Stable isotopes in subtidal food webs: Have enriched carbon ratios in benthic consumers been misinterpreted?

Abstract

We examined the hypothesis that the δ13C of benthic consumers is depleted in offshore, deeper waters where kelp and benthic microalgae are absent by conducting three analyses: (1) we analyzed δ13C (and δ15N) of consumers along a nearshore‐offshore gradient in the field, (2) we analyzed δ13C (and δ15N) of consumers placed in cages at various depths and distances from shore, and (3) we reexamined published stable isotope values for consumers taken at various depths from studies around the world. In all three situations, we did not observe the predicted change in δ13C values of benthic consumers. In both the literature and in our field measurements, the δ13C values for consumers were on average separated from the values for suspended particulate organic matter by 4%, irrespective of depth, indicating that factors other than the feeding on 13C‐rich benthic primary producers may explain the enriched carbon ratios in benthic consumers (e.g., selective feeding on enriched particles). The hypothesis that the high δ13C ratio in suspension and deposit feeders reflects feeding on benthic primary producers is thus tenuous and requires further investigation.