Mapping of geomorphology, seabed substrate and nearshore habitats within the Ascension Island Marine Protected Area

Macdonald, C.; Cooper, R.; Bonde, C.E.; Stewart, H.A.; Baum, D.; Simpson, D.; Simpson, T.; Tieppo, M.. 2025 Mapping of geomorphology, seabed substrate and nearshore habitats within the Ascension Island Marine Protected Area. Edinburgh, UK, British Geological Survey, 52pp. (OR/24/015) (Unpublished)

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Abstract/Summary

Seamounts are important ecosystems as they have a major influence on physical oceanography, which in-turn influences the pelagic and benthic communities found on and around them (e.g., Clark et al., 2010). Ascension Island is a remote volcanic island situated atop one such seamount in the South Atlantic and characterised by its complex geological history and diverse ecosystems. In 2019, the entire Economic Exclusion Zone (EEZ) surrounding Ascension Island was declared a Marine Protected Area (MPA) (National Protected Areas Order, 2019). The area covered by the Ascension Island MPA (AI-MPA) exceeds 445,000 km2, making it one of the largest MPAs globally. Nearshore habitats are a key component of the AI-MPA as they support many unique species and are also the area’s most at risk from anthropogenic development and climate change. This study presents the first detailed seafloor mapping of the AI-MPA, revealing a variety of previously unmapped features. These include submerged lava flows, submarine terraces, or palaeoshorelines, that record evidence of previous sea level changes around the island, submarine landslides, and soft sedimented plains. The new maps give unprecedented insight into the variety of habitats associated with this geodiverse seascape. The resulting maps of seafloor geomorphology, substrate and habitat are based on a combination of semi-automated and manual mapping techniques and expert judgement. Prior to this project, sparce high-resolution bathymetric information was available, therefore these outputs provide urgently needed resources to aide accurate assessment and monitoring of these ecosystems, ultimately supporting improved evidence-based management of the AI-MPA. This report describes the acquisition and data processing methodology for the following datasets: multibeam echosounder (MBES) bathymetry, MBES backscatter intensity, and sampling data comprising both physical seafloor samples and video transects acquired by the British Geological Survey (BGS) (Cooper and Macdonald, 2024) and the Royal Navy (HMS Protector) (Royal Navy, 2021). Additionally, the methodology to produce derived multibeam echosounder products (bathymetry, hillshade, slope, aspect, and rugosity) for display and interpretation purposes is described. Finally, an overview of the approach and results of the complementary map components (i.e. the seabed geomorphology, substrate and habitat interpretations), undertaken by the BGS on behalf of the Ascension Island Government (AIG), is provided. Key findings and future recommendations from this study include: • The higher resolution MBES bathymetry and backscatter intensity data (1 m to 10 m resolution) added significant value to the project allowing for a detailed classification of seabed features and highlighted diverse seafloor features and environment; • Detailed mapping of the geomorphology down to 1000 m was completed at 1:10,000 scale. Mapping identified a range of volcanic, erosion-depositional and coastal features including volcanic knolls, ridges, gullies, channels, and landslide scars. These features are of international scientific importance for our understanding of seamounts and are intrinsically linked to the conservation interests of the MPA; • Six geological substrate classes were identified within the MPA including ‘Sand’, ‘Rock’, ‘Rhodolith’ and ‘Mixed sediment’ which was delineated by estimation of gravel content; • Features, or areas of high seabed rugosity or slope angles, such as the rocky shore habitats, exhibited a higher abundance of marine species. This was also supported by observations from the drop-camera video analysis; • The seabed substrate interpretation was converted into a seabed habitat map using different biological zones delineated by water depth and marine zone. • Future work to ground-truth the physical samples via a dedicated seabed sampling campaign should be considered. Collection of sub-seabed seismic profiles may also provide further insight into the island formation, and morphology of the shelf. Specific studies into the morphometrics and seabed processes, as well as a comparison with other island settings (such as St. Helena) may provide valuable insight into the stability of marine habitats and geohazard potential of the island.

Item Type:	Publication - Report
Funders/Sponsors:	British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed, but not externally peer-reviewed.
Date made live:	19 Mar 2025 14:16 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/539110

Item Type:

Publication - Report

Funders/Sponsors:

British Geological Survey

Additional Information. Not used in RCUK Gateway to Research.:

This item has been internally reviewed, but not externally peer-reviewed.

Date made live:

19 Mar 2025 14:16 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/539110