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

The interactions between phytoplankton nutrition and the response of carbon (C) and nitrogen (N) uptake to irradiance relationships were examined during September 1993 in Monterey Bay, California, an eastern boundary current upwelling regime. Measurements of N uptake and C assimilation rates versus irradiance (V:I and P:I) experiments were performed using trace-level additions of N-15-labeled NO3- and NH4+, and C-14-labeled bicarbonate to water collected from a depth of similar to 30% of surface photosynthetic photon flux density (PPFD). An upwelled water mass was sampled consecutively, with hydrographic stations located at the upwelling site, 48 h later down the horizontal axis of the upwelling plume, and a final time (24 h later) with waters consisting of a mixture of 5-6 day aged upwelled water and warmer surface water from outside the plume. As the water aged, a progressive shift in the rates of C and N utilization occurred, with C assimilation increasing while N uptake rates decreased. At the same time, NH4+ dominated the nitrogenous nutrition in older upwelled water, even in the presence of high concentrations of ambient NO3- Dark-uptake rates for all substrates were uniformly low at all stations; NH4+ uptake demonstrated the least dependence on PPFD. The results of this study demonstrate dramatic changes in the light-mediated response of C and N uptake, resulting in assimilation ratios considerably different from predicted values assuming phytoplankton C:N uptake rates will be proportional to Redfield C:N composition. These data provide clear evidence of physiological changes in the natural planktonic assemblage of this evolving upwelling ecosystem.