Laboratory tests evaluating the effects of cadmium on freshwater protozoan communities

Abstract

AbstractNaturally derived protozoan communities developed on polyurethane foam (PF) substrates were used to evaluate the toxic effects of cadmium. The ability of the communities to resist displacement from a structural equilibrium in acute cadmium exposures was evaluated. The number of species remaining on substrates after 2 d was inversely related to cadmium concentration. The concentration required for a 50% reduction in species number relative to the control (EC50) was 4,000 μg Cd/L. Chronic effects were evaluated by observing the colonization of barren PF islands from a species source (PF epicenter) for 28 d. The species occurring on islands over time and the autotrophic index (AI) of substrate communities were determined. The equilibrium species number on islands declined with increasing cadmium concentrations, and AI increased with cadmium concentration. EC20s for measures of colonization were consistently near 1 μg Cd/L. However, epicenters exposed to 9.5 μg Cd/L were not significantly different from controls in species number or AI. This discrepancy in effect levels between PF islands and epicenters is consistent with a relationship between stress tolerance and successional maturity. Levels of toxicity were consistent and reasonable when compared with other cadmium data. The tests compared favorably to single species toxicity tests in time and cost, yet had the advantages of using indigenous species and incorporating emergent properties of communities, such as predation, competition and succession.