Riverine mixing zones as regions of enhanced methylmercury bioaccumulation in Lake Superior

Abstract

Riverine mixing zones along the southern shore of Lake Superior were sampled in Spring and Fall over a 2 year period (2000-2001) at the following sites: St. Louis River (STL), Fish Creek (FSC), and Tahquamenon River (TAQ). Each nearshore area was sampled along a transect moving from the river mouth, through the mixing zone, toward the offshore waters. Samples for unfiltered, filtered and particulate Hg T and MeHg, SPM, DOC and chl-a were collected from each transect site. The TAQ was re-sampled in Spring 2002 and size-specific seston was also collected from the water column at the mouth and offshore sites. Concentrations of SPM, DOC, unfiltered, filtered and particulate Hg T and MeHg generally decreased between the river mouth and the offshore site of each mixing zone. Decreases in concentrations were presumably the result of dilution as concentrated river water mixed with pristine lake water, or fallout of larger particles with distance. Chl-a appeared to be unaffected by dilution and remained either constant or increased along each transect. Despite the observed decreases in MeHg species in the water column, the amount of MeHg retained on particles always increased along the same transect with a maximum reached in the offshore site. Large increases in the log(K d ) for MeHg, and the biological enrichment in the SPM pool in the offshore waters, suggest that MeHg is strongly bioaccumulated in these riverine mixing zones.