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.     

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.     

Trends in Mysis diluviana abundance in the Great Lakes, 2006–2016

With the large Diporeia declines in lakes Michigan, Huron, and Ontario, there is concern that a similar decline of Mysis diluviana related to oligotrophication and increased fish predation may occur. Mysis density and biomass were assessed from 2006 to 2016 using samples collected by the Great Lakes National Program Office's biomonitoring program in April and August in all five Great Lakes. Summer densities and biomasses were generally greater than spring values and both increased with bottom depth. There were no significant time trends during these 10–11 years in lakes Ontario, Michigan, or Huron, but there was a significant increase in Lake Superior. Density and biomass were highest in lakes Ontario and Superior, somewhat lower in Lake Michigan, and substantially lower in Lake Huron. A few Mysis were collected in eastern Lake Erie, indicating a small population in the deep basin of that lake. On average, mysids contributed 12–18% (spring-summer, Michigan), 18–14% (spring-summer, Superior), 30–13% (spring-summer, Ontario), and 3% (Huron) of the total open-water crustacean biomass. Size distributions consisted of two peaks, indicating a 2-year life cycle in all four of the deep lakes. Mysis were larger in Lake Ontario than in lakes Michigan, Superior, and Huron. Comparisons with available historic data indicated that mysid densities were higher in the 1960s–1990s (5 times higher in Huron, 2 times higher in Ontario, and around 40% higher in Michigan and Superior) than in 2006–2016.     

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.     

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

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

Distribution of the Amphipod Diporeia in Lake Superior: The Ring of Fire

Diporeia, formerly the dominant benthic macroinvertebrate in the Great Lakes, remains a keystone species in Lake Superior. Little is known, however, about fine scale amphipod distributions, especially as influenced by the production, transport and transformation of energy resources. Here, we document the distribution and abundance of Diporeia along 19 transects around the lake's perimeter. Regions of elevated density, averaging 958 ± 408 Diporeia/m² (mean ± S.D.) were observed along all transects, typically within slope habitat (depth of 30–125 m). Waters shoreward (shelf habitat, < 30 m) and lakeward (profundal habitat, > 125 m) of these regions supported significantly lower densities, averaging 239 ± 178/m² and 106 ± 59/m², respectively. Amphipods within regions of elevated density, termed here the Ring of Fire, account for two-thirds of the lakewide population while occupying only one-quarter of the benthic habitat. The Ring of Fire, observed lakewide as a band averaging 14.2 ± 9.4 km in width, is characterized as a region of transitional sediment deposition with gentle slope, proximate to nearshore locations of elevated primary production. Within the Ring of Fire exceptionally high densities are found in the south central region, where the Keweenaw Current and slope bathymetries serve to funnel production from adjoining regions of high production. Density measurements for the 173 stations sampled here are used to estimate lakewide Diporeia standing stock (22.5–37.7 trillion individuals, 4.4–7.4 Gg dry weight, 2.1–3.5 Gg C), individual and biomass density (274–460/m², 0.05–0.09 g DW/m², 0.03–0.04 gC/m²) and areal (0.02–0.03 g C/m²/yr) and total (1.6–2.6 Gg C/yr) production.     

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.     

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.     

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.     

Early observations on an emerging Great Lakes invader Hemimysis anomala in Lake Ontario

Hemimysis anomala, a Ponto-Caspian littoral mysid, is an emerging Great Lakes invader that was discovered in Lakes Michigan and Ontario in 2006. Similar to the native mysid Mysis diluviana, Hemimysis exhibits a diel vertical migration pattern but generally inhabits shallower and warmer waters than M. diluviana. Because basic information on the distribution, habitat use, and biology of Hemimysis in the Great Lakes is scarce, the potential for food web disruption by Hemimysis cannot easily be predicted. Preliminary observations indicate widespread invasion of Hemimysis in Lake Ontario. In this study, we confirm the presence of Hemimysis at sites spanning the northern and southern shores of Lake Ontario and the presence of the individuals during winter months. In one horizontal tow in November 2007, over 26,000 individuals were collected with a length range of 4.4 to 9.0 mm and an average caloric density of 611 cal/g wet weight. The most effective methods for sampling Hemimysis were horizontal tows with either a zooplankton net in the water column or a benthic sled near the lake bottom. Although more quantitative data on the life history and distribution of this species is necessary, our preliminary observations support the prediction that the potential for Hemimysis to impact the nearshore food web in Lake Ontario appears high.     

