Assessment of larval fish assemblages and nursery habitat in the St. Clair River delta

The St. Clair River delta, part of the St. Clair-Detroit River System (SCDRS), is the most fished coastal wetland area in the Laurentian Great Lakes and provides nursery habitat for a variety of fish species; however, few large-scale surveys of larval fish have been performed within the delta since the 1980s. Larval fish, zooplankton, and aquatic plants were sampled at 20 sites from May through July in 2010 and 2011 to characterize shallow channel and backwater delta habitats used by fish. The larval fish assemblage was sampled using active and passive gears (conical nets and light traps) and was dominated by Cyprinidae, Catostomidae, and Gobiidae. The microzooplankton assemblage was composed of rotifers, copepod nauplii, and Dreissena spp. veligers, while the macrozooplankton assemblage was composed of mostly cyclopoids and harpacticoids in May and cladocerans later in the season. Scirpus spp. dominated the plant assemblage in June and was replaced by Chara spp. in July. Seasonal compositional shifts were evident for larval fish, zooplankton, and plant assemblages, and greater densities of microzooplankton and cladocerans were typically found in backwater areas. Assemblage compositions were comparable to those in historical surveys but invasive Gobiidae and Dreissena spp. veligers now represent substantial proportions of the larval fish and zooplankton assemblages, respectively. Due to the high connectivity and advective nature of the SCDRS, understanding the larval fish assemblage dynamics of the delta can help inform estimates of system productivity.     

The Effects of Zebra Mussels on the Lower Planktonic Foodweb in Lake Champlain

Selective grazing by zebra mussels has altered phytoplankton communities in many North American lakes, but the specific changes are not the same in each ecosystem. Because of this variation in response, we investigated the impacts of zebra mussels on the plankton community of Lake Champlain with two objectives: first to determine whether zebra mussels increased the dominance of potentially toxic cyanobacteria in the phytoplankton, and second to explore the impact of zebra mussels on protozoans, rotifers, copepod nauplii, and other microzooplankton in the lower food web. Experiments were conducted in 200-L mesocosms filled with Lake Champlain water filtered through a 150-μm sieve to remove macrozooplankton. Zebra mussels were added to half of the mesocosms while the others were maintained as controls. Over a 96-hour experimental period, we tracked nitrogen and phosphorus concentration, chlorophyll a, microcystin concentration, and both phytoplankton and microzooplankton composition and abundance. We found an increase in SRP and total nitrogen concentration and a decrease in the ratio of TN:TP in the zebra mussel treatments over time. Microcystin was undetectable throughout the experiment using the ELISA assay. Phytoplankton biovolume, including cyanobacteria biovolume, declined significantly in the zebra mussel treatments, as did rotifer, protozoan and nauplii abundance. By both direct (consumption) and indirect (altered nutrient availabilities and increased competition) means, zebra mussels clearly seem capable of strongly influencing the lower planktonic foodweb in the many shallow water habitats of Lake Champlain.     

Dreissenid veliger dynamics along a nearshore to offshore transect in Lake Michigan

Dreissenid mussel veligers compose a substantial component of pelagic biomass in the Great Lakes, yet their dynamics are poorly understood. To evaluate seasonal, spatial, and inter-annual variation in veliger density, we used a 64-μm mesh plankton net (2008, 2013–2016) and a 153-μm mesh plankton net (2007–2016) to collect dreissenid veligers at nearshore (15–25?m depth), transitional (45?m) and offshore (93–110?m) sites in southeast Lake Michigan during March–December. We also evaluated trends in density of recently settled mussels relative to veliger abundance and the density of the standing stock of adult mussels. Veliger density peaked during both summer and fall at all sites, but peak densities in summer were generally higher nearshore, whereas peak densities in the fall were generally higher offshore. The density of veligers in the 153-μm net was overall 28% of that in the 64-μm net, but there was high variability in this comparison among months. Smaller veligers were much more abundant in the 64-μm net, but there was little difference in the size distribution and abundance between nets for the 210–300?μm size classes. Thus, the 153-μm net could still be a useful tool for assessing density trends of larger veligers just prior to their settlement. Newly settled mussels (≤2?mm) were most abundant in summer or fall at the nearshore and offshore sites but were nearly absent at the transitional site despite the high density of veligers there. Factors other than veliger density must play an important role in mussel recruitment.     

