Incorporating a prognostic representation of marine nitrogen fixers into the global ocean biogeochemical model HAMOCC

Abstract

Abstract Nitrogen (N 2 ) fixation is a major source of bioavailable nitrogen to the euphotic zone, thereby exerting an important control on ocean biogeochemical cycling. This paper presents the incorporation of prognostic N 2 fixers into the HAMburg Ocean Carbon Cycle model (HAMOCC), a component of the Max Planck Institute Earth System Model (MPI‐ESM). Growth dynamics of N 2 fixers in the model are based on physiological characteristics of the cyanobacterium Trichodesmium . The applied temperature dependency confines diazotrophic growth and N 2 fixation to the tropical and subtropical ocean roughly between 40°S and 40°N. Simulated large‐scale spatial patterns compare well with observations, and the global N 2 fixation rate of 135.6 Tg N yr −1 is within the range of current estimates. The vertical distribution of N 2 fixation also matches well the observations, with a major fraction of about 85% occurring in the upper 20 m. The observed seasonal variability at the stations BATS and ALOHA is reasonably reproduced, with highest fixation rates in northern summer/fall. Iron limitation was found to be an important factor in controlling the simulated distribution of N 2 fixation, especially in the Pacific Ocean. The new model component considerably improves the representation of present‐day N 2 fixation in HAMOCC. It provides the basis for further studies on the role of diazotrophs in global biogeochemical cycles, as well as on the response of N 2 fixation to changing environmental conditions.