A comparison of mercury cycling in Lakes Michigan and Superior

Mercury cycling in Lake Superior and Lake Michigan was evaluated based on measurements of mercury levels, modeling of evasional fluxes, and development of first-order mass balance models. Total mercury, methylmercury, and dissolved gaseous mercury were measured on sampling cruises in Lake Michigan (2005) and Lake Superior (2006). Average total mercury concentrations in unfiltered surface water were higher in Lake Michigan (420 ± 40 pg/L) compared to Lake Superior (210 ± 20 pg/L). Methylmercury levels were below the detection limit in Lake Michigan. Larger sample volumes were collected to lower detection limits in Lake Superior in 2006 and methylmercury levels averaged 7 ± 6 pg/L. Dissolved gaseous mercury concentrations were also higher in Lake Michigan (27 ± 7 pg/L) compared to Lake Superior (14 ± 8 pg/L). Evasional fluxes were estimated using a two-film model for air-water exchange. The annual evasional flux in Lake Michigan was determined to be ~ 380 kg/yr from Lake Michigan and ~ 160 kg/yr from Lake Superior. Total mercury burdens in each lake were estimated to be ~ 2500 kg in Superior and ~ 2100 kg in Lake Michigan demonstrating that evasional fluxes play an important role in the mass balance of each lake, particularly Lake Michigan. A simple first-order mass balance model demonstrates the importance of air-water exchange and sedimentation as primary removal processes for Hg in each lake. Uncertainties in the mass balance model are highlighted due to lack of key data, particularly in Lake Superior.