Trophic Biomarkers Indicate Coastal Surf Zone Hydrodynamics Affect Resource Assimilation by Mytilus californianus Mussels

Surf zone hydrodynamics influences the delivery of coastal phytoplankton and detritus food subsidies to shore. Variation in surf zone hydrodynamics can cause phytoplankton concentrations to be higher at dissipative than reflective surf zones, altering both the quantity and quality of food for intertidal suspension-feeders. To assess if surf zone-dependent food availability influences suspension-feeder diets, we out-planted Mytilus californianus mussels for a 5-month common garden experiment at sites with dissipative and reflective surf zones at Cape Arago, Oregon. Stable isotopes, gravimetric lipid weights, and fatty acid trophic biomarkers of extracted abductor muscles were used to examine possible diet differences between mussels grown at dissipative and reflective sites. Both δ 15 N‰ and δ 13 C‰ values varied significantly between dissipative and reflective surf zone types, but there was no difference in gravimetric lipid weight between surf zone types. The multivariate fatty acid composition of mussels from dissipative and reflective sites differed; mussels at dissipative sites had a higher proportion of fatty acids indicative of diatoms in their diet (e.g., 16:1ω7, 20:5ω3), whereas mussels at reflective sites had a higher proportion of fatty acids indicative of dinoflagellates (e.g., 22:6ω3). Aspects of our stable isotope and fatty acid data suggest trophic subsidies to mussels were influenced by surf zone hydrodynamics.