Evidence for avoidance of flushing from an estuary by a planktonic, phototrophic ciliate

Abstract

The vertical distribution of the planktonic, phototrophic ciliate Mesodimurn rubrum was monitored during a red-water event in daylight hours, over a complete tidal cycle Theoretical con- siderations based upon the hydrodynamics of the estuary, and upon the behaviour of the ciliate described to date, suggest that the population should not be able to resist flushing losses from the estuary. However, field observations reveal a pattern consisting of aggregation close to the surface during flood tide and high water, but dispersion away from the surface on the ebb tide, thus minimizing flushing losses due to the strong superficial currents Evidence is presented suggesting that the major cue for such migration may be the turbulence generated by shearing of surface currents, or gravita- tional (static) instability of surface water.