HARLEY, C.D.G.*; HELMUTH, B.S.T.: Spatial variation in invertebrate upper limits, thermal stress, and effective tidal height

Physiological stress is generally assumed to play a predominant role in limiting the upper vertical limits of intertidal species. However, several stressors (e.g. temperature, desiccation, limited time for resource acquisition) may operate on any given species, and the specific role of each through space and time are seldom rigorously analyzed with appropriate physical data. We analyzed the relationship between the upper limits of two sessile invertebrates and absolute tidal height (ATH), effective tidal height (ETH, which incorporates wave splash), and temperature. The distributional limits of the mussel Mytilus californianus at the local scale (Tatoosh Island) were tightly correlated with ETH, regardless of ATH or substrate aspect. Maximum temperatures of mussel mimics also expressed relatively low variability through space. These results indicate that emersion time and /or thermal extremes may set the upper limits of mussels on Tatoosh. The upper limits of the barnacle Balanus glandula were compared with ATH, ETH, and temperature at the regional scale (the Northwest Straits, Washington State). At the more wave exposed sites, the upper limit of the barnacles occurred at essentially identical ETHs, although maximum temperature varied greatly between sites. At the more protected sites, the ETH of the upper limit declined with midtidal rock temperatures, indicating that temperature plays an important role at protected sites. Thus, for Balanus, the determinant of the upper limit appears to shift from emersion time at cool, wave-swept sites to temperature at warmer, protected sites. This type of analysis will help physiologists determine when and where various types of stress are likely to influence organismal distributions at a variety of scales.