Fluxes, pools, and turnover of mercury in Swedish forest lakes

Abstract

In boreal forest lakes, high Hg concentrations in fish are common, even in remote areas. Data from recent Swedish surveys were synthesized to assess sources, sinks, fluxes and turnover rates of Hg in headwater lakes of different type, to determine and quantify controlling factors and their relative influence, and to discuss implications for the contamination of fish and potential remedial actions. In humic lakes, annual input of Hg is dominated by the transport from forest soils, whereas in clearwater lakes, direct deposition on the lake surface is often more important. Direct deposition is more likely to dominate during the growing season when the water discharge in forest streams is low. Sedimentation of Hg is largely controlled by the water residence time and is related to water quality. In clearwater lakes, most of the Hg load is deposited in the sediment, which is in contrast to humic lakes, and which partly explains the susceptibility of headwater lakes to atmospheric Hg pollution. The fish community contains about half or more of the methylmercury pool in the water column, and internal recycling in the food web may be important. However, only a minor fraction of the total annual Hg load to lakes is channeled through the fish community. As this allows large variations in the ecological fractionation and bioavailability of Hg, differences in Hg fluxes can only partly explain the wide range of Hg concentrations found in fish from different lakes. The annual Hg load to headwater lakes exceeds the total Hg pool in the water column by a factor of up to 20. Single remedial measures aiming at the removal of Hg from the water column are therefore likely to have little success. Local actions should aim primarily at reducing the bioavailability of the Hg entering the lakes and need to be repeated continuously.