Most flows in the oceans are unsteady. Formally, that statement means that the derivative of at least one velocity component is nonzero. Small organisms experience unsteadiness from at least three broadly important mechanisms. One is turbulent dissipation as it approaches the smallest scales of organized vortices, the so-called Kolmogorov scale, and transitions to viscous dissipation. Because upper mixed and bottom boundary layers are generally turbulent, organisms in these physically and biologically active zones generally experience unsteady flows. Organisms smaller than the Kolmogorov scale in decaying turbulence in a moderately energetic layer will rarely experience constant flow magnitude and direction beyond time scales of a few tens of seconds (Karp-Boss, Jumars and Boss 1996).
A second mechanism is entrainment in the flows produced by suspension feeders. These flows are sometimes inherently unsteady due to beating of appendages, but in any case are unsteady from the perspective of an entrained organism being accelerated into the intake stream. A third mechanism is the approach of any other solid particle; moving particles produce flow perturbations around themselves that are imposed on neighbors as they get close. Consequently, unsteady flows are important to hydrosol filtration (aka suspension feeding), prey detection and capture in general, coagulation, and fertilization.