Mussel selectivity for high-quality food drives carbon inputs into open-coast intertidal ecosystems

Filter-feeding invertebrates consume phytoplankton and detritus and therefore serve as important mediators of the exchange of materials from nearshore pelagic to intertidal benthic ecosystems. Here, we evaluated the linkages between nearshore and intertidal systems on temperate rocky reefs on the coasts of Oregon, USA, and New Zealand’s South Island. We used differences in the concentrations of both nearshore particulate organic carbon and chlorophyll a (chl a), a proxy for phytoplankton availability, at different sites in Oregon and New Zealand to evaluate the influences of suspended particulate organic material (POM) quality and quantity on the rates of carbon inputs associated with intertidal mussels (Mytilus californianus in Oregon and Mytilus galloprovincialis in New Zealand). We also analyzed the carbon stable isotope ratios (δ13C) of intertidal mussels and nearshore POM to examine changes in mussel growth in carbon relative to changes in their potential food sources along gradients of POM quality (i.e. carbon-tochlorophyll ratios, C:chl a). In both Oregon and New Zealand, the δ13C in mussel tissues did not change along a gradient of food quality, whereas the δ13C of the POM declined as food quality declined (i.e. C:chl a increased), suggesting that mussels were selectively consuming high-quality food. We also found that the availability of phytoplankton, a high-quality component of the POM, was a better predictor of mussel growth in carbon (mg C g−1 d−1) than the total concentration of particulate organic carbon, which includes both higher-quality phytoplankton and lower-quality detrital material. Our results highlight the necessity of considering POM quality while evaluating the role of filter-feeders as mediators of carbon inputs into intertidal systems.