Abstract Shell carbonate of Balanus, Diadora, Mytilus and Calliostoma contain relatively light ° 13C-values which suggest that deep ocean waters, enriched with 12C of dissolved inorganic carbon (TDC), well up onto the coast of British Columbia, Canada. Another explanation for the light δ 13C-values of the shells is extensive oxidation of organic carbon in the shallow surface waters along the coast, or kinetic fractionation related to growth rate of the organisms. Organisms that incorporate carbon and oxygen into shell carbonate in isotopic equilibrium with ambient seawater were used to calculate water temperatures. The calculated temperature of 7°C for Balanus suggests that these invertebrates preferentially precipitate calcite during the cooler months of the year, since the yearly temperature range of the water at Cape Beale is between 6.2° and 12.8°C The calculated temperature of 10°C for the limpet implies that this organism calcifies shell carbonate throughout the year. The bivalves Mytilus and Calliostoma with temperatures of 11° and 12°C, respectively, apparently form shells during the warmer summer months of the year. The barnacle Balanus and the limpet Diadora incorporate carbon and oxygen isotopes into shell carbonate in equilibrium with ambient seawater. This observation also applies to Mytilus and Calliostoma from Cape Beale, but not to their representatives from Lucy Island. Anomalous oxygen isotopic values are exhibited by the bivalve Clinocardium at both localities. Similarily, carbon values are anomalous for the carbonate of worm tubes ( Eudistylia). These deviations of the isotopic compositions from expected values are probably related to changes in the nutrient level and/or water temperature.