Accelerational Forces in Breaking Waves: Their Nature and Biological Consequences

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Dennny 9313891 Surf-zone flows on rocky shores are characterized by rapid velocities and large accelerations, with important consequences for wave-swept benthic organisms. In particular, accelerations beneath breaking waves can exceed 40 times gravity, inducing substantial acceleration forces that have been hypothesized to set both distribution limits and mechanical size limits in a variety of intertidal organisms. However, information on surf-zone accelerations is presently inadequate to predict the likelihood of encountering accelerations of given magnitude. In response to these issues, this research project will address four major goals: (1) Surf-zone accelerations will be measured at a representative rocky intertidal site, and their relationship to concurrent, instantaneous velocities will be described. (2) The shape factors that determine the force experienced by an organism encountering a given velocity and acceleration will be measured in the laboratory. Experiments will focus on testing a representative variety of algal species. (3) The ability of the laboratory-derived shape factors to predict forces on algae in the field will be tested by comparing the forces acting on actual plants in the field to forces predicted on the basis of the laboratory data. (4) Using the laboratory and field measurements, the theoretical implications of the accelerational force to wave-swept algae will be explored. To this end, strength distributions of several algal species will be measured, and existing mathematical models will be used to predict survivorship as a function of size and wave exposure. Together, these data will be used to estimate distributional and size limits in these species, results that have significance and applicability to intertidal community ecology in general. Additionally, experiments will be carried out to quantify the effect of turbulence-induced shear stress on fertilization and early development. The majority of intertidal invertebrate an imals reproduce by external fertilization. However, because breaking waves quickly sweep away gametes shed by spawning animals, life in the highly turbulent surf zone poses a severe challenge to reproduction. Recent experiments have shown that turbulent mixing caused by breaking waves rapidly dilutes the gametes, resulting in low fertilization success. But exposure to turbulence has consequences other than gamete dilution: there are some indications that turbulence-induced shear stresses may mechanically damage gametes and could hinder egg-sperm binding. Furthermore even when eggs are fertilized, exposure to turbulence may retard and alter development. Fertilization in the highly turbulent surf-zone will be simulated by exposing gametes and embryos to the high shear stresses developed in liquid sheared between two rotating cylinders (Couette flow). The longer-term effects of the low to moderate levels of shear stress found in the near-shore mixed-layer environment will be studied using a tank in which turbulence is created by vortices shed from an oscillating grid. ***