Sea otter effects on trophic structure of seagrass communities in southeast Alaska

Ww Raymond,Gl Eckert,Awe Galloway,Jb Schram

doi:10.3354/meps13819

Abstract

Previous research in southeast Alaska on the effects of sea otters Enhydra lutris in seagrass Zostera marina communities identified many but not all of the trophic relationships that were predicted by a sea otter-mediated trophic cascade. To further resolve these trophic connections, we compared biomass, carbon (δ13C) and nitrogen (δ15N) stable isotope (SI), and fatty acid (FA) data from 16 taxa at 3 sites with high and 3 sites with low sea otter density (8.2 and 0.1 sea otters km-2, respectively). We found lower crab and clam biomass in the high sea otter region but did not detect a difference in biomass of other seagrass community taxa or the overall community isotopic niche space between sea otter regions. Only staghorn sculpin differed in δ13C between regions, and Fucus, sugar kelp, butter clams, dock shrimp, and shiner perch differed in δ15N. FA analysis indicated multivariate dissimilarity in 11 of the 15 conspecifics between sea otter regions. FA analysis found essential FAs, which consumers must obtain from their diet, including 20:5ω3 (EPA) and 22:6ω3 (DHA), were common in discriminating conspecifics between sea otter regions, suggesting differences in consumer diets. Further FA analysis indicated that many consumers rely on diverse diets, regardless of sea otter region, potentially buffering these consumers from sea otter-mediated changes to diet availability. While sea otters are major consumers in this system, further studies are needed to understand the mechanisms responsible for the differences in biomarkers between regions with and without sea otters.

Highlights

Seagrass ecosystems support diverse communities that are considered to be largely structured by topdown forces (Duffy et al 2014)
Staghorn sculpin differed in stable isotope (SI) and fatty acid (FA) biomarker measures but did not differ in biomass, and butter clams differed in biomass, δ15N, and FA but not δ13C (Table 1)
FA results point to differences in trophic structure not apparent from biomass or SI data alone

Summary

Introduction

Seagrass ecosystems support diverse communities that are considered to be largely structured by topdown forces (Duffy et al 2014). Previous inferences on top-down control in seagrass food webs have usually relied on relative biomass or abundance comparisons to describe these trophic controls. While it is an intuitive and time-tested metric of trophic ecology, biomass may mask weak interactions among seagrass-associated taxa, skewing our perspective of trophic structure. Trophic biomarkers such as stable isotopes (SIs) and fatty acids (FAs) can provide an alternative perspective on trophic structure by focusing on energy transfer from primary producers to consumers. The ratio of the stable isotopes of nitrogen, 15N to 14N, increases with each level of consumption, making it a useful measure of the relative trophic position of species and food chain length (Cabana & Rasmussen 1996, Layman et al 2007)

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