Variability in responses to nutrients and trace elements, and transmissionof stressor effects through an estuarine food web

Aquatic systems are increasingly exposed to multiple stressors from anthrop ogenic sources. These stressors can vary in the consistency and magnitude o f responses they elicit in biota and in how the presence of additional stre ssors modifies their effects. Understanding how the biological environment and temporal dynamics influence responses to stressors, and how stressors i nteract, is important to predicting their effects in the natural environmen t. We examined temporal variability in responses of an experimental estuari ne food web to elevated trace elements and nutrients, as well as non-additi ve effects of the combination of these two stressors. Experiments were cond ucted four times during spring through autumn 1996 in 20 1-m(3) mesocosms. We measured a range of system-, population-, and individual-level parameter s to quantify responses of phytoplankton, bacterioplankton, heterotrophic n anoflagellates, copepods, fish, and benthic invertebrates to trace element and nutrient additions. The response to trace element additions was more variable both temporally a nd among phytoplankton and higher trophic level taxa than was the response to nutrient additions. Most taxa increased, either significantly or showed a trend toward increasing, in response to nutrient additions in all four me socosm runs. In contrast, the direction as well as the magnitude of respons es to trace element additions varied considerably among taxa and experiment al runs. Two distinct types of nutrient x trace element interactions were i mportant. First, temporal dynamics of nutrient ratios appeared to affect th e temporal pattern of toxicity of trace elements to phytoplankton. Second, in the June mesocosm run when trace element additions reduced production, a bundance, or growth of many organisms, these reductions were often proporti onately greater in nutrient addition tanks than where no nutrients were add ed. Our results suggest that considerable temporal and taxonomic variation in responses to trace element loadings are likely to be seen in field setti ngs even under constant loadings to the system and that trace elements may mask the magnitude of the response to high nutrient loadings in eutrophic s ystems. More generally, the presence of multiple stressors may either incre ase or dampen the temporal and spatial variability seen in aquatic systems, depending on the interactions among stressors and the influence of backgro und environmental conditions and sensitive species on the expression of str essor effects.