ECOSYSTEM BEHAVIOR AT BERMUDA STATION-S AND OCEAN WEATHER STATION INDIA - A GENERAL-CIRCULATION MODEL AND OBSERVATIONAL ANALYSIS

A model of biological production in the euphotic zone of the North Atl antic has been developed by coupling a seven-compartment nitrogen-base d ecosystem model with a three-dimensional seasonal general circulatio n model. The predicted seasonal cycles of phytoplankton, zooplankton, bacteria, nitrate, ammonium, primary production, and particle flux hav e been compared to data from Bermuda Station ''S'' and Ocean Weather S tation ''India''. Bearing in mind the simplicity of the model and the paucity of data, the results are encouraging. However, deficiencies in the physical model lead to winter nitrate values at Bermuda being ove restimated, and at both positions the predicted magnitude of the sprin g phytoplankton bloom was too high. Simulations were carried out with different detrital sinking rates and and it was found that a sinking r ate of 10 m d-1 gave the best agreement with observations. The model w as used to investigate the factors affecting the population growth of phytoplankton and it was found that the model supported the generally held theory that the spring bloom is initiated by the cessation of phy sical mixing. After the bloom, phytoplankton are controlled by zooplan kton grazing. At Ocean Weather Station ''India'' the model reproduced the observed high summer nitrate levels and suggested that these high values are caused by a combination of high vertical nitrate transport, ammonium inhibition of nitrate uptake, and zooplankton grazing contro l. The model demonstrated the critical importance of zooplankton in un derstanding ecosystem dynamics and highlights the need for more observ ational data on the seasonal cycles of zooplankton biomass and growth rates.