Ecological determinants of methylmercury bioaccumulation in benthic invertebrates of polar desert lakes

We investigated concentrations of monomethyl-mercury (MMHg) at the base of benthic food webs in six lakes from polar desert (biologically poor and low annual precipitation) on Cornwallis Island (Nunavut, Canada, similar to 75 degrees N latitude). Anthropogenic mercury emissions reach the Arctic by long-range atmospheric transport, and information is lacking on processes controlling MMHg entry into these simple lake food webs, despite their importance in determining transfer to lake-dwelling Arctic char. We examined the influences of diet (using carbon and nitrogen stable isotopes), water depth, and taxonomic composition on MMHg bioaccumulation in benthic invertebrates (Chironomidae and Trichoptera). We also estimated MMHg biomagnification between benthic algae and invertebrates. Similar MMHg concentrations of chironomid larvae in nearshore and offshore zones suggest that benthic MMHg exposure was homogeneous within the lakes. Chironomid delta C-13 values were also similar in both depth zones, suggesting that diet items with highly negative delta C-13, specifically methanogenic bacteria and planktonic organic matter, were not important food (and therefore mercury) sources for profundal larvae. MMHg concentrations were significantly different among two subfamilies of chironomids (Diamesinae, Chironominae) and Trichoptera. Higher MMHg concentrations in Diamesinae were likely related to predation on other chironomids. We found high MMHg biomagnification between benthic algae and chironomid larvae compared with literature estimates for aquatic ecosystems at lower latitudes; thus, benthic processes may affect the sensitivity of polar desert lakes to mercury. Information on benthic MMHg exposure is important for evaluating and tracking impacts of atmospheric mercury deposition and environmental change in this remote High Arctic environment.