Marine intertidal organisms live in a habitat that is characterized by acute environmental gradients influenced by both terrestrial and oceanic conditions. The degree to which organisms extend upshore is generally thought to be set by some aspect of temperature and/or desiccation stress. However, some sites in the Northeast Pacific do not appear to be thermally stressful for intertidal organisms. The mussel, Mytilus californianus, is a dominant species in this region and frequently is used for examining ecological principles. I investigated the importance of temperature in setting patterns of M. californianus zonation across a large latitudinal scale by combining field surveys and physiological studies with a biophysical model of mussel body temperature. M. californianus beds were surveyed at 12 sites ranging from Washington to Southern California (~1500 km of coastline). M. californianus were collected from several survey locations and exposed to high air temperatures in laboratory-controlled conditions to assess mortality for different temperatures. The biophysical model was supplied with remotely sensed, modeled, and reanalyzed physical data to hindcast mussel temperatures for 10 years.

Results/Conclusions

Physiological studies indicated that mussels die when their body temperatures reach approximately 37-38°C for exposures greater than 30 minutes. A synthesis of this information with mussel temperature simulations revealed that high body temperature does not appear to be the mechanism determining M. californianus upper zonation limits at some sites, particularly in Northern and Central California. This result suggests that predictions of changes in zonation as a consequence of climate change will need to be determined on a site-to-site basis for the rocky intertidal habitat in the Northeast Pacific. In addition, there is a need to identify alternative mechanisms driving upper zonation limits.