Small synthetic microcosms have been developed in the authors' laboratory to provide replicate aquatic communities with interspecies competition within primary, secondary, and recycling trophic levels. The microcosms display nutrient depletion, algal competition and succession, and algal depletion through grazing and nutrient depletion. Streptomycin acted as a selective algal toxicant, reducing all indexes of primary production temporarily, and then modifying the algal dominance relationships over the 63-day experiment, although no active streptomycin could be detected by Day 28. Daphnia populations were reduced and ostracod populations markedly increased in the streptomycin-treated microcosms. These results were consistently shown in three successive microcosm experiments. In contrast, malathion caused a temporary reduction in the number of grazers and a concurrent algal bloom. Degradation of malathion was presumed from the recovery of the grazer populations, which was accompanied by the elimination of the algal bloom.

The synthetic microcosms, being composed of distilled water, silica sand, reagent-grade chemicals, and organisms that are easily reared in the laboratory, show promise of providing a nonsite specific, reproducible ecosystem-level bioassay.