Chagos Archipelago - Great Chagos Bank West (BITB43)

Setting

BITB43 is a submerged bank forereef outside the lagoon at Three Brothers, windward on Anderson's field sheet, surveyed 22 March 2015 on a dive to 30.7 m.

General benthic features

Non-coral invertebrates take 35% of the benthos, more than the 30% coral cover, mostly sponges, tunicates, and gorgonian sea fans, conspicuous in nine of the 22 frames (e.g., Image 3, Image 5, Image 8, Image 12, Image 16, Image 21, Image 26).

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Across 28 genera, both leaders are non-Acropora: Porites 6.5% of the benthos and Montipora 6.4%, nearly half the coral, with Acropora a distant third at 3.7%.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 30% coral against 26% algae, so this is a low-algae reef whose free space went to invertebrates rather than weed.
• Transect mean 30 ± 2.6% 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 29% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The depth finding is an Acropora collapse rather than a cover collapse — cover barely moves, 30% to 27%, while Acropora falls twenty-four-fold from 4.7% of the benthos to 0.2%, Porites climbs to a third of all deep coral, and bare substrate triples to 19%; at the shallower stratum invertebrate cover (37%) exceeds coral cover outright.

Stressors

What the data say. Published bleaching is 0% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1).

What Anderson saw. The photographs show three partly bleached branching colonies, each with a sharply bounded group of stark-white branches beside brown, pigmented ones (e.g., Image 10, Image 14, Image 25).

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Polyp texture is still visible and there is no turf on the white branches, so they are bleached rather than dead; it is not a reef-wide field. The sampling sheet logs one sampled colony as 50% diseased or dead and another as sickly, with 50% recently dead (listed below).

No crown-of-thorns or disease is visible in any of the 22 frames (e.g., Image 1, Image 4, Image 6, Image 9, Image 15, Image 19, Image 22, Image 24).

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Anderson's sampling sheet records three pocilloporid colonies sampled here for a collaborator (species by field call): Pocillopora sp. ChJo117 (30.4 m, 50% diseased or dead), ChJo118 (19.7 m, healthy), and ChJo119 (15.8 m, sickly, 50% recently dead). 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). The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and no recently dead coral on the transect lines.

Fish and other fauna

Three gray reef sharks were photographed (e.g., Image 7, Image 18, Image 20).

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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.

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 3.6 km north of North Brother, on the Great Chagos Bank; no bottom or reef edge is visible, so the imagery cannot test the aspect.