Habitats and hydrodynamics on wave-swept rocky shores

Wave-swept shores are an area of intense physical disturbance, as breaking waves create forces that potentially damage or dislodge the organisms inhabiting this region. Previous research has made progress quantifying the forces experienced beneath breaking waves and the effects waves have on organisms and communities. Many investigators have postulated that organisms avoid wave forces by hiding in protected microhabitats such as rock crevices or within mussel beds. I sought to quantify the degree of protection such microhabitats might convey. Looking at forces experienced by objects 1 cm in diameter, I did not find examples of rock topography reducing wave forces. In contrast, I found rock crevices with certain orientation to oncoming waves as regions of enhanced water velocities. In separate experiments, measured wave forces within patches in an artificial mussel bed were reduced near to the mussel bed structure. These results provide a mechanism to explain organisms with low tenacity living within the matrix of mussel beds, as well as grazing halos observed around mussel beds. I also considered the potential for breaking waves to be beneficial to organisms, rather than a negative stress. I quantified the degree to which wave splash up the intertidal zone could reduce exposure to the terrestrial environment at low tide. I found that areas of high wave forces, where waves splashed water higher up the rock surface spent more time inundated by waves than locations 10s of centimeters away. This beneficial effect of waves is another potential mechanism to explain crevice use by intertidal organisms. A final experiment looked at the variation in wave forces at a given location as a function of the rising and falling of tides, finding that wave forces experienced at a location varied with the tide, reaching a maximum when the tide was slightly above the location, and diminishing as the tide rose higher. Together, the chapters of this thesis provide a detailed look the environment in the intertidal zone of wave-swept shores. I discuss how the aspects of the environment considered here structure the habitats in which intertidal communities live.