THE VERTICAL TRAJECTORIES OF MOTILE PHYTOPLANKTON IN A WIND-MIXED WATER COLUMN

Motile phytoplankton representing a diatom, two dinoflagellates and a ciliate were studied using a random walk model in a dynamically active mixing layer where turbulence was driven by a range of wind speeds acting on a weakly stratified water column. The simulations extend the results reported in Yamazaki and Osborn (1989) by incorporating a broad range of more complex phytoplankton behavior and by examining the differential Photosynthetically Active Radiation (PAR) exposure of different individuals within a species population under a range of mixing conditions. The respective motility capabilities of the modeled species clearly influence their vertical distributions under the same mixing conditions even at the highest wind condition considered. The wind energy required to distort the diel vertical migration significantly is related directly to their swimming capability. The portion of the initial phytoplankton population that is mixed to the bottom of the Ekman layer can experience PAR stress. The simulations show that turbulence in the mixing layer does not necessarily create a physiologically uniform state after 24 h; this differs from the conventional wisdom concerning the effect of turbulence in mixed layers. The difference reflects depth-dependent turbulence intensity inferred from the Ekman layer model, as well as the definition of the Lagrangian diffusion coefficient discussed in our study.