Patterns and drivers of δ13C variation in the giant kelp, Macrocystis pyrifera

The giant kelp Macrocystis pyrifera is a major primary producer and supports species-rich and productive marine communities on temperate coastlines. The delta C-13 signature of M. pyrifera tissue is an important potential tool for understanding species interactions within nearshore trophic networks. However, in macroalgae, delta C-13 can vary widely, and M. pyrifera can differ up to 6 parts per thousand 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 delta C-13 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 delta C-13 signatures in M. pyrifera tissues. In the field, data revealed strong correlations between delta C-13 signatures and depth, as well as season and location within a kelp bed. Experimental data demonstrated that the delta C-13 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 delta C-13 enrichment process associated with photosynthate transport. These results facilitate understanding and prediction of the variation in delta C-13 observed in the field, and inform interpretations of delta C-13 signatures in trophic analyses and community interaction studies.