Carbon and nitrogen uptake response to light by phytoplankton during an upwelling event

The interactions between phytoplankton nutrition and the responseof carbon (C) and nitrogen (N) uptake to irradiance relationshipswere examined during September 1993 in Monterey Bay, California,an eastern boundary current upwelling regime. Measurements ofN uptake and C assimilation rates versus irradiance (V:I andP:I) experiments were performed using trace-level additionsof ¹⁵N-labeled NO₃^– and NH₄⁺, and ¹⁴C-labeled bicarbonateto water collected from a depth of {small tilde}30% of surfacephotosynthetic photon flux density (PPFD). An upwelled watermass was sampled consecutively, with hydrographic stations locatedat the upwelling site, 48 h later down the horizontal axis ofthe upwelling plume, and a final time (24 h later) with watersconsisting of a mixture of 5–6 day aged upwelled waterand warmer surface water from outside the plume. As the wateraged, a pro gressive shift in the rates of C and N utilizationoccurred, with C assimilation increasing while N uptake ratesdecreased. At the same time, NH₄⁺ dominated the nitrogenousnutrition in older upwelled water, even in the presence of highconcentrations of ambient NO₃^–. Dark-uptake rates forall substrates were uniformly low at all stations; NH₄⁺ uptakedemonstrated the least dependence on PPFD. The results of thisstudy demonstrate dramatic changes in the light-mediated responseof C and N uptake, resulting in assimilation ratios considerablydifferent from predicted values assuming phytoplankton C:N uptakerates will be proportional to Redfield C:N composition. Thesedata provide clear evidence of physiological changes in thenatural planktonic assemblage of this evolving upwelling ecosystem.