Feeding ecology of Limnocalanus macrurus in the Laurentian Great Lakes

The zooplankton communities of several Laurentian Great Lakes have shifted toward greater biomass of calanoid copepods, particularly Limnocalanus macrurus, since the 1990s. Limnocalanus is an omnivore that feeds on large phytoplankton cells, ciliates, rotifers, and small crustacean zooplankton, especially copepod nauplii, and it may be an increasingly important zooplanktivore in these systems. Although there is previous research examining Limnocalanus predation rates on nauplii, we do not know if the presence of phytoplankton affects predation rates. Our initial experiments confirmed Limnocalanus preference for nauplii over small copepodites. Additional experiments showed that Limnocalanus feeding rates on nauplii decreased by 50% at the highest phytoplankton concentrations tested. Limnocalanus fed more on the larger algae tested (Cryptomonas, Cryptophyta, 40 µm) than on the smaller taxa (Scenedesmus, Chlorophyta, 10 µm). We used stable isotope analysis to infer Limnocalanus trophic position in the five Laurentian Great Lakes by comparing Limnocalanus with simultaneously captured Leptodiaptomus sicilis, another calanoid copepod known to feed on phytoplankton and microzooplankton. This analysis showed Limnocalanus at higher trophic positions in the more oligotrophic lakes Huron, Michigan, and Superior than in lakes Ontario and Erie. Summer Limnocalanus trophic position was inversely related to both the site-specific concentration of algae in the deep chlorophyll layer and a trophic state index based on spring chlorophyll and total phosphorus. Our results indicate that predation by Limnocalanus on zooplankton depends on lake algal abundance, and that feeding rates on nauplii by an individual Limnocalanus adult are likely higher in the more oligotrophic lakes.     

A mesocosm investigation of the effects of quagga mussels (Dreissena rostriformis bugensis) on Lake Michigan zooplankton assemblages

Dreissenid mussels are known to disrupt the base of the food web by filter feeding on phytoplankton; however, they may also directly ingest zooplankton thereby complicating their effects on plankton communities. The objective of this study was to quantify the effects of quagga mussel feeding on the composition and size structure of Lake Michigan zooplankton assemblages. Two mesocosm (six 946 L tanks) experiments were conducted in summer 2013, using quagga mussels and zooplankton collected near Beaver Island, MI, to examine the response of zooplankton communities to the presence and absence of mussels (experiment 1) and varying mussel density (experiment 2). Mesocosms were sampled daily and zooplankton taxa were enumerated and sized using microscopy and FlowCAM® imaging. In experiment 1, the presence of quagga mussels had a rapid negative effect on veliger and copepod nauplii abundance, and a delayed negative effect on rotifer abundance. In experiment 2, mussel density had a negative effect on veliger, nauplii, and copepodite abundance within 24 h. Multivariate analyses revealed a change in zooplankton community composition with increasing mussel density. Ten zooplankton taxa decreased in abundance and frequency as quagga mussel density increased: except for the rotifer Trichocerca sp., treatments with higher mussel densities (i.e., 1327, 3585, and 5389 mussels/m²) had the greatest negative effect on small-bodied zooplankton (≤ 128 μm). This study confirms results from small-scale (≤ 1 L) experiments and demonstrates that quagga mussels can alter zooplankton communities at mesoscales (~ 1000 L), possibly through a combination of direct consumption and resource depletion.     

