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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.

Plant bioassays are used to predict the effects of chemicals on terrestrial systems. However, in tiered-testing schemes, plants are tested only when introduction and fate information suggest that terrestrial exposure is probable. In this paper we discuss how information on chemical introduction and fate may trigger the need for plant toxicity tests, and how terrestrial exposure estimates are obtained and used at the U.S. Food and Drug Administration (FDA). FDA regulates chemicals used for food additives, food packaging and processing, and animal drugs and feed additives. These materials may be introduced into the environment as a result of their manufacture, use, or disposal. Information on introduction rates, environmental partitioning, and transformation is used to estimate the potential for terrestrial exposure. Described in detail are two scenarios for terrestrial exposure from application of wastewater treatment plant sludges to soils and one scenario for direct introductions of animal wastes to soils.

Fish predation has been recognized to influence community composition and structure, but has not been represented in experimental community tests of chemical effects. To investigate the feasibility of including vertebrate predation in 65-L microcosms, four species of small fish (Gasterosteus aculeatus, Cottus asper, Poecilia reticulata, and Pimephales promelas) were tested. These studies suggest that predation can be included in moderately sized microcosms only if exposure is restricted either by limiting the time of fish presence or by allowing the fish access to a limited portion of the microcosm. Predation was simulated by filtration of fixed proportions of the microcosms; no major changes were observed, which suggests that microcosm communities can tolerate removal of at least 25% of selected zooplankton populations per week. Fish predation did not alter the ability to detect direct effects of a selective algicide (streptomycin) on the algae, but did reduce the ability to detect indirect effects on cladoceran and ostracod populations.