Benthic Assemblage Composition of South Atlantic Seamounts

Amelia E. H. Bridges,Kerry L. Howell,Rebecca E. Ross,David K. A. Barnes,James B. Bell

doi:10.3389/fmars.2021.660648

Abstract

Seamounts and oceanic islands rise from the seafloor and provide suitable habitat for a diverse range of biological assemblages including Vulnerable Marine Ecosystems (VMEs). Whilst they have been the focus of some work globally, there has been little description of the biological and physical environments of seamounts in the South Atlantic Ocean. In this study, we characterized benthic assemblage composition from 13 seamounts and oceanic islands spanning 8–40°S within the exclusive economic zones (EEZs) of Ascension Island, Saint Helena and Tristan da Cunha. Drop camera imagery was collected between 170 and 1000 m. All fauna present in images were identified and quantified, and multivariate statistics were used to describe biological assemblages and identify their environmental drivers. Benthic communities of temperate regions (Tristan da Cunha archipelago) were shown to be distinct from those found in the tropics, with latitude and depth identified as key environmental drivers of assemblage composition. Our results are consistent with the current understanding of the biogeography of the South Atlantic, both in terms of the distinction between tropical and temperate regions, and the influence of depth and water mass structure on assemblage distribution. Faunal assemblages are similar to those observed in the North Atlantic in terms of functional groups. VMEs are present within the EEZs of all three territories and are potentially protected from some threats by large marine protected areas (MPAs). Our imagery, data and analyses provide a baseline for south Atlantic seamounts so that future monitoring can establish whether existing protected status is sufficient to conserve both unique biodiversity and considerable potential for vital ecosystem services.

Highlights

Seamounts are features generally defined as rising more than 1000 m off the surrounding seabed and are found in all ocean basins (Clark et al, 2010)
This paper aims to: (1) identify the broadscale environmental drivers of seamount benthic assemblage structure in the South Atlantic; (2) characterize the benthic assemblages of South Atlantic seamounts; and (3) identify Vulnerable Marine Ecosystems (VMEs) protected by each large marine protected areas (MPAs) to support future spatial management
Substituting latitude for surface primary productivity in the available variables list did not significantly change the proportion of variance explained by the five selected variables, it did alter the order of longitude and fine-scale bathymetry position index (FBPI)

Summary

Introduction

Seamounts are features generally defined as rising more than 1000 m off the surrounding seabed and are found in all ocean basins (Clark et al, 2010). In the South Atlantic, the United Kingdom Overseas Territory (UKOT) of Saint Helena, Ascension and Tristan da Cunha is made up of Benthic Assemblage Composition of Seamounts three geographically separated islands/island groups spanning ∼8 to ∼40◦S. These islands function to seamounts in the sense that they provide a hard substrate habitat in an otherwise largely soft substrate deep sea (Rogers, 1994), as well as providing benthic habitat in areas that would otherwise be pelagic and are important in sustaining populations of benthic fauna, when they are part of mid-ocean ridge systems (Priede et al, 2013). This attracts subsequent activity from organisms further up the trophic chain, all resulting in increased carbon flux to depth, which facilitates higher species richness and supports increased benthic biomass (Samadi et al, 2006)

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