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

Recent geochemical estimates of N-2-fixation in the North Atlantic ocean in dicate rates that are significantly higher than those derived from direct o bservations. In this paper different N-2-fixation rate scenarios are explor ed 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 N-2-fixation cycles similar to those observed at BATS. Two solutions are presented, one where the N-2-fixation rate is incr eased enough to reproduce the observed summertime drawdown of DIG, and a se cond where it is tuned to reproduce the observed sediment trap fluxes. The high N-2-fixation solution reproduces the seasonal and interannual variabil ity in DIC concentrations quite accurately and generates N-2-fixation rates that agree with direct rate measurements from 1990 and recent geochemical estimates. However, this solution generates export fluxes that are more tha n 4 times higher than those observed, and predicts the development of DON a nd DOC anomalies in late summer/early fall that have not been observed. In contrast, the low N-2-fixation solution generates trap fluxes that are appr oximately correct, but overestimates the summertime DIC concentrations by 2 0-30 mu mol/kg(-1) Both solutions indicate that there is significant intera nnual variability in N-2-fixation at BATS and that the rates were much lowe r in 1995-1996 than in the previous six years. It is suggested that this va riability is linked to decadal-scale fluctuations in the North Atlantic cli mate. (C) 2001 Elsevier Science Ltd. Ah rights reserved.