Combinations of natural and anthropogenic stressors affect populations of freshwater rotifers

Limitations of current environmental management procedures include the failure to investigate ecological relationships in susceptibility to toxicants. Rotifer populations have been used in single-species laboratory tests to estimate toxicity in freshwater. Typically, such tests expose the rotifer Brachionus calyciflorus to toxicants under optimal conditions for growth. Important events such as food shortage, cyanobacteria blooms, and predation are absent from these tests. In the present work, toxicity tests with B. calyciflorus were designed to incorporate these natural stressors. The objective was to determine if ecological relationships such as predation and natural stressors such as food limitation and toxic cyanobacteria affect the impact of anthropogenic toxicants on rotifer populations. Results indicated that food limitation significantly modifies estimates of toxicity. Population growth thresholds, measured as the amount of food required to maintain rotifer populations, were used as an endpoint to investigate combinations of toxic cyanobacteria and pentachlorophenol or mercury. The cyanobacterium Microcystis aeruginosa was shown to be toxic to B. calyciflorus populations at naturally occurring densities. Strong interactive effects were detected between the toxicity of Microcystis aeruginosa and pentachlorophenol or mercury. Using a well defined rotifer predator-prey system, it was determined that toxicity disrupts this relationship based on predator effects only. It is proposed that single species tests can be used to set safe action levels to protect this ecological relationship.