Correlation between allochthonous subsidy input and isotopic variability in the giant kelp Macrocystis pyrifera in central California, USA

Isotope values of primary producers can vary greatly across space and time due to changes in physiological status and the source of subsidies used in photosynthesis. Forests of the giant kelp Macrocystis occur in temperate oceans around the world and are positioned at the land-sea interface where they may receive subsidies from terrestrial and open ocean sources. In this study, we investigated the spatial and temporal variability in delta C-13 and delta N-15 values of M. pyrifera over 3 yr in Big Sur, California, USA. During each of those 3 yr, delta C-13 and delta N-15 values of M. pyrifera varied significantly by season, with low values occurring in the spring, and high values in the fall. To examine the possible mechanisms driving this seasonal variability, we used step-wise multiple regression analyses to test whether the variability in kelp isotope values was correlated with the variability in nitrate concentration or the delivery of subsidies via upwelling circulation and river discharge. We found that temporal variation in kelp isotope values was most strongly related to the strength of upwelling and the concentration of nitrate in surface waters. The results of this study increase our understanding of the mechanisms that drive the temporal and seasonal isotopic variability in marine primary producers. This study also highlights the importance of measuring isotope variability at the base of the food chain in order to create more robust trophic predictions.