Biological and physical characterization of the seabed surrounding Ascension Island from 100–1000 m

Abstract

Recent studies have improved our understanding of nearshore marine ecosystems surrounding Ascension Island (central Atlantic Ocean), but little is known about Ascension's benthic environment beyond its shallow coastal waters. Here, we report the first detailed physical and biological examination of the seabed surrounding Ascension Island at 100 -1000 m depth. Multibeam swath data were used to map fine scale bathymetry and derive seabed slope and rugosity indices for the entire area. Water temperature and salinity profiles were obtained from five Conductivity, Temperature, Depth (CTD) deployments, revealing a spatially consistent thermocline at 80 m depth. A camera lander (Shelf Underwater Camera System; SUCS) provided nearly 400 images from 21 sites (100 m transects) at depths of 110 -1020 m, showing high variability in the structure of benthic habitats and biological communities. These surveys revealed a total of 95 faunal morphotypes (mean richness .14 per site), complemented by 213 voucher specimens constituting 60 morphotypes collected from seven targeted Agassiz trawl (AGT) deployments. While total faunal density (maximum .300 m 22 at 480 m depth) increased with rugosity, characteristic shifts in multivariate assemblage structure were driven by depth and substratum type. Shallow assemblages ( 100 m) were dominated by black coral (Antipatharia sp.) on rocky substrata, cup corals (Caryophyllia sp.) and sea urchins (Cidaris sp.) were abundant on fine sediment at intermediate depths (250 -500 m), and shrimps (Nematocarcinus spp.) were common at greater depths (.500 m). Other ubiquitous taxa included serpulid and sabellid polychaetes and brittle stars (Ophiocantha sp.). Cold-water corals (Lophelia cf. pertusa), indicative of Vulnerable Marine Ecosystems (VMEs) and representing substantial benthic carbon accumulation, occurred in particularly dense aggregations at ,350 m but were encountered as deep as 1020 m. In addition to enhancing marine biodiversity records at this locality, this study provides critical baseline data to support the future management of Ascension's marine environment.

Extremes in Benthic Ecosystem Services; Blue Carbon Natural Capital Shallower Than 1000 m in Isolated, Small, and Young Ascension Island’s EEZ

“…It is clear from the current study that blue carbon occurring within the top 1000 m of seabed in the Ascension Island EEZ is extremely unevenly distributed across multiple spatial scales, supporting initial observations (Nolan et al, 2017). Within our Ascension EEZ data, the highest levels of variability occurred on the cm to m scale (associated with rugosity).…”

“…These are expensive financially, in time and expertise, and deeper work would require multibeam, deep cameras and limited targeted physical collections (to monitor temporal growth effects) but there are few such vessels passing Ascension Island. Furthermore we could not find multibeam (SWATH) signatures for centers of blue carbon interest (for example Topographic Position Index) at 25 m data gridding scale (Nolan et al, 2017). The resolution of such systems at that depth make them unlikely to provide rapid "remote" monitoring solution.…”

“…We considered three main threats to Ascension Island EEZ's biodiversity and blue carbon ecosystem services, how to monitor for these and any potential mitigation. It was not apparent from our 2015 and 2017 surveys that there were immediate strong impacts or threats (Nolan et al, 2017). However, given the remote nature of the seabed we surveyed, most of the threats are likely to be quite diverse and global in nature.…”

“…High marine biomass is typically associated with coral reefs, which in Ascension EEZ are in shallow warm waters but also in deep cold waters. However, that is only where there is enough hard surface to establish and even then only (perhaps by chance) in some of those areas (Nolan et al, 2017). Where shelf seabeds are in contact with phytoplankton blooms, such as round Ascension Island's coast they can be very important for suspension feeders and their predators (but not so much for many other carbon rich benthos).…”

“…Such extreme locations are often the furthest from anthropogenic impacts and have very high ratios of Exclusive Economic Zones (EEZ) to land area. There are dense patches of life around Ascension Island coast and associated seamounts, in the shallows and important cold coral stands in deeper water (Nolan et al, 2017). The large EEZ around Ascension Island is likely to have considerable natural capital and ongoing ecosystem services derived from it.…”