Impacts of predation by the Eurasian round goby (Neogobius melanostomus) on molluscs in the upper St. Lawrence River

An invasive Eurasian fish, the round goby Neogobius melanostomus, has recently spread from the Great Lakes into the St. Lawrence River. We quantified prey preferences of this benthivore and determined whether its predatory impacts on molluscs in the river are similar to those in the Great Lakes. We measured the size structure of gastropods and dreissenid mussels at 13 St. Lawrence River sites where round goby densities ranged from 0 to 6 m^− 2. For four of these sites, data were available for multiple years before and after invasion. Contrary to studies in the Great Lakes, there were no consistent effects of round goby density on the size structure of dreissenids, although there was an ontogenetic diet shift toward dreissenids. However, the abundance and richness of small gastropods (≤ 14 mm) was negatively correlated with round goby density across all sites, and declined over time at three of four sites sampled before and after invasion. Median gastropod size also declined across sites with increasing round goby density. Gastropods (as well as chironomid larvae, caddisfly larvae, and ostracods) were consistently among the most preferred prey items consumed by gobies, whereas dreissenids (as well as leeches and freshwater mites) were consistently avoided. These results indicate the major role of the round goby in structuring gastropod populations in the St. Lawrence River, and highlight large-scale spatial variation in its predatory impact on dreissenid populations.     

Comparison of thiaminase activity in fish using the radiometric and 4-nitrothiophenol colorimetric methods

Thiaminase induced thiamine deficiency occurs in fish, humans, livestock and wild animals. A non-radioactive thiaminase assay was described in 2007, but a direct comparison with the radioactive ¹⁴C-thiamine method which has been in use for more than 30 years has not been reported. The objective was to measure thiaminase activity in forage fish (alewife Alosa pseudoharengus, rainbow smelt Osmerus mordax, and slimy sculpin Cottus cognatus) consumed by predators that manifest thiamine deficiency using both methods. Modifications were made to the colorimetric assay to improve repeatability. Modification included a change in assay pH, enhanced sample clean-up, constant assay temperature (37 °C), increase in the concentration of 4-nitrothiophenol (4NTP) and use of a spectrophotometer fitted with a 0.2 cm cell. A strong relationship between the two assays was found for 51 alewife (R² = 0.85), 36 smelt (R² = 0.87) and 20 sculpin (R² = 0.82). Thiaminase activity in the colorimetric assay was about 1000 times higher than activity measured by the radioactive method. Application of the assay to fish species from which no thiaminase activity has previously been reported resulted in no 4NTP thiaminase activity being found in bloater Coregonus hoyi, lake trout Salvelinus namaycusch, steelhead trout Oncorhynchus mykiss or Chinook salmon Oncorhynchus tshawytscha. In species previously reported to contain thiaminase, 4NTP thiaminase activity was measured in bacteria Paenibacillus thiaminolyticus, gizzard shad Dorosoma cepedianum, bracken fern Pteridium aquilinum, quagga mussel Dreissena bugensis and zebra mussels D. polymorpha.     

Thiamine concentrations in lake whitefish eggs from the upper Great Lakes are related to maternal diet

Thiamine deficiency is responsible for reproductive impairment in several species of salmonines in the Great lakes, and is thought to be caused by the consumption of prey containing thiaminase, a thiamine-degrading enzyme. Because thiaminase levels are extremely high in dreissenid mussels, fish that prey on them may be susceptible to thiamine deficiency. We determined thiamine concentrations in lake whitefish Coregonus clupeaformis eggs from the upper Laurentian Great Lakes to assess the potential for thiamine deficiency and to determine if thiamine concentrations in lake whitefish eggs were related to maternal diet. Mean thiamine concentrations in lake whitefish eggs were highest in Lake Huron, intermediate in Lake Superior, and lowest in Lake Michigan. Some fish had thiamine concentrations below putative thresholds for lethal and sublethal effects in salmonines, suggesting that some larval lake whitefish may currently be at risk of at least sublethal effects of low thiamine concentrations, although thiamine thresholds are unknown for lake whitefish. Egg thiamine concentrations in lake whitefish eggs were statistically significantly related to isotopic carbon signatures, suggesting that egg thiamine levels were related to maternal diet, but low egg thiamine concentrations did not appear to be associated with a diet of dreissenids. Egg thiamine concentrations were not statistically significantly related to multifunction oxidase induction, suggesting that lower egg thiamine concentrations in lake whitefish were not related to contaminant exposure.     

