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.     

Circular Rep encoding single stranded (CRESS) DNA virus-like sequences detected in quagga mussels (Dreissena rostriformis bugensis) and sediments from the central Lake Michigan benthos

Circular Rep Encoding Single Stranded (CRESS) DNA viruses are a diverse group of viruses that have been identified in both terrestrial and aquatic ecosystems. Recent work in the Laurentian Great Lakes characterized the ecology and diversity of CRESS-DNA viruses associated with amphipods, Diporeia spp. In the last 20 years the Lake Michigan benthos has changed considerably with drastic population declines of Diporeia spp. concurrent with an increase in invasive quagga mussel (Dreissena rostriformis bugensis) abundance. The purpose of this study was to characterize CRESS-DNA virus-like elements (VLES; which could represent complete, partial/defective, endogenized or satellite viruses) associated with both invasive quagga mussels and in sediment collected in central Lake Michigan. Viral metagenomic libraries were prepared from two size classes (>25 mm and <15 mm shell length) of Lake Michigan quagga mussels and for two different sediment layers (136 mm and 290 mm below the lake floor) in a sediment core extracted from the lake. Viral metagenomes were different between quagga mussels and sediment cores. Nine VLE sequences were present in both the quagga mussel tissues and the sediment core layers analyzed. Cs¹³⁷ radiometric dating results indicate that these VLEs were present in the sediment prior to arrival of quagga mussels in Lake Michigan. These data suggest quagga mussels may interact with CRESS-DNA VLEs and algal DNA VLEs historically present in the Lake Michigan benthos. Overall, these data suggest that quagga mussels interacted with CRESS-DNA VLES present in Lake Michigan benthos since at least 1952.     

Abundance and distribution of benthic macroinvertebrates in offshore soft sediments in Western Lake Huron, 2001–2007

Invasive species have had major impacts on the Great Lakes. This is especially true of exotic dreissenid mussels which are associated with decreased abundance of native macroinvertebrates and changes in food availability for fish. Beginning in 2001, we added a benthic macroinvertebrate survey to the USGS-Great Lakes Science Center's annual fall prey fish assessment of Lake Huron to monitor abundance of macrobenthos. Mean abundance of Diporeia, the most abundant benthic taxon in Lake Huron reported by previous investigators, declined greatly between 2001 and 2007. Diporeia was virtually absent at 27-m sites by 2001, decreased and was lost completely from 46-m depths by 2006, but remained present at reduced densities at 73-m sites. Dreissenids in our samples were almost entirely quagga mussels Dreissena bugensis. Zebra mussels Dreissena polymorpha were virtually absent from our samples, suggesting that they were confined to nearshore areas shallower than we sampled. Loss of Diporeia at individual sites was associated with arrival of quagga mussels, even when mussel densities were low. Quagga mussel density peaked during 2002, then decreased thereafter. During the study quagga mussels became established at most 46-m sites, but remained rare at 73-m sites. Length frequency distributions suggest that initial widespread recruitment may have occurred during 2001–2002. Like other Great Lakes, Lake Huron quagga mussels were associated with decreased abundance of native taxa, but negative effects occurred even though dreissenid densities were much lower. Dreissenid effects may extend well into deep oligotrophic habitats of Lake Huron.     

Evidence for Remote Effects of Dreissenid Mussels on the Amphipod Diporeia: Analysis of Lake Ontario Benthic Surveys, 1972–2003

The status of invasive dreissenid mussels (Dreissena polymorpha and D. bugensis) and native amphipods (Diporeia spp.) in Lake Ontario was assessed in 2003 and compared with historical data. D. polymorpha (zebra mussels) were rarely observed in 2003, having been displaced by D. bugensis (quagga mussels). D. bugensis expanded its depth range from 38 m depth in 1995 to 174 m in 2003 and this dreissenid reached densities averaging 8,000/m² at all sites < 90 m. During the same time period, Diporeia populations almost completely disappeared from 0–90 m depth, continuing a declining trend from 1994–1997 reported in previous studies. The average density of Diporeia in the 30–90 m depth interval decreased from 1,380/m² to 63/m² between 1997 and 2003. Prior to 2003, areas deeper than 90 m represented a refuge for Diporeia, but even these deep populations decreased, with densities declining from 2,181/m2 in 1999 to 545/m2 in 2003. Two common hypotheses for the decline of Diporeia in the Great Lakes are food limitation and a toxin/pathogen associated with dreissenid pseudofeces. The Diporeia decline in deep waters preceded the expansion of D. bugensis to these depths, and suggests that shallow dreissenid populations remotely influence profundal habitats. This pattern of decline is consistent with mechanisms that act from some distance including nearshore dreissenid grazing and downslope transport of pseudofeces.     

