Due to increasing human-induced landscape changes around the world, there is a critical need to understand how the movement of subsidies across habitat borders affects species composition, community function, and biogeochemical cycling. In this study I examine how nutrient subsidies from terrestrial and open ocean systems influence nearshore marine community structure and food web dynamics. I measured stable isotope values of δ¹³C and δ¹⁵N in primary producers and primary consumers from kelp forest communities to answer three questions: 1) what is the seasonal variability in nutrient use dynamics of the giant kelp Macrocystis? 2) what are the isotope values and concentrations of the three main food sources – kelp, phytoplankton, and terrestrial organic matter – available to consumers? 3) how do the dynamics of an atypical coastal upwelling system influence the diet composition of consumers? Samples were collected from 2005 to 2008 at six sites in Big Sur, California.  Correlation analyses between isotope values and environmental variables were used to determine the mechanism for seasonal variation in kelp isotope values. Using isotope values and concentrations for the three main food sources, I used the isotope mixing model program MixSIR to determine the contribution of each food source to consumers in the kelp forest community.    

Results/Conclusions

The dramatic seasonal variability in isotope values of Macrocystis is driven by upwelling dynamics that deliver a suite of nutrients to the nearshore system from spring through early summer. The results from the MixSIR program suggest three patterns for kelp forest consumers: 1) terrestrial organic matter contributes very little to consumer diets. 2) Phytoplankton contributions were seasonal and lower than predicted for a coastal upwelling system. 3) Kelp contributed over 60% to the diet of primary consumers across all seasons. Although these results are consistent with studies from Chile, South Africa, and the sub-Antarctic islands, they are anomalous for nearshore communities along the U.S. west coast where phytoplankton tend to dominate the diets of primary consumers. This diet shift along the Big Sur coast is likely to be caused by the combination of highly episodic river flows, steep bathymetry of the nearshore shelf, and dynamic oceanographic circulation patterns. This study highlights the importance of contributions from allochthonous sources to primary producers and the entrainment of autochthonous resources for primary consumers.