Survey of four essential nutrients and thiaminase activity in five Lake Ontario prey fish species

Thiamine deficiency is an impediment to salmonine reproduction in the Great Lakes, but little is known about other measures of dietary quality, such as lipid-soluble vitamins or fatty acids in prey fish. The objective of the present research was to measure selected essential nutrients and thiaminase activity in five Lake Ontario prey fish species (alewife Alosa psuedoharengus, rainbow smelt Osmerus mordax, slimy sculpin Cottus cognatus, threespine stickleback Gasterosteus aculeatus and round goby Neogobius melanostomus). Total thiamine was greater in alewife (13.6 nmol/g) than in the other species (6.2–9.0 nmol/g). In 2006, thiaminase activity was unexpectedly high in goby (12.49 nmol/g/min), sculpin (1.99 nmol/g/min) and smelt (9.24 nmol/g/min). In 2007, thiaminase activity in goby (0.99 nmol/g/min) and smelt (4.94 nmol/g/min) was low compared to 2006, whereas sculpin thiaminase activity was greatest (6.01 nmol/g/min). The causes for this variability are unknown. Thiaminase activity was within the expected range for alewife (4.31–6.31 nmol/g/min) and stickleback (0.06 nmol/g/min). Concentrations of retinoids, carotenoids, vitamin E (tocopherol) and fatty acids also differed among prey fish species. Tocopherol concentrations in goby (12.74 ng/mg), sculpin (25.29 ng/mg), and smelt (22.81 ng/mg) were greater than in alewife (1.59 ng/mg). Goby had the lowest ∑ ω-3 to ∑ ω-6 fatty acid ratio (1.44) when compared to sculpin (2.97) and smelt (2.85). Thiaminase concentrations in alewife and smelt (and possibly goby) suggest that they have the potential to adversely affect natural reproduction in salmonines. Concentrations of carotenoids, retinoids and tocopherol in prey fish appear to be lower than salmonine dietary requirements.     

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.     

The effects of dreissenid mussels on the survival and condition of burrowing mayflies (Hexagenia spp.) in western Lake Erie

Burrowing mayflies (Hexagenia limbata and H. rigida) are once again prominent members of the benthic community in western Lake Erie. However, this community is now dominated by dreissenid mussels. We conducted a laboratory experiment and field sampling to investigate whether survival and condition of Hexagenia were affected by the presence, density, and quality of dreissenid mussels. In a laboratory experiment, Hexagenia survival was higher in microcosms without dreissenid mussels. We also found Hexagenia density to be higher at field sites with low dreissenid density, suggesting that Hexagenia survival is higher in these areas as well. In microcosm treatments with low dreissenid density, Hexagenia survival was higher in treatments with live dreissenids than in treatments containing only dreissenid shells. These findings suggest that while dreissenid shells degrade the quality of soft sediments for Hexagenia, some of the negative effect is offset by the presence of live dreissenids. The positive effect of live dreissenids is likely due to additional food resources made available to Hexagenia by the filtering activity of dreissenids. Neither dreissenid density nor shell “type” (shells alone or live dreissenids in shells) had an effect on Hexagenia condition. Thus, the interactions between these dominant benthic invertebrates are complex. Recovery of Hexagenia populations in western Lake Erie is likely affected by both changing environmental conditions due to anthropogenic activities and the introduction of exotic species into the benthic community. The results are likely to be continued instability of the benthic food web and unpredictable consequences for human utilization of this ecosystem.     

Thiamine concentrations in lake trout and Atlantic salmon eggs during 14 years following the invasion of alewife in Lake Champlain

Thiamine (vitamin B₁) deficiency in Great Lakes salmonines has been linked to consumption of alewife Alosa pseudoharengus. Thiamine deficiency has been recognized as a possible impediment to lake trout Salvelinus namaycush recruitment in the Great Lakes and Atlantic salmon Salmo salar recruitment in the Finger Lakes and Baltic Sea. Alewife invaded Lake Champlain in 2003 which provided an opportunity to investigate changes in thiamine concentrations in salmonine predators during an alewife invasion. We monitored egg unphosphorylated and total thiamine concentrations in lake trout and Atlantic salmon in 2004 and 2007–2019, assessed whether concentrations were associated with mortality, and examined thiaminase activity in alewife. Total thiamine concentrations in lake trout and Atlantic salmon were significantly lower than in 2004 for seven of the ten collection years for lake trout and for nine of the 12 collection years for Atlantic salmon. Mortality and signs of thiamine deficiency were observed in laboratory-reared Atlantic salmon free embryos but not in lake trout. Average thiaminase activity in adult alewife declined from 5200 pmol/g/min in 2006 to 1500 pmol/g/min in 2012. Our results provide further evidence that a diet that includes alewife reduces egg thiamine concentrations in salmonines. This effect was observed within four years of the invasion of alewife.     

