Chagos Archipelago - Great Chagos Bank West (BIEA20)

Setting

Where the bottom is visible, the reef reads as low-relief, gently sloping carbonate pavement rather than a high-relief spur system: broad pale rubble-and-sand flats with colonies set into them singly or in loose bands, and a blocky undercut ledge with dark shaded recesses in Image 1.

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General benthic features

Acropora was the most common coral genus on this station's transects (2.9% of the benthos), followed by Pocillopora (2.4%), Porites (2.2%), Montipora (1.5%), Goniastrea and Leptastrea (0.7% each), and Favites (0.6%); bare substrate covered 7.3%, behind other invertebrates (31%), the largest single living non-coral group (LOF transects; all algae together 49%). Among the algal classes, sediment-laden turf led (26% of the benthos), followed by crustose coralline algae (11%) and turf (6.8%). Where coral does concentrate it is worth looking at. Image 10, Image 15, and Image 43 carry large yellow-tan foliose and columnar Porites; Image 5, Image 9, and Image 35 show brown ridged foliose plates spreading over the pavement, and comparable ridged tan plates appear again in Image 36 and Image 38; Image 38 also holds a tan meandroid brain compatible with Platygyra; and Image 7 – the site's highest-cover benthic frame at 47% live by point count – is a stand of pale tan digitate Porites with a spotted moray eel looking out from between the branches. Beyond those, low massive domes recur, including one large smooth-surfaced Porites dome carrying a red-pink encrusting sponge in Image 40; small branching and corymbose colonies sit over sand in Image 6; mushroom corals lie loose on the pavement in Image 35 and Image 38; and small tables and staghorn thickets occupy the brighter reef top in Image 8.

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Green calcareous algae, pink and maroon coralline crusts, red and orange encrusting sponges, and small fleshy soft-coral tufts are present throughout, and open pavement between colonies is conspicuous in most benthic frames. Black corals are a quiet but repeated feature: antipatharian whips beside the shark in Image 1 and again in Image 6, and bushes in Image 21 and Image 27.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 13%, from the Khaled bin Sultan Living Oceans Foundation's BIOT-1 in situ transect survey (the mean of all of this station's transects); the photographs illustrate the site. Algal cover is 49% of the benthos.
• Transect mean 13 ± 2.1% SE (n = 12 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean, and the published station value is 13% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• 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.
• Frame-to-frame live cover runs from 0% to 47% by point count.
• These products have different sampling footprints and denominators and should not be averaged.

Stressors

Condition – mortality is localized but real. Image 2 is the single most degraded frame at the site: a branching colony gone chalky gray-white, corallite texture still sharp but no live tissue color, ringed by dark maroon crusts and with a green algal film beginning to take it, alongside red-orange sponges.

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The point annotation places two bleached points on this frame. It is either bleached tissue or a recently dead colony; a photograph cannot separate the two, but it is not a healthy pale morph. Image 43 holds a second, much smaller white patch. Elsewhere in the set the corals are pigmented brown, tan, olive, and yellow, and this is a 15 March dive, on the early leg of the expedition. No bleaching percentage is estimated from these images and none is tested against them; Anderson's published per-site percentage bleached is the authoritative figure for this reef. Anderson's sampling sheet records two Pocillopora colonies sampled here, ChJo59 (24.0 m, healthy, some polyps extended) and ChJo60 (20.4 m, all polyps extended, with very subdued verrucae). Anderson's sheet records no P. damicornis or P. acuta at this site; these colonies were collected for a collaborator. Pocillopora acuta colonies from elsewhere in the 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 under 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

One structural fact governs how much weight the image products can carry here: roughly half the photo set is water-column photography. About eighteen of the 41 frames are of a single enormous mixed surgeonfish aggregation in open blue water and contribute no benthic detail at all, and several more are blue-water or distant-fish frames. Those frames are still in the paired denominator.

The single most striking thing about the photo set is that it is not primarily a benthic record. Roughly half the frames – Image 4, Image 16, Image 22, Image 23, Image 24, Image 25, Image 26, Image 30, Image 31, Image 32, Image 34, Image 37, Image 39, Image 41, Image 44, and Image 45 – are given over to an enormous mixed surgeonfish aggregation moving over the reef in open blue water, dark brown and pale-lavender bodies packed shoulder to shoulder and filling the frame in several shots.

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As benthic frames they carry almost nothing.

The macrofauna is notable. Image 1 is a fine shark frame – a whitetip reef shark, close and sharply lit, cruising over white sand beneath a maroon-crusted overhang, its white dorsal and caudal tips unmistakable. A whitetip-type shark appears again low on the slope in Image 21, and Image 20 holds a second, different requiem shark in open blue water – dark caudal margin but a pale first-dorsal tip, so gray reef or silvertip; it is not callable from the frame.

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Beyond the sharks: a giant clam with its mantle open and boldly patterned in Image 42; an adult emperor angelfish in Image 27; a two-spot red snapper in Image 23; a coral hind in Image 39; powder-blue surgeonfish in Image 36; a long-spined urchin in Image 35, with a second urchin frame at Image 37; blue-and-yellow damselfish swarming (e.g., Image 3) and over the reef top in Image 36; and black triggerfish, hogfish, cleaner wrasse, and orange anthias clouds throughout, notably (e.g., Image 5, Image 15, Image 33).

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Sponges register in 16 of the paired frames, urchins in 2, giant clams in 1, sea cucumbers in 1, and anemones in 1.

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

All forty-one habitat frames were reviewed. There are no sampled-colony frames, so the account rests entirely on the wide set. Color is generally good, but several wide frames are hazy and blue-shifted with depth, and no scale bar appears anywhere, so no colony dimension here is measured.

This is also a well-documented methodology site. Eight frames show the survey being made. Image 40 – a camera housing and strobe arm in the foreground aimed squarely at a massive Porites dome, a diver with a slate behind it. Image 13 – a diver with a camera and, lying on the reef beside them, a second twin-cylinder set. Image 8 and Image 11 – divers with housings and twin strobes filming the fish schools. Image 14 and Image 29 – divers working over the rubble plain.

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And two frames that may be the most useful of all: Image 31 carries a distinct white rectangular open frame lying flat on the substrate at lower right, with a thin pale line running away from it across the reef, and Image 36 shows a similar rectangular frame silhouetted in the background at upper center. If they are a photoquadrat and a transect tape, this site holds a direct image of the apparatus behind the 13% figure; the identification is not confirmed.