Eutrophication continues in Lake Erie and low oxygen concentration remains a concern in the central basin of the lake. Summertime dissolved oxygen concentrations can be hypoxic (low dissolved oxygen) and anoxic (dissolved oxygen < 1 mg l^-1) in the hypolimnion. We examined the spatial and temporal patterns of hypoxia in the central basin along a ∼26 km west-east station transect in the western portion of the central basin (depth gradient from 11.4 m in the west to 20 m in the east). Water column properties were monitored using moored instruments (dissolved oxygen, temperature, turbidity, Chlorophyll a fluorescence) and instrument profiling during cruises in 2008 and 2009. Hypoxia was examined using a biologically relevant value of 40% dissolved oxygen saturation (i.e. ∼ 4 mg l^-1) and 25% dissolved oxygen saturation (i.e. ∼ 2.5 mg l^-1) as used by government agencies. Our goals were to determine the onset and location of hypoxia, as well as the frequency of hypoxic events. We observed differences in the spatial and temporal patterns between the two years, related potentially to different water levels, stronger winds, and a smaller hypolimnion and lower thermocline depth in 2009. Near-bottom hypoxia occurred in the east at the end of June and extended westward by the end of July 2008 using 40% saturation; in early July to mid-August using 25% saturation. The onset of hypoxia (40% sat) occurred earlier in the west in 2009 but was similar to 2008 using 25% saturation. Hypoxia was not static, rather there were a total of ∼100 events of both levels of hypoxia, which were of different duration, noted across the transect in both years. Both the frequency and duration of hypoxic events (> 1 min) were higher and longer in 2008, perhaps related to water circulation and the resuspension of bottom sediment by synoptic-scale storms, which coincided with low oxygen events. Understanding the spatial and temporal patterns of hypoxia provides insight into their effects on habitat quality as well as biogeochemical processes in benthic and hypolimnetic environments in Lake Erie.

Little is known about external and internal loading and cycling of bioaccumulative methylmercury in Lake Erie, despite the lake having a world-renowned sport fishery. During the summer/early fall of 2018 to 2021, concentrations and fluxes of total mercury and methylmercury in the water column were examined near the Detroit and Maumee River discharges into western Lake Erie, as well as the junction between Sandusky Bay and central Lake Erie. Average unfiltered total mercury concentrations were similar near the Detroit River (5.4 ± 0.8 pM) and Sandusky Bay inputs (5.3 ± 0.9 pM), which were less than half of those near the Maumee River mouth (11.6 ± 2.8 pM). Similarly, unfiltered methylmercury concentrations near the Detroit River (0.29 ± 0.09 pM) and Sandusky Bay inputs (0.24 ± 0.06 pM) were less than half of those near the Maumee River mouth (0.63 ± 0.21 pM). Potential specific mercury methylation rates measured in central Lake Erie were 0.062 ± 0.027 day^-1, 0.045 ± 0.012 day^–1 near the Sandusky Bay input, and 0.031 ± 0.006 day^–1 at the Detroit River input (Maumee Bay was below detection; rates were not different; Tukey, p > 0.87). Compared to previous work, total mercury concentrations in the western basin observed in this study indicate a decrease of about 3.3% yr^–1, which may reflect positive impacts of state, provincial, and national legislation (U.S. National Clean Water Act 1990, Ohio Clean Air and Water Act 2004, Ontario Clean Water Act 2006). However, methylmercury concentrations have increased in western Lake Erie from 2010 to 2019, which may reflect the impact of legacy mercury pollution.

Few lake-wide, seasonal primary production measurements have been made in Lake Erie. None more recent than 1997 have been reported in the literature. In 2019 we used ¹³C uptake to measure production at 11 stations across the lake during three surveys in May, July, and September and at four of these stations during two additional surveys in April and August. Samples were collected at two depths (2 m and 6 m) and incubated on deck using eight levels of irradiance. We fit the resultant photosynthesis-irradiance curves with models that included the initial slope at low light levels, the maximum photosynthesis rate at light saturation, and when warranted, the negative slope at high irradiance. During May, there was little variation in chlorophyll-normalized volumetric productivity with sample depth, and productivity generally was higher nearshore and in the western basin than it was offshore. The distinction between nearshore and offshore stations was less pronounced in July, but nearshore productivity was higher than offshore in September. Using the photosynthesis data from all five surveys, chlorophyll concentration profiles, and estimated clear-sky solar insolation, we calculated time series of vertically integrated daily production at a western basin station, two central basin stations, and an eastern basin station. Central basin production was highest during August, estimated to be 748 mg C m^-2 d^-1 and 779 mg C m^-2 d^-1 at our two stations. Production peaked in July in the western basin (510 mg C m^-2 d^-1) and September in the eastern basin (421 mg C m^-2 d^-1). Whole-lake seasonal areal phytoplankton photosynthesis was estimated as 1.661E6 tonnes, about half of estimates made in the 1990s using ¹⁴C methods. These results inform our understanding of spatial-temporal trends in Lake Erie productivity and highlight the need for continued measurements of Lake Erie primary production.

