Diagnosis of physical and biological controls on phytoplankton distribution in the Gulf of Maine-Georges Bank region

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1999The linkage between physics and biology is studied by applying a one-dimensional model and a two-dimensional model to the Sargasso Sea and the Gulf of Maine- Georges Bank region, respectively. The first model investigates the annual cycles of production and the response of the annual cycles to external forcing. The computed seasonal cycles compare reasonably well with the data. The spring bloom occurs after the winter mixing weakens and before the establishment of the summer stratification. Sensitivity experiments are also carried out, which basically provide information of how the internal bio-chemical parameters affect the biological system. The second model investigates the effect of the circulation field on the distribution of phytoplankton, and the relative importance of physical circulation and biological sources by using a data assimilation approach. The model results reveal seasonal and geographic variations of phytoplankton concentration, which compare well with data. The results verify that the seasonal cycles of phytoplankton are controlled by both the biological source and the physical advection, which themselves are functions of space and time. The biological source and the physical advection basically counterbalance each other. Advection controls the tendency of the phytoplankton concentration more often in the coastal region of the western Gulf of Maine than on Georges Bank, due to the small magnitude of the biological source in the former region, although the advection flux divergences have greater magnitudes on Georges Bank than in the coastal region of the western Gulf of Maine. It is also suggested by the model results that the two separated populations in the coastal region of the western Gulf of Maine and on Georges Bank are self-sustaining