Chagos Archipelago - Great Chagos Bank West (BIDI09)

Setting

The dive's first two-thirds cross genuinely excellent reef.

General benthic features

The assemblage is massive-led: Porites 16% of the benthos, i.e. 43% of all live coral present, more than twice the next genus. Acropora follows at 7.9%, then Montipora 2.5%, Pocillopora 2.2%, Favia 1.5%, Platygyra and Stylophora 1.1% each, Gardineroseris, Montastrea, and Favites 0.6% each, Goniastrea and Symphyllia 0.5% each, Echinopora 0.4%, and Psammocora, Leptastrea, and Turbinaria 0.3% each, then a long tail — Acanthastrea, Pachyseris, Galaxea, Pectinia, Hydnophora, Pavona, Isopora, Astreopora, and Cyphastrea 0.1% each, Ctenella <0.1% (the Chagos-endemic monotypic genus, not Cynarina), and Millepora, Goniopora, Fungia, Lobophyllia, and Leptoseris <0.1% each. The non-coral substrate is dominated by non-coral invertebrates at 17% — the single largest non-coral category here (all algae together 40%) — then crustose coralline algae 13%, macroalgae 9.4%, turf-with-sediment 8.3%, bare substrate 5.5%, erect algae 5.5%, turf 4.0%, and cyanobacteria <0.1%. Image 11, Image 13, Image 15, Image 18, and Image 31 show large olive and gold tables overlapping across the whole frame with massive Porites wedged between them, and Image 11 puts a fine green meandroid brain in the foreground. Image 17 and Image 19 are dominated by broad, smooth, pale-green massive Porites plates carrying purple and crimson encrusting sponges — the photographic counterpart of the transect's 16% Porites.

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Massive corals are a real structural component rather than an accent: broad low Porites-type mounds and plate-lobe complexes fill the foreground of Image 11, Image 17, and Image 19, some with maroon-red encrusting patches and mauve sponges set into their surfaces (e.g., Image 17), and a rounded meandroid brain sits beside a yellow-green table in Image 11. Smaller structural elements fill the pavement between them — digitate and corymbose Acropora, Pocillopora-type bushes, encrusting and foliose colonies, brown leather corals in Image 11 and Image 15, thick pink and mauve crustose coralline crusts throughout, and dark blue-gray tubular sponges in Image 22.

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Image 22 is the single most diverse benthic frame — a mid-slope pavement packed with meandroid brains, Pocillopora, purple digitate Acropora, faviid domes, pink crustose coralline algae, and encrusting sponges, with a peacock grouper, blue-spotted and unmistakable, resting at frame left.

The last three frames are a different reef entirely. Image 7, Image 20, and Image 27 show pale rubble and sand plains carrying only small, widely spaced corymbose and digitate colonies over dominantly bare substrate.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 38%, 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 40% of the benthos. Surveyed 12 March 2015 on the western rim of the Great Chagos Bank at Danger Island (6.37934°S, 71.23359°E; station CTD temperature 29.2°C, salinity 34.2).
• Live hard coral 38% against algal cover 40% of the benthos, across 30 scleractinian genera plus the hydrocoral Millepora.
• Transect mean 38 ± 5.3% SE (n = 22 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 38% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• That Porites-over-Acropora ranking is the transect's own account of the site.
• Full per-genus and substrate values are in the benthic data for this site (Mayfield & Dempsey 2026, Oceans, supplemental 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.
• Quality is good — clear water, mostly well-composed wide reef views — with five exceptions: Image 9 is badly backlit and reads almost black over the substrate, Image 36 carries a strong magenta cast, and Image 5, Image 25, and Image 34 are so densely filled with fish that little benthos shows behind them.
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• No bleached percentage is derived here from any photograph. This site's published per-site bleached fraction is a separate, authoritative number from CoralNet analysis of the expedition's survey images, trained on Anderson's point annotations (Mayfield & Dempsey 2026, suppl. S1) and is not confirmed, contradicted, or revised by an image.

Stressors

Bleaching is not a feature of these frames. Image 21 is the one clear case — a bone-white branching colony on the reef below a rudderfish school — and the same low-relief pale colony appears again in Image 30; nothing else in the set reads as bleached tissue.

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The recurring bright white patches in Image 24, Image 26, and Image 38 are clean carbonate sand and rubble, not bleached coral — the kind of feature a mask model may misclassify.

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Some tables carry small dead or pale-dulled sectors, and the sparse pavement in three of the frames is old, algal-filmed substrate, but there is no storm rubble field, no cyanobacterial mat, and no crown-of-thorns scar anywhere in the set. Anderson's sampling sheet records six Pocillopora colonies sampled here at 9.2–29.2 m (ChJo25–ChJo30; species not settled in the field, either P. verrucosa or P. meandrina for most), none recorded as 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 under 0.1% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

What dominates the impression of this site is planktivore biomass. Fusiliers — brilliant blue with dark dorsal stripes — fill at least fourteen frames (e.g., Image 6, Image 10, Image 11, Image 16, Image 23, Image 24, Image 25, Image 26, Image 28, Image 30, Image 32, Image 35, Image 36, Image 38), in schools of many hundreds that in Image 30 and Image 36 largely obscure the reef behind them.

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Layered above and among them are large dark-bodied rudderfish or snappers (e.g., Image 5, Image 13, Image 21, Image 34), and blue-striped snapper — bright yellow with electric blue lines — in Image 6, Image 23, and Image 26. Larger individuals appear repeatedly: a gray jobfish crossing frame in Image 10 and again in Image 16, a red-orange hogfish or wrasse in Image 35, red bigeyes in Image 21 and Image 35, a large pale-bodied fish over the reef in Image 30, Moorish idols and a powder-blue surgeonfish. Thousands of planktivores stacked over the reef, several large-bodied predators in frame, and no fishing gear or damage anywhere stand out in the photographs.

Comparisons with other data

The XL Catlin Seaview Survey covered a nearby station (transect 36002) in 2015, 0.26 km from the pin; we could not confirm that it is the same reef. Within 5 km, two XL Catlin Seaview Survey stations recorded station-year means of 6.3–25% hard coral cover in 2015; the nearest, XL Catlin Seaview Survey at transect 36002, 0.26 km away, read 25% in 2015.

Other notes

Thirty-four habitat frames from the 12 March dive; there are no sampled-colony frames, so this is a habitat account throughout.

Survey-methodology imagery — eight frames. Image 28 and Image 38 show a diver in yellow fins carrying and then placing a large white PVC quadrat frame over the tabular plain with dense fusilier schools filling the water; Image 32 has the quadrat frame at the upper-left edge mid-deployment with line running across the substrate below, and the transect bar visible in-frame beside a large giant clam, its fluted shell wedged among Porites plates. Image 6 catches a diver deploying a transect reel with a slate; Image 16 shows a diver with camera alongside a white transect bar laid across the reef; Image 9 and Image 23 show divers running line at depth, with Image 23 placing a diver with line in the middle distance over the fusilier-covered slope.

No frame carries a scale bar or a depth stamp, so colony dimensions, relief, and depth zonation cannot be read from this set; the only scale objects are the divers and the PVC quadrat and transect bar in the methodology frames. Fish identifications above are held at body-type or hedged genus level.