Characteristics of a refuge for native freshwater mussels (Bivalvia: Unionidae) in Lake St. Clair

The Lake St. Clair delta (∼ 100 km²) provides an important refuge for native freshwater mussels (Unionidae) wherein 22 of the ∼ 35 historical species co-occur with invasive dreissenids. A total of 1875 live unionids representing 22 species were found during snorkeling surveys of 32 shallow (∼ 1 m) sites throughout the delta. Richness and density of unionids and zebra mussel infestation rates varied among sites from 3 to 13 unionid species, 0.02 to 0.12 unionids/m², and < 1 to 35 zebra mussels/unionid, respectively. Zebra mussel infestation of unionids in the delta appears to be mitigated by dominant offshore currents, which limit densities of zebra mussel veligers in nearshore compared to offshore waters (13,600 vs. 28,000/m³, respectively). Glycogen concentrations in the tissues of a common and widespread species in the delta (Lampsilis siliquoidea) suggest that zebra mussels may be adversely affecting physiological condition of unionids in a portion of the Lake St. Clair delta. Physiological condition and community structure of unionids within the delta may also be influenced by differences in food quantity and quality resulting from the uneven distribution of water flowing from the St. Clair River. The delta likely supports the largest living unionid community in the lower Great Lakes and includes several species that have been listed as Endangered or Threatened in Canada and/or the state of Michigan, making it an important refuge for the conservation of native unionids.     

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.     

Changes in the Dreissenid Community in the Lower Great Lakes with Emphasis on Southern Lake Ontario

A field study was conducted in the lower Great Lakes to assess changes in spatial distribution and population structure of dreissenid mussel populations. More specifically, the westward range expansion of quagga mussel into western Lake Erie and toward Lake Huron was investigated and the shell size, density, and biomass of zebra and quagga mussel with depth in southern Lake Ontario in 1992 and 1995 were compared. In Lake Erie, quagga mussel dominated the dreissenid community in the eastern basin and zebra mussel dominated in the western basin. In southern Lake Ontario, an east to west gradient was observed with the quagga mussel dominant at western sites and zebra mussel dominant at eastern locations. Mean shell size of quagga mussel was generally larger than that of zebra mussel except in western Lake Erie and one site in eastern Lake Erie. Although mean shell size and our index of numbers and biomass of both dreissenid species increased sharply in southern Lake Ontario between 1992 and 1995, the increase in density and biomass was much greater for quagga mussels over the 3-year period. In 1995, zebra mussels were most abundant at 15 to 25 m whereas the highest numbers and biomass of quagga mussel were at 35 to 45 m. The quagga mussel is now the most abundant dreissenid in areas of southern Lake Ontario where the zebra mussel was once the most abundant dreisenid; this trend parallels that observed for dreissenid populations in the Dneiper River basin in the Ukraine.     

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.     

Changes in the Freshwater Mussel Community of Lake St. Clair: from Unionidae to Dreissena polymorpha in Eight Years

To determine density changes in both the zebra mussel, Dreissena polymorpha, and native mussels, Unionidae, in Lake St. Clair, surveys were conducted in 1990, 1992, and 1994 and compared to a similar survey in 1986 when no D. polymorpha was found. Collection methods were the same each year; divers used the quadrat method to collect 10 replicate samples at 29 sites located throughout the lake. The total number of unionids collected declined from 281 in 1986, to 248 in 1990, 99 in 1992, and 6 in 1994, while the number of species collected in each of the four respective years was 18, 17, 12, and 5. The decline in the unionid community occurred gradually over this time period as the D. polymorpha population expanded from the southeast region of the lake to the northwest region. Mean density and biomass of D. polymorpha throughout the lake was 1,700 m⁻² and 4.7 gDW m⁻² in 1990, 1,500 m⁻² and 3.5 gDW m⁻²in 1992, and 3,200 m⁻² and 3.1 gDW m⁻² in 1994. The density increase can be attributed to the expansion of the population into the northwest region, while the decrease in biomass was mostly a result of a decline in the weight per unit length. Mean biomass of the D. polymorpha population in 1994 was actually lower than the mean biomass of unionids in 1986; however, based on literature-derived filtering rates, the filtering capacity of the D. polymorpha population in 1994 was 12 times greater than the filtering capacity of the unionid community in 1986. This increase has likely led to reported changes in the Lake St. Clair ecosystem (increased water clarity, increased plant growth, and shifts in fish communities).     

