Response of predatory zooplankton populations to the experimental acidification of Little Rock Lake, Wisconsin

Abstract

To assess the effects of lake acidification on large predatory zooplankton, we monitored population levels of four limnetic taxa for 6 years in a lake with two basins, one of which was experimentally acidified (2 years at each of three levels: pH 5.6, 5.2 and 4.7). Concentrations of phantom midge ( Chaoborus spp.), the most abundant large predator, remained similar in the treatment and reference basins until the fourth year (pH 5.2) when they increased in the treatment basin. In contrast, Epischuru lacustris and Leptodora kindtii disappeared from limnetic samples, and water mites declined to near zero upon acidification. Treatment basin populations of E.Iacustris declined sharply during the second year of acidification. The nature of the decline suggested sensitivity of an early life stage during the first year at pH 5.6. Leptodora kindtii showed no population response at pH 5.6, but declined to essentially zero at pH 5.2. Treatment basin populations of water mites fluctuated until declining in the fifth and sixth years (pH 4.7). These changes indicate a variety of direct and indirect responses to lake acidification.