δ13C and δ15N of particulate organic matter in the Santa Barbara Channel: drivers and implications for trophic inference

We investigated the extent to which temporal variation in the stable isotope composition of suspended particulate organic matter (POM) was explained by phytoplankton biomass and production at a southern California coastal reef and kelp forest and further offshore in the Santa Barbara Channel.  On the reef, δ¹³C_POM values were positively related to chlorophyll a concentration and phytoplankton production, which explained 62% of the variability in δ¹³C_POM. These relationships were weaker offshore, where variation in δ¹³C_POM was better explained by the abundance of dinoflagellates. As we predicted based on patterns of generally higher phytoplankton biomass and productivity along the shallow shelf, reef δ¹³C_POM values were typically ¹³C-enriched relative to values offshore. We used the relationship between chlorophyll a and reef δ¹³C_POM to estimate phytoplankton d¹³C and the contribution of terrestrial C to coastal POC immediately following a rain event. These calculated terrestrial contributions explained 88% of the variability in freshwater runoff (indicated by salinity).  δ¹⁵N_POM values varied across the year in association with changes in dissolved inorganic N nutrient pools. These results show that trophic studies of coastal marine ecosystems, at least off Santa Barbara, can use POM stable isotope values to represent phytoplankton when freshwater runoff is low. This finding simplifies the use of stable isotopes to infer trophic relationships in southern California kelp forests. Coastal food web studies, particularly those examining kelp contributions, have typically used offshore POM isotope values to represent inshore phytoplankton.  Our results show this assumption may bias results of food web mixing models.