Cyanobacterial bioactive metabolites (cyanotoxins/cyanopeptides) inhibit enzymes such as proteases, carboxypeptidases, and phosphatases that disrupt metabolic processes relevant to human and ecosystem health. From surface water samples collected in 2017 (n = 44), 2018 (n = 100), and 2019 (n = 171), we report concentrations of more than 20 biologically active cyanobacterial compounds in the western basin of Lake Erie, one of the five Laurentian Great Lakes with multinational jurisdiction. Toxic microcystins LA and LR as well as anabaenopeptin B and F were most frequently detected at >90%. Microcystins RR and YR and anabaenopeptin A were detected at relatively high frequency at >80% in 2019. There was strong correlation among arginine-containing microcystin variants (RR, YR, HtyR, LR, HilR, WR, D-Asp3-LR, Spearman's Rho > 0.80) but not with arginine-lacking LY, LW, LF. Anabaenopeptin F, with the arginine sidechain, also correlated with these same arginine-containing microcystin variants (Spearman's Rho > 0.70). In 2019, only 4% of lake water samples exceeded the recreational water guideline for total microcystins (10 µg l^-1) set by Health Canada with the maximum concentration ∼26 µg l^-1. The drinking water guideline (1.5 µg l^-1) was exceeded in 34% of lake water samples but treated water was not tested. Although maximum total anabaenopeptins concentration was almost four times higher (almost 100 µg l^-1) than microcystins, no anabaenopeptin guideline values have been derived as their impact is currently considered to be ecological rather than toxic to humans/animals. These results can be used to evaluate risk from cyanobacterial bioactive metabolites, including toxins, in Lake Erie and aid binational lake management and policy development in the Great Lakes basin.

Dreissenid mussels arrived at the Waverly Shoal located above the inflow of the Niagara River in 1989, initiating marked changes in the water and sediment chemistry, and benthic invertebrate community composition at an Ontario Ministry of the Environment, Conservation and Parks monitoring station. Here we examine change in the nearshore of eastern Lake Erie until 2019, inferred from monitoring at this station. Dreissenid numbers peaked in 1991, exceeding 200,000 individual m^-2, and remained above 20,000 individual m^-2 until 2004 after which numbers progressively declined. In 1993, the population transitioned from mixed Dreissena polymorpha and D. bugensis, to D. bugensis in subsequent years. The decrease in particle size and increase in organic content of surficial sediment which began the year after mussel arrival has persisted until present time. The low concentrations of trace metals and PAHs at the station increased slightly after the physical alteration in bed sediments and has either not changed or declined. After an initial increase in Secchi depth, water clarity changed little over post invasion years, with prevailing moderate water clarity interspersed with periods of bed resuspension and high turbidity. Calcium concentration in the water column, which fell dramatically after the arrival of dreissenids, has gradually increased in recent years. Chlorophyll a and total phosphorus levels indicating oligo-mesotrophic conditions have not varied systematically over the years, other than a modest decline in chlorophyll a after dreissenid arrival. Benthic invertebrate assemblages have gone through multiple alterations, with shifting abundance of amphipods, oligochaetes, chironomids, gastropods and sphaeriids attributed to invasive species rather than the physical environment. Collectively, the data suggests the nearshore ecosystem has shifted in benthic productivity and trophic transfers mediated by the benthos and invasive species, with water and sediment quality appearing to not vary beyond the range driven by inherently fluctuating physical conditions.

Larval fish growth and survival could be limited or reduced due to patchiness of zooplankton densities, even in productive aquatic systems. Recent declines in Lake Whitefish (Coregonus clupeaformis) populations prompted research to identify underlying mechanisms controlling survival at early life stages. In Lake Erie, the bottleneck window controlling year-class strength of Lake Whitefish likely occurs during the first growing season, suggesting that availability of important prey could influence year-class strength. Therefore, spatial and temporal larval Lake Whitefish distribution, diet, and prey utilization were evaluated in western Lake Erie. The pelagic Lake Whitefish larval period in the western basin extends from April 1 to May 15 with most larvae concentrated nearshore at the surface both day and night. Cyclopoid copepods were the most important prey item; however, calanoid copepods and Cladocera were consistently consumed, indicating that copepods and Cladocera were important larval Lake Whitefish prey items. Copepod and Cladocera biomass were the highest nearshore, overlapping with the highest larval Lake Whitefish densities. However, the amount of food consumed by larvae was consistent in all areas, suggesting that offshore areas in western Lake Erie with relatively low zooplankton biomass harbor enough food to satiate larval Lake Whitefish. Therefore, it is unlikely that prey availability limits survival through means of starvation during the larval phase.

The early-life history stages of fish are sensitive to environmental change and therefore can indicate habitat quality as well as help predict recruitment of resident and transient fishes. In 2019, as part of the Lake Erie Cooperative Science and Monitoring Initiative, we conducted a lake-wide assessment of the ichthyoplankton community in U.S. nearshore waters and international offshore waters. The goal of this work was to characterize the larval fish community across the lake and assess species composition, phenology, and distribution of larvae. Ichthyoplankton were sampled weekly using bongo nets at ports beginning at the Detroit River and along the southern shore of Lake Erie to Dunkirk, NY, and less frequently in the Niagara River and offshore areas. Larval fish were present from March 26 through August 29, 2019. The first taxon to emerge was Lake Whitefish in all basins, followed by Walleye, Yellow Perch, and catostomids, depending on port. Mean total density peaked in mid-June due to high catches of Gizzard Shad, Morone spp., and Freshwater Drum in the western basin. Few fish were collected in the offshore sites. Taxa richness, diversity, and larval density were higher in the western basin and lower in the central and eastern basins, generally following the productivity gradient. This was the first study to provide a comprehensive community assessment of the ichthyoplankton community of Lake Erie and can provide a baseline to assess future change, especially in community composition or phenology, of larvae which are likely to respond to climate and habitat change.