Graduate Thesis Or Dissertation
 

Oceanographic influences on rocky intertidal communities : coastal upwelling, invertebrate growth rates, and keystone predation

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  • The influence of large-scale processes on natural communities has become one of the central issues of modern ecology. I combined field and laboratory studies to investigate the effects of variation in coastal upwelling on rocky intertidal communities along the central Oregon coast. I examined whether the growth of intertidal barnacles (Balanus glandula and Chthamalus dalli) at two sites was linked to nearshore phytoplankton abundance. Short-term growth rates were low during a persistent upwelling event and increased two to three-fold during the subsequent relaxation of upwelling. Although this increase coincided with a large phytoplankton bloom at one site, parallel increases in growth occurred at a second site where no bloom was recorded. Published analyses of barnacle stomach contents suggest that zooplankton may be an important component of barnacle diets. Barnacle growth may thus be enhanced during upwelling relaxations through the combined benefits of more phytoplankton, more zooplankton, and warmer water temperatures. In laboratory studies, I tested the hypothesis that upwelling-related variation in water temperature regulates the feeding, growth, and energetics of two intertidal predators, the sea star Pisaster ochraceus and whelk Nucella canaliculata. Sea stars and whelks maintained at 9°C generally consumed 30% fewer mussels than those at 12°C. Animals that were exposed to cyclic temperatures (alternating between 12°C and 9°C every two weeks) tended to grow faster than those held at constant temperatures. Consumers in upwelling systems may thus feed intensely during periods of warmer water while benefiting from reduced metabolic costs during cold-water intrusions. I conducted field experiments to quantify the effect of small changes in water temperature on the interaction between a keystone predator, Pisaster ochraceus, and the intertidal mussel Mytilus californianus. Per capita and per population effects of sea stars on transplanted mussels were sharply reduced during a persistent cold-water upwelling event. Interannual variation in the frequency and intensity of upwelling may thus alter community dynamics through effects on this keystone interaction. Taken together, these studies suggest that upwelling-related variation in nearshore conditions can profoundly influence intertidal systems at the organismal, population, and community level.
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