Assessment of remnant unionid assemblages in a selection of Great Lakes coastal wetlands

Populations of native unionids have been in steady decline over the past century. The invasion of dreissenid mussels (Dreissena polymorpha and Dreissena rostriformis bugensis) in the mid-1980's impacted already imperiled unionid populations by greatly increasing their regional extinction rates. A selection of Great Lakes coastal wetlands around Michigan was surveyed to locate remnant populations of native unionids. Physical and chemical parameters were measured in coastal wetlands to evaluate the importance of these habitat parameters to remnant unionid assemblages. We assessed fouling rates by dreissenids on unionids and used artificial substrates to estimate dreissenid colonization densities. Live unionids were found in coastal wetlands of the Les Cheneaux Islands, the Lake St. Clair delta, and North Maumee Bay with significantly higher unionid fouling in the Les Cheneaux Islands compared to the other two sampling areas (F_2,76 = 4.97, p = 0.0095). No live unionids were documented in Beaver Island, Garden Island, Grand Traverse Bay, or Saginaw Bay wetlands. Dreissena colonization densities on artificial substrates averaged 19,213 m^− 2 at one site in North Maumee Bay, and 10,425 m^− 2 in Saginaw Bay, but no colonization occurred in the wetlands of Beaver Island, Garden Island, the Les Cheneaux Islands, or Grand Traverse Bay while Dreissena presence in the open water of these regions was evident. Dreissena colonization densities on artificial substrates increased with measures of anthropogenic disturbance and decreased with higher water level fluctuations and aerial exposure. Specific conductance, turbidity, and magnitude of water level fluctuations were important predictors of Dreissena colonization on artificial substrates.     

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.     

Freshwater mussel community response to warm water discharge in western Lake Erie

Native unionid mussels are endangered in the Laurentian Great Lakes due to habitat degradation and biofouling by invasive dreissenids. However, a robust community was discovered living within the thermal discharge of a power plant at Oregon, Ohio, on the south shore of Lake Erie. Our study compared this community to nearby communities outside the thermal plume, and examined habitat characteristics that may affect unionids. Unionids were sampled from the exposed lake bed at three sites during a seiche in 2011: (1) within the thermal plume, (2) at Bayshore Park (2.0 km east of the plant), and (3) at the University of Toledo's Lake Erie Center (4.0 km east). In 2010, sediment samples were collected along a 2 km transect extending east from the plant discharge roughly parallel to the south shore of Lake Erie. Results indicated that the community within the thermal plume had higher densities, higher diversity (H′), more small individuals but overall larger sizes than communities outside the plume. Both the rate and intensity of fouling by dreissenids were lower within the plume. Dry mass of coarse surface sediment and sediment organic matter content were negatively related to distance from the plant (R² = 0.497, and 0.479, respectively). An unexpected discovery was that the bulk of the coarse sediment was comprised of shell material from Asian clams and dreissenid mussels, suggesting a contribution of these exotic species to sediment accumulation. In total, our results suggest that several habitat characteristics close to the power plant are favorable to unionids.     

Diet Choice by the Exotic Round Goby (Neogobius melanostomus) as Influenced by Prey Motility and Environmental Complexity

This laboratory study examined the influence of substratum complexity and water clarity/visibility on non-indigenous round goby (Neogobius melanostomus) diet choice between dreissenid mussels (Dreissena polymorpha and D. bugensis, 6 to 9 mm length) and the exotic amphipod Echinogammarus ischnus. When both prey items were offered simultaneously in bare 20-L aquaria holding clear ambient water, 6.5 to 8-cm round gobies chose primarily amphipods (> 85% of diet numerically) and consumed fewer dreissenids (< 2/h) than when mussels were offered alone (5.2/h). Round gobies could ingest substantially more biomass when feeding on a mixed diet (∼17 to 24 mg/h dry weight, not including dreissenid shells) or on amphipods alone (∼26 mg/h), than feeding on dreissenids alone (∼12 mg/h). Longer handling time of mussels may thus have influenced the round gobies’ preference for amphipods. Added substrata (stones or gravel) and/or diminished visibility (turbid water or darkness) shifted round goby diet markedly towards sessile dreissenids as motile amphipods found refuge. Two-way ANOVA indicated that both substratum and water clarity/visibility significantly influenced round goby diet, but did not interact. It is possible that the large contribution of dreissenids to round goby diet in the Great Lakes may not necessarily reflect a preference for them, but rather lower encounter rates with more profitable prey.     