Influences on Bythotrephes longimanus life-history characteristics in the Great Lakes

We compared Bythotrephes population demographics and dynamics to predator (planktivorous fish) and prey (small-bodied crustacean zooplankton) densities at a site sampled through the growing season in Lakes Michigan, Huron, and Erie. Although seasonal average densities of Bythotrephes were similar across lakes (222/m² Erie, 247/m² Huron, 162/m² Michigan), temporal trends in abundance differed among lakes. In central Lake Erie where Bythotrephes' prey assemblage was dominated by small individuals (60%), where planktivorous fish densities were high (14,317/ha), and where a shallow water column limited availability of a deepwater refuge, the Bythotrephes population was characterized by a small mean body size, large broods with small neonates, allocation of length increases mainly to the spine rather than to the body, and a late summer population decline. By contrast, in Lake Michigan where Bythotrephes' prey assemblage was dominated by large individuals (72%) and planktivorous fish densities were lower (5052/ha), the Bythotrephes population was characterized by a large mean body size (i.e., 37–55% higher than in Erie), small broods with large neonates, nearly all growth in body length occurring between instars 1 and 2, and population persistence into fall. Life-history characteristics in Lake Huron tended to be intermediate to those found in Lakes Michigan and Erie, reflecting lower overall prey and predator densities (1224/ha) relative to the other lakes. Because plasticity in life history can affect interactions with other species, our findings point to the need to understand life-history variation among Great Lakes populations to improve our ability to model the dynamics of these ecosystems.     

Long-term Trends in Benthic Macroinvertebrate Populations in Lake Huron over the Past Four Decades

Surveys of the benthic macroinvertebrate community were conducted in the main basin of Lake Huron in 2000 and 2003, and in Georgian Bay and North Channel in 2002. Results were compared to surveys conducted in the 1960s and early 1970s. Although data of earlier surveys were inconsistent, our best estimates suggest that total density of the four major benthic taxa (Diporeia spp., Oligochaeta, Sphaeriidae, and Chironomidae) in the main basin declined dramatically between the early 1970s and 2000. Populations of all major taxa continued to decline between 2000 and 2003, particularly Diporeia and Sphaeriidae. Diporeia was rare or absent in the southern end of the lake and in some nearshore areas in 2000, and by 2003 was not found at depths < 50 m except in the far northeastern end of the lake. Densities of the major taxa in Georgian Bay and North Channel in 2002 were not different from densities in 1973 despite differences in survey methods. A limited study in southern Georgian Bay, however, found that densities of both Diporeia and Sphaeriidae declined to zero at most sites between 2000 and 2004. The population of Dreissena polymorpha was stable in all lake areas, but Dreissena bugensis increased, particularly at the 31–50 m depth interval in the main basin. Since there were no extensive surveys in Lake Huron in the period between nutrient abatement (late 1970s) and the establishment of Dreissena (early 1990s), it is difficult to determine relative roles of these events on observed declines. However, since phosphorus loads have been stable since the early 1980s, declines between 2000 and 2003 can likely be attributed to Dreissena.     

