whitefish Coregonus clupeaformis in Lake Huron during 2002–04. Age-0 lake whitefish (73–149 mm TL) ate mainly large-bodied cladoceran zooplankton in the summer (July–mid September). Medium lake whitefish (≤ 350 mm TL excluding age-0) generally ate softbodied macroinvertebrates, especially Chironomidae larvae and pupae, in the spring (mid May-June). Zooplankton, if eaten, were generally most important in the summer. Molluscs were generally a minor part of medium lake whitefish diets. Large lake whitefish (> 350 mm) mainly ate molluscs, particularly quagga mussels (Dreissena bugensis), despite geographic differences in mussel abundance. Large-bodied crustaceans (Diporeia spp., Mysis relicta, Isopoda) were a minor part of large lake whitefish diets. Lake whitefish demonstrated a flexible feeding strategy, with individual specialization on some prey and generalized feeding on others. The size of benthic prey (Diporeia spp., Chironomidae, and Dreissena spp.) eaten increased with fish size and influenced the energetic value of prey for medium and large lake whitefish. The type of prey eaten affected the food and energy intake differently for each size class of lake whitefish. Age-0 lake whitefish that ate mainly zooplankton had more food and energy in stomachs than fish eating shelled prey or other macroinvertebrates. On the other hand, food weight in stomachs did not differ across prey groups for medium fish, but energy in stomachs was lowest for fish that ate shelled prey. For large lake whitefish, there was no difference in food weight or energy in stomachs for different prey groups.     

Genetic diversity of lake whitefish in lakes Michigan and Huron; sampling,standardization, and research priorities

We combined data from two laboratories to increase the spatial extent of a genetic data set for lake whitefish Coregonus clupeaformis from lakes Huron and Michigan and saw that genetic diversity was greatest between lakes, but that there was also structuring within lakes. Low diversity among stocks may be a reflection of relatively recent colonization of the Great Lakes, but other factors such as recent population fluctuation and localized stresses such as lamprey predation or heavy exploitation may also have a homogenizing effect. Our data suggested that there is asymmetrical movement of lake whitefish between Lake Huron and Lake Michigan; more genotypes associated with Lake Michigan were observed in Lake Huron. Adding additional collections to the calibrated set will allow further examination of diversity in other Great Lakes, answer questions regarding movement among lakes, and estimate contributions of stocks to commercial yields. As the picture of genetic diversity and population structure of lake whitefish in the Great Lakes region emerges, we need to develop methods to combine data types to help identify important areas for biodiversity and thus conservation. Adding genetic data to existing models will increase the precision of predictions of the impacts of new stresses and changes in existing pressures on an ecologically and commercially important species.     

Potential Strategies for Recovery of Lake Whitefish and Lake Herring Stocks in Eastern Lake Erie

Lake Erie sustained large populations of ciscoes (Salmonidae: Coregoninae) 120 years ago. By the end of the 19^th century, abundance of lake whitefish (Coregonus clupeaformis) had declined drastically. By 1925, the lake herring (a cisco) population (Coregonus artedii) had collapsed, although a limited lake herring fishery persisted in the eastern basin until the 1950s. In the latter part of the 20^th century, the composition of the fish community changed as oligotrophication proceeded. Since 1984, a limited recovery of lake whitefish has occurred, however no recovery was evident for lake herring. Current ecological conditions in Lake Erie probably will not inhibit recovery of the coregonine species. Recovery of walleye (Sander vitreus) and efforts to rehabilitate the native lake trout (Salvelinus namaycush) in Lake Erie will probably assist recovery because these piscivores reduce populations of alewife (Alosa psuedoharengus) and rainbow smelt (Osmerus mordax), which inhibit reproductive success of coregonines. Although there are considerable spawning substrates available to coregonine species in eastern Lake Erie, eggs and fry would probably be displaced by storm surge from most shoals. Site selection for stocking or seeding of eggs should consider the reproductive life cycle of the stocked fish and suitable protection from storm events. Two potential sites in the eastern basin have been identified. Recommended management rocedures, including commercial fisheries, are suggested to assist in recovery. Stocking in the eastern basin of Lake Erie is recommended for both species, as conditions are adequate and the native spawning population in the eastern basin is low. For lake herring, consideration should be given to match ecophenotypes as much as possible. Egg seeding is recommended. Egg seeding of lake whitefish should be considered initially, with fingerling or yearling stocking suggested if unsuccessful. Spawning stocks of whitefish in the western basin of Lake Erie could be utilized.     

