Effects of experimental acidification on zooplankton populations: A multiple-scale approach
The two basins of Little Rock Lake were divided and one basin (treatment) was acidified from pH 6.1 to 4.7, pH was lowered 0.5 units every two years. ,1 examined zooplankton population changes with acidification combining observations in the lake, field enclosures, and laboratory bioassays. I focused in two rotifers species, Keratella cochlearis and Keratella taurocephala. I tested whether laboratory bioassays could predict zooplankton responses observed in the field. Keratella cochlearis abundance and birth rates decreased with acidification. In contrast K. taurocephala abundance increased and this was associated with declines in mortality rate. Neither K. cochlearis nor K. taurocephala reproduction was sensitive to pH changes in laboratory bioassays using abundant high-quality food. Keratella coch/earis survivorship was higher at pH 6.1 than pH 5.1, while K. taurocephala survivorship was similar at pH 6.1 and 4.7. In situ manipulations of food and pH indicated that K. cochlearis reproduction tended to be higher under reference basin pH and food conditions. No relationship was observed between K. taurocephala reproduction and pH conditions, however, reproduction was higher in enclosures with reference basin food. Laboratory bioassays and field studies taken together indicate that the decrease in K. coch/earis abundance was caused by the combined effects of higher mortality at low pH and lower reproduction caused by changes in food availability with acidification. However the K. taurocephala increase could not be explained by these two mechanisms. Kerate/la taurocepha/a morphology changed with acidification. Body and spine lengths decreased in the treatment basin at pH 5.1 and 4.7. In induction experiments shifts in morphology were induced by exposing individuals to reference basin water and phytoplankton. In a feeding selectivity experiment the long-spined morph was better protected than the short-spined morph against predation by Asplanchna. These morphological changes may be a response to changes in predation with acidification. The community ingestion rate declined in the treatment basin at pH 5.5 and initially at pH 5.1, caused by abundance declines of Epischura /acustris, Leptodora kindtii, Asplanchna, Diacyclops thomasi and Mesocyclops edax. However, the community ingestion rate increased during the late stages at pH 5.1 and pH 4.7 when abundances of Chaoborus punctipennis and Tropocyclops prasinus increased.