The giant kelp Macrocystis pyrifera is a major primary producer and supports species‐rich and productive marine communities on temperate coastlines. The δ13C signature of M. pyrifera tissue is an important potential tool for understanding species interactions within nearshore trophic networks. However, in macroalgae, δ13C can vary widely, and M. pyrifera can differ up to 6‰ within a single individual, greatly complicating the designation of a primary end member in isotope mixing models. Abiotic factors such as irradiance, dissolved inorganic carbon concentration, water motion, and water temperature can drive δ13C variation, either by altering the supply of total CO2 available to individual tissues, or by inducing facultative carbon concentrating mechanisms. In this study, we combine a controlled laboratory experiment with field sampling to better understand and predict the abiotic drivers of δ13C signatures in M. pyrifera tissues. In the field, data revealed strong correlations between δ13C signatures and depth, as well as season and location within a kelp bed. Experimental data demonstrated that the δ13C of incubated tissue was strongly driven by irradiance but not flow velocity, suggesting that enrichment in M. pyrifera tissue is the result of a light‐induced metabolic process. We also confirmed a physiological δ13C enrichment process associated with photosynthate transport. These results facilitate understanding and prediction of the variation in δ13C observed in the field, and inform interpretations of δ13C signatures in trophic analyses and community interaction studies.