TURBULENT TRANSPORT OF SUSPENDED PARTICLES AND DISPERSING BENTHIC ORGANISMS - HOW LONG TO HIT BOTTOM

Turbulence plays an important role in the transport of particles in ma ny aquatic systems. In addition to various types of inorganic sediment (silt, sand, etc.), these particles typically include bacteria, algae , invertebrates, and fine organic debris. In this paper, we focus on o ne aspect of turbulent particle transport; namely, the average time re quired for a suspended particle to reach the bottom of a waterbody fro m a specified initial elevation. This is the mean hitting-time problem , and it is important in determining, for example, the effect of turbu lence on downstream transport of organic particles, dispersal times an d dispersal distances of benthic invertebrates, and the utility of swi mming by neutrally buoyant dispersal propagules. We approach this prob lem by developing a stochastic diffusion model of particle transport c alled the Local Exchange Model, which is an extension of a model posed by Denny & Shibata (1989) in an earlier study of the same problem. We show how the mean hitting-time of the Local Exchange Model varies wit h factors such as a particle's fall velocity and the shape of the vert ical profile in turbulent mixing. We also show how the mean hitting-ti me is related to both the vertical profile in current velocity and the vertical profile in concentration of suspended particles, and how the se relationships can be exploited in testing the model. Among other th ings, our results predict that, with the sole exception of neutrally b uoyant particles that do not swim downward, there is always a region o f the water-column in which turbulence increases rather than decreases the mean hitting-time. We discuss the significance of this and other results for dispersal by benthic organisms. (C) 1997 Academic Press Li mited.