Macrophyte contributions to Mytilus californianus diets in southern California as determined by carbon and nitrogen stable isotope ratios

delta13C and delta15N signatures for macrophytes, and for phytoplankton, and estuarine particulate organic matter were determined and used to analyze inputs to the diets of intertidal mussels (Mytilus californianus) at five mainland and two island sites in Southern California. delta13C and delta15N values differed significantly between mainland and island sites for both M. californianus muscle and digestive gland tissues. delta 13C values also were slightly more enriched for mussels from mainland sites with persistent local kelp beds. delta13C and delta 15N values were more depleted and more temporally variable in digestive glands compared with muscle tissues. Isotope biplots and a two-source mixing model showed that macrophyte-derived carbon contributed greatly to the diets of mainland mussels, even at sites with low macrophyte standing stocks, and accounted for a minimum of 39 to 60 % of the carbon content of their muscle tissues. In contrast, phytoplankton provided the largest source of carbon to the diets of mussels collected at offshore Santa Catalina Island. delta 15N values revealed that estuarine and terrestrial sources did not make large contributions to mussel diets in the region. My results demonstrate the importance of macrophyte production to filter-feeding mussels and provide evidence of differences in island and mainland food webs in the southern California region.