A Comparison of Mixed Flask Culture and Standardized Laboratory Model Ecosystems for Toxicity Testing

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Abstract
Two microecosystem protocols, the standardized aquatic microcosm (SAM) method developed by Dr. Frieda Taub and associates (1982) and the mixed flask culture (MFC) method of Dr. John Leffler (1981), were compared on the basis of their responses to copper sulfate. These protocols differed in microcosm structure, age, and the variables monitored. Two complete experiments were conducted with each method so that repeatability, as well as sensitivity and variability, could be evaluated. Copper caused significant reductions in pH, oxygen gain, and oxygen loss at concentrations down to 500 μg/L (the lowest test concentration) in the SAM tests and 320 μg/L in the MFC tests. Lower copper concentrations (36 and 70 μg/L) in the MFC tests caused significant increases over control levels for several system-level variables. Although responses were similar with both systems, the SAM procedure provided considerably more insight into the changes in population densities and nutrient cycling responsible for the observed ecosystem level changes. This additional information was not obtained without cost, however. The SAM protocol was much more labor intensive and required about six times more laboratory effort than did the MFC protocol. The SAM systems, on the basis of their higher coefficients of variation (CV) and higher minimum detectable differences (MDD), appeared to be theoretically less able to demonstrate significant toxic effects. This higher variability was at least partially offset by scheduling toxicant addition during the more sensitive early stages of microcosm development, since the frequency with which statistically significant effects were observed at comparable treatment levels suggested no inherent difference in sensitivity between the two systems. The SAM results were more consistent, with microcosms developing and responding to copper in a similar manner in both runs. In contrast, the microcosms in the MFC experiments showed major differences in both development and sensitivity between runs. Both methods were found to have strengths and weaknesses. The SAM protocol was more repeatable and provided a much more complete picture of toxicant effects, but was extremely labor intensive. The MFC protocol lacked repeatability and provided no information to aid in interpreting ecosystem-level effects, but was capable of displaying sensitivity equal to that of the SAM procedure and required a comparatively small time investment. We believe a useful new microcosm protocol could be developed by incorporating some of the strengths of the SAM system into a mixed flask type system.