Chagos Archipelago - Great Chagos Bank West (BITB42)

Setting

BITB42 is an intermediate-exposure submerged bank forereef outside the lagoon at Three Brothers, surveyed 22 March 2015 at 25 m.

General benthic features

This is a sediment-loaded reef rather than a coralline-paved one, with 16.7% non-coral invertebrates (e.g., Image 1, Image 5, Image 8, Image 22, Image 31, Image 34).

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Across 26 genera, Acropora leads at 13.0% of the benthos, 36% of all coral, with Porites 9.5% and Montipora 4.3% making up three-quarters of the coral between them.

Coral cover

Formal transects recorded 36.2% coral against 33.3% algae — 63rd of 109 Chagos sites for cover, 78th for algae — but the algal budget is the departure: turf-with-sediment alone takes 14.8% of the benthos, more than the coralline algae at 8.6%, which is 89th of 109. The depth profile peaks in the middle and reshuffles at the bottom: 29.0% coral at the shallowest stratum with 45% algae and only four genera recorded, a 42.0% Acropora-dominated mid-depth peak where the genus holds 46% of the coral, then 37.6% at the deepest where Acropora collapses to 18% of coral, genus count reaches 23, and a quarter of the benthos is bare rock. Image methods read 13 points below the transects (CoralNet 23.3%, Coralscapes 22.4%) and should not be averaged. Coral cover: 36%. Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (Mayfield & Dempsey 2026, Oceans, supplemental S1), against algal cover of 33.3% of the benthos. The transect figure is this site's authoritative cover value; no image-derived product overrides it. Across methods the same reef reads: transects 36.2% · CoralNet points 23.3% · Coralscapes masks 22.4% (46 frames). These products have different sampling footprints and denominators and should not be averaged.

Stressors

Bleaching on the canonical % bleaching (2nd training) column is 7.73%, 52nd of 110 dives, mid-range and identical to the first run. Thirty of 51 frames were reviewed: two colonies show sharply demarcated bone-white skeleton, still clean and unfouled, though one still cannot separate severe bleaching from very recent tissue loss; other frames show much older, already overgrown mortality. No crown-of-thorns or disease; one frame holds a long-spined Diadema urchin that an automated pass could mistake for one.

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

No prior survey data from this site. If you would like to share yours, please email us.