Tidal dynamics, topographic orientation, and temperature-mediated mass mortalities on rocky shores

Temperature is among the main structuring agents on rocky intertidal shores. Although infrequent mortality events associated with high temperatures have been observed in several intertidal taxa, careful documentation of these events is rare, and small-scale variability in mortality patterns remains poorly understood. In Bodega Bay, California, USA, 2 mortality events occurred on exposed rocky shores during the spring of 2004 when low tides occurred around mid-day. Limpets Lottia scabra were killed during unseasonably warm weather in mid-March, In late April, record-high temperatures resulted in widespread mortality of the mussel Mytilus californianus. Levels of mortality for both species were closely associated with small-scale variability in temperature, which in turn was closely associated with Substratum orientation. Invertebrates occupying surfaces facing the sun when stress was most intense were much more likely to die than those living on surfaces angled >= 45 degrees away. Because the within-day timing of thermal stress varied seasonally, the highest mortalities were recorded on southwest-facing surfaces on the March afternoon low tide and on southeast-facing surfaces on the April morning low tide. Limpets and mussels showed little mortality during the events that were harmful to the other taxon, suggesting that these different taxa respond to different aspects of their thermal environment, If climate change results in more frequent and more severe episodes of thermal stress, future ecological change may also be heavily dependent on tidal dynamics and small-scale variation in substrate orientation.