Community conditioning confirmed in a multiple stressor microcosm

The authors tested the community conditioning hypothesis by evaluating the effects of multiple stress events on aquatic community dynamics in a Standard Aquatic Microcosm (SAM). Community conditioning suggests that changes in community dynamics are the result of the history of the community, specifically, the conditioning of the treated community by stress events. Consequently, systems exposed to different stressors would be expected to exhibit different dynamics. In an extended 91-day SAM, two of four treatments were dosed with the turbine fuel, JP-8, one week into the experiment. Two treatments were later exposed to the heat stress, one that had received jet fuel and one that had not. Biological, physical and chemical data were subjected to conventional univariate statistical analyses including ANOVA. Data were also analyzed with multivariate techniques including nonmetric clustering and association analysis, an artificial intelligence approach that enabled the detection of community dynamics not detected with conventional methods. The results clearly support the community conditioning hypothesis as the four treatments were distinguishable at the end of the experiment. This design is particularly important since little information is available regarding the impacts of sequential stresses on communities. The results also demonstrate the importance of indirect effects on community dynamics. Furthermore, the experiment demonstrates that microcosms, like natural communities, are complex systems and clearly historical. The community conditioning hypothesis supports the growing argument against the attainment of recovery in ecosystems following a disturbance. Such research is applicable to the refinement of ecological risk assessment