An integrated biological pump model from the euphotic zone to the sediment: a 1-D application in the Northeast tropical Atlantic

A coupled one-dimensional biogeochemical/physical model is developed to fol low the organic matter fluxes from the upper ocean to the sea floor. The bi ogeochemical model is a nitrogen-based seven-compartment model including nu trients, phytoplankton, zooplankton, two pools of dissolved organic matter, and two size classes of detrital material. Particle dynamics are considere d through the water column as well as organic matter deposition and mineral ization in the superficial sediments. The model is applied at the EUMELI ol igotrophic site (21 degreesN, 31 degreesW) where different seasons were sam pled in 1991-1992 and sediment trap data collected continuously over the sa me period. The model, forced with the reanalyzed ECMWF fluxes for these yea rs, reproduces satisfactorily the weak seasonal variability of phytoplankto n concentration as well as the exported nitrogen fluxes. Annual primary pro duction (65 g C/m(2)/yr) is sustained mainly by remineralization of DON and zooplankton excretion. Export production at 150 m is ensured by large part icles, the DON export contributing only 31% of the total export. The POC ex port represents 1.3% of the primary production. Including nutrient horizont al advection in the model to mimic any lateral Ekman transfer from the enri ched neighboring subtropical gyre (5.5 mmol N/m(2)/yr over the first 150 m estimated from optimization) induces an annual primary production of 73 g C /m(2)/yr, closer to Morel et al.'s (Deep-Sea Res. I 43(8) (1996) 1273-1304) estimate (110 g C/m(2)/yr). Estimated mean carbon fluxes at 1000 and 4400 m depth compare well with sediment trap data, 2 mg C/m(2)/d and 1 mg C/m(2) /d, respectively. Remineralization and disaggregation are the dominant proc esses below 150 m, aggregation playing a minor role. Observed continuous pa rticulate organic matter fluxes over both years show a more variable evolut ion than the modeled one. This could be due to mesoscale circulation in the area, or subduction of water masses from the Mauritania upwelling. The mod eled seasonal variability of dissolved matter fluxes at the water-sediment interface is very weak, as expected. (C) 2001 Elsevier Science Ltd. All rig hts reserved.