Kelp forests are highly productive coastal habitats that serve as biodiversity hotspots and provide valuable ecosystem services. Despite being one the largest marine biomes, kelp forests have been drastically understudied relative to other marine systems. Notably, while the role of kelp as habitat-forming, or ‘foundation species’, is well-documented, a comprehensive understanding of kelp forest food web structure is lacking, particularly regarding the importance of kelp-derived energy/nutrients to consumers. Here, we provide a biogeographic perspective on the energetic underpinning of kelp forests based on published literature. We targeted studies which used geochemical proxies – stable isotope analysis – to examine the transfer of carbon from kelp to local consumers. These studies (n=94) were geographically skewed, with >40% from Northern European Seas and Temperate Northeast Pacific. Quantitative estimates for the percentage of kelp energy (or kelp+macroalgae if sources were pooled) incorporated by local consumers came from 43 publications, which studied 141 species and 35 broader taxonomic groups. We examined these data for trends among functional groups and across upwelling regimes. No patterns are evident at present, perhaps due to the paucity or variability of available data. However, energetic subsides from kelps clearly support a wide range of diverse taxa around the globe. We also characterized biogeographic patterns in d¹³C values of kelps and particulate organic matter (POM, a phytoplankton proxy), to evaluate potential limitations of stable isotope analysis in disentangling the relative contributions of pelagic vs. benthic resources to coastal food webs. Globally, kelps and POM differed by >4.5‰, but there was substantial variation among regions and kelp species. Accordingly, we discuss advances in stable isotope techniques which are facilitating more precise analysis of these complex energetic pathways. We end by proposing four main avenues of critical future research that will shed light on the resilience of these communities to global change.

This dataset is a compilation of isotopic data from published literature. See manuscript for literature review and data processing methods.

Data A1. This file contains all of the data compiled by the authors for the current review and used to produce the analyses in the described manuscript. For a full reference list refer to the main text as well as Supplementary materials Appendix 1.

The file “Kelp_POM.csv” presents all isotope values, including the bulk tissue d¹³C and d¹⁵N values as they are originally presented in the cited manuscripts. Where spreadsheet cells are left blank indicates data that was not reported in the original citation.

The file “Kelp_Consumer.csv” presents the estimated mean or median contribution from kelp (or a combined kelp and macroalgae endmember) for consumers from all cited manuscripts. This includes associated metadata and methodological details for each species and study. Where spreadsheet cells are left blank indicates data/methods that was not reported in the original citation. As described in the main text, to avoid biasing analyses based on repeated measures from a small number of studies, where multiple values were recorded for the same species, we averaged the contribution estimates based on the ecological/geographic distinctions made in the original study. For example, Markel and Shurin (2015) and Ramshaw (2012) present estimates for various species in different ecological regions of British Columbia: Kyuquot Sound, an area with sea otters present, and Barkley Sound where sea otters are absent. For these studies we thus binned species data separately for Kyuquot and Barkley Sound. Details for all such bins can be found in the ‘Notes’ column.