SEASONAL TRENDS IN MERCURY BIOACCUMULATION IN ZEBRA MUSSELS (DREISSENA POLYMORPHA) IN A TEMPERATE EUTROPHIC LAKE

Abstract

The zebra mussel (Dreissena polymorpha) invasion in North America began in the 1980’s, has led to the mussels becoming a significant part of the diet of many species of fish and birds as they spread to new water bodies. As such, they represent a new route of trophic introduction of mercury (Hg) in many North American lakes. Studies in estuaries and oligotrophic lakes have observed various seasonal patterns in zebra mussel tissue Hg concentration. However, there has never been a published study observing seasonal trends in Hg concentrations in zebra mussel tissue in an urban eutrophic dimictic lake. Hg concentrations in phytoplankton and zooplankton spike following fall turnover in eutrophic dimictic lakes due to MeHg produced in the hypolimnion during summer stratification mixing through the water column, making it accessible to the aquatic food web, including zebra mussels. We conducted a study examining seasonal trends in Biological Total Hg (BTHg) concentrations in zebra mussel tissue from summer through late fall. We collected 73 samples (n=73) across over 13 sampling events with 2 size classes (6-13 mm and 13-20 mm). We found that BTHg concentrations in zebra mussel tissue were highest in the summer when mussels were most metabolically active, decreased as water temperatures dropped in the early fall, and then increased post- turnover despite continued drop in water temperature. These findings demonstrate that seasonal changes such as temperature and turnover events are important to consider when studying zebra mussel Hg bioaccumulation in eutrophic lakes.