Effects of pelagic food-web interactions and nutrient remineralization on the biogeochemical cycling of carbon: a modeling approach

The operation of the oceans biological CO2 pump depends on both the structure of the pelagic food web and remineralization processes in the water column. We have developed a novel pelagic ecosystem model to study the effects on carbon export of food-web interactions in the euphotic zone and remineralization processes over the entire water column. The one-dimensional model consists of 10 state variables that span the herbivorous and microbial food webs. It is forced by solar radiation, vertical mixing, and the nitrate concentration in deep water. According to the model, adjusted against a CJGOFS data set, up to 52% of the nitrate-based phytoplankton production is processed by the microbial food web before being exported from the euphotic zone. Remineralization of dissolved organic carbon and suspended particles in the water column is a key control on carbon export, and up to 77% of the total material exported from the euphotic zone is remineralized in a layer located between the bottom of the euphotic zone and the annual maximum depth of the surface mixed layer. Nitrification of ammonia released within this layer satisfies most of the biological demand for nitrate in the euphotic zone. This places limitations on the use of new production as usually determined at sea (i.e. based on the uptake of nitrate) to estimate carbon export towards the deep.