The importance of kelp to an intertidal ecosystem varies by trophic level: insights from amino acid δ 13 C analysis

A fundamental question in ecology is understanding how energy and nutrients move through and between food webs, and which sources of production support consumers. In marine ecosystems, these basic questions have been challenging to answer given the limitation of observational methods. Stable isotope analysis of essential amino acids ( EAA δ 13 C) has great potential as a tool to quantify energy and nutrient flow through marine food webs; however, it has been primarily utilized at large spatial scales. Here, we used EAA δ 13 C analysis to test for connectivity between adjacent subtidal and intertidal components of a nearshore ecosystem in south central Alaska. We measured δ 13 C of six EAA from four marine producer groups: subtidal kelp ( Laminaria sp.), offshore particulate organic matter ( POM ), and intertidal red ( Neorhodomela sp.) and green ( Ulva sp.) algae. In addition, we sampled four intertidal invertebrate consumer species spanning a range of trophic/functional groups: Mytilus sp., Strongylocentrotus droebachiensis , Nucella sp., and Pycnopodia helianthoides . Using canonical analysis of principal coordinates ( CAP ) and isotope mixing models (Mix SIAR ), we tested for differences among producer EAA δ 13 C fingerprints and quantified the contribution of producer EAA to consumers. We compared these results to previously published EAA δ 13 C data on marine producers to examine the generality of this technique. We found the EAA δ 13 C fingerprints of subtidal kelps ( Laminaria ), Ulva, and Neorhodomela were highly distinct from one another. Further, our measured EAA δ 13 C patterns for kelp and red algae matched those previously reported from other localities, suggesting unique and universal EAA δ 13 C signatures for these groups. However, CAP could not distinguish between microalgae ( POM ) and Ulva , possibly due to similar biochemical pathways for the synthesis of EAA . Using these producer fingerprints, we found upper trophic‐level invertebrate consumers, Nucella and Pycnopodia , derived more than 60% of their essential amino acids from subtidal kelps. In contrast, the sampled primary consumers in the system, Mytilus and Strongylocentrotus , relied more heavily on Ulva and/or offshore POM . Our results provide evidence for connectivity between two adjacent nearshore ecosystems and exemplify EAA δ 13 C as a powerful new tool in tracing energy and nutrient flow within and among marine food webs.