Assessing the importance of land and marine sources of organic matter to kelp forest food webs

Abstract

We used stable isotope analysis to investigate the relative importance of marine and terrestrial sources of organic matter to the food web of 4 nearshore reefs in the Santa Barbara Channel (California, USA) over a 4 yr period that included substantial variability in the availability of marine and terrestrial organic matter. We measured stable C and N isotope values of ocean-and landderived sources of organic matter and of different types of consumers on reefs that varied in exposure to freshwater runoff and in biomass of giant kelp. 13 C values of suspended particulate organic matter (POM) on reefs tended to decrease following periods of significant rainfall at the reef most influenced by freshwater runoff, to increase with phytoplankton standing crop at all reefs, and generally to decline in both wet and dry years in late fall-early winter at all reefs. Stable isotope values of reef consumers indicated little direct use of terrestrially-derived POM. However, a pattern of 15 N-enrichment in 2 common benthic-feeding species, the sea urchin Strongylocentrotus purpuratus and the annelid Diopatra ornata, with increasing influence of runoff indicated that terrestrially-derived nitrogen may enter the food web indirectly through a trophic intermediate (e.g. microbes, algae). The importance of giant kelp to the reef food web varied with consumer feeding mode. In contrast to previous studies, 13 C values suggested little use of kelp-derived material by suspension-feeders, based on the similarity in isotope values of these consumers among reefs when kelp biomass was low, and the absence of a directional shift in isotope values that would indicate the use of more 13 C-enriched kelp when kelp biomass was higher at 2 of the reefs. However, isotope values for herbivores were generally 13 C-enriched relative to suspension-feeders, reflecting the use of local giant kelp or other 13 C-enriched benthic algal production. Thus, spatial and temporal fluctuations in the biomass of giant kelp would be least likely to impact the food resources of suspension-feeders and have greatest effect on benthic herbivores.