Is reduced benthic flux related to the Diporeia decline? Analysis of spring blooms and whiting events in Lake Ontario

Benthic monitoring by USGS off the southern shore of Lake Ontario from October 1993 to October 1995 provides a detailed view of the early stages of the decline of the native amphipod Diporeia. A loss of the 1994 and 1995 year classes of Diporeia preceded the disappearance of the native amphipod at sites near Oswego and Rochester at depths from 55 to 130 m. In succeeding years, Diporeia populations continued to decline in Lake Ontario and were nearly extirpated by 2008. Explanations for Diporeia's decline in the Great Lakes include several hypotheses often linked to the introduction and expansion of exotic zebra and quagga mussels (Dreissena sp.). We compare the timeline of the Diporeia decline in Lake Ontario with trends in two sources of organic matter to the sediments — spring diatom blooms and late summer whiting events. The 1994–95 decline of Diporeia coincided with localized dreissenid effects on phytoplankton in the nearshore and a year (April 1994 to May 1995) of decreased flux of organic carbon recorded by sediment traps moored offshore of Oswego. Later declines of profundal (> 90 m) Diporeia populations in 2003 were poorly associated with trends in spring algal blooms and late summer whiting events.     

Diporeia site preference in Lake Superior: Food or physical factors?

Vital to the Lake Superior food web, the amphipod Diporeia remains the dominant macroinvertebrate in Lake Superior despite drastic population declines throughout the rest of the Laurentian Great Lakes. Diporeia is most abundant in the slope region of the lake at water depths between 30 and 125 m. It has been hypothesized that this depth range is preferred because of elevated primary production and deposition within this zone. This hypothesis of food driving habitat preference has not been directly tested. Here we used 120-hour preference-avoidance trials to record Diporeia choice of sediments from different water depths, seasons, and other treatments. Most preferences were weak to absent; however, Diporeia strongly preferred sediment from 30- and 60-m water depths over deeper or shallower sites. Contrary to the hypothesis about food driving habitat choice, chemical characteristics did not explain this strong preference. Grain size variation was the only measured variable that was consistent between the sites preferred by Diporeia and different from unpreferred sites. Both the 30- and 60-meter sites contained predominantly medium silt but had a wider range in grain sizes. These results indicate that physical habitat characteristics may have a stronger bearing on Diporeia habitat preference than food availability and may account for their distribution in the lake. The results also may imply that the role of dreissenid mussels as ecosystem engineers altering sediment physical characteristics may be important where they are abundant.     

Dreissenid mussels from the Great Lakes contain elevated thiaminase activity

We examined thiaminase activity in dreissenid mussels collected at different depths and seasons, and from various locations in Lakes Michigan, Ontario, and Huron. Here we present evidence that two dreissenid mussel species (Dreissena bugensis and D. polymorpha) contain thiaminase activity that is 5–100 fold greater than observed in Great Lakes fishes. Thiaminase activity in zebra mussels ranged from 10,600 to 47,900 pmol g^− 1·min^− 1 and activities in quagga mussels ranged from 19,500 to 223,800 pmol g^− 1·min^− 1. Activity in the mussels was greatest in spring, less in summer, and least in fall. Additionally, we observed greater thiaminase activity in dreissenid mussels collected at shallow depths compared to mussels collected at deeper depths. Dreissenids constitute a significant and previously unknown pool of thiaminase in the Great Lakes food web compared to other known sources of this thiamine (vitamin B₁)-degrading enzyme. Thiaminase in forage fish of the Great Lakes has been causally linked to thiamine deficiency in salmonines. We currently do not know whether linkages exist between thiaminase activities observed in dreissenids and the thiaminase activities in higher trophic levels of the Great Lakes food web. However, the extreme thiaminase activities observed in dreissenids from the Great Lakes may represent a serious unanticipated negative effect of these exotic species on Great Lakes ecosystems.     