Crustacean zooplankton available for larval walleyes in a Lake Michigan embayment

Adequate densities of zooplankton prey are critical for growth and survival of larvae of many fish species. Little information exists on the density of zooplankton in Great Lakes inshore areas during early spring, when larvae of important fishes rely on zooplankton. Reduced age-0 walleye recruitment and the absence of data on zooplankton availability for larval walleyes in northern Green Bay, Lake Michigan, led us to assess zooplankton densities during this critical spring period. We conducted biweekly vertical plankton tows in 2014–2016 near reefs and river plumes used by spawning walleyes for periods when larval walleyes were expected to be relying on zooplankton prey. Densities of zooplankton were well below literature values identified for good growth and survival of larval walleyes, averaging 1.5 individuals L⁻¹ for all taxa and 0.12 individuals L⁻¹ for large-bodied taxa across all sites and sampling dates. Various factors could contribute to the low density of zooplankton observed. We found low but significantly higher densities of cyclopoid copepods, nauplii, Bosmina, and total zooplankton at river mouth sites compared to open water sites. These results suggest that food availability for larval walleye in our study area was severely limiting which is consistent with the paucity of strong year classes observed since 2000. We suspect northern Green Bay has limited potential for producing strong year classes of walleyes under such conditions. Fishery managers working in unproductive waters should consider assessing the zooplankton community during critical periods to identify potential bottlenecks to reproductive success and larval fish survival.     

Importance of biotic interactions in large rivers: an experiment with planktivorous fish,dreissenid mussels and zooplankton in the St. Lawrence River

Physical conditions are usually considered pre‐eminent in controlling river plankton, but biotic interactions may be important in slackwater areas. To begin testing this general hypothesis, we conducted a 12‐day, predator–prey experiment in 3500 litre mesh enclosures in a slackwater area of the St. Lawrence River using planktivorous, juvenile yellow perch (Perca flavescens) and Dreissena mussels. Results generally supported our hypotheses that: (1) perch would directly suppress large zooplankton via predation but benefit microzooplankton through indirect interactions; and (2) dreissenids would directly depress rotifer densities via predation and have indirect negative effects on macrozooplankton. Based on gut contents of experimental fish, cladocera were the principal prey of smaller yellow perch (c. 46–50 mm), followed by copepods, ostracods, and rotifers. Larger juvenile perch (c. 67–73 mm) fed almost exclusively on copepods and ostracods. Densities of calanoid copepodids, nauplii, and some cyclopoid copepods (Diacyclops thomasi) were significantly depressed by perch, and adult Eurytemora affinis (99% of adult calanoids) essentially disappeared from fish enclosures. Despite being a favourite prey item of small perch, densities of the small cladoceran Bosmina (Sinobosmina) spp. were significantly higher when fish were present (150% greater than control densities on Day 12). Densities of the very abundant rotifer Polyarthra were >300% greater in fish enclosures than controls by Day 12, and the rotifers Synchaeta and predaceous Ploesoma were significantly more abundant in the presence of perch. Increases in rotifers and cladocera suggest indirect, positive effects of fish related to significantly higher phytoplankton biomass or decreased densities of predaceous copepods. Densities of eight of ten zooplankton groups examined declined significantly when mussels were present, and calanoid copepodids also declined but not significantly. Chlorophyll‐a concentrations were slightly lower in mussel enclosures. This evidence suggests biotic interactions play important roles among potamoplankton in slackwater habitats, but river‐wide implications of these findings require further study. Copyright © 2003 John Wiley & Sons, Ltd.     

Brachionus leydigii (Monogononta: Ploima) reported from the western basin of Lake Erie

Several species of non-indigenous planktonic invertebrates have historically been introduced to the Laurentian Great Lakes. Previous introductions of non-indigenous planktonic invertebrates to the Great Lakes have been crustacean zooplankton, specifically Cladocera and Copepoda. This report documents the first known occurrence of Brachionus leydigii var. tridentatus (Zernov, 1901) in Lake Erie and possibly the first detection of a non-indigenous rotifer species in the Laurentian Great Lakes. The specimen was collected from a U.S. EPA monitoring station in the western basin of Lake Erie on April 4, 2016.     

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.     

