Chagos Archipelago - Great Chagos Bank West (BITB46)

Setting

BITB46 is a submerged bank reef, leeward on Anderson's field sheet, inside the Three Brothers lagoon, surveyed 23 March 2015 on a dive to 27.3 m: 32% coral against 52% algae (e.g., Image 17, Image 23, Image 30, Image 33).

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General benthic features

Crustose coralline algae alone is 22% of the benthos, so about two-fifths of the algal cover is hard pink-mauve crust rather than fleshy algae, and the photographs show the crust binding rubble and dead framework (e.g., Image 1, Image 10, Image 12, Image 22, Image 24).

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In Image 8, maroon, irregularly branching strands, consistent with red algae, lie draped across coralline-crusted rubble and small colonies; they are not netting.

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Across 22 genera, Porites dominates at 13% of the benthos, 39% of every coral point and about twice the next genus.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Cover more than halves between the two strata, from 35% to 17%, while Acropora collapses 95% from 7.4% of the benthos to 0.4% and Porites comes to hold 58% of the deep stratum's coral, but that stratum rests on two transects, so it is descriptive rather than a demonstrated depth effect.
• Transect mean 32 ± 4.2% SE (n = 11 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 26% (Mayfield & Dempsey 2026, Oceans, suppl. S1).

Stressors

Published bleaching is 2.02% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). None of the 29 frames shows an acute bleaching field; one very large table carries sharply bounded partial mortality across 40% of its plate, already colonized by coralline algae and so an older lesion (e.g., Image 4).

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No crown-of-thorns, disease lesion, or predation is visible in the photographs, though Anderson's sheet notes one sampled colony with a purple lesion and another as "possibly diseased". Anderson's sampling sheet records two colonies sampled here: Seriatopora hystrix ChSe37 (24.9 m, healthy) and ChSe38 (23.8 m, small, young, very pale; logged as >90% bleached or dead). It also records three other pocilloporids sampled for a collaborator (species by field call): Pocillopora sp. ChJo126 (19.3 m, healthy), ChJo127 (15.4 m, healthy, one purple lesion), and ChJo128 (14.2 m, 20% bleached, possibly diseased). 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 no cyanobacteria and no recently dead coral.

Fish and other fauna

The photographs show a gray reef shark and a whitetip reef shark, surgeonfish and fusilier schools, a baitfish cloud, and a giant clam (e.g., Image 3, Image 5, Image 6, Image 9, Image 11, Image 13, Image 14, Image 16, Image 18, Image 20, Image 21, Image 25, Image 26, Image 27, Image 29, Image 32).

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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', '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 4.7 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.