Diagnosing the contribution of phytoplankton functional groups to the production and export of particulate organic carbon, CaCO3, and opal from global nutrient and alkalinity distributions

[ 1] We diagnose the contribution of four main phytoplankton functional groups to the production and export of particulate organic carbon (POC), CaCO3, and opal by combining in a restoring approach global oceanic observations of nitrate, silicic acid, and alkalinity with a simple size-dependent ecological/biogeochemical model. In order to determine the robustness of our results, we employ three different variants of the ocean general circulation model (OGCM) required to transport and mix the nutrients and alkalinity into the upper ocean. In our standard model, the global export of CaCO3 is diagnosed as 1.1 PgC yr(-1) ( range of sensitivity cases 0.8 to 1.2 PgC yr(-1)) and that of opal as 180 Tmol Si yr(-1) ( range 160 to 180 Tmol Si yr(-1)). CaCO3 export is found to have three maxima at approximately 40 degrees S, the equator, and around 40 degrees N. In contrast, the opal export is dominated by the Southern Ocean with a single maximum at around 60 degrees S. The molar export ratio of inorganic to organic carbon is diagnosed in our standard model to be about 0.09 ( range 0.07 to 0.10) and found to be remarkably uniform spatially. The molar export ratio of opal to organic nitrogen varies substantially from values around 2 to 3 in the Southern Ocean south of 45 degrees S to values below 0.5 throughout most of the rest of the ocean, except for the North Pacific. Irrespective of which OGCM is used, large phytoplankton dominate the export of POC, with diatoms alone accounting for 40% of this export, while the contribution of coccolithophorids is only about 10%. Small phytoplankton dominate net primary production (NPP) with a fraction of similar to 70%. Diatoms and coccolithophorids account for about 15% and less than 2% of NPP, respectively. These diagnosed contributions of the main phytoplankton functional groups to NPP are also robust across all OGCMs investigated. Correlation and regression analyses reveal that the variations in the relative contributions of diatoms and coccolithophorids to NPP can be predicted reasonably well on the basis of a few key parameters.