Slimy sculpin depth shifts and habitat squeeze following the round goby invasion in the Laurentian Great Lakes

The collapse of Diporeia spp. and invasions of dreissenid mussels (zebra, Dreissena polymorpha; quagga, D. bugensis) and round goby (Neogobius melanostomus) have been associated with declines in abundance of native benthic fishes in the Great Lakes, including historically abundant slimy sculpin (Cottus cognatus). We hypothesized that as round goby colonized deeper habitat, slimy sculpin avoided habitat competition, predation, and aggression from round goby by shifting to deeper habitat. Accordingly, we predicted increased depth overlap of slimy sculpin with both round goby and deepwater sculpin (Myoxocephalus thompsonii) that resulted in habitat squeeze by both species. We used long-term bottom trawl data from Lakes Michigan, Huron, and Ontario to evaluate shifts in slimy sculpin depth and their depth overlap with round goby and deepwater sculpin. Lake Huron most supported our hypotheses as slimy sculpin shifted to deeper habitat coincident with the round goby invasion, and depth overlap between slimy sculpin and both species recently increased. Slimy sculpin depth trends in Lakes Michigan and Ontario suggest other ecological and environmental factors better predicted sculpin depth in these lakes.     

A short-term look at potential changes in Lake Michigan slimy sculpin diets

Diporeia hoyi and Mysis relicta are the most important prey items of slimy sculpins (Cottus cognatus) in the Great Lakes. Slimy sculpins were collected from dreissenid-infested bottoms off seven Lake Michigan ports at depths of 27–73 m in fall 2003 to study their lake-wide diets. Relatively large dreissenid biomass occurred at depths of 37- and 46-m. Quagga mussels (Dreissena bugnesis) composed at least 50% of dreissenid biomass at Manistique, Saugatuck, and Sturgeon Bay. Mysis accounted for 82% of the sculpin diet by dry weight at eastern Lake Michigan while Diporeia composed 54–69% of the diet at western Lake Michigan and dominated the diets of slimy sculpins at all sites deeper than 46 m. In northern Lake Michigan, this diet study in new sites showed that slimy sculpin consumed more prey with low energy contents, especially chironomids, than Mysis and Diporeia in shallow sites (depth <55 m). We recommend diet studies on sedentary benthic fishes to be conducted along perimeters of the Great Lakes to observe changes in their diets that may be impacted by changing benthic macroinvertebrate communities.     

Dreissenid mussels are not a “dead end” in Great Lakes food webs

Dreissenid mussels have been regarded as a “dead end” in Great Lakes food webs because the degree of predation on dreissenid mussels, on a lakewide basis, is believed to be low. Waterfowl predation on dreissenid mussels in the Great Lakes has primarily been confined to bays, and therefore its effects on the dreissenid mussel population have been localized rather than operating on a lakewide level. Based on results from a previous study, annual consumption of dreissenid mussels by the round goby (Neogobius melanostomus) population in central Lake Erie averaged only 6 kilotonnes (kt; 1 kt = one thousand metric tons) during 1995–2002. In contrast, our coupling of lake whitefish (Coregonus clupeaformis) population models with a lake whitefish bioenergetics model revealed that lake whitefish populations in Lakes Michigan and Huron consumed 109 and 820 kt, respectively, of dreissenid mussels each year. Our results indicated that lake whitefish can be an important predator on dreissenid mussels in the Great Lakes, and that dreissenid mussels do not represent a “dead end” in Great Lakes food webs. The Lake Michigan dreissenid mussel population has been estimated to be growing more than three times faster than the Lake Huron dreissenid mussel population during the 2000s. One plausible explanation for the higher population growth rate in Lake Michigan would be the substantially higher predation rate by lake whitefish on dreissenid mussels in Lake Huron.     

