Many of the dense patches of copepods and other zooplankton found near Georges Bank may be created and sustained by integrated physical/biological mechanisms that depend both on the specific swimming behavior of the organisms and on the nature of the surrounding physical flow field. We describe a numerical model of one such mechanism, in which depth-keeping swimming behavior on the part of an organism combines with a convergent flow field near a small-scale front to create dense patches of the organism on one side of the front. In particular, we investigate ways in which changing the intensity of vertical diffusivity in the flow field changes the structure and degree of concentration of the resulting patch
.
The results can be summarized in terms of two dimensionless parameters of the model: a diffusivity parameter, which governs the strength with which animals near the top and bottom of the patch resist the effects of vertical diffusion; and an elasticity parameter, which measures the degree to which the plankton patch re-forms to create a thin, horizontal layer when subjected to distorting shear flows. Increasing the diffusivity parameter decreases the final concentration of plankton in the patch and causes the patch to acquire a distinct ``tail" on one side. Decreasing the elasticity parameter also decreases the final concentration of plankton in the patch, and causes the patch to form in a lozenge-like shape aligned roughly parallel to the slope of the front.