A 3-D prognostic numerical model study of the Georges bank ecosystem. Part II: biological–physical model

A three-dimensional ecosystem-physical model is presented for the Gulf of Maine (GOM), including Georges Bank (GB) and the Nantucket Shoals (NS). The coupling of a simple nutrient-phytoplankton–zooplankton model to a detailed physical model forced by the M2 tides generated patterns of plankton and nutrients that agreed closely with data. High phytoplankton biomass developed in regions of strong vertical mixing, particularly on GB and the eastern flank of the NS. Low concentrations were seen in the GOM, where a subsurface chlorophyll maximum developed at 20 m, coincident with the top of the nutricline. High surface phytoplankton concentrations in the GOM, and low concentrations on the top of GB led us to reject several parameterizations of the biological model. A horizontally dependent euphotic depth was required for an accurate simulation of GB and the GOM with a single biological parameter set. High f-ratios developed in the frontal regions, implying a vertical flux of nutrients from below the euphotic zone. Biological patterns were dominated by vertical mixing induced by the tidal forcing, though removing advection from the simulations led to a decrease in the regions of high f-ratio. A surprising feature of the model was the robustness of the simulated patterns of phytoplankton and nutrients, in spite of large changes in biological parameters and the removal of advection from the model. This reinforces the hypothesis of the overall dominance of vertical mixing in structuring the plankton in and around GB.