Long-term trends of Lake Michigan benthos with emphasis on the southern basin

Lake Michigan benthic macrofauna have been studied for almost a century, allowing for a unique analysis of long-term changes in community structure. We examined changes in abundances of three major taxonomic groups of benthic macroinvertebrates (Diporeia, Oligochaeta, and Sphaeriidae) in southern Lake Michigan from 1931 to 2015 and identified the most likely causes for these changes. Abundances of all three groups increased during 1931–1980 with the bulk of these increases occurring in nearshore (≤50 m) waters and coincident with increased loading of phosphorus (P) and subsequent increased primary production. Abundances of all three taxa declined during 1980–2000 again mostly in nearshore waters and coincident with decreased P loading. The quagga mussel (Dreissena rostriformis bugensis) invasion was associated with a further decline in phytoplankton primary production during 2000–2015. Both Diporeia and Sphaeriidae declined in abundance during that time, with Diporeia exhibiting the more pronounced decrease of the two groups. In contrast, Oligochaeta increased in abundance during 2000–2015. The quagga mussel has become, by far, the most abundant benthic macroinvertebrate species in terms of density and biomass. Overall, the primary driver of changes in the abundances of the three major taxa during this 85-year period appeared to be changes in phytoplankton primary production due to changing P loadings and, later in the time series, Dreissena filtering. The dreissenid mussels invasions coincided with a rapid decline of Diporeia abundance, but the mechanism of this negative effect remains unidentified. In contrast, Oligochaeta likely benefited from the quagga mussel invasion, perhaps via quagga-generated food supply.     

Quagga mussel (Dreissena rostriformis bugensis) selective feeding of phytoplankton in Saginaw Bay

Experiments from May to December measuring selective grazing and egestion of different phytoplankton taxa in natural Saginaw Bay (Lake Huron) seston by shallow-water morph quagga mussels (Dreissena bugensis rostriformis) showed that the mussels were highly selective filter feeders and that their net clearance rates on different species ranged widely, resulting in food consumption that was strongly driven by seasonal phytoplankton dynamics. Overall, net clearance rates by quagga mussels on the entire phytoplankton assemblage were similar to those observed for zebra mussels (Dreissena polymorpha) during the 1990s. Phytoplankton taxon, rather than size, was more important to food selection since quagga mussels cleared similar sized but different species of algae at very different rates. In contrast to many studies with zebra mussels, larger-sized algae such as Dinobryon divergens, Aulacoseira italica, Fragilaria crotonensis, and Anabaena were cleared at high rates and rejected at lower rates than many smaller species within the same broad taxonomic group. We suspect that these differences between dreissenid species do not stem from species differences but methodological factors and phytoplankton composition of systems studied. Small-sized diatoms, green algae with thick cell walls (Scenedesmus and Oocystis), and colonial cyanobacteria with gelatinous sheaths (Aphanocapsa, Chroococcus, and Microcystis) were cleared at low rates and rejected in high proportion in pseudofeces or feces during all seasons. We describe the likely mechanisms of pre- and post-ingestive behavior that explain these differences, which relate to phytoplankton size, morphology, cell wall characteristics, and chemical composition. Changes in the Great Lakes phytoplankton communities are consistent with mussel grazing preferences.     

Changes in Benthic Invertebrate Communities of South Bay,Lake Huron Following Invasion by Zebra Mussels (Dreissena polymorpha), and Potential Effects on Lake Whitefish (Coregonus clupeaformis) Diet and Growth

In this study we evaluated changes in benthic invertebrate communities of South Bay, Lake Huron following the invasion of zebra mussels (Dreissena polymorpha) and considered the implications for diets and growth of whitefish (Coregonus clupeaformis), a commercially important fish in the Great Lakes. Of the ten benthic invertebrate groups identified prior to invasion (1980–81), only densities of Diporeia and Oligochaeta have changed since the appearance of the zebra mussel, and only Diporeia and Chironomidae changed in relative abundance. These changes are similar to those observed in other areas of the Great Lakes, with the exception of an increase in Oligochaeta density. Post-invasion (2002–03) shallow-water communities appear to be more homogeneous, dominated by zebra mussels and Isopoda, whereas deep-water sites are more heterogeneous due to the loss of Diporeia. Additional data on Diporeia density for several years between 1959 and 2004 indicated that current low densities are not typical of South Bay. Based on changes in the benthic communities and published literature on whitefish diets, we predict that unless whitefish are able to switch to Mysis as an alternative to Diporeia, post-invasion whitefish diets will only contain a maximum of 57 to 84% of their former energy content. These predictions are likely underestimates, as they do not take into account increased energy costs associated with reductions in total invertebrate density at historical foraging depths.     

