COS 31-2
Multiple, divergent effects of a ubiquitous foundation species on organism-scale climate and thermal risk
Like terrestrial forests in miniature, marine foundation species in rocky intertidal habitats alter organism-scale climatic conditions, including aerial temperature and humidity during low tide. These physiologically important factors can vary profoundly across space even within a habitat, and substantially different conditions may arise at the top surface or canopy compared to the interior. Such differences have important implications for understanding current and future climate-driven stress and mortality risk for resident species. Using a widespread marine foundation species – the California mussel, Mytilus californianus – we quantified the extent to which upper surface and interior mussel bed microhabitats altered temperature maxima and relative humidity conditions compared to exposed bedrock habitats during low tide emersion, using sensors deployed in each microhabitat over 15 months at Bodega Marine Reserve (CA, USA). We also conducted field and laboratory experiments to determine how each microhabitat influenced mortality risk for the three most abundant invertebrate species inhabiting the beds: juvenile mussels, porcelain crabs (Petrolisthes cinctipes), and predatory isopods (Cirolana harfordi).
Results/Conclusions
We found large, and physiologically important, differences between physical conditions at the bed surface versus the interior, despite the close proximity of surface and interior bed microhabitats, which were separated by an average of six centimeters. Temperatures were substantially reduced in the mussel bed interior, remaining well below sub-lethal stress thresholds (~25°C) even on the hottest days. Humidity levels also remained near saturation. In contrast, peak aerial temperatures at the mussel bed surface were typically 10 to 15 degrees hotter, often surpassing even exposed bedrock temperatures, and frequently exceeding lethal thresholds for inhabitant species. These physical differences strongly affected inhabitant mortality risk and habitat use in all three species. Juvenile mussels under 8 mm in length, adult P. cinctipes, and C. harfordi were unable to survive bed surface conditions on warm, sunny days due to desiccation (80 to 100% mortality), but all species had >99% survival inside the bed. Our findings have direct implications for fine-scale distributions and thermal risk for the hundreds of invertebrate species residing in mussel beds. These results also underscore the potential for a single habitat-forming species to have complex physical effects on organism-scale climate.