Cadmium concentrations constituting a threat to aquatic ecosystems were predicted from data collected at two levels of biological hierarchy. A population-level estimate was derived from single-species toxicity test data, and a community-level estimate was derived from toxicity tests on protozoan communities. Estimates were compared with each other and with an ecosystem-level estimate derived from reports of ecological health and ambient cadmium levels in rivers, lakes, and streams.
Estimates of permissible acute concentrations differed by an order of magnitude. Single-species toxicity test data suggested that 42 μg Cd/L would affect 5% of taxa. The corresponding estimate from the community-level test was 459 μg Cd/L. Similar estimates of permissible chronic concentrations were not significantly different (0.82 and 0.20 μg Cd/L, single-species and community-level tests, respectively). Both estimates of permissible chronic concentrations fell within a rational range, the minimum defined by median cadmium concentrations in healthy aquatic systems (0.05 μg Cd/L) and the maximum defined by median cadmium concentrations in damaged systems (9.2 μg Cd/L).