Watershed influences on the transport, fate and bioavailability of mercury in Lake Superior: Field measurements and modelling approaches: Fid-Bau Portal

Watershed influences on the transport, fate and bioavailability of mercury in Lake Superior: Field measurements and modelling approaches

Back, Richard C. / Hurley, James P. / Rolfhus, Kristopher R.

Abstract

The issuance of fish consumption advisories in US states bordering Lake Superior has heightened the need for understanding the biogeochemical cycling and transformations of mercury in this great lake. Major routes of mercury (Hg) transport to lakes include atmospheric deposition (wet and dry), direct discharges and riverine (watershed) inputs. The specific objectives of this ongoing study are to: (i) determine the speciation and bioavailability of Hg transported to Lake Superior (ii) determine the importance of watershed‐specific characteristics that control physical and chemical forms of Hg; (iii) identify key mechanisms controlling Hg bioavailability and speciation in near‐shore zones relative to open lake regions; and (iv) provide process‐level information to compliment concurrent development of Hg fate and transport models of the Lake Superior ecosystem. Three tributaries of Lake Superior were chosen for detailed field study based on previous results and particular watershed characteristics. Mixing zones represent a potentially important zone of Hg entry into the food chain, and were sampled for biota, water and sediment in transects between the tributary mouth and the lake end member. Sampling of open‐water sites was conducted during Environmental Protection Agency‐sponsored cruises on the research vessel Lake Guardian. Results from our work on Lake Superior tributaries demonstrate that dissolved organic carbon and methylmercury (MeHg) export was greatest from watersheds containing wetlands. In Lake Superior, concentrations of Hg species were small at pelagic stations (total mercury (HgT) averaged 0.49 ng/L, MeHg averaged 6.4 pg/L). In contrast, MeHg concentrations in tributaries ranged from 100 to 250 pg/L. Watershed sites dominated by wetlands exhibited the greatest MeHg concentrations, occasionally in excess of 12 ng/L. Samples of bulk zooplankton ranged from 20 to 130 ng/g dry weight HgT and from 15 to 45 ng/g dry weight MeHg, and demonstrated typical bioacculmulation of mercury as well as distinct seasonality in concentrations.