Convergence of trophic state and the lower food web in Lakes Huron,Michigan and Superior

Signs of increasing oligotrophication have been apparent in the open waters of both Lake Huron and Lake Michigan in recent years. Spring total phosphorus (TP) and the relative percentage of particulate phosphorus have declined in both lakes; spring TP concentrations in Lake Huron are now slightly lower than those in Lake Superior, while those in Lake Michigan are higher by only about 1 μg P/L. Furthermore, spring soluble silica concentrations have increased significantly in both lakes, consistent with decreases in productivity. Transparencies in Lakes Huron and Michigan have increased, and in most regions are currently roughly equivalent to those seen in Lake Superior. Seasonality of chlorophyll, as estimated by SeaWiFS satellite imagery, has been dramatically reduced in Lake Huron and Lake Michigan, with the spring bloom largely absent from both lakes and instead a seasonal maximum occurring in autumn, as is the case in Lake Superior. As of 2006, the loss of cladocerans and the increased importance of calanoids, in particular Limnocalanus, have resulted in crustacean zooplankton communities in Lake Huron and Lake Michigan closely resembling that in Lake Superior in size and structure. Decreases in Diporeia in offshore waters have resulted in abundances of non-dreissenid benthos communities in these lakes that approach those of Lake Superior. These changes have resulted in a distinct convergence of the trophic state and lower food web in the three lakes, with Lake Huron more oligotrophic than Lake Superior by some measures.     

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.     

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

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

Status of the amphipod Diporeia spp. in Lake Superior, 2006–2016

The amphipod Diporeia spp. has historically been an important component of the benthic food web of the Laurentian Great Lakes. The Great Lakes Water Quality Agreement included its population density as an indicator of ecological condition for Lake Superior, with target values of 220–320 m^?2 in nearshore areas (≤100 m depth) and 30–160 m^?2 in offshore areas (>100 m). To assess the status of Diporeia in Lake Superior, we used a probability-based lake-wide survey design to obtain estimates of Diporeia density and biomass in 2006, 2011 and 2016. A PONAR grab sampler was used to collect Diporeia at 50–53 sites each year, with approximately half in the nearshore (<100 m depth) region of the lake and half in the offshore. The mean area-weighted lake-wide density was 395 ± 56 (SE) m^?2 in 2006, 756 ± 129 m^?2 in 2011, and 502 ± 60 m^?2 in 2016. For all years, both density and biomass were greater in the nearshore than in the offshore stratum. The densities for 2006–2016 were 3–5 times higher than those reported from a lake-wide survey conducted in 1973 by the Canada Centre for Inland Waters. The severe declines in Diporeia populations observed in the other Great Lakes during recent decades have apparently not occurred in Lake Superior. Further research is needed to understand spatial and temporal variability of Diporeia populations in Lake Superior to enhance the utility of Diporeia density as an indicator of benthic condition.     

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

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

Distribution,seasonality and putative origin of the non-native red alga Bangia atropurpurea (Bangiales,Rhodophyta) in the Laurentian Great Lakes

Bangia atropurpurea was first observed in Lake Erie in 1964 and subsequently spread to the lower Laurentian Great Lakes by the mid to late 1970s. The present study was initiated to examine the recent distribution of B. atropurpurea in the Great Lakes, the seasonal variation of the alga and the putative origin of this species based on DNA sequence analysis. From surveys in 1995 and 2002, this species has clearly spread, with newly identified populations observed in Lakes Huron, Michigan, Georgian Bay and the St. Lawrence River. Morphological analyses showed that Great Lakes populations from individual lakes or neighboring populations did not group together in cluster analyses. Correlation analysis, however, revealed significant relationships between the presence or absence of Bangia among the studied sites with pH and specific conductance as those locations that had stable populations had a mean pH and conductance of 8.2 and 353 μS·cm^− 1 respectively. Seasonal variation in morphology of a population from Burlington, Ontario (Lake Ontario, Canada) was examined monthly for one year. Maximum filament length occurred in April, with the greatest diameter and archaeospore production observed in May. Significant correlations were also noted for many morphological characteristics with water temperature, population height on the shoreline relative to the waterline and total phosphorus. Collections of B. atropurpurea analyzed from the Great Lakes were observed to be identical in sequence to collections of European freshwater Bangia in the cox2–3 gene spacer, the nuclear internal transcribed spacers (ITS 1 and 2) and the 5.8S rRNA gene (between the small and large subunits of the rRNA gene). These results suggest a recent European origin; however, further global collections of B. atropurpurea and microsatellite analyses are necessary to confirm this hypothesis.