The Zebra Mussel Dreissena polymorpha Found in North America in 1986 and 1987

The zebra mussel Dreissena polymorpha was first detected in the western basin of Lake Erie, Ontario, Canada, on natural gas wellheads and well markers between April and November 1986. It was found again in 1987 on the north shore of Lake Erie in a water treatment plant, and in vessel fouling. The population increased in Lake Erie in 1988. Dreissena may have spread from Lake Erie to Lake St. Clair, where it was then discovered on 1 June 1988.     

Twenty five years of changes in Dreissena spp. populations in Lake Erie

Lake Erie has the longest history of colonization by both Dreissena polymorpha and Dreissena rostriformis bugensis in North America and is therefore optimal for the study of long-term dynamics of dreissenid species. In addition, the morphometry of Lake Erie basins varies dramatically from the shallow western to the deep eastern basin, making this waterbody a convenient model to investigate patterns of Dreissena distribution, as well as interspecies interactions among dreissenids. We compare our data on the distribution, density and wet biomass of both dreissenid species in Lake Erie collected in 2009 and 2011–2012 with previous data. We found that Dreissena spp. distribution in Lake Erie varied depending on the time since the initial invasion, collection depth, and lake basin. In 2009–2012, zebra mussels were smaller than in 1992 and were consistently smaller than quagga mussels. During 2009–2012, quagga mussels were found at all depths and in all basins, while zebra mussels were common in the western basin only, and in the central and eastern basins were limited to shallow depths, resulting in an almost complete replacement of D. polymorpha with D. rostriformis bugensis. In the shallowest western basin of Lake Erie, zebra mussels represented > 30% of the combined dreissenid density even after more than 20 years of coexistence, providing strong evidence that, even in lakes as large as Lake Erie (or at least its western basin), D. polymorpha may sustain a significant presence for decades without being displaced by quagga mussels.     

Sensitivity to aerial exposure: potential of system-wide drawdowns to manage zebra mussels in the Mississippi River

We report the results of five experiments performed during periods of lowered river water level in Pool 26 of the Mississippi River. Four experiments compared survivorship of zebra mussels (Dreissena polymorpha) and unionid mussels (Unionidae). Under mild spring conditions (March), survivorship of zebra mussels was similar to that of unionid mussels, but during summer (July) survivorship of zebra mussels was lower than that of unionid mussels. Survivorship of zebra mussels was greatest when attached to native unionids, compared with detached zebra mussels and zebra mussels attached rocks. A fifth experiment compared survivorship of 10 species of unionid mussels after 24 hours of aerial exposure. In general, survivorship of thick-shelled species, such as Amblema plicata, was greater than survivorship of thin-shelled species, such as Potamilus ohiensis. The experiments conducted suggest that drawdowns during warm summer conditions could have a profound, negative influence on zebra mussel demography and distribution. In contrast, unionid mussel survival was unaffected by aerial exposure of up to 24 hours during a midsummer drawdown. © 1997 John Wiley & Sons, Ltd.     

Recovery of Burrowing Mayflies (Ephemeroptera: Ephemeridae: Hexagenia) in Western Lake Erie

Burrowing mayflies (Hexagenia spp.) are native to western Lake Erie and were abundant until the 1950s, when they disappeared due to degraded water and sediment quality. Nymphs were absent from the sediments of most of western Lake Erie after the 1950s, although small, widely disjunct populations apparently persisted near shore. Sediment samples collected in 1993 revealed several small populations near the western and southern shores and beyond the mouths of the Detroit and Maumee rivers. A larger population was found in the southern island area, but nymphs were absent in the middle of the basin. By 1995, nymphs had spread throughout the western half and eastern end of the basin but remained absent from the middle of the basin. These data indicate that Hexagenia began recolonizing nearshore areas before offshore areas. Increasingly large swarms of winged Hexagenia on shore and over the lake between 1992 and 1994 further indicate that mayflies are recolonizing the basin. Factors that have permitted Hexagenia recovery in western Lake Erie probably include improved sediment and water quality attributed to pollution abatement programs implemented after the early 1970s, and perhaps environmental changes in the early 1990s attributed to effects of the exotic zebra mussel (Dreissena polymorpha)     

Sediment Mixing by Lampsilis Radiata Siliquoidea (Mollusca) from Western Lake Erie

The sediment reworking activities of an abundant Lake Erie unionid bivalve, Lampsilis radiata siliquoidea, have been studied by field observations and laboratory experiments. Unionid burrowing in laboratory microcosms increased sediment water content 10-20%, decreased water content variability, homogenized sedimentary structures, and increased tenfold the volume of oxidized sediment over that in microcosms with no unionids. Incomplete mixing of sediment took place to a depth of 10 cm during burrowing by L. r. siliquoidea. Burrowing by other unionids may extend the depth of maximum reworking to 20 cm. Unionid burrowing, feeding, and respiratory activities may alter the profiles of various elements and radionuclides associated with sediment particles and alter the location and intensity of microbial activity in sediments.     

