Consequences of wave-induced water motion to nearshore macroalgae.

摘要

Details of one aspect of the interaction of wave-induced water motion with benthic organisms are explored, in particular the potential of large water accelerations produced by breaking waves to place size constraints on intertidal seaweeds. A simple theoretical model is developed that predicts "optimal" sizes of plants based on their strength, morphology, and hydrodynamic environment, and these predicted optimal sizes are compared to sizes of plants in nature. Results imply that the dimensions of intertidal macroalgae might be limited by forces associated with fluid accelerations. However, subsequent data show this hypothesis to be overly simplistic. Although measurements of surf-zone flows verify that exceptional accelerations indeed occur in intertidal regions, additional recordings of forces imposed on organisms by breakers suggest that large hydrodynamic accelerational forces do not in fact act commonly on intertidal plants and animals. Thus it appears unlikely that wave-produced fluid accelerations actually play a major role in limiting the sizes of littoral organisms. A more detailed analysis suggests that the absence of large water acceleration-induced forces arises a consequence of the short temporal and (in particular) small spatial scales of surf-zone accelerations. Field measurements also indicate that many of the largest loads imposed on intertidal organisms are "impact" type forces associated with waves crashing directly on emersed plants and animals. Consequences of such forces are explored through the use of a numerical model that tracks the propagation of brief pulses through an idealized viscoelastic "organism." Results support the concept that low stiffness in macroalgae provides an alternative low-strength strategy for coping with large but transient forces. Additional implications of low stiffness, in particular the tendency of seaweeds to deflect in flow, are further examined in a case study of two subtidal kelps. Here it is shown that inertial forces associated with changes in the momentum of swaying plants can strongly influence the stresses they experience. Estimates of turbulence intensity and energy dissipation rate are also extracted from the flow recordings and potential biological ramifications of the extreme turbulence characteristic of the surf zone are discussed.