Phytoplankton trends in the Great Lakes, 2001–2011

We describe recent trends in phytoplankton composition and abundance in the Laurentian Great Lakes using synoptic spring (April) and summer (August) sampling events from 2001 through 2011, a period of rapid shifts in pelagic food webs and water quality. Data analysis identified qualitative and quantitative changes in algal densities, biovolume, and taxonomic composition of assemblages. Since 2001, Lake Superior has changed subtly with an increase in small-celled blue-green algae in spring and a recent decline in summer centric diatoms, possibly a result of lake warming and changes in water quality. Spring phytoplankton declines mainly attributed to diatoms occurred in Lakes Huron and Michigan, a probable result of invasions by non-native dreissenids that have reduced pelagic nutrients and selectively consumed certain taxa. The decline in Lake Huron's spring phytoplankton biovolume was earlier and more severe than that in Lake Michigan, despite a faster and more abundant dreissenid invasion in Lake Michigan. Lake Erie's central basin had a notable increase in spring centric diatoms (largely Aulacoseira), while the whole of Lake Erie shows a summer increase in cyanobacteria, complementing that found in coastal regions. The composition of Lake Ontario's species assemblage shifted, but little overall change in algal abundance was observed with the exception of higher summer densities of cyanophytes. Additional mechanisms for shifts in the pelagic primary producers are described or hypothesized in the context of concurrent shifts in water quality and invertebrate populations. Tracking these trends and explaining driving factors will be critical to the management of lake conditions.     

Six decades of Lake Ontario ecological history according to benthos

The Laurentian Great Lakes have experienced multiple anthropogenic changes in the past century, including cultural eutrophication, phosphorus abatement initiatives, and the introduction of invasive species. Lake Ontario, the most downstream lake in the system, is considered to be among the most impaired. The benthos of Lake Ontario has been studied intensively in the last six decades and can provide insights into the impact of environmental changes over time. We used multivariate community analyses to examine temporal changes in community composition over the last 54 years and to assess the major drivers of long-term changes in benthos. The benthic community of Lake Ontario underwent significant transformations that correspond with three major periods. The first period, termed the pre/early Dreissena period (1964–1990), was characterized by high densities of Diporeia, Sphaeriidae, and Tubificidae. During the next period defined by zebra mussel dominance (the 1990s) the same groups were still prevalent, but at altered densities. In the most recent period (2000s to present), which is characterized by the dominance and proliferation of quagga mussels deeper into the lake, the community has changed dramatically: Diporeia almost completely disappeared, Sphaeriidae have greatly declined, and densities of quagga mussels, Oligochaeta and Chironomidae have increased. The introduction of invasive dreissenids has changed the Lake Ontario benthic community, historically dominated by Diporeia, Oligochaeta and Sphaeriidae, to a community dominated by quagga mussels and Oligochaeta. Dreissenids, especially the quagga mussel, were the major drivers of these changes over the last half century.     

Depth drives growth dynamics of dreissenid mussels in Lake Ontario

Understanding dreissenid mussel population dynamics and their impacts on lake ecosystems requires quantifying individual growth across a range of habitats. Most dreissenid mussel growth rates have been estimated in nutrient rich or nearshore environments, but mussels have continued to expand into deep, cold, low-nutrient habitats of the Great Lakes. We measured annual quagga mussel (Dreissena rostriformis bugensis) growth at 15 m, 45 m, and 90 m in Lake Ontario using caged mussels near Oswego, New York, USA from June 2018 to May 2019. Quagga mussel growth (starting size 12 mm) was greatest at 15 m (mean shell length increase = 10.2 mm), and was lower at 45 m (5.9 mm) and 90 m (0.7 mm). Caged mussels were obtained from near the 90-m site and those reared at 15 and 45 m developed thicker shells than those that were caged at 90 m. We observed relatively high colonization by quagga and, to a lesser degree, zebra mussels (Dreissena polymorpha) at 15 m, very few colonizers at 45 m, and none at 90 m. Higher growth potential, but low natural mussel densities observed at 15 m and 45 m suggest factors other than growth limit dreissenid abundance at these depths. The relatively slow dreissenid growth rates observed in offshore habitats are consistent with the gradual abundance increases documented in these zones across the Great Lakes and suggest new mussels that become established in these habitats may contribute to ecosystem effects for decades.     

