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The response of phytoplanktonic growth to experimental nutrient addition in a tributary of the Chesapeake Bay estuary varied with season and nutrient availability. In bioassay experiments employing outdoor 0.5-m3 continuous cultures freshly inoculated with natural phytoplankton from the Patuxent River estuary, supplements of N, either as ammonium or nitrate, enhanced growth greatly during the low-flow, late-summer season — a period when N:P ratios of dissolved inorganic nutrient standing stocks are characteristically below 5:1 (by atoms). Growth response to N addition was very rapid (1 d after the start of an experiment), implying that phytoplankton in the bioassay were N limited when removed from the estuary. PO43− addition enhanced phytoplankton growth during the late-winter, high-flow season, when N: P ratios (as previous) typically exceed 90:1 (by atoms), but the response lagged enrichment by at least 4 d and biomass levels achieved in these cultures were less than one third of those achieved in the N-enriched cultures during the late summer. The great seasonal variability in river flow, nutrient regimes, and the response of natural phytoplankton assemblages to nutrient enrichment in the Patuxent, and similar estuaries, suggest that management strategies to improve "water quality" will need to consider N as well as P additions.

Aquatic systems are increasingly exposed to multiple stressors from anthropogenic sources. These stressors can vary in the consistency and magnitude of responses they elicit in biota and in how the presence of additional stressors modifies their effects. Understanding how the biological environment and temporal dynamics influence responses to stressors, and how stressors interact, is important to predicting their effects in the natural environment. We examined temporal variability in responses of an experimental estuarine food web to elevated trace elements and nutrients, as well as non-additive effects of the combination of these two stressors. Experiments were conducted four times during spring through autumn 1996 in 20 l-m 3 mesocosms. We measured a range of system-, population-, and individual-level parameters to quantify responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, copepods, fish, and benthic invertebrates to trace element and nutrient additions.The response to trace element additions was more variable both temporally and 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 mesocosm runs. In contrast, the direction as well as the magnitude of responses to trace element additions varied considerably among taxa and experimental runs. Two distinct types of nutrientϫtrace element interactions were important. First, temporal dynamics of nutrient ratios appeared to affect the temporal pattern of toxicity of trace elements to phytoplankton. Second, in the June mesocosm run when trace element additions reduced production, abundance, or growth of many organisms, these reductions were often proportionately greater in nutrient addition tanks than where no nutrients were added. Our results suggest that considerable temporal and taxonomic variation in responses to trace element loadings are likely to be seen in field settings even under constant loadings to the system and that trace elements may mask the magnitude of the response to high nutrient loadings in eutrophic systems. More generally, the presence of multiple stressors may either increase or dampen the temporal and spatial variability seen in aquatic systems, depending on the interactions among stressors and the influence of background environmental conditions and sensitive species on the expression of stressor effects.Stressors vary considerably in the specificity of their effects; some stressors may affect nearly all organisms within AcknowledgmentsWe thank B. Albright, D. Butera, L. Cole, D. Connell, R. DeKorsey, T. Huber, A. Imirie, S. Sellner, J. Smallwood, M. Ward, M. Weinstein, T. Wiegner, and W. Yates for their help in running experiments and data analysis; S. Nixon, S. Grainger, and E. Buckley for suggestions on mesocosm design and unpublished data from their seagrass mesocosm experiments; M. Bundy for advice on copepod feeding strategies; E. Perry f...

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