Benthic Assemblage Composition of South Atlantic Seamounts

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.

Climate Mitigation through Biological Conservation: Extensive and Valuable Blue Carbon Natural Capital in Tristan da Cunha’s Giant Marine Protected Zone

“…Tristan da Cunha’s coastal carbon stock of kelp is small at an estimated 60 tC ( Table 6 ), but surrounded by deep-water, its export (>200 tC) has higher chances of sequestration [ 6 ]. Although Tristan da Cunha’s small kelp forests are important for the islands’ economy, coastal biodiversity [ 30 ], and the most productive and efficient blue carbon sources, they are dwarfed by the magnitude of deep-water seabed communities [ 11 ]. Foley et al [ 12 ] and Jobstvogt et al [ 13 ] specifically highlight the potential of both biological and economic value of deep cold water coral-dominated systems, but being beyond Earth Observation (satellite) imagery this is difficult, expensive and effort intensive to do—So it has rarely been done.…”

“…The combination of these techniques can enable estimates of carbon storage held by zoobenthos in a standardized and comparable method [ 25 , 26 , 27 ]. The main body of work was examining 987 seabed images, collected across four islands and five seamounts [ 11 ]. From these we identified each organism in each image to one of thirteen functional groups to calculate a density of each.…”

“…All biological specimens visible in images were identified to one of 47 categories spread across 13 functional groups (recorded to at least Phylum and Class level, see Supplementary material and Table S1 ). This classification scheme for deep-water biota has advantages in reduced image processing time, minimising invasive, destructive sampling (to attain actual specimens of each species) and allows comparability with other deep-water, regional data sets (e.g., from Ascension seamounts, [ 11 , 26 ]). Assessment of biodiversity using functional groups has also been shown to have strong taxonomic surrogacy [ 25 , 27 ].…”

“…The efficiency of kelp to sequester carbon to deep-water around coasts can be globally significant [ 7 , 8 ]. Remote islands, with small, wave-battered coastlines, have little traditionally viewed blue carbon habitats but they can have considerable cold water coral (CWC) reefs [ 9 , 10 , 11 ]. These remote islands are further from most direct anthropogenic pressures, such as commercial fishing and terrestrial pollution.…”

“…Where this upwelling reaches the photic zone then productivity can be considerably higher than in the surrounding ocean [ 24 ]. All of these areas host CWC reef, and other VMEs, to different degrees, regulated by the habitat preferences of the different coral species present, such as Desmophyllum pertusum and Solenosmilia variabilis reefs, and Caryophyllid cup corals [ 11 ]. To date, little is known about the magnitude, variability or value of the ecosystem services provided within the Very Large Marine Protected Area (VLMPA) around Tristan da Cunha.…”