Elevated Methylmercury in Water and Aquatic Food Webs in Lagoons

We assessed mercury contamination of water, sediment, and selected aquatic biota from two lagoons in the Apostle Islands National Lakeshore (southern Lake Superior). These nearshore lagoon systems appear to be mercury-sensitive environments with unusually high rates of production of methylmercury, a highly toxic compound that readily bioaccumulates in aquatic food webs. Concentrations of methylmercury in oxic, near-surface water from these systems were unusually high (averaging 1.3 ng L in Outer Lagoon and 2.5 ng L in Stockton), with observed values greatly exceeding reported concentrations for Lake Superior and inland lakes of the region. Methylmercury concentrations in bulk samples of zooplankton from these systems were also high (averaging 481 ng g dry weight in Outer Lagoon and 270 ng g in Stockton) and substantially exceeded values reported in the literature or measured in samples taken from regional inland lakes. In contrast, concentrations of total mercury in surface water (averaging 4.5 ng L in Outer and 10 ng L in Stockton) and sediment from these systems were not unusual, and were within or near the range of observed values for regional lakes. The fraction of total mercury present as methylmercury in sediment was much less than that in water, leading us to hypothesize that the high concentrations of aqueous methylmercury in the lagoons resulted from high rates of mercury methylation in the water column. Concentrations of total mercury in whole northern redbelly dace Phoxinus eos, a prey fish sampled from the Stockton lagoon, averaged 0.105 μg g (parts per million) wet weight and ranged from 0.070 to 0.193 μg g. Total mercury in axial muscle tissue of northern pike Esox lucius from Outer lagoon averaged 0.78 μg g wet weight and ranged from 0.25 to 1.15 μg g, exceeding the U.S. Environmental Protection Agency methylmercury criterion of 0.3 μg g wet weight in 15 of 17 fish analyzed. We conclude that these lagoon systems are mercury-sensitive environments that actively produce methylmercury, a toxic compound that may be bioaccumulating to harmful concentrations in organisms in upper trophic levels of food webs in these lagoon systems.