David C. Depew

Toxicity of dietary methylmercury to fish: Derivation of ecologically meaningful threshold concentrations

Threshold concentrations associated with adverse effects of dietary exposure to methylmercury (MeHg) were derived from published results of laboratory studies on a variety of fish species. Adverse effects related to mortality were uncommon, whereas adverse effects related to growth occurred only at dietary MeHg concentrations exceeding 2.5 µg g(-1) wet weight. Adverse effects on behavior of fish had a wide range of effective dietary concentrations, but generally occurred above 0.5 µg g(-1) wet weight. In contrast, effects on reproduction and other subclinical endpoints occurred at dietary concentrations that were much lower (<0.2 µg g(-1) wet wt). Field studies generally lack information on dietary MeHg exposure, yet available data indicate that comparable adverse effects have been observed in wild fish in environments corresponding to high and low MeHg contamination of food webs and are in agreement with the threshold concentrations derived here from laboratory studies. These thresholds indicate that while differences in species sensitivity to MeHg exposure appear considerable, chronic dietary exposure to low concentrations of MeHg may have significant adverse effects on wild fish populations but remain little studied compared to concentrations in mammals or birds.

The re-eutrophication of Lake Erie: Harmful algal blooms and hypoxia

Lake Erie supplies drinking water to more than 11 million consumers, processes millions of gallons of wastewater, provides important species habitat and supports a substantial industrial sector, with >

Derivation of screening benchmarks for dietary methylmercury exposure for the common loon (Gavia immer): Rationale for use in ecological risk assessment

50 billion annual income to tourism, recreational boating, shipping, fisheries, and other industries. These and other key ecosystem services are currently threatened by an excess supply of nutrients, manifested in particular by increases in the magnitude and extent of harmful planktonic and benthic algal blooms (HABs) and hypoxia. Widespread concern for this important international waterbody has been manifested in a strong focus of scientific and public material on the subject, and commitments for Canada-US remedial actions in recent agreements among Federal, Provincial and State agencies. This review provides a retrospective synthesis of past and current nutrient inputs, impairments by planktonic and benthic HABs and hypoxia, modelling and Best Management Practices in the Lake Erie basin. The results demonstrate that phosphorus reduction is of primary importance, but the effects of climate, nitrogen and other factors should also be considered in the context of adaptive management. Actions to reduce nutrient levels by targeted Best Management Practices will likely need to be tailored for soil types, topography, and farming practices.

Spatiotemporal patterns of water quality in Lake Ontario and their implications for nuisance growth of Cladophora.

The current understanding of methylmercury (MeHg) toxicity to avian species has improved considerably in recent years and indicates that exposure to environmentally relevant concentrations of MeHg through the diet can adversely affect various aspects of avian health, reproduction, and survival. Because fish-eating birds are at particular risk for elevated MeHg exposure, the authors surveyed the available primary and secondary literature to summarize the effects of dietary MeHg on the common loon (Gavia immer) and to derive ecologically relevant toxic thresholds for dietary exposure to MeHg in fish prey. After considering the available data, the authors propose three screening benchmarks of 0.1, 0.18, and 0.4 µg g(-1) wet weight MeHg in prey fish. The lowest benchmark (0.1 µg g(-1) wet wt) is the threshold for adverse behavioral impacts in adult loons and is close to the empirically determined no observed adverse effects level for subclinical effects observed in captive loon chicks. The remaining benchmarks (0.18 and 0.4 µg g(-1) wet wt) correspond to MeHg levels in prey fish associated with significant reproductive impairment and reproductive failure in wild adult loons. Overall, these benchmarks incorporate recent findings and reviews of MeHg toxicity in aquatic fish-eating birds and provide the basis for a national ecological risk assessment for Hg and loons in Canada.

Modelling mercury concentrations in prey fish: Derivation of a national-scale common indicator of dietary mercury exposure for piscivorous fish and wildlife

ABSTRACT This study, motivated by a resurgence in Cladophora, investigates changes in the nutrient environment in the littoral zone of Lake Ontario. We measured nutrient concentrations from 2004 to 2008 at two littoral zone (2–12 m) sites on the north shore of Lake Ontario where Cladophora has experienced a resurgence and compared concentrations with data collected in the late 1970s. Spring total phosphorus (TP) and soluble reactive P (SRP) concentrations have significantly declined at these two sites. Furthermore, P loading from the major tributaries to our study sites declined between 1964 and 2008. Upwelling events were not detectably associated with increases in P concentrations at our sites. We conclude that a recent upsurge in nuisance Cladophora, at least at these sites, cannot be explained by deteriorating littoral zone water quality in terms of P concentrations or by changes in catchment loading. For additional context, we also examined trends in coastal (14–20m) and offshore (>50m) nutrients using Environment Canada epilimnetic surveillance data, 1975–2008. Significant declines in TP and SRP concentrations have occurred in north coast waters, concurrent with declines in the offshore. However, nutrient concentrations, notably spring SRP, have not decreased among south coast stations, potentially reflecting greater coastal entrapment of catchment-derived waters. We infer that EC-monitored north coast stations reflect integrated interannual water quality, while south coast stations are more strongly influenced by catchment loading. The effects of higher nutrient concentrations along the south coast, which co-occur with lower water transparency, on benthic algal growth have yet to be determined.

