Elevated methylmercury in High Arctic Daphnia and the role of productivity in controlling their distribution

Mercury is a contaminant of concern in polar regions due to long-range atmospheric transport of this metal from southern latitudes followed by intense deposition on snow. We surveyed zooplankton in 16 lakes and ponds in the Canadian Arctic Archipelago (74-76°N) to determine methylmercury (MeHg) content and the role of environmental characteristics and taxonomic composition on accumulation processes. Zooplankton communities containing Daphnia (mainly D. middendorffiana) had on average five times the MeHg content of copepod-dominated communities. The percent biomass of Daphnia best explained MeHg variation in bulk zooplankton compared with water chemistry and morphometric variables. Water-column concentrations of MeHg were low at most study sites (mainly [less-than or equal to]0.07 ng L⁻¹), and Daphnia strongly bioaccumulated mercury through species-specific processes. As we observed Daphnia in more productive water bodies (i.e., ponds, a eutrophied lake), we then tested the role of productivity in determining the distribution of this keystone herbivore using a broad-scale literature dataset of 47 High Arctic lakes (65-77°N). Daphnia density was positively related to the amount of organic carbon in the water column in both dissolved and particulate fractions [dissolved organic carbon (DOC) partial [graphic removed] , P < 0.001; particulate organic carbon (POC) partial [graphic removed] , P=0.032]. The strong influence of DOC suggests that bacterial production is an important energy source for Arctic Daphnia. Our findings indicate that productivity influences the MeHg content of zooplankton communities through its control of species composition; specifically, low productivity limits the presence of mercury-rich Daphnia in many copepod-dominated lakes of the High Arctic. Aquatic productivity is expected to increase with climate warming, and we present a conceptual model that predicts how environmental drivers could extend the distribution of Daphnia in lakes and alter the movement of mercury in food webs of the Canadian High Arctic.