Metamorphosis in chironomids, more than mercury supply, controls methylmercury transfer to fish in High Arctic lakes.

Lake-dwelling Arctic char (Salvelinus alpinus) are monitored internationally as a sentinel species for effects of atmospheric mercury deposition on Arctic fresh waters. We investigated the control of mercury supply and biological processes on the methylmercury (MeHg) content of their main food, aquatic chironomids, in 22 lakes and ponds in the Canadian High Arctic. Total mercury (THg) concentrations in sediment (corrected for organic matter content) increased with drainage basin size, suggesting a gradient in mercury loading among study sites. MeHg concentrations in sediment and water were low and relatively uniform along this THg supply gradient suggesting MeHg production in High Arctic lakes is weakly coupled to inorganic mercury supply. Metamorphosis was a key biological process that concentrated MeHg in adult chironomids 1.7-2.9 times more than in immature stages. Drainage basin size, environmental mercury levels, and habitat characteristics were also significant factors but they explained less variation in chironomid MeHg concentration than their degree of maturity. Chironomid larvae, pupae, and adults are distinct mercury sources for fish, and we provide evidence from nitrogen stable isotopes and published feeding studies that suggest differential consumption of these stages may affect MeHg uptake by Arctic char. We conclude that biological and food web processes have a greater impact on MeHg transfer to fish than atmospheric mercury deposition in High Arctic lakes.