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Understanding the dynamics of upwelling systems, especially the interactions between nutrients and light, has benefited from the application of models of varying complexity. Validation of such models using unialgal cultures or field observations has often proven difficult, but short-term incubations of contained natural assemblages and use of instantaneous physiological indicators offer an alternative approach. In May and June 1996, phytoplankton communities deep in the euphotic zone were sampled from nearly identical physical environments. Replicate samples (20 l volume) were incubated on deck at 50% surface irradiance with either no nutrient additions (Controls) or additions of 20 μM nitrate (Enrichments). Over 24 h, variable fluorescence (Fv:Fm), nitrate reductase activity (NR), nutrients, chlorophyll a and particulate C and N were monitored. Initial chlorophyll a (∼3 μg l-1), phosphate (∼0.2 μM), nitrate (∼1.5 μM) and silicate (∼3 μM) were similar in both months. Changes in NR and Fv:Fm indicated clear physiological responses to changes in irradiance and added nitrate that differed between months. In May, Controls and Enrichments responded in the same way. Fv:Fm stayed constant (0.5), chlorophyll a increased slightly, and NR activity increased markedly in all samples. In contrast, in June, treatments responded quite differently. Fv:Fm was near the theoretical maximum (0.7–0.8) initially and remained constant in Enrichments, but fell sharply in Controls. Declines in controls were also seen for chlorophyll a, and NR activity. Thus, the addition of 20 μM nitrate had a significant effect even though ambient levels of nitrate (>1 μM) should not have been limiting. Small (<20 μm) flagellates predominated in the May samples, but in June large and chain-forming centric diatoms constituted a significant proportion of the phytoplankton community. We conclude that the response of a phytoplankton community to environmental changes can depend on factors that are poorly represented by bulk measurements of chlorophyll, nutrients and particulate elements. [ABSTRACT FROM AUTHOR]