- Ocean Acidification (OA) has emerged as a major threat to marine ecosystems, particularly regarding calcifying organisms. A growing body of literature describing laboratory investigations into pH stress indicates broadly deleterious effects for calcifiers, but responses vary greatly across taxa and can be influenced by variations in other environmental characteristics. Scaling laboratory results to ecological performance is critical for understanding the impacts of OA on marine communities. One method that can be useful for elucidating these ecological impacts is to study organisms and communities in environments that naturally vary in pH. The California Current Large Marine Ecosystem (CCLME) is one such ecosystem in which pH varies both in space and over time, bathing intertidal communities in a mosaic of pH conditions. The dynamics of the CCLME during the upwelling season also drive biologically-relevant variation in productivity and temperature.
My dissertation leverages variation in oceanographic processes along the CCLME to explore the potential for OA impacts on rocky intertidal community distributions and the performances of two major space occupiers in rocky intertidal communities. In Chapter 2, a series of large scale community surveys along the CCLME were re-examined to ask if distributions of calcifiers and their mineral forms differ whether these differences are linked to environmental conditions. The patterns of differential calcifier abundances that emerge may better inform studies into the potential community impacts of OA by highlighting regions where calcifiers are relatively diverse or replete. Although these patterns are partially driven by complex interactions among temperature, phytoplankton productivity and upwelling, much of the spatial variation in calcifier abundance remains unexplained, suggesting the need to better characterize the pH environment along this oceanographically-complex region.
In Chapter 3, I explored the relative influence of the pH mosaic along the CCLME on performance of the California mussel, Mytilus californianus. When considered along with other known stressors such as temperature and chlorophyll-a variations, pH meaningfully contributed toward explaining variation in mussel growth, condition and shell thickness. Contrary to expectation, some aspects of mussel performance were enhanced at comparatively low pH sites. The potential implications of this work include mediation of pH stress by other environmental factors, energetic trade-offs between calcified and soft tissue development, a life history transition toward increased resilience, and genotypic or persistent phenotypic differences that integrate exposure history.
In Chapter 4, I investigated the relative influence of natural pH variation on performance of the purple sea urchin, Strongylocentrotus purpuratus. As with the California mussel study in Chapter 3, aspects of sea urchin calcified structures were enhanced, not suppressed as expected, at sites with comparatively low pH, after considering the effects of algal consumption and mean temperature. The combined results of Chapters 3 and 4 underscore the complex interactions between multiple environmental stressors and organismal physiology, highlight the biological relevance of pH on ecological performance, and suggest that life in areas already subject to natural pH variation may have the adaptive capacity to persist under future conditions.