Chagos Archipelago - Great Chagos Bank West (BIDI13)

General benthic features

The bottom in these frames is low-relief pavement and rubble rather than coral framework. Image 1, Image 3, and Image 20 are the three cleanest wide views and all show the same thing: an expanse of pale, eroded consolidated carbonate and coarse rubble crusted with turf and pink and mauve coralline algae, carrying small, well-spaced colonies – pale tabular and foliose plates, low tan knobby and encrusting heads, a dark blue-gray massive at the bottom edge of Image 20 and another in Image 3, and scattered clumps of green macroalgae. Image 1 is dominated by a broad flat dark-brown mass low in the frame whose surface is entirely given over to turf, reading as old framework rather than living tissue.

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Nothing in these frames stands more than a colony's height above the substrate.

Two frames record the seagrass beds Anderson's sampling sheet notes for this dive ('seagrass meadows'; colonies interspersed among seagrass patches). Image 12 and Image 18 show olive-to-dark-brown strap-like blades covering the seafloor over pale rubble, with only sparse coralline patches showing through and a single small tan coral head at the base of Image 12.

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Where coral persists, the close frames are the informative ones. Image 10 packs the most into one field of view: a dark, camouflaged grouper wedged in a hollow, a powder-blue surgeonfish, yellow anthias, ruffled cream-yellow foliose colonies, brown massive Porites and small olive-brown massives, and extensive red and pink crustose coralline algae. Image 8 is a clean close view of a brown, densely verrucose branching colony – Pocillopora or Stylophora, not separable here – set against a gray-white massive whose fine, even corallites are compatible with Porites.

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Montipora was the most common coral genus on this station's transects (2.3% of the benthos), followed by Pocillopora (1.8%), Porites (1.1%), and Millepora, Acropora, Goniastrea, and Astreopora (0.3% each); bare substrate covered 7.5%, behind other invertebrates (26%), the largest single living non-coral group (LOF transects; all algae together 60%). Among the algal classes, crustose coralline algae led (24% of the benthos), followed by sediment-laden turf (20%) and turf (7.0%).

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 7.2%, 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 60% of the benthos.
• Together, the coral and algal figures describe the site: this is an algal-dominated stretch of reef with coral persisting in patches.
• Transect mean 7.2 ± 1.0% SE (n = 14 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 7.8% (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.
• These products have different sampling footprints and denominators and should not be averaged.

Stressors

Image 19 is the condition frame: two large brown knobby massive colonies sit either side of a distinct dark brown dead patch of unclear age, surrounded by white coralline-crusted substrate, with a single small yellow branching recruit compatible with Acropora settled nearby – loss and recruitment inside one field of view.

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The dead patch is a real photographic record, but its cause cannot be diagnosed from an image. No unambiguously bleached colony appears in any of the sixteen frames. The corals that are present read as normally pigmented brown, tan, olive, and cream. The one ambiguous feature is a small cluster of bright white branch tips at the upper right of Image 8, which at this magnification cannot be separated from ordinary bright growth tips; it is reported as seen and diagnosed no further. No bleaching percentage is read off this set and none is tested against it. Anderson's published per-site percentage bleached is the authoritative figure for this reef; the photographs record only that whitening was not conspicuous in the frames taken on this dive, while Anderson's sampling sheet records three of the five Pocillopora colonies sampled here as bleached (20%, 5%, and 50%) and two as healthy. The dead patch in Image 19 is mortality of some kind, not bleaching. The Living Oceans Foundation transects here recorded under 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

Fauna – a grazer-dominated dive, which fits an algae-dominated site. Dense schools of surgeonfishes and unicornfishes fill the water column in Image 1, Image 9, and Image 14, mostly as silhouettes against the surface, and parrotfishes work the pavement in Image 1 and Image 6.

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A whitetip reef shark cruises low over the rubble ridge in Image 6. Image 2 is a clear fish photograph: an oriental sweetlips, sheltering under a coralline- and sponge-crusted ledge, white with crisp black longitudinal stripes and yellow, black-spotted fins – an unambiguous species-level identification. Image 5 is a deeply shadowed undercut with a single small yellow fish.

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Three frames record jellyfish: a small translucent medusa resting against algal turf on pale sand in Image 11, a large bell with trailing oral arms drifting past a diver's yellow fins in Image 15, and a third in open blue water near the surface in Image 4.

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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.

Dive record

Anderson sampled five pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora sp. ChJo35 (collected 10:23 at 20.9 m; 20% bleached, likely P. verrucosa and common at the site, bleached part sampled), ChJo36 (10:30, 20.6 m; 5% bleached, likely P. verrucosa, bleached part sampled; 'seagrass meadows, though some corals'), ChJo37 (10:36, 15.6 m; healthy, likely P. verrucosa; colonies interspersed among seagrass patches), ChJo38 (10:42, 14.6 m; 50% bleached, P. verrucosa or P. meandrina, bleached part sampled), and ChJo39 (10:53, 11.5 m; healthy, P. verrucosa or P. meandrina, 'found amidst seagrass meadows'). The dive notes record surge and a mild current, soft corals and sponges in place of coral in places, and no P. damicornis, P. acuta, or Seriatopora at the site. 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).

Other notes

Sixteen habitat frames survive from the dive: Image 1, Image 2, Image 3, Image 4, Image 5, Image 6, Image 8, Image 9, Image 10, Image 11, Image 12, Image 14, Image 15, Image 18, Image 19, and Image 20. There are no sampled-colony frames. Visibility and natural color are good here, but the effective benthic sample is smaller than 16: three frames (e.g., Image 4, Image 11, Image 15) are jellyfish and water-column subjects, two (e.g., Image 9, Image 14) are fish silhouettes against the surface with the bottom barely lit, and Image 5 is a deeply shadowed undercut with essentially no benthic information. The genuinely informative benthic frames are Image 1, Image 3, Image 6, Image 8, Image 10, Image 12, Image 18, Image 19, and Image 20. No quadrat, transect line, or scale object appears anywhere.