ABSTRACT: The relationship between nearshore food availability and physiological state of the suspension-feeding mussel Mytilus californianus was examined along a wave-exposure gradient at 2 rocky intertidal sites on the Oregon coast, USA, differing innearshore concentrations of phytoplankton. The RNA:DNA ratio (an indirect measure of protein synthetic capacity) of adductor muscle was used as an indicator of dietary status and growth. The activities of 2 metabolic enzymes, citrate synthase (CS) andmalate dehydrogenase (MDH), were measured for gill (CS) and adductor (MDH) as indicators of metabolic activity. Mussels were sampled at wave-exposed and wave-protected areas at Strawberry Hill (SH), a site with relatively high phytoplanktonconcentrations, and Boiler Bay (BB), a site with relatively low phytoplankton concentrations. Individuals at wave-exposed areas at both SH and BB had higher RNA:DNA ratios in spring and summer than those at wave-protected areas. RNA:DNA ratios in springand summer were higher at SH than at BB, and these differences were especially pronounced following coastal upwelling events. Increases in RNA:DNA ratio during phytoplankton blooms after upwelling suggest that mussels respond rapidly to short-term changesin nearshore oceanographic conditions. Both CS and MDH activities were higher at SH than at BB many times throughout the year, suggesting metabolic activity was higher overall for SH mussels. Reciprocal transplant experiments were performed to evaluatethe degree of physiological plasticity in relation to site- and exposure-related factors (e.g. phytoplankton concentration, heat and/or desiccation stress, wave forces). Between-site transplants were done between wave-exposed areas at SH and BB;within-site transplants were done between areas of different wave-exposure at each of the 2 sites. RNA:DNA ratios of transplanted mussels converged on those of the site or area to which they were transplanted, suggesting that mussel physiology is highlyplastic in response to site and area-related variations in environmental factors. The strong apparent response of mussels to variations in phytoplankton concentration suggests that physiological indices may be useful tools in assessing the magnitude of'bottom-up' effects in rocky intertidal communities.