Production and Respiration in Lake Erie Plankton Communities

The National Descriptive Model for Mercury in Fish (NDMMF) was applied to a Canada - wide dataset of fish mercury (Hg) concentrations to derive a common indicator of dietary methylmercury (MeHg) exposure (HgPREY) to piscivorous fish and wildlife. The NDMMF provided unbiased parameter estimates and strong spatial biases in prediction error were not apparent. Prediction error was comparatively higher when sample sizes were small and events with high Hg concentrations. Estimates of HgPREY from 1936 locations between 1990 and 2010 averaged 0.09 ± 0.07 μg g(-1) (wet wt) and increased from west to east in a manner consistent with independent measures of MeHg exposure in piscivorous wildlife and conceptual models of aquatic ecosystem sensitivity to Hg methylation and bioaccumulation. The HgPREY dataset offers an approach to evaluate the risk of MeHg exposure to piscivorous fish and wildlife on a continental scale.

Spatial patterns of methylmercury risks to common loons and piscivorous fish in Canada.

ABSTRACT Planktonic community metabolism (photosynthesis and respiration) was assessed in the oligotrophic east basin of Lake Erie, from November 2001 to October 2002 using O2 and 14C methods. Areal gross production (AGP; mmol O2 m−2 d−1) exceeded areal respiration (AR; mmol O2 m−2 d−1) in the surface mixed layer for 69% of the observations during the study period. The median AGP:AR for the entire study period was 1.32. A significant positive relationship between AGP and AR existed, but AGP explained only 25% of the variation in AR. AGP:AR varied seasonally, being below 1.0 in the fall/winter of 2001 and in early spring 2002. High (>> 1.0) AGP:AR was observed in late spring (May) and AGP:AR remained > 1.0 for most of the summer stratified period (July–September). AGP:AR was > 1.0 in the fall of 2002, but the magnitude was less than observed during spring. The results supported traditional concepts of the seasonal production and consumption cycles in planktonic communities of large oligotrophic lakes. Parallel incubations of 14C uptake and gross O2 production determined with the light and dark bottle method revealed a mean experimental gross photosynthetic quotient (PQG) of 1.29 ± 0.48, indicating that the 14C method used here had a variable but, on average, close relationship to gross production as it is commonly measured.

Nearshore–offshore differences in planktonic chlorophyll and phytoplankton nutrient status after dreissenid establishment in a large shallow lake

Deposition of inorganic mercury (Hg) from the atmosphere remains the principle source of Hg contamination for most aquatic ecosystems. Inorganic Hg is readily converted to toxic methylmercury (MeHg) that bioaccumulates in aquatic food webs and may pose a risk to piscivorous fish and wildlife. We conducted a screening-level risk assessment to evaluate the extent of risk to top aquatic piscivores: the common loon (Gavia immer), walleye (Sander vitreus), and northern pike (Esox lucius). Risk quotients (RQs) were calculated on the basis of a dietary Hg exposure indicator (HgPREY) modeled from over 230,000 observations of fish Hg concentrations at over 1900 locations across Canada and dietary Hg exposure screening benchmarks derived specifically for this assessment. HgPREY exceeded benchmark thresholds related to impaired productivity and behavior in adult loons at 10% and 36% of sites, respectively, and exceeded benchmark thresholds for impaired reproduction and health in fishes at 82% and 73% of sites, respectively. The ecozones of southeastern Canada characterized by extensive forest cover, elevated Hg deposition, and poorly buffered soils had the greatest proportion of RQs > 1.0. Results of this assessment suggest that common loons and piscivorous fishes would likely benefit from reductions in Hg deposition, especially in southeastern Canada.

Nearshoreoffshore comparison of chlorophyll a and phytoplankton production in the dreissenid-colonized eastern basin of Lake Erie

Abstract In Lake Simcoe, a large lake in southern Ontario, Canada, with more than 50% of its surface area <15 m deep, dreissenid mussels are abundant in the extensive nearshore zone but not offshore. We hypothesized that mussel grazing would depress chlorophyll a (Chl-a) concentrations in the nearshore compared to the offshore while alleviating nearshore phosphorus (P) deficiency through nutrient regeneration. During both years of our study Chl-a concentration and other indicators of phytoplankton biomass, including particulate carbon (C), nitrogen, P, and silicon, were lower in the nearshore areas of Lake Simcoe where the exotic invader Dreissenia polymorpha was in contact with overlying epilimnetic water. In the first year of our study, grazing and associated nutrient regeneration activity seemed to reduce P deficiency in phytoplankton in the dreissenid-impacted shallow locations. In the second year, however, phytoplankton in the nearshore dreissenid-affected areas remained as strongly P deficient as phytoplankton in offshore waters physically separated from dreissenid grazing. Photoacclimation in the nearshore phytoplankton was evident in higher particulate C:Chl-a ratios and higher effective absorptive cross section of photosystem II (σPS II) throughout the stratified sampling season compared to offshore phytoplankton. A multiple linear regression utilizing the mean light intensity in the mixed layer as well as total P (TP) resulted in better predictions of Chl-a than TP alone. We conclude that, in shallow lakes where transparency is strongly impacted by dreissenid grazing, the comparison of Chl-a–TP relationships over time will require accounting for the effect of changing transparency.