Mercury Bioaccumulation in Lacustrine Fish Populations Along a Climatic Gradient in Northern Ontario, Canada

Mixed Model Approaches Can Leverage Database Information to Improve the Estimation of Size-Adjusted Contaminant Concentrations in Fish Populations

Concentrations of bioaccumulative contaminants in fish increase with their size and age; thus, research and monitoring of these contaminants in fish across space and time can be confounded by size covariation. To account for this, size-standardization of contaminant concentrations within fish samples is a common practice. Standardized concentrations are often estimated using within-sample regression models, also known as power series regression (referred to here as sampling event regressions, or SERs). This approach requires higher sample sizes than mixed effect models (MEMs), which are suited for this application but are not as commonly used. Herein we compare SERs to three MEM approaches; restricted maximum likelihood, Bayesian inference via Markov chain Monte Carlo (MCMC), and approximate Bayesian inference with nested Laplace approximation (INLA). We did this for two contaminants: mercury (Hg), a contaminant known to bioaccumulate, and arsenic (As), where the bioaccumulative potential is less understood. The MEM approaches generated size-standardized concentrations for small populations (e.g., <5 fish) and/or populations that lacked the range of sizes required for SER estimates, with comparable residual and root mean squared error to SER estimates. INLA was determined to be the best method in most cases because it was computationally less intensive than other approaches and showed consistent performance across a range of scenarios with sample-size limitations. Additionally, we provided example code for prediction using the R-INLA package to enable use and application in fisheries' contaminant monitoring and research.

Increased mercury concentrations in walleye and yellow perch in lakes invaded by zebra mussels

Zebra mussels (Dreissena polymorpha) are invasive species that alter ecosystems and food webs with the potential to affect aquatic mercury cycling and bioaccumulation in fishes, although the effect of zebra mussels on fish tissue mercury has not been tested in inland lakes. We assessed differences in fish tissue mercury concentrations and food webs in Minnesota lakes with and without zebra mussels while controlling for other lake and watershed characteristics. Mercury concentrations in adult walleye (Sander vitreus) and yellow perch (Perca flavescens) were 72 % and 157 % higher, respectively, in lakes containing zebra mussels compared to uninvaded lakes. Mercury in young of year (age-0) fish was also elevated, with mercury concentrations 97 % and 82 % higher in age-0 walleye and yellow perch, respectively, in zebra mussel lakes. Walleye mercury concentrations exceeded 0.22 ppm - a threshold triggering more restrictive human consumption advisories for sensitive populations - at a 23 % smaller size, and average-sized walleye (420 mm) exceeded this threshold at a rate of 77 % in invaded lakes, compared to 35 % in uninvaded lakes. Walleye and yellow perch relied more on littoral resources in lakes with zebra mussels but did not feed at meaningfully higher trophic levels. Increased fish tissue mercury in lakes invaded by zebra mussels have consequential implications for fisheries and human health.

Can remotely sensed catchment to lake area ratios predict mercury levels in subarctic fishes?

Mercury concentrations ([Hg]) in fish reflect a complex array of interacting biogeochemical and ecological variables. In northern regions where fish are a critical subsistence food, understanding and predicting fish [Hg] can be particularly difficult, largely due to a paucity of comprehensive data associated with the logistical challenges of field sampling. Building on previous work where we elucidated causal relationships between fish [Hg] and a variety of catchment, water quality, and ecological variables in subarctic lakes, we investigated whether using only ratios of catchment area to lake area (CA:LA) can predict [Hg] in northern freshwater fish species. As CA:LA can be sensed remotely, they may be more feasible and practical to obtain than field data in far northern regions. Our study included thirteen remote lakes that represent a CA:LA gradient of 6.2-423.5 within an ∼66,000 km2 subarctic region of Northwest Territories, Canada. We found that size-standardized [Hg] in three widespread fish species, including Lake Whitefish (Coregonus clupeaformis), Walleye (Sander vitreus), and Northern Pike (Esox lucius), were significantly and positively related to CA:LA (p < 0.007, r2 = 67-80%), indicating higher fish [Hg] in smaller lakes surrounded by relatively larger catchments. Our findings provide compelling evidence that remotely sensed CA:LA can be used to predict [Hg] in northern fishes and aid in prioritizing understudied and subsistence fishing lakes of the Canadian subarctic for [Hg] monitoring programs.