Size-selective predation by round gobies (Neogobius melanostomus) on dreissenid mussels in the field

Although numerous studies have shown that round gobies (Neogobius melanostomus) prey on dreissenid mussels (Dreissena polymorpha and Dreissena bugensis), there is an apparent shortage of detailed field studies on the subject. The 5-month field study described here quantifies predation by round gobies on dreissenids in Presque Isle Bay, Lake Erie. Dreissenids dominated the diet of round gobies, composing 92% of the prey items recovered. Over half of the 3870 valves (1935 mussels) recovered from 155 round gobies were crushed, while the remainder were swallowed whole. Crushed dreissenids were larger than those swallowed whole, and the tendency to crush dreissenids did not vary among three length classes of round goby. Round goby length was positively related to average size of dreissenids consumed, average size of whole and crushed dreissenids, largest whole dreissenid consumed, and largest crushed dreissenid consumed. Indices of selectivity revealed similarly shaped curves for three length classes of round gobies, a shift toward larger dreissenid size classes with an increase in round goby length, and peak preferences for 8-11 mm dreissenids. Factors such as gape limitation, availability and accessibility of differently sized dreissenids, forces generated while removing mussels from the substrate and crushing them, and caloric content of dreissenids all likely play roles in the observed size-selectivity and differential processing of dreissenids. Although factors influencing size-selectivity are not completely understood, the observed preference of round gobies for dreissenids near the size when they are first reproductive could impact the demography of dreissenid populations.     

Lakewide dominance does not predict the potential for spread of dreissenids

In recent years, quagga mussels (Dreissena rostriformis bugensis) have almost completely replaced zebra mussels (Dreissena polymorpha) in the Lower Great Lakes. As recreational boats are the main vector of spread for dreissenids in North America, this study examined whether lakes Erie and Ontario could still be sources for the spread of zebra mussels. In the summer–fall of 2010, the abundance of each species of Dreissena on 196 boats from 5 marinas in lakes Erie and Ontario was examined. Additional samples of Dreissena in 2010–2012 were collected in tributaries, bays, and in the upper littoral zones of these lakes. A total of 77 boats were fouled by Dreissena, and of those 61 were fouled by both species, 13 were fouled just by zebra mussels, and only 3 were fouled solely by quagga mussels. Although quagga mussels compose ~ 99% of dreissenids in eastern Lake Erie and in Lake Ontario, on boats at most marinas sampled, zebra mussels were usually more abundant and significantly larger than quagga mussels. Refugia for zebra mussels were found in bays, tributaries, and upper littoral zones with high wave activity. Thus, although quagga mussels are now more abundant than zebra mussels within the Lower Great Lakes, these waterbodies still have the potential to be a source for the spread of zebra mussels, and for some vectors, the propagule pressure from zebra mussels is likely greater than that from quagga mussels.     

Seasonal abundance,community composition,and silica content of diatoms epiphytic on Cladophora glomerata

The establishment of dreissenid mussels in the Great Lakes has been implicated in causing a resurgence of benthic macroalgae, notably Cladophora glomerata (L.) Kützing. The seasonal proliferation and nutrient retention of epiphyton on Cladophora may be important to littoral zone processes. In this study, peak diatom epiphyte density (601 diatoms μg^− 1 dry mass) co-occurred with peak Cladophora biomass. The exceptionally adhesive Cocconeis pediculus came to dominate the epiphyton to the near exclusion of all other diatoms by early summer, indicating powerful constraints on the epiphyte assemblage. There was a strong relationship between epiphytic silica content and epiphyte abundance, but the relationship was different between seasons. In the spring and fall when Cladophora was growing vigorously, silica content of the diatom epiphytes was 20.1–25.6 pmol Si diatom^− 1. In the summer, during Cladophora senescence, silica content was 6.5–10.0 pmol Si diatom^− 1 and valves were visibly thinner. These observations suggest that diatom epiphytes may be limited by Si during peak Cladophora biomass. Areal estimates of silica content of Cladophora epiphyton was strongly related with Cladophora biomass: at 2 m depth, silica content increased from 22.7 mmol Si m^− 2 in the spring to 490 mmol Si m^− 2 during peak Cladophora biomass. Silica content can be a valuable proxy for diatom epiphyte abundance if the vitality of the Cladophora substrate is considered. Future work needs to assess the seasonality of pelagic versus benthic silica demand and the interannual variability of epiphyte silica content to assess how changes in Cladophora biomass may affect nearshore Si cycling.     

