Flow-induced aggregation of plankton at a front: a 2-D Eulerian model study

We use a simple 2-D Eulerian numerical model to illustrate a mechanism whereby depth-keeping swimming behavior on the part of an organism, combined with a convergent flow field at a surface front, can create dense populations. In this model, a steady flow field and vertical diffusivity are prescribed, along with an initial plankton distribution. The plankton swim vertically with speeds that depend only on depth, where the form of that dependence may reflect such factors as the vertical variation in light level or the concentration of some prey organism. The Eulerian nature of the model enables diffusion to be treated explicitly. The numerical model describes the evolution of an initial plankton patch in time. A parameter study illustrates the roles played by diffusion, water velocity and details of the animals’ swimming behavior on patch evolution. Output from the model is compared with data taken at a dense plankton patch observed near a small-scale front in the northern Great South Channel in early June 1989.