Hydrodynamic forces and surface topography: Centimeter-scale spatial variation in wave forces

We measured the variability in hydrodynamic forces among locations separated by only centimeters along three horizontal transects on a steep rocky shore. Our results extend previous work showing that wave forces have the characteristics of 1/f-noise, in which variability in wave forces decreases exponentially with decreasing scales of measurement. Furthermore, our results suggest that protection from hydrodynamic forces is not a certain consequence of a rugose substratum, suggesting that investigators should directly test (rather than assume) smallscale topographic protection from hydrodynamic forces. The intertidal zone of wave-swept rocky shores is one of the most physically stressful environments on Earth. Perhaps the most apparent physical stress factors—at least to the human observer—are the violent hydrodynamic forces imposed by breaking waves. On exposed shores, water velocities from breaking waves routinely reach 10 m s21, while storm waves can produce water velocities of greater than 25 m s21 (Denny et al. 2003). The high water velocities in the intertidal zone can impose large hydrodynamic forces on objects they encounter (e.g., Koehl 1984; Denny 1988), and many investigators have examined dislodgement and breakage of organisms subjected to breaking waves (e.g., Shanks and Wright 1986; Trussell et al. 1993; Denny 1995; Gaylord 1999). Amid these potentially dangerous hydrodynamic forces, wave-swept rocky shores provide habitat for rich and highly diverse communities of species (Ricketts and Calvin 1939; Evans 1947; Stephenson and Stephenson 1972) which have been the subject of intense study, both for their own sake (Dayton 1971; Sousa 1979; Lohse 1993) and as a model system for community ecology in general (Wethey 1985; Gaines and Bertness 1993; Underwood 2000). Understanding the degree to which physical factors such as wave exposure (Trussell 1997a; Gaylord 1999; Rilov et al. 2004) or temperature (Helmuth 1998; Somero 2002) determine where organisms live has been a common theme in many intertidal studies. For example, distributions of intertidal organisms are often patchy (Paine and Levin 1981); if aspects of the physical environment determine the limits of habitable space for certain species, then understanding the patchiness of environmental conditions can potentially help to explain the patchy distributions of organisms. On the rugose rock surfaces of wave-swept shores, interactions between substratum topography and wave-induced flow may create such a spatially variable environment.