Climatic, abiotic and biotic drivers of mercury in freshwater fish in northern ecoystems (Climer)

Mercury (Hg) pollution is a global environmental problem. Northern lakes have been contaminated with Hg from long-range transported air pollution and local industrial sources. Even though industrial emissions of Hg have been strongly reduced in Northern countries, global emissions of Hg are still on the rise. Because of its volatility, Hg can be transported across national, continental and even hemispheric boundaries, implying that reduction of Hg pollution requires an international approach. In the environment, formation of highly toxic methyl-Hg (MeHg) is what causes most concern because of its effective bioaccumulation in fish, thereby posing health risk for both wildlife and humans. But how does Hg make it all the way from the atmosphere to fish? And how sensitive is that pathway to environmental change? In the Climer project, we have investigated key factors that drive transport of Hg from soils via surface waters into aquatic foodwebs. We were especially interested in the role of dissolved organic matter (DOM). In recent decades, DOM in boreal waters has doubled or tripled as a combined effect of cleaner air (reduced acid deposition) and climate change. DOM impacts Hg in multiple, partially counteracting ways such as promoting transport, changing bioavailability and reducing loss of MeHg by sunlight. We collated fish Hg records from five decades of monitoring in 3000 lakes across Fennoscandia (Norway, Sweden, Finland and the Russian Murmansk area), and found strong Hg declines in perch and pike. The health advisory limit (0.5 mg Hg/kg) for freshwater fish is exceeded in 25% of all lakes sampled after 2000, but far more lakes now have fish with Hg levels below the health limit than in the 1970s. We observed that the reduction in Hg was similar in lakes mainly impacted by Hg from air pollution and in lakes with nearby industrial sources of Hg, suggesting that overall drivers common to both lake types may be responsible. In the boreal lake Langtjern in southern Norway, long-term records of insects (chironomid larvae) showed a very similar decline in Hg as the pike and the perch in the Fennoscandian database. This could imply a causal link between the lower and the higher foodweb. The observed decline in MeHg in the insects coincides with a strong decline in sulfate deposition and higher DOM in this lake, and we suspect that these changes in water chemistry may have altered exposure of the food web to Hg (e.g. through changes in Hg methylation or availability for uptake into the food web). In a paired-lake study, with high and low DOM lakes positioned along climate and deposition gradient from south to north Scandinavia, we tested hypotheses on the importance of diet, DOM and food web characteristics on Hg bioaccumulation. As we expected, high DOM lakes had higher Hg in water and the food web, indicating that DOM transports Hg from sources on land and into the food chain. Also, zooplankton and fish in high DOM lakes showed clear signs of higher body contents of food with poor nutritional quality (more food derived from bacteria and terrestrial organic material, and less from nutrient-rich algae). We observed similar patterns in a broader 25-lake survey. We suggest that fish in brown lakes are a more contaminated and less nutritious food source to wildlife and human consumers. We also found that fish in subarctic lakes generally contain lower Hg than in boreal lakes, even though fish in colder regions grow more slowly and can live longer than in a warmer climate. Low growth rates and a long life span are usually combined with high fish Hg accumulation. We conclude that along the climate gradient, exposure to Hg beats temperature as a driver for fish Hg accumulation. Exposure is a function of deposition and DOM, and especially DOM is sensitive to environmental change, implying that if lake browning occurs in subarctic areas, fish Hg could increase. In summary, our project contributes to a better risk assessment of Hg contamination of freshwater fish in northern regions. The remarkable decline in fish Hg since the 1970s in all of Fennoscandia is not fully understood, but it appears that cleaner air (sulfate deposition) and lower dispersal of Hg in the environment (by air, and from industries that release Hg to water) both helped to lower fish Hg levels. Climate impacts are harder to demonstrate but warming could impact growth rates, while a wetter climate will lead to browning. Prediction of future Hg levels in fish, given that environmental change impacts Hg in fish in multiple ways, is challenging. Global solutions to Hg pollution are sought through the Minamata Convention, hopefully leading to lower Hg deposition in Northern regions. We recommend more research on interactions between changes in water chemistry and climate for availability and transport of Hg. Furthermore, we suggest international guidelines for monitoring of Hg in fish, to allow for a higher consistence of temporal records.

Scientific: increased understanding of transport of Hg from air to catchments to food webs, in relation to perceived drivers of change: air quality (emissions of pollutants) and climate. Promotion of international collaboration on mercury cycling in northern ecosystems (Norway, Sweden, Finland, Canada). Societal: 1. A better basis for health advisories for fish mercury (using the collated fish database) 2. Improved monitoring guidelines for monitoring of aquatic endpoints under the Minamata Convention (we contributed to chapters of reports prepared under the Minamata Convention) 3. Dissimination of results to the bodies under the Convention of Long-Range Transported Air Pollution (UNECE). Here, effect-based policy is key, and documentation of environmental effects of policies to reduce emissions of air pollutions is central. 4. Dissemination to national environmental authorities through workshop and planned popular scientific publications.

Mercury (Hg) in freshwater fish, especially in its organic and most toxic form methyl-Hg (MeHg), has shown increases in recent decades, leading to increased risk of Hg exposure for fish consumers (including wildlife and humans). The provision of healthy and uncontaminated fish for humans and wildlife by surface waters is under pressure. Key questions regarding Hg levels in biota relate to identification of driving factors of Hg in surface waters such as catchment Hg export, factors stimulating or limiting MeHg production, degradation, transfer into the food web and trophic magnification. Current trends of large scale environmental change such as climate change, reduced sulfur deposition and surface water browning may have direct and indirect impacts on food web Hg exposure and bioaccumulation. Here, we propose to test impacts of such large scale environmental change on Hg in aquatic food webs. We will investigate spatial patterns and temporal trends in Hg in food webs across climatic and depositional gradients by 1) sampling fish and lower trophic levels in boreal and subarctic lakes; 2) analyzing temporal trends in Hg in fish in existing time-series (and adding to those time-series by targeted sampling) in relation to climate, deposition and water quality; 3) studying key processes in catchments, lakes and food webs in greater detail and using obtained insights in process-based models of catchment and lake Hg cycling and trophic transfer of Hg; 4) building on expertise and existing data within the project international project consortium in order to improve the quality and reliability of predictions of food web exposure to Hg and food web Hg levels in boreal lakes. The overarching aim of the project is to better assess present and future risk of Hg levels in biota and associated changes in ecosystem value from a toxicological and fisheries perspective.