Deconstructing the relative influence of upwelling-driven ocean acidification on field performance of the California mussel Mytilus californianus along the California Current Large Marine Ecosystem

Background/Question/Methods

The rapid expansion of laboratory studies addressing ocean acidification (OA) , described as declining marine pH driven by oceanic absorption of increasing atmospheric carbon dioxide, indicate broadly deleterious effects for marine organisms, particularly those that rely on calcification processes. Growing evidence also supports combined effects of OA and other environmental stressors. A critical next step toward understanding OA impacts on natural communities is to scale these laboratory studies to ecological performance, where increased environmental heterogeneity may mediate responses.  Our study leverages the upwelling-driven pH mosaic along the California Current Large Marine Ecosystem to deconstruct the relative influences of pH, temperature, and food availability on size and growth performance of a spatially and functionally dominant intertidal inhabitant, the heavily-calcified California mussel Mytilus californianus. We measured seasonal performance on tagged adult mussels both in the intertidal and on nearshore moorings at 8 sites between central Oregon and southern California over the 2011-2012 upwelling seasons. Sites were chosen to coincide with the first network of pH sensors at the scale of a Large Marine Ecosystem, providing the context for comparing pH influences among other environmental stressors.

Results/Conclusions

Our study reveals ecologically relevant scales of among-site and interannual variations in adult California mussel growth and size. Mussel performance was enhanced, not suppressed as expected, at comparatively low pH sites. Including local pH conditions with the previously documented influences of variations in biomimetic temperature and food availability enhances the explanatory power of models that describe the observed performance differences.  Performance also differed between mussels placed in the intertidal and those kept fully submerged on nearshore moorings, indicating the additional stressor of emersion time.  Population source was important, with responses differing between mussels from local populations and those from a common source population that were distributed among study sites.  Mussel performance therefore partially depends on genetic or persistent phenotypic differences driven by historical exposure. In light of other work by collaborators showing variable effects of pH on larval and juvenile mussels, our results demonstrating mediated effects of natural pH exposure on adults suggest a transition during life history that leads to greater resilience and potential adaptive capacity to ocean acidification by adult California mussels.