Carbon cycling in the upper waters of the Sargasso Sea: I. Numerical simulation of differential carbon and nitrogen fluxes

A complex ecosystem model is developed for the area around Bermuda in the S argasso Sea. The model is physically driven by seasonal changes in spectral light, temperature, and water column mixing. Autotrophic growth is represe nted by four functional groups of phytoplankton. The groups have light and nutrient utilization characteristics that reflect those of Prochlol ococcus , Synechococcus and Chromophycota species. The model includes differential carbon and nitrogen cycling, nitrification, and nitrogen fixation to effect realistic allochthonous and autochthonous nutrient sources to the euphotic zone. This simulation yields realistic seasonal and vertical (I) successio n of phytoplankton functional groups' biomass, productivity, and pigments; (2) profiles of dissolved inorganic carbon, nitrate, and ammonium; and (3) fluxes of carbon dioxide at the air-sea botmdary and particulate carbon and nitrogen settling losses, when compared to the JGOFS BATS site. The addition of local nitrification, differential carbon and nitrogen remin eralization, and nitrogen fixation removes the need for an unrealistically high upward vertical flux of nitrate to mimic the productivity and chloroph yll a stocks. The explicit numerical description of carbon and nitrogen uti lization by heterotrophic bacteria simulated a population that was not nitr ogen-limited in these waters. Instead, the heterotrophic bacteria community was limited by energy resources in the form of DOC, and was a nitrogen sou rce for the autotrophic community through the excretion of excess NH4 from the labile DOM energy source. Numerical descriptions of ecosystems based so lely on nitrogen dynamics, or fixed carbon to nitrogen ratios, may yield an inaccurate prediction of carbon and nitrogen fluxes, and fail to properly predict the carbon cycle. O 1998 Elsevier Science Ltd. All rights reserved. A complex ecosystem model is developed for the area around Bermuda in the S argasso Sea. The model is physically driven by seasonal changes in spectral light, temperature, and water column mixing. Autotrophic growth is represe nted by four functional groups of phytoplankton. The groups have light and nutrient utilization characteristics that reflect those of Prochlorococcus, Synechococcus and Chromophycota species. The model includes differential c arbon and nitrogen cycling, nitrification, and nitrogen fixation to effect realistic allochthonous and autochthonous nutrient sources to the euphotic zone. This simulation yields realistic seasonal and vertical (1) succession of phytoplankton functional groups' biomass, productivity, and pigments; ( 2) profiles of dissolved inorganic carbon, nitrate, and ammonium; and (3) f luxes of carbon dioxide at the air-sea boundary and particulate carbon and nitrogen settling losses, when compared to the JGOFS BATS site. The additio n of local nitrification, differential carbon and nitrogen remineralization , and nitrogen fixation removes the need for an unrealistically high upward vertical flux of nitrate to mimic the productivity and chlorophyll a stock s. The explicit numerical description of carbon and nitrogen utilization by heterotrophic bacteria simulated a population that was not nitrogen-limite d in these waters. Instead, the heterotrophic bacteria community was limite d by energy resources in the form of DOG, and was a nitrogen source for the autotrophic community through the excretion of excess NH, from the labile DOM energy source. Numerical descriptions of ecosystems based solely on nit rogen dynamics, or fixed carbon to nitrogen ratios, may yield an inaccurate prediction of carbon and nitrogen fluxes, and fail to properly predict the carbon cycle. (C) 1998 Elsevier Science Ltd. All rights reserved.