Chagos Archipelago - Great Chagos Bank West (BITB56)

Setting

BITB56 is a submerged bank reef east of South Brother in the Three Brothers group, leeward on Anderson's field sheet, surveyed 27 March 2015 on a dive to 25.7 m.

General benthic features

Porites is by far the dominant coral genus, holding nearly half of all recorded coral, with Acropora, Montipora, Pocillopora, Favites, and Favia trailing well behind (e.g., Image 1, Image 3, Image 9, Image 11).

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Coral cover and diversity peak at mid-depth and collapse at the deepest transect to a near two-genus assemblage of Porites and Acropora.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 25% live coral cover against a much higher 48% algal cover, making this an algae-dominated hardground of open pavement and rubble rather than a continuous coral wall (e.g., Image 4, Image 6, Image 12, Image 13, Image 16).
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• Transect mean 25 ± 3.0% SE (n = 13 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 21% (Mayfield & Dempsey 2026, Oceans, suppl. S1).

Stressors

Published bleaching was 16% (CoralNet analysis of the survey images, not a diver's field estimate; Mayfield & Dempsey 2026, suppl. S1); no bleached colony is visible in the 13 frames, and the three colonies Anderson sampled here were 5–10% pale, bleached, or sickly (see Dive record below). A spiny lobster sheltered beneath a ledge of large plating corals (e.g., Image 2, Image 5, Image 8, Image 15).

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The Living Oceans Foundation transects here recorded no 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 27 March 2015 entered the water at 11:05 and lasted 45 minutes, to a maximum depth of 25.7 m (the field sheet); the notes describe an “unspectacular site” with a strong current and Halimeda-dominated ground at 15 m. Anderson sampled three pocilloporid colonies on this dive for a collaborator: ChJo150 (collected 11:09 at 21.3 m; Pocillopora sp., a large, flattened colony partly dead for months or years, 5% pale or sickly; the sickly part was sampled), ChJo151 (11:16, 25.1 m; P. verrucosa, large and healthy overall, some polyps extended; a pale branch, 5% of the colony, was sampled), and ChJo152 (11:29, 15 m; P. verrucosa, large, 10% bleached or sickly, common at the site, especially at 15 m; the healthy portion was sampled). The notes record no Pocillopora damicornis, P. acuta, or Seriatopora hystrix. 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 2.5 km east of South Brother, at the eastern edge of the imaged area; no bottom or reef edge is visible, so the imagery cannot test the aspect.