Hypolimnetic methylmercury and its uptake by plankton during fall destratification: A key entry point of mercury into lake food chains?

Abstract

Seasonal concentration changes in monomethylmercury (MMHg) and total Hg (T-Hg) were determined in Devil's Lake, Wisconsin, a lake with a mercury consumption advisory for walleye. Mercury dynamics were studied in water and several lower food chain fractions during the ice-free seasons in 1994 and 1995, and limited data for Hg in water were collected in 1993. MMHg concentrations increased in hypolimnetic waters before turnover each year, although maximum concentrations declined from 1993 to 1995. As the hypolimnion eroded, MMHg concentrations increased in both particulate matter and Daphnia. Maximum concentrations were obtained near the time of complete mixis. The magnitude of this increase correlated with the mass of hypolimnetic MMHg that had built up prior to turnover in 1994 and 1995. Hg concentrations in yearling planktivorous fish exhibited a decline corresponding to the decline in hypolimnetic MMHg from 1993 to 1995. Our results suggest that fall destratification represents an important time period for entry of MMHg to the food chain of lakes exhibiting a hypolimnetic MMHg buildup.