Chagos Archipelago - Great Chagos Bank West (BITB45)

Setting

BITB45 is a submerged bank reef inside the Three Brothers lagoon, leeward on Anderson's field sheet, surveyed 23 March 2015 on a dive to 25.7 m.

General benthic features

What replaces algae is open ground: 16% bare substrate and 13% turf-on-sediment, so the reef reads as coral thickets and plate stands set on white carbonate sand (e.g., Image 1, Image 2, Image 8, Image 12, Image 15, Image 16).

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Across 23 genera, Acropora leads at 18% of the benthos and Porites at 13%, four genera holding 90% of the coral and evenness at just 0.54 (e.g., Image 5, Image 6, Image 9).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 45% coral against just 30% algae, and fleshy turf is only 3.8% of the benthos.
• Transect mean 45 ± 3.0% SE (n = 15 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean, and the published station value, which weights each depth equally, is 46% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• In the depth profile, Acropora climbs from 16% of the benthos shallow to 27% deep, 57% of that stratum's coral, while algae halves and bare substrate nearly triples to 33% — but it is descriptive only: the deep stratum rests on three transects reading 69%, 36%, and 36% and yet supplies half the headline number.
• Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (the mean of all of this station's transects), against total algal cover (all algal groups) of 30% of the benthos.
• Image-based estimates (CoralNet, DINOv3, CoralSCOP, and Claude) are listed in the method table for comparison only; the transect figure is this site's cover value.

Stressors

Published bleaching is 7.4% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). All 13 frames were reviewed and show pallor without a demonstrated event: pale mottling and white patches on one large table, unresolved from a single still, plus genuine old partial mortality on another (e.g., Image 3).

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No crown-of-thorns, disease front, or predation is visible in the photographs; one of the three colonies Anderson sampled here was 5% bleached or diseased (possible disease; see Dive record below). The Living Oceans Foundation transects here recorded 0.4% cyanobacteria and no recently dead coral.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. If you would like to share yours, please email us.

Dive record

Anderson's morning dive on 23 March 2015 entered the water at 11:29 and lasted 46 minutes, to a maximum depth of 25.7 m (the field sheet). The notes record an incredibly strong current (surfacing with only 20 bar ended the dive before exploring depths shallower than 15 m) and “beautiful corals.” Three pocilloporid colonies were sampled on this dive for a collaborator (Pocillopora sp.): ChJo123 (collected 11:41 at 19.8 m; 5% bleached or diseased; the unhealthy part was sampled), ChJo124 (11:51, 15.5 m; large and healthy), and ChJo125 (12:00, 15.5 m; healthy, likely P. verrucosa, with part of the colony dead for years). The notes record no Pocillopora damicornis, P. acuta, or Seriatopora at the site. Pocillopora acuta colonies from this archipelago were analyzed in Thomas et al. (2026, Global Ecology and Biogeography 35:e70305, https://doi.org/10.1111/geb.70305).

Other notes

Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'submerged bank', 'leeward'); the expedition's 933-site table classes the station as 'protected' (bank reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) shows open water at the pin about 10 km north-northeast of North Brother, on the Great Chagos Bank; no bottom or reef edge is visible, so the imagery cannot test the aspect.