Modeling the effect of nitrogen fixation on carbon and nitrogen fluxes at BATS

Hood, Raleigh R., Bates, Nicholas R., Capone, Douglas G. and Olson, Donald B. (2001) Modeling the effect of nitrogen fixation on carbon and nitrogen fluxes at BATS. Deep Sea Research Part II: Topical Studies in Oceanography, 48 (8-9), 1609-1648. (doi:10.1016/S0967-0645(00)00160-0).

Abstract

Recent geochemical estimates of N2-fixation in the North Atlantic ocean indicate rates that are significantly higher than those derived from direct observations. In this paper different N2-fixation rate scenarios are explored using a one-dimensional, biogeochemical model that includes an explicit representation of Trichodesmium. This model reproduces most of the observed interannual variability in phytoplankton production and generates seasonal Trichodesmium biomass and N2-fixation cycles similar to those observed at BATS. Two solutions are presented, one where the N2-fixation rate is increased enough to reproduce the observed summertime drawdown of DIC, and a second where it is tuned to reproduce the observed sediment trap fluxes. The high N2-fixation solution reproduces the seasonal and interannual variability in DIC concentrations quite accurately and generates N2-fixation rates that agree with direct rate measurements from 1990 and recent geochemical estimates. However, this solution generates export fluxes that are more than 4 times higher than those observed, and predicts the development of DON and DOC anomalies in late summer/early fall that have not been observed. In contrast, the low N2-fixation solution generates trap fluxes that are approximately correct, but overestimates the summertime DIC concentrations by 20–View the MathML source. Both solutions indicate that there is significant interannual variability in N2-fixation at BATS and that the rates were much lower in 1995–1996 than in the previous six years. It is suggested that this variability is linked to decadal-scale fluctuations in the North Atlantic climate.

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