The C isotopic composition in macroalgae (δ¹³C) is highly variable and its prediction is very complex relative to terrestrial plants. To contribute to the knowledge on the variations and determinants of δ¹³C-macroalgal, we analyzed a large stock of specimens varying in taxa and morphology and inhabiting shallow marine habitats from the Gulf of California Ecoregion featured by distinctive environmental conditions. A large δ¹³C variability (-34.61‰ to -2.19‰) was observed, mostly explained on the life form (taxonomy, morphology and structural organization), and modulated by the interaction between habitat features and environmental conditions. The intertidal zone specimens had less negative d¹³C values than in subtidal zone. Except pH, environmental conditions of the seawater do not contribute to the δ¹³C variability. Specimens of the same taxa showed δ¹³C similar patterns, to increase or decrease, with latitude (21º-30°N). d¹³C-macroalgal provides information on the inorganic carbon source used for photosynthesis (CO₂ diffusive entry vs HCO₃^- active uptake). Most species showed a δ¹³C belong into a range that indicates a mix of CO₂ and HCO₃^- uptake; the HCO₃^- uptake by active transport is widespread among GCE macroalgae. About 20-34% species depending on cutoff limits for CCM presence showed at least one specimen with δ¹³C>-10‰, suggesting that potentially could have highly efficient CCM. Ochrophyta presented a high number of species with d¹³C>-10‰, suggesting widespread HCO₃^- use by non-diffusive mechanisms. Few species belonging to Rhodophyta relied on CO₂ diffusive entry (d¹³C<-30‰) exclusively. δ¹³C provides useful information about the physiological and environmental status of macroalgae.