A horizontally resolving physical-biological model of nitrate concentration and primary productivity in the Strait of Georgia

Model simulations revealed that wind mixing was the dominant physical mechanism that added nitrate to the surface layer and subsequently enhanced primary productivity in the Strait of Georgia. Simulations of high Fraser River runoff showed that the enhanced stability of the water column in the vicinity of the riverine plume made wind mixing of nutrients into the surface layer more difficult. We propose that this increase in stability results in an earlier onset of the spring bloom in regions influenced by Fraser River runoff. During the summer, an increase in the buoyancy of surface water due to the freshwater plume reduces nitrate concentration in the surface layer and thereby limits primary production in the plume area. The reduced impact of wind events on nitrate fluxes is the result of a greater energy requirement to break down the more buoyant surface layer. Results indicate that during the fall, when light is again limiting and surface nitrate concentrations increase due to wind mixing by fall storms, the freshwater runoff from the Fraser River results in a more stable water column (similar to the spring situation) in the southern Strait, resulting in the potential for a fall bloom.