Generic mixed-flask culture microecosystems derived from small lake and pond planktonic communities were used to evaluate the effects of selected alcohols, aniline derivatives, and aromatic amides on pH and dissolved oxygen: ecosystem-level state variables which reflect energy flow and nutrient cycling processes within the systems. Using changes in these variables as indicators of effect, the relative toxicity rankings of compounds within each group were determined and compared with the toxicity rankings established by fathead minnow acute toxicity.