EFFECTS OF SIZE AND STIFFNESS ON THE HYDRODYNAMICS OF SESSILE MARINE ORGANISMS

M. A. R. Koehl
Department of Integrative Biology
University of California
Berkeley, CA 94720-3140

Sessile marine organisms often depend on ambient currents for transport of food or nutrients, removal of wastes, and dispersal of gametes or young, yet they also risk being dislodged or broken by hydrodynamic forces. These sessile organisms range in stiffness from rigid stony corals to bendable gorgonians and kelp, to very flexible hydroids and algae. We have been investigating how, in unidirectional currents versus in waves, the stiffness and size of such benthic organisms affect: 1) the water motion and mass transport experienced by different regions of sessile macrophytes or animal colonies, and 2) the hydrodynamic forces they experience. Lab and field experiments have shown how the degree to which flexible organisms move with the flow, as well as their whiplashing and fluttering, affect the spatial distribution along a colony or plant of the local water velocities with respect to the organisms' surfaces, which in turn can affect the rates of processes such as suspension feeding or uptake of dissolved substances. Furthermore, passive reconfiguration of flexible organisms by moving water reduces the hydrodynamic forces they must resist. Our work revealed the mechanisms underlying the surprising result that unidirectional currents are more mechanically stressful to flexible organisms than are ocean waves, whereas the opposite is true for stiff creatures. Although hydrodynamic forces on stiff organisms rise as they increase in size, flexible bodies in the oscillating flow of waves experience no increase in force as they grow beyond a critical length.

The local flow encountered by individuals on a branching colony can be affected by other branches on the colony; similarly, the flow experienced by a sessile colony or macrophyte is affected by the spatial structure of the surrounding canopy of neighboring organisms. Field studies have shown that both rigid and flexible canopies slow ambient water flow, but enhance water mixing. Whether or not the size and shape of a sessile organism affects the transport and mixing of materials it releases (e.g. larvae, wastes, chemical cues) depends on ambient waves or currents and surrounding canopy. Furthermore, the consequences of colony size and of upstream neighbors to suspension feeding depend on flow habitat.