Simulation of Carbon-Nitrogen Cycling
During Spring Upwelling in the Cariaco Basin

John J. Walsh,1 Dwight A. Dieterle,1 Frank E. Muller-Karger,1
Richard Bohrer,1 W. Paul Bisset,2 Ramón J. Varela,3 Ruben Aparicio,4
Rafael Díaz,4,1 Robert Thunell,5 Gordon Taylor,6 Mary I. Scranton,6
Kent A. Fanning1 and Edward T. Peltzer7

1: Department of Marine Science, University of South Florida, St. Petersburg, FL, USA.
2: Florida Environmental Research Institute, Florida Aquarium, Tampa, FL, USA.
3:
 
Estación de Investigaciones Marinas, Fundación La Salle de Ciencias Naturales,
Punta de Piedras, Estado Nueva Esparta, Venezuela.
4: Instituto Oceanográfico, Universidad de Oriente, Nucleo de Sucre, Cumaná, Venezuela.
5: Department of Geology, University of South Carolina, Columbia, SC, USA.
6: Marine Sciences Research Center, State University of New York at Stony Brook, NY, USA.
7: Monterey Bay Aquarium Research Institute, PO Box 628, Moss Landing, CA, USA.

Journal of Geophysical Research (1999) 104: 7807-7825.

Received: 22 July 1998.
Revised: 1 December 1998.
Accepted: 15 December 1998.
Published: 15 April 1999.


ABSTRACT

Coupled biological-physical models of carbon-nitrogen cycling by phytoplankton, zooplankton, and bacteria assess the impacts of nitrogen fixation and upwelled nitrate during new production within the shelf environs of the Cariaco Basin. During spring upwelling in response to a mean wind forcing of 8 m/s, the physical model matches remote sensing and hydrographic estimates of surface temperature. Within the three-dimensional flow field, the steady solutions of the biological model of a simple food web of diatoms, adult calanoid copepods, and ammonifying/nitrifying bacteria approximate within ~9% the mean spring observations of settling fluxes caught by a sediment trap at ~240 m, moored at our time series site in the basin. The models also estimate within ~11% the average C-14 net primary production and mimic the sparse observations of the spatial fields of nitrate and light penetration during the same time period of February-April. Stocks of colored dissolved organic matter are evidently small and diazotrophy is minimal during spring. In one summer case of the model with weaker wind forcing, however, the simulated net primary production is 14% of that 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 biological model, attributed to use of a single phytoplankton group.

© 1999 by the American Geophysical Union.


Acknowledgements

This research was supported by NSF grants OCE-9216626 to FMK and JJW, OCE-9415790 to MIS and GTT, and OCE-9401537 to RT as part of the CARIACO project. Additional funds were provided by NASA grants NAG5-6449 to JJW and NAG5-6448 to FMK and by the Venezuelan CONICIT to RJV and RA.


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