Chagos Archipelago - Great Chagos Bank West (BIDI16)

General benthic features

Porites alone holds 19% of the benthos, which is 73% of all live coral on the transects. Pocillopora is a distant second at 2.6%, then Acropora and Montipora 0.9% each, Leptastrea 0.6%, Ctenella 0.5% (the Chagos-endemic monotypic genus, not Cynarina), Stylophora 0.4%, Pavona 0.3%, Favia and Favites 0.2% each, Goniastrea, Galaxea, Cantharellus, Alveopora, Goniopora, and Turbinaria 0.1% each, Millepora, Astreopora, Pachyseris, Fungia, Lobophyllia, Cyphastrea, and Echinophyllia <0.1% each, and Platygyra <0.1%. The substrate is dominated by turf-with-sediment at 21%, then bare substrate 17%, crustose coralline algae 10%, non-coral invertebrates 8.6%, erect algae 8.1%, turf 5.8%, macroalgae 2.5%, and cyanobacteria 0.2%. That profile — one massive genus carrying seven-tenths of the coral, almost no Acropora, and a sixth of the benthos bare — is a back-reef signature, and it is the transect's own account of the site. It is worth holding alongside the photographs, because the two most striking photographic subjects at BIDI16 are meandroid brain corals and Platygyra is one of the rarest genera on the transects here: photographic salience and transect abundance are different quantities, and the transect is the one that measures the reef. Quality is good for a flat reef: Image 1, Image 4, Image 5, Image 11, and Image 15 are wide views (e.g., Image 5 densely covered), Image 3 and Image 12 are close down-views of the substrate, Image 2, Image 6, and Image 9 are close single-colony subjects, Image 16 is hazy and strongly purple-cast with little usable benthic content, and Image 4 is distant and color-shifted.

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No scale object, quadrat, or transect line appears anywhere in the set.

The reef in these frames is flat, open, and built of small discrete colonies rather than framework. Image 1, Image 4, Image 5, Image 11, and Image 15 all look across a broad, near-horizontal bottom of consolidated white carbonate pavement, coralline rubble, and clean sand, crusted pink and mauve, carrying scattered to moderately dense small colonies — corymbose and branching Acropora, brown Pocillopora, olive plating Montipora or Porites, domal Porites, cup corals, low foliose plates, dark green to olive encrusting sheets, and free-living discs lying loose on the substrate — each colony separated from the next by bare crusted rock rather than merging into continuous framework. Relief is minimal: almost nothing in the set rises more than a colony's height above the bottom, and Image 4 shows the plain running away flat to the limit of visibility. Image 3 is the best single illustration — a downward view resolving perhaps forty distinct colonies of a dozen growth forms in one frame, individually separated by bare pavement, and a useful image for any colony-density, colony-size, or recruitment analysis. Image 12 is the same substrate seen close, and Image 13 adds large lobed and columnar Porites — the genus the transects say carries 73% of the coral here. Image 12 also holds a close study of a broad olive-green encrusting-to-submassive sheet with polyp detail visible and a small dead patch at its center.

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The site's outstanding photographic subjects are its brain corals. Image 6 and Image 9 are near-spherical meandroid colonies, dark chocolate-brown with tightly and evenly sinuous valleys, each sitting isolated on clean white sand and photographed from close to directly above. The pale substrate acts as a natural backdrop. Two other single-colony subjects stand out: Image 2, an open-branched brown acroporid photographed from above beside a dark green corymbose colony, and the large olive-green encrusting-to-submassive lobate colony spreading across the pavement in Image 12.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 27%, 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 48% of the benthos. Surveyed 14 March 2015 at Danger Island on the western Great Chagos Bank (6.34088°S, 71.25667°E; station CTD temperature 29.2°C, salinity 34.3).
• Live hard coral 27% against algal cover 48% of the benthos, across 23 scleractinian genera plus the hydrocoral Millepora.
• Transect mean 27 ± 2.0% 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 30% (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.
• No percentage is derived from either observation, and neither qualifies this site's published bleached fraction, which 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 revised by a photograph.
• Elsewhere the small white cup-shaped objects recurring on the pavement in Image 3, Image 12, and Image 13 are unresolved — cup corals, ascidians, or foraminifera.

Stressors

Image 3 contains, at lower center, a small bright bone-white branching colony sitting on the pavement among fully pigmented neighbors, unmistakable against them. Image 2 shows an acroporid whose branches read as uniformly brown with a few small white flecks and patches on the branch surfaces. Both are recorded as seen. No storm rubble, no cyanobacterial mat, and no crown-of-thorns scar appears in the set. Anderson's sampling sheet logs one of the two colonies sampled here as 10% bleached (see Dive record below). The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and no recently dead coral.

Fish and other fauna

The site is quiet, consistent with a sheltered back-reef. Snappers or rudderfish hang in mid-water in Image 16, a hazy, purple-cast frame, alongside an angelfish; damselfishes and small dark planktivores work the colonies in Image 3 and Image 13; and a spotted holothurian lies on the pavement in Image 15.

Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. Within 5 km, one independent station, XL Catlin Seaview Survey at transect 36002, 4.98 km away, read 25% in 2015.

Dive record

Anderson sampled two pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora sp. ChJo47 (collected 10:39 at 19.8 m; healthy) and ChJo48 (10:47, 20.8 m; 10% bleached, P. verrucosa or P. meandrina, bleached part sampled). The dive notes record a very gradual slope, clear water, and a current, 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

Thirteen habitat frames from the 14 March dive; no sampled-colony frames exist for this site, so this is a habitat account throughout. Two divers with bubble trails cross the blue in Image 8 above the same flat pavement.

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There is no scale object, quadrat, or transect line in any frame, so no colony dimension here is measured. Image 16 is hazy enough that its bottom carries little usable benthic information and Image 4's distance and color cast make condition unreadable beyond the foreground. The frames cannot establish depth zonation, cannot be used to check the algal fraction, and cannot substitute for the transect in any respect.