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.     

Decadal changes in the benthic invertebrate community in western Lake Erie between 1981 and 2004

Faunal surveys of western Lake Erie showed a dramatic change in the species composition and abundance of the invertebrate mud-bottom community. Abundances of formerly dominant ostracods, tubificid and naidid oligochaetes, and chironomids declined 85%, 87%, 80%, and 72%, respectively, between 1982 and 2003. The majority of the declines occurred between 1982 and 1993, when dreissenid abundances were increasing rapidly. Hexagenia nymphs became important members of the infaunal community after 1993. Faunal declines were uncorrelated to weather-related variables. The frequency and magnitude of thermal stratification with accompanying hypoxic conditions during the study period remained within the normal range of variation observed in western Lake Erie. Significant wind events and storm driven seiches that might cause benthos mortality were not correlated with the decadal patterns of faunal decline. Nor were the faunal declines associated with the historical decrease in organic loading, because most of the decrease occurred prior to the study period. Nearly all abundant species declined significantly between 1982 and 1993, but deposit-feeding fauna, eutrophic indicator species, small organisms, and nearshore mud-bottom stations adjacent to dreissenid mussel habitat were most severely affected. These patterns suggest that a systemic change in the trophic structure of western Lake Erie occurred, due mostly to the invasion of dreissenid mussels nearshore and an accompanying diversion of organic matter away from deeper offshore muds. Trophic group amensalism may operate differently in marine and freshwater habitats, although dreissenids may exert ecosystem effects in the Great Lakes similar to oysters in Chesapeake Bay.     

Eastern banded killifish (Fundulus diaphanus diaphanus) in Lake Michigan and connected watersheds: The invasion of a non-native subspecies

Biological invasions can produce severe ecological impacts at both large spatial scales between distantly related species and also smaller spatial scales between conspecifics. We investigated here a potential intraspecific invasion within Lake Michigan and adjacent waters. Banded killifish (Fundulus diaphanus) experienced population increases and spread into novel habitats over recent decades in this region, but managers and policy makers were uncertain if these fish were native western banded killifish (F. d. menona) or instead an invasion by non-native eastern banded killifish (F. d. diaphanus). We applied mitochondrial DNA (mtDNA) barcoding and population genetic analyses to investigate the identity of these fish. We found that new banded killifish populations were the eastern subspecies, including mtDNA haplotypes from the remote mid-Atlantic region, suggesting some introductions from anthropogenic pathways (e.g., live bait trade) rather than only spread from connected, downstream waters (e.g., Lake Erie). Further, population genetic analyses identified eastern banded killifish in our focal region as having low genetic diversity relative to their native range, and relative to western banded killifish populations. Lastly, we found non-native eastern banded killifish were associated with Lake Michigan and connected waters, rather than isolated kettle lakes, and primarily occurred at clearer sites potentially affected by dreissenid mussel invasions. More research is needed on the causes and consequences of eastern banded killifish invasions in Lake Michigan and adjacent waters, including an emphasis on their risk to the conservation of western banded killifish.     

Phytoplankton Composition and Biomass in the Offshore Waters of Lake Erie: Pre- and Post-Dreissena Introduction (1983–1993)

