Potentially important food sources to consumers on shallow subtidal reefs include phytoplankton-dominated seston, kelp-derived detritus, and for locations adjacent to sources of freshwater runoff, terrestrially-derived material. The SBC LTER is using stable carbon, nitrogen and deuterium isotope ratio analysis to evaulate the relative contribution of these sources to reef food webs. We collect seasonal samples from five core SBCLTER research reefs (Arroyo Hondo, Naples, Arroyo Burro, Goleta bay and Carpinteria), and areas adjacent. We routinely collect several types of samples: from giant kelp (Macrocystis pyrifera), red, brown and green algae, terrestrial material (Oak leaves), stream, sediment and ocean water particulate organic material, and a benthic polychaete worm (Diopatra). This information will be used to evaluate whether these isotopic values differ enough from one another to permit the use of mixing models to estimate the contribution of each source to the reef food web.