Simulation of carbon-nitrogen cycling during spring upwelling in the Cariaco Basin

Coupled biological-physical models of carbon-nitrogen cycling by phytoplank ton, zooplankton, and bacteria assess the impacts of nitrogen fixation and upwelled nitrate during new production within the shelf environs of the Car iaco Basin. During spring upwelling in response to a mean wind forcing of 8 m s(-1), the physical model matches remote-sensing and hydrographic estima tes of surface temperature. Within the three-dimensional flow field, the st eady solutions of the biological model of a simple food web of diatoms, adu lt calanoid copepods, and ammonifying/nitrifying bacteria approximate withi n similar to 9% the mean spring observations of settling fluxes caught by a sediment trap at similar to 240 m, moored at our time series site in the b asin. The models also estimate within similar to 11% the average C-14 net p rimary production and mimic the sparse observations of the spatial fields o f nitrate and light penetration during the same time period of February-Apr il. Stocks of colored dissolved organic matter are evidently small and diaz otrophy is minimal during spring. In one summer case of the model with weak er wind forcing, however, the simulated net primary production is 14% of th at measured in August-September, while the predicted detrital flux is then 30% of the observed. Addition of a cyanophyte state variable, with another source of new nitrogen, would remedy the seasonal deficiencies of the biolo gical model, attributed to use of a single phytoplankton group.