Phytoplankton was collected in all basins of Lake Erie during 42 cruises during the spring and summer from 1983 to 1993—a period that spans the Dreissena mussel invasion. Two potential impacts of Dreissena on the phytoplankton community of the western, central, and eastern basins of Lake Erie were evaluated: Was selective feeding occurring as observed in Saginaw Bay and were reductions in biomass evident in the offshore regions of the three basins of Lake Erie? In the western basin, significant summer decreases in Chlorophyta, Bacillariophyta, Cyanobacteria, and total phytoplankton biomass were observed after Dreissena introduction. Similarly in the spring, Bacillariophyta and total phytoplankton biomass and chlorophyll a concentrations decreased significantly. Since several divisions of phytoplankton did not decrease in phytoplankton biomass in the western basin, and spring Cyanobacteria biomass increased significantly while other divisions decreased in biomass, selective feeding on the phytoplankton community was suggested. Where significant reductions in biomass were observed in the offshore waters of the western basin, they were approximately 50% of the reduction observed at the nearshore sites in Lake Erie by other workers.Dreissena impact on the phytoplankton community of the pelagic waters of the central and eastern basin appeared to be minimal. Pre- and post-Dreissena total phytoplankton biomass and chlorophyll a concentrations were not significantly different or increased significantly after the Dreissena invasion. Biomass of several divisions of phytoplankton significantly increased after Dreissena introduction in the central and eastern basins. These included Bacillariophyta (central basin), Cyanobacteria (central and eastern basin), Chrysophyta (eastern basin), Chlorophyta biomass (eastern basin) and phytoplankton biomass (central basin) and chlorophyll a (central basin) in the spring, and Chrysophyta (eastern basin) and Cryptophyta biomass (central basin) in the summer. Generally, a reduction in phytoplankton biomass would be expected as a result of Dreissena grazing, not an increase in biomass. Dreissena-mediated changes in phytoplankton have generally occurred in shallow, well-mixed lakes, ponds, and embayments, not in deeper waters such as the central and eastern basins of Lake Erie.     

The Invasive New Zealand Mud Snail (Potamopyrgus antipodarum) in Lake Erie

The New Zealand mud snail (Potamopyrgus antipodarum) is an invasive species in Europe, Japan, Australia, and North America. In the western United States it is a species of special concern where population densities in some rivers and streams are very large (∼300,000 per m²) and considerable ecological effects of its presence have been reported. Much less about the effects of this species is known in the Great Lakes, where the snail was found in Lake Ontario and the St. Lawrence River in 1991. Here we report the occurrence of the snail in Lake Erie. Two P. antipodarum were collected in 18 m deep water (sampling range 5–18 m) in Lake Erie off shore of Presque Isle State Park near Erie, Pennsylvania in the summer of 2005 and others were collected off of Sturgeon Point in Lake Erie (sampling range 5–20 m) south of Buffalo, NY and in the central basin of Lake Erie (18 m) in 2006. This finding demonstrates that this species continues to expand its range in the Great Lakes. The range expansion increases the likelihood that it may become established in rivers and streams emptying into the Great Lakes where higher densities and greater ecological damage may result.     

Sediment Contamination in Lake Erie: A 25-Year Retrospective Analysis

Lake Erie sediment surveys were conducted in 1997 and 1998 to characterize spatial and temporal trends in contamination and for comparison with historical levels to assess the degree of improvement in environmental quality since the advent of measures to reduce impacts from sources. These surveys were also designed to assist in identification of possible sources of contamination and areas where contamination exceeded Canadian sediment quality guidelines for protection of aquatic biota. Encouragingly, lakewide contaminant concentrations were found to have significantly decreased from levels observed in samples collected in 1971 in previous Environment Canada surveys. The lakewide average polychlorinated biphenyl (PCB) sediment concentrations decreased from 136 ng/g in 1971 to 43 ng/g in 1997. This decreasing temporal trend was also evidenced by contaminant profiles of core samples from all three major basins. There was a lakewide spatial trend in increasing sediment contamination from the eastern basin to the western basin, and from the north-central basin to the south-central basin. Sediments in many areas of Lake Erie still exceeded Canadian Federal and Provincial sediment quality guidelines. However, exceedences of sediment guidelines describing contaminated environments in 1997/98 were largely restricted to the western basin and the southern portion of the central basin. Exceedences of Canadian Sediment Quality probable effects guidelines were most numerous for dioxins and furans (40%) followed by mercury (6%). The Canadian threshold effects guideline for PCBs (34.1 ng/g) and the Provincial lowest effect guideline (70 ng/g) were exceeded at 52% and 22% of the sites, respectively. Mercury, PCBs and polychlorinated dibenzo-p-dioxins and dibenzofurans are responsible for fish consumption advisories in Lake Erie.     

