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 δ¹³C) 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 δ¹³C analysis to test for connectivity between adjacent subtidal and intertidal components of a nearshore ecosystem in south central Alaska. We measured δ¹³C 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 (MixSIAR), we tested for differences among producer EAA δ¹³C fingerprints and quantified the contribution of producer EAA to consumers. We compared these results to previously published EAA δ¹³C data on marine producers to examine the generality of this technique. We found the EAA δ¹³C fingerprints of subtidal kelps (Laminaria), Ulva, and Neorhodomela were highly distinct from one another. Further, our measured EAA δ¹³C patterns for kelp and red algae matched those previously reported from other localities, suggesting unique and universal EAA δ¹³C 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 δ¹³C as a powerful new tool in tracing energy and nutrient flow within and among marine food webs.

The data included were used to produce all analyses and outputs presented in ECS18-0380. For sample location codes, please contact corresponding author.

DESCRIPTION OF FILE(S) USE: This file contains all of the data used to produce the analyses in the described manuscript. The file “ECS18-0380_Bulk_Data.csv” presents the bulk δ¹³C and δ¹⁵N isotope values. The file “ECS18-0380_EAA_Data.csv” presents essential amino acid δ¹³C generated by the authors for the current study. Finally, the table “ECS18-0380_Larsen_Data.csv” presents data originally sources from Larsen et al. 2013 that were used as a comparison to the newly presented data.

ECS18-0380_Bulk_Data.csv – Bulk δ¹³C and δ¹⁵N values and associated weight percent [C]:[N] ratios (figure 1) for all macroalgae and macroinvertebrate samples from Amalik Bay site, as well as all 2012 and 2013 offshore POM samples.

ECS18-0380_EAA_Data.csv – Corrected EAA δ¹³C values for Amalik Bay producers and consumers (figure 2). See supplementary data S1 for details on the correction of raw EAA δ¹³C values and see Methods for sample collection details. POM = Offshore particulate organic matter. Abbreviations for essential amino acids are as follows: isoleucine (Ile), leucine (Leu), lysine (Lys), phenylalanine (Phe), threonine (Thr), valine (Val). For two samples, KATM-AM-LAM-2012-03 and KATM-AM-ULV-2012-05, we were not able to obtain Phe δ¹³C values. For analysis of these samples we thus used the mean Phe δ¹³C value for other samples in these respective groups. 

ECS18-0380_Larsen_Data.csv – Data from Larsen et al. 2013 used here in comparison with sampled Amalik Bay producers. The column ‘ID’ corresponds exactly to those ID’s used in the Larsen et al. 2013 paper. An asterisk (*) before a location indicates that the individual was a laboratory culture sample. Abbreviations for essential amino acids are as follows: isoleucine (Ile), leucine (Leu), lysine (Lys), phenylalanine (Phe), threonine (Thr), valine (Val).