WETHEY, D S; Univ. of South Carolina: Wave forces in the surf zone: implications for population dynamics and recruitment

The striking spatial patterns of settlement of invertebrate larvae and distributions of adults on wave-exposed rocky shores are difficult to understand, when one considers the extreme hydrodynamic forces exerted on larvae and adults. In order to understand these forces on spatial scales relevant to invertebrates, I deployed 3-axis force sensors in the surf zone on several shores, and made continuous recordings of wave heights and forces at 10 Hz for a 12 month period. The record includes several million waves at each shore, and their associated forces on sensors within and on top of mussel beds. The sensors were the size and shape of adult mussels Mytilus edulis. These observations were scaled from the size of the mussel sensor to the size of barnacle larvae in order to examine implications of the forces for recruitment. There are several important results of this analysis. 1) Not all waves of the same size generate forces of the same magnitude. There is a strong probabilistic component to the hydrodynamic forces exerted by waves of the same size, with only a small fraction of the waves generating big forces. Therefore one cannot assume that the average force is representative of the environment experienced by organisms in the surf zone. 2) Neighbors create an enormous hydrodynamic refuge. Drag forces within mussel beds are negligible, and only lift forces are important. This same phenomenon will scale down to roughness scales equivalent to the larvae themselves, so small surface topography can create a very effective refuge from wave forces. 3) Larval adhesion in barnacles is adequate to resist erosional forces associated with all but the most extreme wave conditions.