Seasonal and size-specific distribution of methylmercury in seston and zooplankton of two contrasting Great Lakes embayments

The use of a large volume sieve system is described which allows quantification of mass, pigments, and methylmercury (MeHg) in five distinct size classes of suspended material: 243 mum, 112-243 mum, 63-112 mum, 35-63 mum, and < 35 mum. These stacked sieves were used in concert with standard zooplankton tows to examine the MeHg distribution, with respect to particle size, zooplankton species and season, in eutrophic Green Bay (Lake Michigan) and oligotrophic Whitefish Bay (Lake Superior). The two sites, sampled during spring (May), summer (July), and fall (October) of 1998 differed in both the size distribution of methylmercury and in the seasonal pattern of concentrations and size distributions. In Green Bay, where suspended particulate mass is 5 to 15 times higher than Whitefish Bay, significant mass of methylmercury was found in size fractions greater than 35 mum, particularly in the summer, when larger phytoplankton were present. In Whitefish Bay, on the other hand, suspended particulate mass is low, and particulate methylmercury was found almost exclusively in the < 35 mum size fraction with the seasonal peak in the spring.