Phytoplankton trends in the Great Lakes, 2001–2011

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

Distribution and abundance of larval lake whitefish (Coregonus clupeaformis) in Stokes Bay,Lake Huron

Lake whitefish (Coregonus clupeaformis) have been widely studied across the Laurentian Great Lakes. However, there are major gaps in our understanding of factors that affect larval distribution and abundance. The goal of this study was to investigate the distribution and abundance of larval lake whitefish in a Great Lakes embayment using Stokes Bay, Lake Huron as a case study. We collected plankton samples and environmental data from mid-spring to early summer during 2011 and 2012. Ichthyoplankton tows in 2011 revealed densities that are among the highest to be reported in Great Lakes studies. Overall there was little relationship between environmental variables (temperature, dissolved oxygen, conductivity, and depth) and larval lake whitefish distribution and abundance. Ichthyoplankton tows in 2012 revealed a virtual absence of larval lake whitefish during the entire sampling season; unseasonably warm conditions during spring 2012 likely had an important effect on larval survival.     

Historical trophic position of Limnocalanus macrurus in Lake Michigan

In Lake Michigan, oligotrophication and invasive species proliferation have led to dramatic changes in the planktonic food web. Limnocalanus macrurus is a native, glacial relict predatory copepod whose trophic role within the zooplankton community may help us better understand the dynamics behind those changes. We applied nitrogen isotope spectrometry to zooplankton from Lake Michigan to quantify the yearly and seasonal position of Limnocalanus and other taxa in the historical Lake Michigan planktonic food web. We found that Limnocalanus was positioned about one-half trophic level above Bythotrephes and about two levels above Daphnia during the summer in 1989, 1993, 1995 and 1997. It was unlikely that adult Limnocalanus encountered Bythotrephes during summer months because of vertical segregation during thermal stratification. Instead, Limnocalanus probably had greater access to copepod prey such as Diaptomus and Epischura. Limnocalanus became isotopically lighter seasonally in relation to Daphnia and either shifted its diet to one consisting of more phytoplankton, or its prey (e.g., Diaptomus) shifted to a more phytoplankton food base, thus indirectly resulting in decreased trophic status of Limnocalanus. This study serves as a historical, foundational basis for zooplankton food web relations in Lake Michigan that complements similar investigation in Lake Huron. Comparing the historical to the recent zooplankton food web may now elucidate how invasive species such as Bythotrephes and quagga mussels have altered zooplankton communities and bioenergetic relationships within the Great Lakes.     

Long-term patterns in Lake Champlain's zooplankton: 1992–2010

We examined patterns in Lake Champlain zooplankton abundance from 1992 to 2010 using summer data from five study sites. Rotifer abundance (#/m³) for many common taxa such as Polyarthra, Kellicottia, and Keratella declined lakewide in the mid-1990s which coincided with the invasion of zebra mussels (Dreissena polymorpha) into Lake Champlain. The only rotifer to increase in density following zebra mussel invasion was Conochilus which is a colonial species. Long-term shifts in copepod and cladoceran community composition can be attributed to the arrival of another invasive species in 2004–2005, the alewife (Alosa pseudoharengus). Our results support previous findings that alewife predation can impact larger bodied zooplankton within temperate lake systems. Following alewife invasion into Lake Champlain, body length of Leptodiaptomus and Daphnia retrocurva decreased to a size at or below known alewife feeding preferences. In addition, smaller bodied copepods (primarily Diacyclops thomasi) have increased in abundance since alewife invasion while juvenile copepods have declined. Our results suggest that post-alewife zooplankton patterns are most likely due to alewife size-selective feeding strategies. Observed long-term changes in zooplankton community structure have potential implications for the lake's food web dynamics, particularly recent declines in large bodied zooplankton which may release smaller plankton from top-down control.     

Spatial variation in zooplankton community structure is related to hydrologic flow units in the Missouri river,USA

Zooplankton are a vital link in the food webs of large rivers, and their communities are shaped by both local environmental features and advection. In the Missouri River, flow characteristics naturally change along its length, but human modifications to facilitate commercial transport have altered natural flow in many sections of the river. We evaluated the effect of flow on zooplankton community structure at multiple spatial scales, and used multivariate analyses to evaluate the relative importance of flow and local abiotic environment on these communities. During July–September 2005, zooplankton samples and physico‐chemical measures were collected from the Missouri River main channel at 78 sites over a 2831 km range (Montana to Missouri). We identified a total of 30 cladoceran species, 22 copepod species and 27 rotifer genera, and we detected highly significant differences in zooplankton community structure among hydrologically distinct flow units and larger spatial zones. At the local scale, crustacean zooplankton and rotifers responded differently in the analyses. For copepods and cladocerans, distance from the nearest upstream reservoir explained more of the overall community pattern of the river than any other combination of environmental factors, reflecting the influence of dams on the zooplankton community of the Missouri River. For rotifers, a combination of flow characteristics due to impoundment and channelization and local environment (temperature) was important. Our study indicates that, because of the overwhelming effect of flow on zooplankton communities, hydrology must first be taken into account before zooplankton can be used as bioindicators of other environmental stresses. Copyright © 2009 John Wiley & Sons, Ltd.     

