Chagos Archipelago - Great Chagos Bank West (BITB41)

Setting

BITB41 is a submerged bank reef outside the lagoon, windward on Anderson's field sheet, at Three Brothers, surveyed 22 March 2015 on a dive to 30.2 m.

General benthic features

Across 28 genera, the assemblage is overwhelmingly Porites-led at 19% of the benthos, 43% of every coral point and four times the next genus, while Acropora is third at 3.1% (e.g., Image 2, Image 5, Image 8, Image 15, Image 16).

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One frame captures a coral-on-coral overgrowth margin (e.g., Image 3).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 43% coral against 32% algae; crustose coralline algae is 8.3% of the benthos and turf 8.6%, while bare substrate takes 9.2% and non-coral invertebrates 16% (e.g., Image 1, Image 4, Image 6, Image 11, Image 12).
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• Transect mean 43 ± 3.3% SE (n = 14 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 42% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• The three depth strata are nearly identical in cover — 44%, 41%, 41% — yet individual transects range 28% to 73%, so the spread between transects (45 points) is more than ten times the spread between strata, and no cover trend can be defended; composition does change, Acropora falling to zero at the deepest stratum while Pachyseris rises from absent to 7.0% and bare substrate climbs to 25%.
• 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 32% 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 7.65% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). In the 13 frames paling is real but patchy: a stark-white branching colony with green algae only at its base, and white branch tips on an otherwise pigmented branching colony, but no site-wide field, no crown-of-thorns, and no disease (e.g., Image 9, Image 13).

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One of the four colonies Anderson sampled here was 10% bleached or pale (see Dive record below). The Living Oceans Foundation transects here recorded 0.1% 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 22 March 2015 entered the water at 08:41 and lasted 50 minutes, to a maximum depth of 30.2 m (the field sheet); the notes describe fairly murky water, an octopus, and sharks. Anderson sampled four pocilloporid colonies on this dive for a collaborator (Pocillopora sp.): ChJo110 (collected 08:54 at 25.4 m; large and healthy, the local morphology common at the site), ChJo111 (09:09, 15.9 m; healthy, with a green fluorescent hue), ChJo112 (09:12, 16.0 m; a huge, healthy colony of candelabrum form), and ChJo113 (09:20, 16.2 m; 10% bleached or pale; the bleached part was sampled). 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', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (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 13 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.