Widespread infection of lake whitefish Coregonus clupeaformis with the swimbladder nematode Cystidicola farionis in northern lakes Michigan and Huron

We estimated the prevalence, intensity, and abundance of swimbladder nematode infection in 1281 lake whitefish (Coregonus clupeaformis) collected from four sites in northern lakes Huron (Cheboygan and DeTour Village) and Michigan (Big Bay de Noc and Naubinway) from fall 2003 through summer 2006. Morphological examination of nematode egg, larval, and mature stages through light and scanning electron microscopy revealed characteristics consistent with that of Cystidicola farionis Fischer 1798. Total C. farionis prevalence was 26.94%, while the mean intensity and abundance of infection was 26.72 and 7.21 nematodes/fish, respectively. Although we detected C. farionis in all four stocks that were examined, Lake Huron stocks generally had higher prevalence, intensity, and abundance of infection than Lake Michigan stocks. A distinct seasonal fluctuation in prevalence, abundance, and intensity of C. farionis was observed, which does not coincide with reported C. farionis development in other fish species. Lake whitefish that were heavily infected with C. farionis were found to have thickened swimbladder walls with deteriorated mucosa lining, which could affect swimbladder function. Whether C. farionis infection may be negatively impacting lake whitefish stocks in the Great Lakes is unclear; continued monitoring of C. farionis infection should be conducted to measure responses of lake whitefish stocks to infection levels.     

Genome-wide genetic diversity may help identify fine-scale genetic structure among lake whitefish spawning groups in Lake Erie

In Lake Erie, lake whitefish Coregonus clupeaformis supported lucrative fisheries before populations were decimated by overfishing and water quality degradation. In recent years, there has been a renewed interest in lake whitefish and management of the fishery they support. Lake whitefish spawn on several reefs throughout Lake Erie, but the relative recruitment dynamics and contributions of spawning groups to the fishery are not well understood. Modern high-throughput sequencing approaches offer new opportunities to census population diversity and to identify subtle differences among closely related populations. We used high-throughput sequencing data to evaluate the genetic structure and diversity of lake whitefish collected opportunistically across broad spatial scales in Lake Erie. Using RAD-capture (Rapture), we sequenced and genotyped individuals (N = 88) from the west, central, and east basin of Lake Erie at 120,268 single nucleotide polymorphisms (SNPs). Lake whitefish from Niagara and Crib Reefs (west basin) diverged from the three collections. Interestingly, these were the only lake whitefish collected during the act of spawning (late November), and all other fish were collected pre-spawn (August-early November). These results suggest that some lake whitefish spawning reefs may be reproductively isolated, though definition of these groups into stocks will require more intentional sampling during the act of spawning.     

Trophic connections in Lake Superior Part II: The nearshore fish community

We use detailed diet analyses of the predominant planktivorous, benthivorous and piscivorous fish species from Lake Superior to create a nearshore (bathymetric depths < 80 m) fish community food web. The food web was based on analysis of 5125 fish stomachs collected seasonally (spring, summer, fall) from 9 nearshore sites in 2005. Based on mass of prey items, nearshore diets across all sites and seasons were similarly structured with a dominance of macroinvertebrates (Mysis diluviana and Diporeia spp). Although the piscivorous fishes like lean lake trout (Salvelinus namaycush) fed to a lesser extent on Diporeia and Mysis, they were still strongly connected to these macroinvertebrates, which were consumed by their primary prey species (sculpin spp., rainbow smelt Osmerus mordax, and coregonines). The addition of Bythotrephes to summer/fall cisco and lake whitefish diets, and the decrease in rainbow smelt in lean lake trout diets (replaced by coregonines) were the largest observed differences relative to historic Lake Superior diet studies. Although the offshore food web of Lake Superior was simpler than nearshore in terms of number of fish species present, the two areas had remarkably similar food web structures, and both fish communities were primarily supported by Mysis and Diporeia. We conclude that declines in Mysis or Diporeia populations would have a significant impact on energy flow in Lake Superior. The food web information we generated can be used to better identify management strategies for Lake Superior.     

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.     

