Predicting wave exposure in the rocky intertidal zone: Do bigger waves always lead tolarger forces?

Abstract

Hydrodynamic forces from breaking waves are among the most important sources of mortality in the rocky intertidal zone. Information about the forces imposed by breaking waves is therefore critical if we are to interpret the mechanical design and physiological performance of wave-swept organisms in an ecologically and evolutionarily relevant context. Wave theory and engineering experiments predict that the process of wave breaking sets a limit on the maximum force to which organisms can be subjected. Unfortunately, the magnitude of this limit has not been determined on rocky shores. To this end, at a moderately exposed shore in central California, we measured the maximum hydrodynamic forces imposed on organism-sized benthic objects and related these forces to nearshore significant wave heights. At 146 of 221 microsites, there was a significant and substantial positive correlation between force and wave height, and at 130 of these microsites, force increased nonlinearly toward a statistically defined limit. The magnitude of this limit varied among sites, from 19 to 730 newtons (N). At 37 other sites, there was no significant correlation between surf zone force and wave height, indicating that increased wave height did not translate into increased force at these sites either. At only 16 sites did force increase in proportion to wave height without an apparent upper bound. These results suggest that for most microsites there is indeed a limiting wave height beyond which force is independent of wave height. The magnitude of the limit varies substantially among microsites, and an index of local topography was found to predict little of this variation. Thus, caution must be exercised in any attempt to relate observed variations in ocean ''waviness'' to the corresponding rates of microsite disturbance in intertidal communities.Rocky intertidal invertebrates and algae live in a world of extreme environmental severity, and the risk of damage or dislodgment from wave-generated forces is thought to be among the most important determinants of survival in this habitat (e.g., Dayton 1971; Levin and Paine 1974;Koehl 1979; Paine 1979; Paine and Levin 1981;Sousa 1984;Denny 1987Denny , 1988Carrington 1990Carrington , 2002Bertness et al. 1991;Hunt and Scheibling 1996;Blanchette 1997). Quantifying the hydrodynamic forces acting on organisms, and how they vary in space and in time, is therefore key to understanding the evolutionary and ecological consequences of morphological design and the subsequent effects of wave-driven forces on the dynamics of intertidal ecosystems (Denny 1988;Koehl 1996;Denny and Wethey 2001;Carrington 2002).The use of engineering theory to study these issues has proven fruitful and has led to a deeper understanding of how intertidal organisms are able to withstand the rigors of waveexposed shores (e.g., Koehl 1979;Denny 1988;Carrington 1990Carrington , 2002 Bell and Gosline 1996;Denny et al. 1998;Gaylord 2000). Understanding how organisms resist damage and dislodgment, however, also requires an estima...

Intra- and interspecific variation of attachment strength in sea urchins

“…The only other safety factors reported for sea urchins were measured for Strongylocentrotus droebachiensis and amounted to about 10 (Siddon & Wittman 2003). The large difference between the 2 studies may come from the fact that Siddon & Wittman (2003) measured water velocity directly onsite, while our values were extrapolated from wave heights, and there is sometimes a poor correlation between significant wave height and onshore water velocity (Helmuth & Denny 2003). On the other hand, the population of Paracentrotus lividus from Cabo Raso was intertidal and thus fully exposed, whereas the population of S. droebachiensis was subtidal (Siddon & Wittman 2003).…”

Failure by fatigue in the field: a model of fatigue breakage for the macroalga Mazzaella, with validation

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Precisely proportioned: intertidal barnacles alter penis form to suit coastal wave action

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