The model runs clearly demonstrate that the temporal variability of winds is responsible for significant changes in plume structure and transport over time scales as short as a few days. Strong wind events, particularly upwelling favorable, appear capable of enhancing mixing to levels such that the alongshore transport of plume waters is strongly retarded, and the freshwater and Alexandrium are significantly dispersed in both the cross-shore and vertical directions. Given the combination of both elevated mixing rates and temporarily arrested of the downcoast transport, upwelling winds seem most capable of dispersing concentrations of Alexandrium cells. The idealized simulations also emphasize that the timing of cell release with respect to wind events is critical in predicting algal bloom dynamics. Studies are continuing with the idealized model to identify the locations of strongest mixing during large wind events.
Preliminary results suggest intensified mixing occurs at the offshore edge of the plume.
This behavior would suggest that red tide cell growth may be largest in the frontal regions of the plume given the elevated fluxes of nutrients associated with the amplified mixing. However, any increases in the population density due to growth at the offshore mixing zone may be outweighed by subsequent dispersion of the population during strong upwelling conditions. This also emphasizes the need to understand the timing and source conditions associated with release of toxic cells. The combined timing of buoyancy and wind forcing and cell release will determine the ultimate toxicity and location of high concentrations. In addition to the idealized modeling, we have also numerically hindcasted the 1993 and 1994 field seasons using realistic bathymetry, measured wind, river and heat flux data, and boundary conditions derived from in situ observations.