Historical changes and current status of crayfish diversity and distribution in the Laurentian Great Lakes

Despite increasing recognition of the importance of invertebrates, and specifically crayfish, to nearshore food webs in the Laurentian Great Lakes, past and present ecological studies in the Great Lakes have predominantly focused on fishes. Using data from many sources, we provide a summary of crayfish diversity and distribution throughout the Great Lakes from 1882 to 2008 for 1456 locations where crayfish have been surveyed. Sampling effort was greatest in Lake Michigan, followed by lakes Huron, Erie, Superior, and Ontario. A total of 13 crayfish species occur in the lakes, with Lake Erie having the greatest diversity (n = 11) and Lake Superior having the least (n = 5). Five crayfish species are non-native to one or more lakes. Because Orconectes rusticus was the most widely distributed non-native species and is associated with known negative impacts, we assessed its spread throughout the Great Lakes. Although O. rusticus has been found for over 100 years in Lake Erie, its spread there has been relatively slow compared to that in lakes Michigan and Huron, where it has spread most rapidly since the 1990s and 2000, respectively. O. rusticus has been found in both lakes Superior and Ontario for 22 and 37 years, respectively, and has expanded little in either lake. Our broad spatial and temporal assessment of crayfish diversity and distribution provides a baseline for future nearshore ecological studies, and for future management efforts to restore native crayfish and limit non-native introductions and their impact on food web interactions.     

The benthic community of the Laurentian Great Lakes: Analysis of spatial gradients and temporal trends from 1998 to 2014

We used the results of seventeen years of Great Lakes benthic monitoring conducted by the U.S. EPA's Great Lakes National Program Office to describe the spatial and temporal patterns of benthic communities, assess their status, trends, and main drivers, and to infer the potential impact of these community changes on ecosystem functioning. Benthic abundance and diversity were higher at shallow (<70?m in depth) stations with chlorophyll concentrations above 3?μg/L than at deeper sites (<1?μg/L). We infer that lake productivity, measured by chlorophyll was likely the major driver of benthic abundance and diversity across lakes. Consequently, benthic diversity and abundance were the highest in the most productive Lake Erie, followed by lakes Ontario, Michigan, Huron, and Superior. Multivariate analysis distinguished three major communities shared among lakes (littoral, sublittoral, and profundal) that differed in species composition and abundance, functional group diversity, and tolerance to organic pollution. Analysis of temporal trends revealed that the largest changes occurred in profundal communities, apparent in significant shifts in dominant taxa across all lakes except Lake Superior. In lakes Michigan, Huron, and Ontario, the former dominant Diporeia was replaced with Dreissena and Oligochaeta. Profundal species, with the exception of dreissenids, became less abundant, and their depth distribution has shifted. In contrast, density and diversity of native littoral and sublittoral communities increased. The invasion of dreissenids was among the most important drivers of changes in benthic communities. Continued monitoring is critical for tracking unprecedented changes occurring in the Great Lakes ecosystem.     

Infection of Lake Whitefish (Coregonus clupeaformis) with motile Aeromonas spp. in the Laurentian Great Lakes

Infections with motile Aeromonas species were detected in lake whitefish collected over a one-year period from four stocks within lakes Michigan and Huron, USA. Sixty-nine isolates were recovered from the kidneys and swim-bladders of sixty-four infected fish. Representative isolates were Gram-negative bacilli that produced cytochrome oxidase, grew in the absence of salt, were facultative anaerobes, and were resistant to the vibriostatic agent 2,4-diamino,6,7-di-isopropylpteridine. Phenotypic characterization placed twenty-two isolates into the A. hydrophila complex, twelve into the A. sobria complex, and one into the A. caviae complex, while six isolates were characterized as A. allosaccharophila, two as A. veronii bv. veronii, and one as A. popoffi. The prevalence of infection by motile aeromonads varied by site and season, with lake whitefish sampled in the summer having a significantly higher prevalence. Clinical signs in lake whitefish infected only with Aeromonas spp. included congestion and hemorrhaging in the fins and musculature; generalized pallor; congestion, hemorrhaging, and multifocal necrotic foci within the liver; moderate to severe splenomegaly; congestion and swelling of the kidneys; ascites within the peritoneal cavity; and hemorrhagic enteritis. This study provides evidence on the wide spread prevalence of motile aeromonad infections in lake whitefish stocks inhabiting northern lakes Michigan and Huron.