Changes in trace metal concentrations in lake water and biota during experimental acidification of Little Rock Lake, Wisconsin, USA

Abstract

Little Rock Lake, a small (18 ha), low-alkalinity (25 μeq litre −1, pH 6·1) seepage lake in northern Wisconsin, was divided into two basins by a flexible, inert barrier and, beginning in spring 1985, the north basin was acidified in three 2-year steps to pH 5·6, 5·1 and 4·7. The annual average pH of the reference basin remained near 6·1. As part of a comprehensive programme to determine the chemical and biological responses to acidification, minor metals (Al, Fe, Mn) and trace metals (Cd, Cu, Pb, Zn) in lake water (0·4 μm pore filtered samples), periphyton, zooplankton, and yellow perch ( Perca flavescens) were measured. At pH 5·6, dissolved Mn and Fe increased in the acidified basin. At pH 5·1 and 4·7, dissolved Al, Fe, Mn, Cd and Zn were elevated in the acidified basin. At pH 4·7, dissolved Pb in the acidified basin became elevated over reference basin levels. Dissolved Cu remained similar in both basins down to pH 4·7. Cd burdens in periphyton collected on artificial substrates were lower in the treatment basin at pH 5·1 (1·8 μg g −1 dry wt.) than in the reference basin at pH 6·1 (7·5 μg g −1 dry wt.), but Al and Fe burdens in periphyton were similar in both basins. Likewise, Cd levels in muscle tissue of perch from the treatment basin at pH 4·7 were lower (26 ng g −1 dry wt.) than in the reference basin at pH 6·1 (36 ng g −1 dry wt.); Al and Fe burdens were similar in perch muscle tissue from both basins. Levels of Cd and Fe in zooplankton from the acidified basin at pH 4·7 were ≊2× higher than in animals from the reference basin. In both basins of the lake, Al and Cd levels in lake biota decreased with increasing trophic level, demonstrating that food chain biomagnification does not occur for these metals.