Heavy metals in sediments and uptake by burrowing mayflies in western Lake Erie basin

During the past two decades, burrowing Hexagenia mayflies have returned to the western basin of Lake Erie. Because of their importance as a prey resource for higher trophic levels and their extensive residence time in potentially contaminated sediment, Hexagenia may be a source of heavy metal transfer. To better understand the distribution and transfer of heavy metals in sediment and mayflies, sediment and mayfly nymphs were collected from 24 locations across the western basin of Lake Erie in May 2007. Following USEPA protocols, samples were analyzed for 16 elements using ICP-OES or ICP-MS. Metal concentrations in the sediments exceeded the Threshold Effect Level for at least one metal at all sample sites. Sediment heavy metal distribution profiles indicate metal concentrations are correlated with organic matter content, and the highest heavy metal concentrations were found in the central deeper region of the western basin where organic content in the sediments was greatest. Hexagenia were distributed throughout the western basin, with greatest density (1350/m²) within the Detroit River plume. The Cd and Zn levels in mayflies were on average approximately 4 and 2 times greater, respectively, than sediment levels, and the Cd concentrations in the sediments exceeded the Threshold Effect Level at 27 of 28 sites and exceeded the Probable Effect Level at 9 of 28 sites. Spatial representation of heavy metal concentrations in mayflies exhibited a similar pattern to the spatial distribution of heavy metals and organic matter in the sediments with higher concentrations of metals found in mayflies residing in the central deeper region of the western basin.     

The Asian cyclopoid copepod Mesocyclops pehpeiensis Hu 1943 reported from the western basin of Lake Erie

The Asian cyclopoid copepod Mesocyclops pehpeiensis Hu, 1943 has been reported as an introduced species at several locations in the western hemisphere. In the United States, reports of this exotic species are restricted to localities in Louisiana, Mississippi, and Washington D.C. This report documents a new record of occurrence for M. pehpeiensis from the western basin of Lake Erie. The detection of M. pehpeiensis in Lake Erie constitutes the first record of this species from the Laurentian Great Lakes, and the northernmost record in the western hemisphere. The species was found in 2016, 2017 and 2018, including females with egg sacs, and can therefore be considered established in the area. The occurrence of M. pehpeiensis in Lake Erie suggests that this Asian copepod may be more widely distributed in North America than is currently understood.     

Contaminants Associated with Suspended Sediments in Lakes Erie and Ontario, 1997–2000

Sediment traps were installed at individual index stations in the western basin of Lake Erie and the Mississauga (central) basin of Lake Ontario, and refurbished seasonally during the period 1997–2000. In Lake Ontario, sediment down flux rates and corresponding contaminant down flux rates were highest in winter and increased with depth due to the influence of resuspended bottom sediments. Sediment down flux rates in western Lake Erie (22 to 160 g m⁻² d⁻¹) were far greater than in Lake Ontario (0.19–3.0 g m⁻² d⁻¹). Suspended material in western Lake Erie was characterized as predominately resuspended bottom sediments; down flux rates were roughly 5- to 10-fold higher in spring and fall, compared to summer. Suspended sediment concentrations of PCBs and other organochlorine contaminants, represented by both annual means and individual seasonal values, were higher in Lake Ontario throughout the duration of the study, compared to Lake Erie. The mean annual concentration of PCBs in suspended sediments over the period 1997–2000 was 330 ng/g in western Lake Erie and 530 ng/g in Lake Ontario. Based on a comparison with historical data from Lake Ontario, mean contaminant concentrations over the period 1997–2000 for PCBs, hexachlorobenzene, and mirex corresponded to decreases of 38%, 74%, and 40%, respectively, since the mid-1980s. Corresponding down flux rates for PCBs, hexachlorobenzene, and mirex decreased by approximately 70%, 90%, and 80%, respectively, since the 1980s.     

Historical Changes in the Distribution of Threatened Channel Darter (Percina copelandi) in Lake Erie with General Observations on the Beach Fish Assemblage

Laurentian Great Lakes beach fish assemblages and the factors influencing their composition have been rarely investigated. In this study, we investigated whether north shore Lake Erie beach fish assemblages, and the distribution of the channel darter (Percina copelandi), a threatened species in Canada, have changed since the late 1940s. Over this time period, Lake Erie has been severely altered by the combined effects of eutrophication, overexploitation of fishery resources, habitat degradation, and invasive species. Seining data from 34 north shore beach sites indicate that a large decline in species richness has occurred, and that several introduced species are present. Three fishes of federal conservation concern and four species of recreational and commercial importance, previously captured from central and eastern Lake Erie basin beaches, were absent. This included the channel darter, which was collected from only one of six historical collection sites, indicating a substantial decline in its Lake Erie distribution. Potential causes of this decline include eutrophication-induced ecosystem changes, the effect of extensive shoreline modification on beaches, and the invasive round goby (Neogobius melanostomus). Nearshore bottom trawls of Long Point Bay indicate that, since the establishment of round goby, concurrent short-term declines in the abundance of two other native benthic fishes (johnny darter Etheostoma nigrum, and logperch P. caprodes) have occurred.