Effects of experimental acidification on Rotifer population dynamics in Little Rock Lake, Wisconsin, USA
Although zooplankton species distribution are clearly influenced by pH conditions, limited information exists on the details on response and the mechanisms by which changes in pH affect zooplankton populations. Surveys have shown decreases in zooplankton diversity and shifts in the species composition with decreasing pH. Individual zooplankton species exhibit a variety of patterns associated with pH, some are reduced, some increased, and others remain unaffected as pH declines (SPRULES 1975, ROFF \& KWIATKOWSKI 1977, YAN \& STRUS 1988, CONFER et al. 1983, SIEGFRIED et al. 1984, MAClSSAC et al. 1987). Several experimental studies on scales ranging from laboratory assays (DAVIS \& OZBuRN 1969, WALTON et al. 1982, HAVAS \& HUTCHINSON 1983, HAVAS \& LIKENS 1985) to whole-lake manipulations (MALLEYet al. 1982, SCHINDLER et al. 1985) have reported the effect of acidification on crustaceans, however no experimental studies on rotifers exist. As a first step in identifying mechanisms by which pH reduction affects zooplankton, we report population dynamics of two rotifer species, Keratella cochlearis and Keratella taurocephala before and after the controlled acidification of one basin of Little Rock Lake, Wisconsin, U.S.A. We address two questions: 1) Do abundance patterns change in conjunction with acidification? 2) Can changes in abundances be attributed to differences in birth rate or death rate? We selected Keratella cochlearis and Ktaurocephala because they are two of the most abundant taxa in the zooplankton community of Little Rock. In addition these rotifers have shown contrasting distribution patterns in relation to pH conditions. K. cochlearis tend, to be present in lakes with pH higher than 5.5, while K. taurocephala is a predominant species in lakes with pH lower than 5.5 (RoFF \& KwAITKOWSKI 1977, SIEGFRIED et a!. 1984, MAcisAAc eta!. 1987). As part of a whole-system experiment we have been monitoring rotifer populations in Little Rock Lake over a five-year period that includes a one-year baseline period and two, two-year periods in which one lake basin was acidified, first from pH 6.1 to 5.6 and then to 5.1. We have analyzed population parameters for these rotifers in both basins using abundance and egg ratio information. Such population analyses can provide insight into the mechanisms by which pH shifts affect zooplankton by identifying whether changes in birth or death rates are influencing populations.