Lake acidification strongly affects zooplankton, microscopic animals which consume algae and are a major food source for fish. Acidification selectively eliminates large zooplankton, which are the most effective grazers of algae and the preferred food source for fish. The small zooplankton which dominate in acid lake are ineffective in both roles. The objective of this study is to determine why some zooplankton are acid-sensitive, while others are acid-tolerant. The hypothesis to be tested is that high acidity and aluminum (a toxic element which becomes concentrated in acid lakes) hinder normal osmoregulation (water and ion balance) in acid-sensitive zooplankton, but not in acid-tolerant zooplankton. When animals cannot osmoregulate, they lose essential ions like sodium. Consequently, they experience dysfunction of their nervous and muscular systems and die. Experiments will be performed in which several common North American zooplankton (including both acid-sensitive and acid- tolerant species) will be exposed to lakewater at various combinations of acid and aluminum levels, covering the range which occurs during lake acidification. After exposures of varying duration, animals will be removed from the treatments, weighed, and their body sodium content assayed using a standard technique called neutron activation analysis. Mortality rates will also be determined, and vital staining will be done to determine the extent and location of aluminum binding to tissues. Statistical analyses will quantify the effects of pH and aluminum on body sodium content of the test animals, and the effects of sodium losses on their survival.
