The relative influence of upwelling-driven ocean acidification on growth of the California mussel, Mytilus californianus, along the California Current Large Marine Ecosystem

Background/Question/Methods

Ocean Acidification (OA), described as declining marine pH driven by oceanic absorption of increasing atmospheric carbon dioxide, has emerged as a major anthropogenic threat to a broad spectrum of marine biota, with rippling effects on biodiversity, ecosystem functioning and services to humanity. A growing body of laboratory studies has elucidated deleterious effects of forecasted OA on calcifying organisms, but relatively few studies have investigated the effects of natural pH variation on in situ communities. Such studies can suggest the capacity of organisms to acclimate or adapt to OA.  Our study leverages the upwelling-driven pH mosaic along the California Current Large Marine Ecosystem as a natural laboratory for assessing the relative influence of pH variations among a suite of other impactful environmental characteristics on growth of a spatially and functionally dominant intertidal inhabitant, the California mussel Mytilus californianus.  Growth was measured on tagged mussels at 8 sites between central Oregon and southern California during the 2011 and 2012 upwelling seasons. Sites were chosen to coincide with the first coastal network of pH sensing at the scale of a Large Marine Ecosystem, providing the context for comparing the relative influence of pH compared to biomimetic mussel body temperature, primary productivity and upwelling.

Results/Conclusions

Our study reveals local and regional-scale variations in California mussel growth. Both the mean and degree of variation in pH contributed to explaining variations in mussel growth. Counter to expectation, growth was not reduced at low-pH sites across the regional upwelling mosaic. The overall strength of explanatory influence by pH was comparable to the influences of biomimetic mussel temperature, primary productivity and local upwelling, underscoring the value in assessing the separate and combined influences of multiple climate stressors. Mussel growth did not differ between those that originated from the transplant site (i.e. local mussels) compared to those from the common site. Thus, local and common-source mussels do not appear to differ in their responses to local pH either due to genetic differences or persistent phenotypic differences (age, prior exposure before collection, etc). In light of other work by collaborators showing strong negative effects of pH reductions on mussel larvae, our results suggest a transition during mussel life history that leads to greater resilience to ocean acidification by adult mussels.