The influence of upwelling-driven ocean acidification on growth of the California mussel, Mytilus californianus

Background/Question/Methods

Ocean acidification (OA) has emerged as a major threat to marine biodiversity, ecosystem function and services. Calcifying organisms are likely to be particularly sensitive to OA but few studies have examined their responses to variations in pH and carbonate chemistry in natural communities. Such studies can suggest the capacity of organisms to acclimate or adapt to OA, and may suggest possible cascading ecological effects. Coastal upwelling systems can serve as natural laboratories for assessing such biological responses to future global OA projections. These systems, including the California Current Large Marine Ecosystem (CCLME), are characterized by seasonal shoaling of deeper carbon-dioxide-rich waters, bathing nearshore biological communities in low-pH conditions. The goal of this study was to quantify growth of the adult California mussel Mytilus californianus at 8 sites (2 in each of 4 regions) along the CCLME from central Oregon to southern California during the 2011 upwelling season. Mussels were transplanted at each site from both the local transplant-site population and a common population originating at Bob Creek, OR. Mussel growth was measured both in the intertidal zone and subtidally on nearshore moorings, with pH sensors deployed at each location.  These sensors provided the context for comparing the relative influence on mussel growth of variation in carbonate chemistry among a suite of other environmental factors.

Results/Conclusions

Our study reveals both local and regional-scale variations in California mussel growth. These differences are partially consistent with the hypothesis of growth being influenced by the alongshore pH mosaic and are further influenced by water temperature and primary productivity (consistent with earlier work that revealed strong temperature and food effects on mussel growth). Mussel growth at a given site did not differ between paired intertidal zone and subtidal deployments on moorings, suggesting that submergence time influenced growth less than the general oceanographic context.  Mussel growth also 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). Combined with other work showing strong negative effects of pH reductions on mussel larvae, our results suggest that adult mussel growth may be severely impacted by future OA conditions.