Biogeochemical cycling in the oligotrophic ocean: Redfield and non-Redfield models

The assumption of fixed elemental ratios in ocean biogeochemical models was tested using the Hawaii Ocean Time-Series data set from the subtropical North Pacific Ocean, where nutrient-starvation is a permanent condition for near-surface phytoplankton populations. Two three-element (C, N, P) ecosystem models were coupled to a mixed-layer model, an inorganic carbon chemistry model, and dynamic pools of dissolved organic C, N, and P. One model has fixed Redfield ratios for phytoplankton (constant ratio model, CRM), whereas the other has varying ratios (variable ratio model, VRM). The results suggest that the ecosystem is strongly phosphorus limited, but would be nitrogen limited in the absence of dinitrogen fixation (DNF), and may be nitrogen limited in the lower part of the euphotic zone. Known sources of phosphorus appear to be close to those required to sustain observed levels of export, given plasticity of elemental ratios. The vertical profile and seasonal time course of primary production of organic carbon are simulated well by the VRM but poorly by the CRM, as are surface concentrations of dissolved organic carbon. The seasonal cycles of air-sea CO, flux and export of organic carbon by sedimentation are similar in the two models, but there is a slight but persistent bias toward greater downward fluxes in the VRM. The ability of oceanic phytoplankton to adapt to P stress by reducing cellular requirements can therefore potentially enhance the oceanic sink for atmospheric carbon over vast areas of low-latitude ocean, but there is potential for saturation of this enhancement if DNF increases.