Dense concentrations of zooplankton are often observed near surface fronts; several mechanisms have been proposed to explain this phenomenon. We present a two-dimensional Eulerian numerical model of one such mechanism: physical convergence in the surface flow, combined with depth-keeping swimming behavior on the part of the animals. In this model a steady-state flow field is prescribed, along with the initial distribution of the zooplankton. The animals swim vertically with speeds that depend only on depth, but the form of that depth-dependence may take into account such factors as the vertical variation in light level or in the concentration of some prey organism. It is assumed that the animals' horizontal swimming speed is insignificant compared to the fluid's horizontal velocity.
We show model results for a variety of swimming behaviors and initial distributions of zooplankton. In general, the zooplankton are found to become highly concentrated on the lighter side of the front, and at a depth slightly below the depth they would swim to if the fluid had zero velocity. We compare these results with observations of high copepod concentrations near the leading edge of a buoyant plume in the western Gulf of Maine.