The influence of intertidal location and temperature on the metabolic cost of emersion in Pisaster ochraceus
Résumé
Vertical zonation within the intertidal results from an interaction between the physical environment and an organism's physiological limits. Bioenergetic costs of emersion are likely to vary based on an organism's vertical location in the intertidal. The present study quantified the metabolic costs of microhabitat choice in the important intertidal predator Pisaster ochraceus. Rates of oxygen consumption (VO2) were measured at a range of ecologically relevant temperatures in both water and air. In both media, rates increased with increasing temperature but, at any given temperature, aerial VO2 was approximately 50% that of aquatic VO2. These rates, along with biomimetic body temperature data from two field sites in Bodega Bay, California, were used to estimate the metabolism of sea stars at different vertical locations over a 10-day period in the summers of 2006, 2007, and 2010. Results suggest that vertical location would have a much smaller effect on sea star VO2 than would inter-annual temperature differences. The influence of higher body temperatures experienced by sea stars at low tide in the mid-high intertidal, as compared to the low intertidal, was almost negligible because aerial VO2 was lower than aquatic VO2. By contrast, the higher average water temperature experienced by sea stars in 2006 yielded a 50% higher metabolic cost relative to sea stars in 2007 and 2010. These results suggest that energetic demands of intertidal organisms can vary markedly according to global environmental fluctuations such as El Niño and Pacific Decadal Oscillations.