Dreissena veligers in western Lake Superior – Inference from new low-density detection

The notion that Lake Superior proper is inhospitable to dreissenid mussel survival has been challenged by recent finds on shipwrecks and rocky reefs in the Apostle Islands region. Motivated by concerns surrounding these finds, we conducted an intensive sampling campaign of Apostle Islands waters in 2017 to understand Dreissena prevalence and distribution. The 100-site effort combined random and targeted sites and collected zooplankton, benthos, video, environmental DNA, and supporting water quality data. We did not find settled Dreissena in any video footage or benthos samples, and quantitative PCR applied to eDNA samples was negative for Dreissena. Dreissena veligers were found in almost half the zooplankton samples but at orders of magnitude lower densities than reported from other Laurentian Great Lakes. Veligers were most prevalent around the western islands and associated with shallower depths and slightly higher phosphorus and chlorophyll, but did not spatially match known (still very localized) settled Dreissena colonies. This is the first study to conduct veliger-targeted sampling in western Lake Superior and the first to report consistent detection of veligers there. We speculate that these Apostle Islands veligers are not a new locally-spawned component of the zooplankton community, but instead are transported from an established population in the St. Louis River estuary (~100 km away) by longshore currents; i.e., low-density propagule pressure that may have been present for years. Small-mesh zooplankton data collected along a gradient from the Apostle Islands to the St. Louis River estuary and enumerated with thorough veliger searching would help elucidate these alternatives.     

Invasive bighead and silver carp effects on zooplankton communities in the Illinois River,Illinois, USA

Aquatic invasive species introductions are a global environmental concern. Negative effects of invasive species are often manifested in alterations of food web structure and through competition with and predation upon native species. The Illinois River, Illinois, USA harbors invasive, planktivorous bighead, Hypophthalmichthys nobilis, and silver carp, Hypophthalmichthys molitrix, and can be a model ecosystem to test for their effects on zooplankton communities. We tested for bighead and silver carp effects on zooplankton communities pre- and post-establishment within one reach of the Illinois River and among river reaches that varied in abundances of these invasive fishes. The establishment of bighead and silver carp was associated with increased rotifer abundances, while cladoceran and copepod abundances were reduced relative to pre-establishment. Cladoceran and copepod abundance and biomass were negatively associated with bighead and silver carp abundances among reaches. Total zooplankton and rotifer abundance and biomass were positively associated with bighead and silver carp abundances. Our results suggest that bighead and silver carp have changed the zooplankton community of the Illinois River which may have implications for the food web, native species, and other ecosystems poised to be invaded, such as the Laurentian Great Lakes.     