Global change effects on biogeochemical mercury cycling

Past and present anthropogenic mercury (Hg) release to ecosystems causes neurotoxicity and cardiovascular disease in humans with an estimated economic cost of $117 billion USD annually. Humans are primarily exposed to Hg via the consumption of contaminated freshwater and marine fish. The UNEP Minamata Convention on Hg aims to curb Hg release to the environment and is accompanied by global Hg monitoring efforts to track its success. The biogeochemical Hg cycle is a complex cascade of release, dispersal, transformation and bio-uptake processes that link Hg sources to Hg exposure. Global change interacts with the Hg cycle by impacting the physical, biogeochemical and ecological factors that control these processes. In this review we examine how global change such as biome shifts, deforestation, permafrost thaw or ocean stratification will alter Hg cycling and exposure. Based on past declines in Hg release and environmental levels, we expect that future policy impacts should be distinguishable from global change effects at the regional and global scales.

Age, body size, growth and dietary habits: What are the key factors driving individual variability in mercury of lacustrine fishes in northern temperate lakes?

Fish total mercury concentration ([THg]) has been linked to various fish attributes, but the relative importance of these attributes in accounting for among-individual variation in [THg] has not been thoroughly assessed. We compared the contributions of ontogeny (age, length), growth (growth rate, body condition), and food web position (δ¹³C, δ¹⁵N) to among-individual variability in [THg] within populations of seven common fishes from 141 north temperate lakes. Ontogenetic factors accounted for most variation in [THg]; age was a stronger and less variable predictor than length for most species. Adding both indices of growth and food web position to these models increased explained variation (R²) in [THg] by 6-25% among species. Fish [THg] at age increased with growth rate, while fish [THg] at length decreased with growth rate, and the effect of body condition was consistently negative. Trophic elevation (inferred from δ¹⁵N) was a stronger predictor than primary production source (inferred from δ¹³C) for piscivores but not benthivores. Fish [THg] increased with δ¹⁵N in all species but showed a more variable relationship with δ¹³C. Among-individual variation in [THg] is primarily related to age or size in most temperate freshwater fishes, and effects of growth rate and food web position need to be considered in the context of these ontogenetic drivers.

Beyond the Corporatization of Death Systems: Towards Green Death Practices

One less explored area of research concerns the response to the ecological crisis through environmentally sustainable death practices, which we broadly define in this paper as ‘green death practices’. In this paper, interdisciplinary research and scholarship are utilized to critically analyze death practices, and to demonstrate how contemporary Westernized death practices such as embalming, traditional burial, and cremation can have harmful environmental and public health implications. This paper also investigates the multi-billion-dollar funeral industry, and how death systems which place economic growth over human wellbeing can be socially exploitative, oppressive, and marginalizing towards recently bereaved persons and the environment. Death-care as corporatized care is explicitly questioned, and the paper provides a new social vision for death systems in industrialized Western societies. Ultimately, the paper advocates for how green death practices may offer new pathways for honoring our relationships to the planet, other human beings, and even our own deepest values.

Current understanding of the ecological risk of mercury from subsea oil and gas infrastructure to marine ecosystems

Many oil and gas fields are nearing production cessation and will require decommissioning, with the preferred method being complete infrastructure removal in most jurisdictions. However, decommissioning in situ, leaving some disused components in place, is an option that may be agreed to by the regulators and reservoir titleholders in some circumstances. To understand this option's viability, the environmental impacts and risks of any residual contaminants assessed. Mercury, a contaminant of concern, is naturally present in hydrocarbon reservoirs, may contaminate offshore processing and transmission infrastructure, and can biomagnify in marine ecosystems. Mercury's impact is dependent on its speciation, concentration, and the exposure duration. However, research characterising and quantifying the amount of mercury in offshore infrastructure and the efficacy of decontamination is limited. This review describes the formation of mercury-contaminated products within oil and gas infrastructure, expected exposure pathways after environmental release, possible impacts, and key research gaps regarding the ecological risk of in situ decommissioned contaminated infrastructure. Suggestions are made to overcome these gaps, improving the in situ mercury quantification in infrastructure, understanding environmental controls on, and forecasting of, mercury methylation and bioaccumulation, and the cumulative impacts of multiple stressors within decommissioned infrastructures.

