A SEASONAL 3-DIMENSIONAL ECOSYSTEM MODEL OF NITROGEN CYCLING IN THE NORTH-ATLANTIC EUPHOTIC ZONE

A seven-component upper ocean ecosystem model of nitrogen cycling cali brated with observations at Bermuda Station ''S'' has been coupled to a three-dimensional seasonal general circulation model (GCM) of the No rth Atlantic ocean. The aim of this project is to improve our understa nding of the role of upper ocean biological processes in controlling s urface chemical distributions, and to develop approaches for assimilat ing large data sets relevant to this problem. A comparison of model pr edicted chlorophyll with satellite coastal zone color scanner observat ions shows that the ecosystem model is capable of responding realistic ally to a variety of physical forcing environments. Most of the discre pancies identified are due to problems with the GCM model. The new pro duction predicted by the model is equivalent to 2 to 2.8 mol m-2 yr-1 of carbon uptake, or 8 to 12 GtC/yr on a global scale. The southern ha lf of the subtropical gyre is the only major region of the model with almost complete surface nitrate removal (nitrate<0.1 mmol m-3). Despit e this, almost the entire model is nitrate limited in the sense that a ny addition of nitrate supply would go predominantly into photosynthes is. The only exceptions are some coastal upwelling regions and the hig h latitudes during winter. where nitrate goes as high as approximately 10 mmol m-3.