Wind chop and ship wakes determine hydrodynamic stresses on larvae settling on different microhabitats in fouling communities

Abstract

Fouling communities living on hard surfaces in harbors are model systems for studying larval recruitment and ecological succession. Although they live in protected harbors, fouling communities are exposed to waves due to wind chop and ship wakes. We studied how superimposing waves onto unidirectional currents affects hydrodynamic stresses experienced by larvae settling into different microhabitats within rugose fouling communities. We exposed fouled plates in a flume to turbulent water currents and waves mimicking those measured across fouling communities in Pearl Harbor, Hawaii, USA, and used laser-Doppler velocimetry to measure water velocities on the scale of larvae (500 µm from surfaces) at specific positions within each community chosen to represent a wide range of microhabitat types. These data were used to determine in stantaneous hydrodynamic stresses encountered by larvae and to calculate larval settlement probabilities. Local topography was more important than successional stage in determining hydro - dynamic stresses on the scale of larvae. Increasing current velocity reduced settlement probabili- ties, with the largest effects on a flat unfouled surface and on microhabitats on the tops of fouling organisms. Wind chop and ship wakes reduced the probability of larval settlement at all current speeds and in all microhabitats, with the most pronounced effects on sites atop fouling organisms. Episodic peak stresses can be orders of magnitude higher than mean stresses, so using instanta- neous stresses to calculate settlement probability yields a lower value than is predicted using mean stress. The factor by which the use of mean stress over-estimates settlement probability depends on both microhabitat and flow conditions.