A deterministic model for N2 fixation at stn. ALOHA in the subtropical North Pacific Ocean

Abstract

Marine N2 fixation by diazotrophic microorganisms is a key process in biogeochemical cycling and yields an important input of new nitrogen into the tropical and subtropical surface ocean. However, it is only poorly accounted for in current numerical models. We present a simple biological model that explicitly includes N2 fixation by diazotrophic phytoplankton. The model employs a mechanistic parameterization of N2 fixation based on physiological responses of Trichodesmium to physical conditions of the environment. The model is conceived to allow shifts in nitrogen versus phosphorus control of the plankton community by resolving the biogeochemical cycles of both elements. Typical N : P ratios were assigned to the different functional groups to capture variations in the N : P stoichiometries of inorganic and organic matter pools. The biological model was coupled to a 1D physical model of the upper ocean. A simulation was performed at stn. ALOHA (22°45′N, 158°W) in the subtropical North Pacific Ocean, where intense blooms of Trichodesmium occurred during the last decade. The model captures essential features of the biological system including the vertical structure and seasonal course of chlorophyll, the seasonal cycle and interannual differences in diazotrophic biomass, the mean vertical particle flux, and an oscillation in the relative importance of nitrogen versus phosphorus. We regard this model as a step towards a mechanistic tool to assess the magnitude of marine N2 fixation and to explore hypotheses on its effect on carbon sequestration from the atmosphere.