Modeling the distribution of Trichodesmium and nitrogen fixation in the Atlantic Ocean

Abstract

In this paper we use a coupled, 3‐dimensional, biological‐physical model, which includes an explicit, dynamic representation of Trichodesmium, to predict the distribution of Trichodesmium and rates of N2‐fixation in the tropical and subtropical Atlantic Ocean. It is shown that the model reproduces the approximate observed meridional distribution of Trichodesmium in the Atlantic and elevated concentrations in specific coastal and open ocean regions where this organism is known to occur. The model also appears to reproduce the observed seasonality of Trichodesmium populations at higher latitudes (highest concentrations in summer and fall), but this seasonal cycle may be too pronounced at low latitudes. High and persistent Trichodesmium concentrations and rates of N2‐fixation are generated by the model in the Gulf of Guinea off of Africa. This unexpected finding appears to be confirmed by historical measurements. In general, increased Trichodesmium concentrations develop in regions where the mixed layer is relatively thin (resulting in high mean light levels) and dissolved inorganic nitrogen (DIN) concentrations and phytoplankton biomass are low for extended periods of time. The model‐predicted Trichodesmium distributions are therefore very sensitive to the fidelity of the physical model's representation of mixed layer depth variability, and upwelling intensity, and the biological model's estimated DIN and phytoplankton concentrations. The model generates a three‐step successional sequence where (1) high DIN concentrations due to upwelling and/or mixing stimulate phytoplankton growth, followed by (2) Trichodesmium growth after DIN depletion and phytoplankton decline, followed by (3) enhanced phytoplankton growth due to new nitrogen inputs from N2‐fixation. This sequence develops in response to seasonal variations in mixing in the southwestern North Atlantic and in response to upwelling along the coast of Africa and the equator. We interpret this sequence as representing a diatom‐ Trichodesmium‐flagellate succession, which is consistent with observed species successions off of northwest Africa and in the Gulf of Mexico. The results presented in this paper lead us to conclude that our model includes the primary factors that dictate when and where Trichodesmium and N2‐fixation occurs in the Atlantic. Moreover, it appears that our model reproduces some of the major effects that diazotrophically‐derived inputs of new nitrogen have on the pelagic ecosystem.