Seasonal Variation of Mercury Associated with Different Phytoplankton Size Fractions in Lahontan Reservoir, Nevada

Abstract

Sampling is conducted during 2006 in Lahontan Reservoir, Nevada to investigate seasonal variation of total mercury (THg) and methylmercury (MeHg) partitioning in different phytoplankton size fractions as a function of point source (fluvial) mercury (Hg) loads, reservoir residence time, and algal growth. Carson River Hg inputs into the reservoir are extremely dynamic with spring loads two orders of magnitude larger than summer loads. Chlorophyll a measurements show two periods of algal growth. A small amount of algal growth occurs March to May. A second more substantial bloom occurs in the late summer, which is dominated by large, filamentous algae. THg concentrations (Cb) and partitioning coefficients (Kd) in total suspended particulate matter (SPM) are highest when fluvial inputs of Hg-contaminated sediment are large and are not necessarily associated with living biomass. However, MeHg Kd in the small size fraction is indirectly related to fluvial loads and more strongly associated with living biomass in the later portion of the summer when algal growth occurs and reservoir residence times are longer. Data suggest size distinction is important to MeHg partitioning in the reservoir. Lumping all sizes into a single SPM sample will bias the analysis toward low MeHg Cb and low MeHg Kd in late summer when Aphanizomenon flos-aquae dominates the phytoplankton assemblage.