SANFORD, E.: The effect of variation in water temperature on the keystone predator Pisaster ochraceus
Ecologists and physiologists have been drawn to the significant influences of aerial heat stress on intertidal organisms. Temporal variation in water temperature is an additional, often overlooked, factor that may have important effects on the physiology and ecology of intertidal species. Along the Pacific coast of North America, coastal upwelling generates short-term fluctuations in water temperature that appear to affect rates of feeding and growth in a keystone predator, the sea star Pisaster ochraceus. I held laboratory sea stars at a constant 9°C, a constant 12°C, and a treatment that simulated episodic upwelling by cycling between 14-day periods of 12°C and 9°C. Unsatiated sea stars held at 9°C consumed 30% fewer mussels than those in warmer tanks, but grew at similar rates in 9°C and 12°C tanks. This suggests that reduced consumption under cold conditions was balanced by reduced metabolic costs. In contrast, there appeared to be an energetic advantage to living in the thermal regime characteristic of episodic upwelling. Sea stars alternately exposed to 12°C and 9°C had a significantly higher growth rate, conversion efficiency, and storage of reserves in the pyloric ceca than individuals in the constant 12°C tanks. At the community level, field experiments conducted on the Oregon coast demonstrated that rates of Pisaster predation on the competitively dominant mussel species were sharply reduced during episodes of cold-water upwelling. By linking temperature and keystone predation, these experiments suggest a pathway through which spatial and temporal variation in upwelling may influence the dynamics of rocky intertidal communities.
