Plankton Production over Georges Bank: A 3-D Coupled Physical-Biological Modeling Study

Changsheng Chen, Peter J. S. Franks, and Robert Beardsley

3-D coupled physical-biological model of the Gulf of Maine was developed. The physical model was the modified Blumberg and Mellor's (BM) turbulent-closure primitive equation model, and the biological model was the Nutrient-Phytoplankton-Zooplankton (NPZ) model. The numerical domain covered the entire region of the Gulf of Maine and included the Scotian Shelf to the northeast and the New Jersey shelf to the southwest. This domain was enclosed by an open boundary which runs along the 4000-m isobath off Georges Bank and intersected with the coast at 60o 24' W, 45° 42' N in Nova Scotia and at 74° 48' W, 39° 12' N in New Jersey. Numerical grids were generated by the orthogonal curvilinear coordinate transformation in the horizontal and the -coordinate transformation in the vertical. Horizontal resolutions were 1.5 to 2.5 km over Georges Bank and in the interior region of the Gulf of Maine and 4 to 20 km near the open boundary. Vertical resolution was

A prognostic run of the coupled biological and physical model with the summertime stratification and NPZ distributions clearly showed the dominant influence of tidal mixing on the 3-D lower trophic food web patterns over Georges Bank (see Figure 1). Tide mixed water vertically on the top of the bank, creating a tidal mixing front around the 40-m isobath on the northern flank and the 50 to 60-m isobath on the southern flank. Correspondingly, biological fields were homogenized inside the front, while a high concentration of phytoplankton develops around the front. The biomass of biological variables in the model agreed well with recent GLOBEC interdisciplinary measurements over Georges Bank. The diurnal heat flux variation was added into the coupled biological and physical model. The results can be viewed in the 3-D animation at [This is not available at the moment - 06/03/03 - DMO] A good agreement also was found in the cross-bank distribution of NPZ between 2-D and 3-D experiments. This implied that a 2-D model has captured the basic dynamics of biological and physical interaction processes controlling cross-bank distributions of NPZ at the center of the bank.