Impact of triphenyltin acetate in microcosms simulating floodplain lakes. II. Comparison of species sensitivity distributions between laboratory and semi-field.

The study objectives were to shed light on the types of freshwater organism that are sensitive to triphenyltin acetate (TPT) and to compare the laboratory and microcosm sensitivities of the invertebrate community. The responses of a wide array of freshwater taxa (including invertebrates, phytoplankton and macrophytes) from acute laboratory Single Species Tests (SST) were compared with the concentration-response relationships of aquatic populations in two types of freshwater microcosms. Representatives of several taxonomic groups of invertebrates, and several phytoplankton and vascular plant species proved to be sensitive to TPT, illustrating its diverse modes of toxic action. Statistically calculated ecological risk thresholds (HC5 values) based on 96 h laboratory EC50 values for invertebrates were 1.3 microg/l, while these values on the basis of microcosm-Species Sensitivity Distributions (SSD) for invertebrates in sampling weeks 2-8 after TPT treatment ranged from 0.2 to 0.6 microg/l based on nominal peak concentrations. Responses observed in the microcosms did not differ between system types and sampling dates, indicating that ecological threshold levels are not affected by different community structures including taxa sensitive to TPT. The laboratory-derived invertebrate SSD curve was less sensitive than the curves from the microcosms. Possible explanations for the more sensitive field response are delayed effects and/or additional chronic exposure via the food chain in the microcosms.