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 structu re of the pelagic food web and remineralization processes in the water colu mn. We have developed a novel pelagic ecosystem model to study the effects on carbon export of food-web interactions in the euphotic zone and reminera lization 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 co ncentration in deep water. According to the model, adjusted against a CJGOF S data set, up to 52% of the nitrate-based phytoplankton production is proc essed by the microbial food web before being exported from the euphotic zon e. 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 to tal material exported from the euphotic zone is remineralized in a layer lo cated between the bottom of the euphotic zone and the annual maximum depth of the surface mixed layer. Nitrification of ammonia released within this l ayer satisfies most of the biological demand for nitrate in the euphotic zo ne. This places limitations on the use of new production as usually determi ned at sea (i.e. based on the uptake of nitrate) to estimate carbon export towards the deep. (C) 1999 Published by Elsevier Science Ltd. All rights re served.