Chagos Archipelago - Great Chagos Bank West (BITB44)

Setting

BITB44 is a submerged bank reef, leeward on Anderson's field sheet, inside the Three Brothers lagoon, surveyed 23 March 2015 on a dive to 18.5 m (e.g., Image 4, Image 5, Image 8, Image 13, Image 16).

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General benthic features

Across 25 genera the reef is a two-genus system with a long thin tail: Porites at 15% of the benthos, 38% of all coral, and Acropora at 11%, the two holding 67% between them (e.g., Image 7, Image 11, Image 28, Image 36, Image 37).

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The strata differ far more in composition than in cover, 39% shallow against 42% deep, but the deeper one becomes close to a monoculture — Porites rising to 58% of its coral while Acropora nearly halves, genus count falling from 25 to just 9, algal cover halving, and bare substrate tripling to 27%.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 39% coral against 34% algae, with an open 11% bare substrate (e.g., Image 9, Image 10, Image 19, Image 38).
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• Transect mean 39 ± 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 41% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• That contrast rests on only two transects, one of which recorded 20% dead coral, the highest reading at either depth here.
• 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 34% 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 5.98% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). Two colonies show textbook partial bleaching, a branching colony with half its branch mass bone-white against intact brown tissue and a corymbose colony with discrete white branch clusters, but no site-wide field (e.g., Image 3, Image 40).

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Broad areas of dead coralline-encrusted pavement carry dense juvenile Acropora recruits, so the reef is visibly recolonizing lost framework (e.g., Image 26, Image 30).

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No crown-of-thorns, disease lesion, or predation is visible in the photographs, though Anderson's sheet logs two of the colonies sampled as "bleached/diseased"; a lobster aggregation was photographed (e.g., Image 15, Image 29).

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Anderson's sampling sheet records two colonies sampled here: Seriatopora hystrix ChSe35 (17.5 m, healthy) and ChSe36 (18.5 m, sickly, 50% bleached). It also records three other pocilloporids sampled for a collaborator (species by field call): Pocillopora sp. ChJo120 (14.6 m, 30% bleached or diseased), ChJo121 (14.6 m, large, healthy), and ChJo122 (14.5 m, 15–20% bleached or 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.

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 12 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.