Chagos Archipelago - Great Chagos Bank West (BITB42)

Setting

BITB42 is a submerged bank forereef outside the lagoon, windward on Anderson's field sheet, at Three Brothers, surveyed 22 March 2015 on a dive to 21.7 m.

General benthic features

Non-coral invertebrates cover 16% of the benthos (e.g., Image 1, Image 5, Image 8, Image 22, Image 31, Image 34).

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Across 26 genera, Acropora leads at 16% of the benthos, 39% of all coral, followed by Porites (9.3%) and Montipora (4.8%); the three genera make up nearly three-quarters of the coral.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 40% coral against 30% algae, and turf-with-sediment alone takes 11% of the benthos, more than the coralline algae at 8.6%.
• Transect mean 40 ± 3.5% SE (n = 16 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 36% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• The depth profile peaks in the middle and reshuffles at the bottom: 29% coral at the shallowest stratum with 45% algae and only four genera recorded, a 42% Acropora-dominated mid-depth peak where the genus holds 46% of the coral, then 38% at the deepest where Acropora collapses to 18% of coral, genus count reaches 23, and a quarter of the benthos is bare rock.
• 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 30% 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 7.73% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). Two colonies show sharply demarcated bone-white skeleton, still clean and unfouled, though one cannot separate severe bleaching from very recent tissue loss (e.g., Image 37, Image 53); other frames show much older, already overgrown mortality.

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Anderson's sampling sheet records three pocilloporid colonies sampled here for a collaborator (species by field call): Pocillopora sp. ChJo114 (20.8 m, 80% dead or bleached, healthy part sampled), ChJo115 (20.5 m, large, healthy), and ChJo116 (about 15 m, appears healthy in the photograph). 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). No crown-of-thorns or disease is visible in the photographs; one frame holds a long-spined urchin. The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

Gray reef sharks appear in four frames (e.g., Image 2, Image 4, Image 11, Image 12).

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