Use of microcosm and fish toxicity data to select mesocosm treatment concentrations.

Data from Mixed Flask Culture (MFC) microcosms and fish single species toxicity tests were used to select treatment concentrations for littoral enclosure studies. Selected concentrations were evaluated by comparing the predicted responses to those observed in littoral enclosure studies with a pyrethroid pesticide, esfenvalerate (Lozano et al. 1989) and an organophosphate pesticide, azinphos-methyl (Knuth et al. 1992). Comparisons of the effects predicted by these laboratory studies and effects that occurred in the two littoral enclosure trials provided useful information on the accuracy associated with each prediction. Although the approach did not always predict the exact magnitude or breadth of response found in the littoral enclosures, response levels for each of the selected treatment concentrations were adequate to describe the dose-response relationship of the littoral ecosystem for the two pesticides. The evaluation suggests that selection of treatment concentrations for the two littoral enclosure studies could have been improved by using this approach.

The effects of atrazine on microcosms developed from four natural plankton communities.

Comparisons were made among Leffler microcosms developed from four different natural communities and exposed to 0, 20, 100, 200, 500, 1,000, and 5,000 micrograms/L atrazine, a commonly used herbicide. Atrazine reduced net primary productivity, pH, and net productivity/respiration ratios in all four microcosm communities. In three of the four communities, the lowest observed (P less than 0.05) effect concentration (LOEL) was 100 micrograms/L. In the fourth community the LOEL was 200 micrograms/L atrazine. The sensitivity and accuracy of bioassays with four different microcosm communities were evaluated by comparing results with values reported for acute and chronic single species bioassays, other types of microcosms, and experimental ponds exposed to similar concentrations of atrazine. The ranges of sensitivity noted in these experiments were less than the range reported for single species bioassays using common test organisms and similar to those reported for other microcosms. The similarity between Leffler microcosm results and the responses reported for the experimental ponds suggests that the Leffler microcosms accurately reflected concentrations causing ecosystem level changes in the experimental ponds.