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

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 degrees S and 40 degrees 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.