O’DONNELL, Michael, J.; Hopkins Marine Station, Stanford University: Big Breaking Waves Bashing Small Sessile Stuff

Hydrodynamic forces imposed by breaking waves are one of the most obvious stresses in the intertidal zone of wave-swept shores. Previous studies have indicated that surface topography can influence the distributions of organisms by providing protection from extreme hydrodynamic forces, particularly for those organisms that are small relative to the bumps and cracks of the rock surfaces. Although many investigators have suggested that surface features should provide a reduction in hydrodynamic forces, this has not been directly tested at an appropriately small size scale. I have deployed dynamometers that record the maximum forces experienced by objects approximately 1cm in diameter. These data show that there are significant in wave forces among locations separated by only a few centimeters, but that rock crevices actually enhance wave velocity, rather than providing protection from it. Drag-measurement spheres routinely experienced water velocities in excess of 10m/sec, suggesting that organisms of this size do not experience any reduction in hydrodynamic forces simply by being close to the rock surface. The hydrodynamic environment that is relevant to many small organisms on rocky shores is highly complex, rarely predictable, and strongly influenced by local substratum topography.