DOI: 10.5061/dryad.f7m0cfxwg
title: The importance of kelp to an intertidal ecosystem varies by trophic level: insights from amino acid δ13C analysis

description: 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.
description: The data included were used to produce all analyses and outputs
 presented in ECS18-0380. For sample location codes, please contact
 corresponding author. <b>DESCRIPTION OF FILE(S)
 USE: </b>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 δ<sup>13</sup>C and
 δ<sup>15</sup>N isotope values. The file
 “ECS18-0380_EAA_Data.csv” presents essential amino acid
 δ<sup>13</sup>C 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. <b>ECS18-0380_Bulk_Data.csv</b> – Bulk δ<sup>13</sup>C and δ<sup>15</sup>N 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. <b>ECS18-0380_EAA_Data.csv</b> – Corrected EAA<sub> </sub>δ<sup>13</sup>C values for Amalik Bay producers and consumers (figure 2). See supplementary data S1 for details on the correction of raw EAA<sub> </sub>δ<sup>13</sup>C 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 δ<sup>13</sup>C values. For analysis of these samples we thus used the mean Phe δ<sup>13</sup>C value for other samples in these respective groups.  <b>ECS18-0380_Larsen_Data.csv </b>– 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).