Biological and physical processes controlling the spring phytoplankton bloom dynamics on Georges Bank
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A lower trophic level food web model was developed to examine the biological and physical processes controlling the spring phytoplankton bloom dynamics on Georges Bank. It is a 9-component nutrients-phytoplankton-zooplankton-detritus model including 3 nutrients (nitrate, ammonia and silicate), 2 phytoplankton (large- and small-size groups), 2 zooplankton (large- and small-size groups), 1 detrital- organic nitrogen and 1 detrital silicon. This biological model was coupled with state of the art structured- and unstructured-grid coastal ocean models (ECOM-si and FVCOM) and was used to examine impacts of physical processes on the ecosystem dynamics on Georges Bank. To distinguish the roles of light intensity and light attenuation, current advection, tidal mixing/rectification, wind-induced mixing/advection, and buoyancy fronts as well as remote in ow in the formation of the spring phytoplankton bloom on Georges Bank, numerical experiments were conducted through 1-dimensional (1-D), 2-dimensional (2-D), and 3-dimensional (3-D) approaches. The 1-D experiments were performed for a fixed location at which all biological and physical variables were assumed to be uniform in the horizontal but not in the vertical. The 2-D experiments featured a transect across Georges Bank in which the along-isobath variation for all the variables were ignored. The 3-D experiments were focused on the in uence of the \cross-over" event through the Northeast Channel in the Scotian Shelf on the formation of the dense spring bloom over the southeastern edge of Georges Bank. The 1-D model results clearly suggest that the biological and physical mecha- nism for the spring phytoplankton bloom significantly differs between the shallow and deeper regions of Georges Bank. In the shallow, well-mixed central bank, the timing and duration of the spring bloom are determined by light intensity and its downward penetration while the bloom intensity is regulated by initial nutrient con- centration and zooplankton grazing pressure. In deeper water (> 60 m), given the same conditions of light intensity/attenuation, initial nutrient concentration and zoo- plankton grazing pressure as those in the shallow, well-mixed region, the timing of the spring bloom is closely linked to the seasonal development of stratification. The dense phytoplankton biomass forms as the seasonal vertical stratification develops. The 2-D model results show that the biological and physical processes governing the spring phytoplankton bloom in the well-mixed region remain the same as in the 1-D case. However, in deeper water, the timing, location, and duration of the bloom are in uenced strongly by on-bank nutrient supply over the anks of Georges Bank. In particular, once the tidal mixing front is established, a \second bloom" can form near the front as a result of the up-frontal nutrient ux driven by the secondary ow. A 3-D model experiment detected that the 1999 March bloom event captured in SeaWiFS images on the southeastern ank of Georges Bank was typical of features driven by strongly coupled biological and physical processes. It was in uenced by (1) transport of the Scotian Shelf Water, (2) wind- and tidal-induced vertical mixing and surface cooling, and (3) on-bank intrusion of the salinity-dominated shelf break front. With a sufficient supply of nutrients from the slope, the bloom could occur due to rapid in situ growth of phytoplankton near the shelf break front. This exper- iment also suggests that an accurate simulation of the spatial distribution of water temperature and salinity (in particular, the location of the shelf break front) and \cross-over" water transport is a prerequisite to capture the spring bloom over the southeastern and southern edges of Georges Bank.