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.     

Bio-optical properties and primary production of Lake Michigan: Insights from 13-years of SeaWiFS imagery

Thirteen years of SeaWiFS data (1998–2010) from the early spring isothermal period (March–April) were used to determine trends of water attenuation coefficient (K_dPAR), chlorophyll a (Chl a), Photosynthetic Available Radiation (PAR), and modeled primary production in southern Lake Michigan. Surface PAR values remained unchanged between 1998 and 2010, but there was an 18–22% drop in K_dPAR during the March/April isothermal period as water clarity increased. This transparency increase was accompanied by a 41–53% decline in Chl a concentration (μg · L^− 1) and a 42–46% decline in modeled primary production (Great Lakes Primary Production Model). These changes were most pronounced in 2001–2003 which coincided with the period of initial colonization of the quagga mussels. Statistically significant spatial differences were noted in Chl a (μg · L^− 1) concentrations between mid-depth (z = 30–90 m deep), and offshore (z > 90 m deep) waters. Chl a concentrations in the mid-depth region (30–90 m) decreased at a higher rate compared to offshore waters (> 90 m) likely as a result of filtration activities of quagga mussel.     

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.     

Non-native Dreissena associated with increased native benthic community abundance with greater lake depth

Although the typical interaction between non-native invasive species and native species is considered to be negative, in some cases, non-native species may facilitate native species. Zebra and quagga mussels (Dreissena spp.) are aggressive invaders in freshwater systems, and they can alter energy flow by diverting nutrients from pelagic to benthic food-webs. In the last two decades, quagga mussels have largely replaced zebra mussels in shallow regions of the Laurentian Great Lakes and colonized deeper waters previously devoid of all dreissenids. Here, we aim to characterize potential positive effects of dreissenids in relation to depth on the benthic community in lakes Michigan and Huron. For this study, we used benthic survey data collected from Lake Michigan in 2015 and Lake Huron in 2017 and annual U.S. EPA Great Lakes National Program Office Long-term Biology Monitoring Program data for both lakes from 1998 to 2019. Benthic species richness and abundance (excluding dreissenids) in both lakes were almost three-fold higher in the nearshore (<70 m) compared to offshore (>70 m) communities. We found that, even though abundance of benthic invertebrates decreased with increased depth, total benthos density and biomass were higher in the presence than in the absence of quagga mussels in both lakes. Moreover, increased quagga mussel density and biomass with depth offset the lower benthos density and biomass at deeper depths, and samples with dreissenids had high densities of oligochaetes in both nearshore and offshore communities. These patterns are consistent with facilitative effects of quagga mussels on both shallow and deep-water benthic communities.     

Diet,trophic position of upper St. Lawrence River round goby giants reveals greater dependence on dreissenids with increasing body size

Round gobies (Neogobius melanostomus) from the upper St. Lawrence River (USLR) have an abundance of some of the largest individuals recorded from the Great Lakes (>230-mm total-length). We found a distinct separation in diet and isotopic signatures (δ¹⁵N and δ¹³C) between round goby classified as small (≤130-mm; n = 63) and large (>130-mm total-length; n = 75) from USLR coastal bays. At small sizes, round gobies had variable diets indicative of generalist and opportunistic feeding on native and non-indigenous benthic prey. Between 100 and 130-mm total-length, signatures of assimilated carbon (δ¹³C) indicated a directed shift towards a dreissenid-centric diet and once larger than 130-mm total-length, round gobies appeared to feed proportionally more on dreissenid mussels. We also found that large round gobies fed proportionally more on Hydrobiidae than small round gobies. A weak negative relationship between δ¹⁵N (indicative of trophic position) and round gobies total length resulted where smaller round gobies had slightly higher δ¹⁵N values than larger conspecifics. Round gobies larger than 180-mm total-length were common in nearshore habitats (≤2-m) during the spring, and dreissenid mussels and Hydrobiidae were the most frequently encountered prey. Our results demonstrate elevated reliance on dreissenids as round goby increased body size, but the diversity of prey suggest more complex trophodynamic pathways associated with coastal bay habitats.