Associations between Breeding Marsh Bird Abundances and Great Lakes Hydrology

We used Great Lakes hydrologic data and bird monitoring data from the Great Lakes Marsh Monitoring Program from 1995–2002 to: 1) evaluate trends and patterns of annual change in May-July water levels for Lakes Ontario, Erie, and Huron-Michigan, 2) report on trends of relative abundance for birds breeding in Great Lakes coastal marshes, and 3) correlate basin-wide and lake-specific annual indices of bird abundance with Great Lakes water levels. From 1995–2002, average May, June, and July water levels in all lake basins showed some annual variation, but Lakes Erie and Huron-Michigan had identical annual fluctuation patterns and general water level declines. No trend was observed in Lake Ontario water levels over this period. Abundance for five of seven marsh birds in Lake Ontario wetlands showed no temporal trends, whereas abundance of black tern (Chlidonias niger) declined and that of swamp sparrow (Melospiza georgiana) increased from 1995–2002. In contrast, abundances of American coot (Fulica americana), black tern, common moorhen (Gallinula chloropus), least bittern (Ixobrychus exilis), marsh wren (Cistorthorus palustris), pied-billed grebe (Podilymbus podiceps), sora (Porzana carolina), swamp sparrow, and Virginia rail (Rallus limicola) declined within marshes at Lakes Erie and Huron/Michigan from 1995–2002. Annual abundances of several birds we examined showed positive correlations with annual lake level changes in non-regulated Lakes Erie and Huron/Michigan, whereas most birds we examined in Lake Ontario coastal wetlands were not correlated with suppressed water level changes of this lake. Overall, our results suggest that long-term changes and annual water level fluctuations are important abiotic factors affecting abundance of some marsh-dependent birds in Great Lakes coastal marshes. For this reason, wetland bird population monitoring initiatives should consider using methods in sampling protocols, or during data analyses, to account for temporal and spatial components of hydrologic variability that affect wetlands and their avifauna.     

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: I. Introduction and Phytoplankton Results

The Great Lakes National Program Office of the U.S. EPA has been conducting biological monitoring of the Laurentian Great Lakes since 1983. This paper presents synoptic survey data of phytoplankton communities from all five lakes. These communities were highly diverse, each lake typi-cally supporting over 100 species during both the spring and summer surveys. Much of that diversity was contributed by diatoms, which dominated the plankton of all lakes except Lake Superior in the spring. Summer communities shifted away from diatoms, toward chrysophytes in the upper lakes and chloro-phytes in the lower lakes. Ordination analyses indicated the close similarity of communities in the upper lakes, in particular Lakes Huron and Michigan, and a diverse range of communities in Lake Erie. Floristically, Lake Ontario was fundamentally different from all other lakes.     

European Valve Snail Valvata piscinalis (Müller) in the Laurentian Great Lakes Basin

Previously reported from the lower Great Lakes basin and St. Lawrence and Hudson rivers, the nonindigenous gastropod Valvata piscinalis was found for the first time in Superior Bay (Minnesota) of Lake Superior, Lake Michigan (Wisconsin), and Oneida Lake (New York) of the Lake Ontario basin. This snail was not abundant in Lakes Superior and Michigan, whereas in eutrophic Oneida Lake it reached a maximum density of 1,690 individuals/m² (mean density = 216 individuals/m²). Human-mediated disturbances could facilitate the range extension of this snail by providing dispersal opportunities (e.g., canals, shipping traffic) or increasing nutrients (e.g., eutrophication). A native of the Palaearctic region, V. piscinalis has colonized sites across the Great Lakes basin, suggesting that it will likely become common in disturbed Great Lakes areas.     

A brief history of the U.S. EPA Great Lakes National Program Office's water quality survey

The U.S. Environmental Protection Agency Great Lakes National Program Office (GLNPO) water quality survey (WQS) constitutes the longest-running, most extensive monitoring of water quality and the lower trophic level biota of the Laurentian Great Lakes, and has been instrumental in tracking shifts in nutrients and the lower food web over the past several decades. The initial impetus for regular monitoring of the Great Lakes was provided by the 1972 Great Lakes Water Quality Agreement (GLWQA) which asked the parties to develop monitoring and surveillance programs to ensure compliance with the goals of the agreement. The resulting monitoring plan, eventually known as the Great Lakes International Surveillance Plan (GLISP), envisioned a nine-year rotation of intensive surveys of the five lakes. A broadening of the scope of the GLWQA in 1978 and the completion of the first nine-year cycle of sampling, prompted reappraisals of the GLISP. During this pause, and using knowledge gained from GLISP, GLNPO initiated an annual WQS with the narrower focus of tracking water quality changes and plankton communities in the offshore waters of the lakes. Beginning in 1983 with lakes Erie, Huron, and Michigan, the WQS added Lake Ontario in 1986 and Lake Superior in 1992, evolving into its current form in which all five lakes are sampled twice a year. The WQS is unique in that all five lakes are sampled by one agency, using one vessel and one principal laboratory for each parameter group, and represents an invaluable resource for managing and understanding the Great Lakes.