| A two-phase set of experiments was conducted to address some of the problems inherent in ecological screening of toxic substances in aquatic microcosoms, and to test hypotheses concerning the response of ecosystems to perturbations. Phase I was a 4 x 4 factorial experiment with static microcosms designed to test the "subsidy-stress" hypothesis (Odum et al. 1979), and focused on the interactive effects of cadmium and nutrient enrichment. Phase II was a 2 x 4 factorial experiment with flowthrough microcosms designed to test the "biomass-increment" hypothesis (Vitousek 1977), and focused on temporal aspects of system behavior (especially out/input for several elements) in response to phosphorus enrichment and chronic versus acute cadmium perturbations.Conclusions relevant to toxicity screening in microcosms are: (1) Of the variables measured, community metabolism, community composition by trophic groups, and output/input rations for NO(,3)-N, Mn and Fe, provided the most consistent indicators of system response to cadmium. (2) Nutrient enrichment and phosphorus limitation significantly influenced cadmium effects on most of the variables studied. (3) Pulsed cadmium inputs early in succession significantly affected system response to cadmium pulses later in succession.A hierarchical procedure for screening a suspected toxic substance is discussed, which includes: (1) static microcosms, (2) flowthrough microcosms, and (3) microcosm subsamples from specific natural ecosystems. Each step results in increased information about effects of a toxicant and each step more closely approximates natural ecosystems.Phase I results supported the subsidy-stress hypothesis with respect to cadmium inputs: Increasing cadmium concentrations (0, 1, 10, 100 ppb) caused a decrease in the P/R ratio, a decrease in grazing herbivores, and an increase in nighttime respiration and the abundance of fungii, all indicators of system stress. Since net daytime production and nighttime respiration increased with nutrient enrichment, there was no nutrient stress effect even at the highest level. There was a significant interaction effect of cadmium and nutrients, with high nutrient levels reducing the stress effect of cadmium. Phase II results generally supported the biomass increment hypothesis with the exception that the element most limiting in system inputs (phosphorus) showed little or no output response to cadmium perturbations. Instead, NO(,3)-N, present in abundance relative to biotic demand, showed the greatest response, possibly as a result of selective cadmium effects on nitrogen metabolism. The retention pattern for continuous, low concentration cadmium inputs was similar to that of essential elements. Cadmium may have accumulated to a toxic threshold in some of the microcosms. Pulsed, high concentration cadmium inputs had significant effects on system behavior, depending on timing of inputs. The general community level response to cadmium perturbations was a shift from a grazing to a detritus food chain. |