Environmental and biological factors are joint drivers of mercury biomagnification in subarctic lake food webs along a climate and productivity gradient

Subarctic lakes are getting warmer and more productive due to the joint effects of climate change and intensive land-use practices (e.g. forest clear-cutting and peatland ditching), processes that potentially increase leaching of peat- and soil-stored mercury into lake ecosystems. We sampled biotic communities from primary producers (algae) to top consumers (piscivorous fish), in 19 subarctic lakes situated on a latitudinal (69.0-66.5° N), climatic (+3.2 °C temperature and +30% precipitation from north to south) and catchment land-use (pristine to intensive forestry areas) gradient. We first tested how the joint effects of climate and productivity influence mercury biomagnification in food webs focusing on the trophic magnification slope (TMS) and mercury baseline (THg baseline) level, both derived from linear regression between total mercury (log₁₀THg) and organism trophic level (TL). We examined a suite of environmental and biotic variables thought to explain THg baseline and TMS with stepwise generalized multiple regression models. Finally, we assessed how climate and lake productivity affect the THg content of top predators in subarctic lakes. We found biomagnification of mercury in all studied lakes, but with variable TMS and THg baseline values. In stepwise multiple regression models, TMS was best explained by negative relationships with food chain length, climate-productivity gradient, catchment properties, and elemental C:N ratio of the top predator (full model R² = 0.90, p < 0.001). The model examining variation in THg baseline values included the same variables with positive relationships (R² = 0.69, p = 0.014). Mass-standardized THg content of a common top predator (1 kg northern pike, Esox lucius) increased towards warmer and more productive lakes. Results indicate that increasing eutrophication via forestry-related land-use activities increase the THg levels at the base of the food web and in top predators, suggesting that the sources of nutrients and mercury should be considered in future bioaccumulation and biomagnification studies.

Regulation of glutathione-dependent antioxidant defense system of grass carp Ctenopharyngodon idella under the combined stress of mercury and temperature

In this study, we investigated the combined effects of temperatures fluencies and mercury (Hg) on glutathione-dependent antioxidant system in fish, by measuring the oxidative stress indicator (LPO, lipid peroxidation) and the parameters involved in the glutathione-related antioxidant defense system (GPx, glutathione peroxidase; GR, glutathione reductase; GST, glutathione S-transferase; GSH, reduced glutathione), as well as the expression of related genes in grass carp, Ctenopharyngodon idella. Fish (45.37 ± 3.58 g) were exposed to 10 test groups, e.g., 15 °C with/without Hg, 20 °C with/without Hg, 25 °C with/without Hg, 30 °C with/without Hg, 35 °C with/without Hg for 4 weeks. Three-way ANOVA was used to analyze the correlation between the measured parameters and experimental conditions (water temperature, Hg exposure, exposure time, and their interactions.). Our results show that there is no interaction between mercury and low temperature, but the combined effect at high temperature has been confirmed, which indicated the glutathione-dependent enzyme system in grass carp has a complex regulatory mechanism with temperature fluctuations. In the actual field monitoring, it is necessary to consider the impact of extreme temperature on the toxicity of pollutants in the aquatic ecosystem.

How Do Indirect Effects of Contaminants Inform Ecotoxicology? A Review

Indirect effects in ecotoxicology are defined as chemical- or pollutant-induced alterations in the density or behavior of sensitive species that have cascading effects on tolerant species in natural systems. As a result, species interaction networks (e.g., interactions associated with predation or competition) may be altered in such a way as to bring about large changes in populations and/or communities that may further cascade to disrupt ecosystem function and services. Field studies and experimental outcomes as well as models indicate that indirect effects are most likely to occur in communities in which the strength of interactions and the sensitivity to contaminants differ markedly among species, and that indirect effects will vary over space and time as species composition, trophic structure, and environmental factors vary. However, knowledge of indirect effects is essential to improve understanding of the potential for chemical harm in natural systems. For example, indirect effects may confound laboratory-based ecological risk assessment by enhancing, masking, or spuriously indicating the direct effect of chemical contaminants. Progress to better anticipate and interpret the significance of indirect effects will be made as monitoring programs and long-term ecological research are conducted that facilitate critical experimental field and mesocosm investigations, and as chemical transport and fate models, individual-based direct effects models, and ecosystem/food web models continue to be improved and become better integrated.