Summer Food Habits of Pumpkinseeds (Lepomis gibbosus) and Bluegills (Lepomis macrochirus) in Presque Isle Bay,Lake Erie

Zebra mussels (Dreissena polymorpha) and quagga mussels (D. bugensis) have received much attention since they were first reported in the Great Lakes. Predation by fishes may be an important factor in regulating dreissenid populations, but the extent to which fish prey on them is not entirely clear. Pumpkinseeds (Lepomis gibbosus) are known to be effective predators of mollusks, but bluegills (Lepomis macrochirus) do not generally prey heavily on mollusks. Analysis of stomach contents of pumpkinseeds and bluegills collected from Presque Isle Bay of Lake Erie (Erie, Pennsylvania) revealed considerable differences in the diets of the two species. Specifically, dreissenids were the most important food item in the diet of pumpkinseeds and composed 63% of the volume of their diet. In contrast, dreissenids were relatively unimportant in the diet of bluegills and composed only 2.3% of the volume of their diet. Although bluegills and pumpkinseeds differed considerably in their consumption of dreissenids and other prey items, no clear differences in PCB concentrations were detected between the two species. Thus bluegills must acquire most of their PCBs from prey other than dreissenids.     

Lake Champlain offshore benthic invertebrate community before and after zebra mussel invasion

Benthic invertebrates are important bio-indicators of water quality and play a significant role in aquatic systems. Lake Champlain has limited benthic invertebrate data which hinders development of food web models, assessment of invasive species impacts, and evaluation of management actions. In June 2016, we assessed benthic invertebrates along three transects in the main basin of Lake Champlain ranging from 5 to 100?m, and then compared results to densities from a limited survey in 1991 prior to the zebra mussel (Dreissena polymorpha) invasion. In 2016, total biomass and density were 1–2 orders of magnitude greater at 5?m than at 20–100?m. Zebra mussels, chironomids, oligochaetes, and gastropods were dominant at 5?m, and oligochaetes and sphaeriids were dominant at 20–100?m. Total density at the 5-m site was 94% lower in 2016 compared to 1991, but similar at the 100-m site. Diporeia, while abundant in many freshwater bodies, is historically rare in Lake Champlain and was not detected in our sampling. Because Lake Champlain benthic invertebrate densities are low and display dissimilar distributions to the Great Lakes, we hypothesize the offshore fish community is likely much more reliant on pelagic rather than benthic production. Although the current composition and biomass suggest the benthic community in Lake Champlain may not be greatly impacted by an invasion of quagga mussel (D. rostriformis bugensis), the potential for quaggas to re-route energy from pelagic to benthic habitats, as it has in the Great Lakes, could limit the Lake Champlain offshore fish community.     

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.     

Quantifying a shift in benthic dominance from zebra (Dreissena polymorpha) to quagga (Dreissena rostriformis bugensis) mussels in a large,inland lake

Dreissenid mussels are aggressive invasive species that are continuing to spread across North America and co-occur in the same waterbodies with increasing frequency, yet the outcome and implications of this competition are poorly resolved. In 2009 and 2015, detailed (700 + sample sites) surveys were undertaken to assess the impacts of invasive dreissenid mussels in Lake Simcoe (Ontario, Canada). In 2009, zebra mussels were dominant, accounting for 84.3% of invasive mussel biomass recorded. In 2015, quagga mussels dominated (88.5% of invasive mussel biomass) and had expanded into profundal (> 20 m water depth) sites and onto soft (mud/silt) substrates with a mean profundal density of 887 mussels/m² (2015) compared to ~ 39 mussels/m² in 2009. Based on our annual benthos monitoring, at a subset of ~ 30 sites, this shift from zebra to quagga mussels occurred ~ 2010 and is likely related to a population decline of zebra mussels in waterbodies where both species are present, as recorded elsewhere in the Great Lakes Region. As the initial invasion of dreissenid mussels caused widespread ecological changes in Lake Simcoe, we are currently investigating the effects this change in species dominance, and their expansion into the profundal zone, will have on the lake; and our environmental management strategies. Areas of future study will include: changes in the composition of benthos, fish, or phytoplankton communities; increased water clarity and reduction of the spring phytoplankton bloom; energy/nutrient cycling; and fouling of anthropogenic in-lake infrastructures (e.g. water treatment intakes) built at depths > 25 m to avoid previous zebra mussel colonization.