WOLCOTT, Bryce D.; Univ. of California, Berkeley: Surviving the maelstrom: morphology, size, and danger of dislodgement of intertidal algae

Wave-induced water motion has been shown to influence both the size and shape of organisms on rocky shores. The morphology of many seaweeds, for example, varies with flow conditions, resulting in a reduction of the mechanical stress imposed on individuals at exposed sites. In addition to the magnitude of hydrodynamic forces, thallus morphology may influence rates of dessication and heating at low tide, light interception, and nutrient uptake, potentially creating trade-offs for algae during growth. Here, as part of a larger study investigating these trade-offs, I examine the influence of wave-induced flow on the morphology and size-dependent survivorship of Pelvetiopsis limitata, a common intertidal seaweed. Individuals were collected from an exposed and a moderately protected site for measurements of mass, surface area, length, and holdfast area. The breaking strength of stipe tissue and the force to dislodge thalli in the field were also measured. Surprisingly, no differences were found between the sites in any of these parameters. To examine the mechanical consequences of having the same morphology at sites exposed to different levels of wave action, I constructed a mathematical model of the forces on the algae in oscillatory flow, estimating the load imposed on the stipe as a function of thallus size and ambient flow. Results from the model indicate that algae across the size range observed in the field exhibit a very low probability of dislodgement at either site in a year of typical wave exposure for the area. This is consistent with the morphological and mechanical data in suggesting that, despite the extreme flows common at the exposed site, P. limitata are too small for morphology to have an ecologically-relevant effect on mechanical performance in waves.