Rotifer responses to increased acidity: long-term patterns during the experimental manipulation of Little Rock Lake

Little Rock Lake, Wisconsin, U.S.A. has been the site of a whole-ecosystem experiment since 1983. It was divided into a treatment basin that was acidified in three, two-year stages and a reference basin. The rotifer community in the treatment basin exhibited a variety of responses to the manipulation. Many species decreased in abundance under reduced pH conditions but other rotifers increased at the same time such that there were ultimately increases with acidification in total rotifer biomass, and quite conspicuously, in the proportion that rotifers comprised of total zooplankton biomass. Ten rotifer species decreased at some stage during the acidification (e.g., Kellicottia longispina, Asplanchna priodonta and Keratella cochlearis) while four species increased dramatically (e.g., Synchaeta sp. and Keratella taurocephala ). Similarity indices and total rotifer biomass differences measured between the two basins exhibited very different temporal patterns of response to acidification. Similarity decreased regularly beginning with the earliest stages of acid additions while biomass was nearly the same between the basins until the late stages of the experiment. Comparisons with other nearby lakes indicate, however, that acid conditions are not the only factors generating among-lake differences in rotifer community characteristics. Changes observed with acidification in Little Rock Lake were such that its total rotifer biomass grew more similar to that in a nearby acidic-bog lake and different from that in a near-neutral-pH lake. At the same time, abundance patterns for individual rotifer species in Little Rock Lake were not particularly similar to those in the other lakes. It appears that, although they are important, acid conditions alone can not account for all observed rotifer community differences among lakes. Higher proportions of rotifer biomass and high populations of K. taurocephala do seem to be common features of many low pH habitats.