Chagos Archipelago - Great Chagos Bank West (BIDI14)

Setting

Leeward lagoonal patch reef (Anderson's field sheet) in the Danger Island group, 6.40987°S, 71.25622°E · station CTD 29.4°C · salinity 34.1.

General benthic features

Nineteen scleractinian genera are non-zero, and the reef is led by massive and small branching taxa rather than by tables: Porites 5.6% of the benthos, Pocillopora 4.5%, Acropora 3.3% — those three alone carry 13 of the 17 percentage points of live coral. The tail runs Favia 1.4%, Montipora 0.5%, Stylophora 0.6%, Galaxea 0.3%, Platygyra 0.2%, Astreopora 0.1%, Goniastrea 0.0%, Ctenella 0.1% (the Chagos-endemic monotypic genus), Echinopora 0.1%, Cyphastrea 0.1%, Favites 0.1%, Pavona 0.1%, Symphyllia <0.1%, and Coscinaraea, Leptoseris, and Pachyseris at <0.1% each. The substrate side of the same transects is dominated by crustose coralline algae at 31% — the single largest benthic category at this site, well ahead of the coral — with non-coral invertebrates 19%, turf 12%, sediment-laden turf 6.2%, bare substrate 5.2%, erect algae 5.1%, macroalgae 4.3%, and cyanobacteria 0.2%. A heavily coralline-encrusted hardground carrying widely spaced colonies is what the transect describes, and it is exactly what the photographs show. Ten of the thirteen are reef frames; three (e.g., Image 4, Image 12) are water-column images of open blue with fish in it and contribute nothing benthic. The photographic genus inventory runs to seven scleractinian groups plus one hydrocoral — Acropora (digitate/corymbose and plating), Pocillopora, Porites (massive domes), meandroid brains, Favites/Goniastrea, Montipora (encrusting), Astreopora, and smooth yellow colonies — all growth-form calls from photographs, given as “compatible with”.

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The substrate in Image 1, Image 5, Image 9, and Image 11 is a pink and mauve crustose-coralline hardground strewn with rubble, carrying widely spaced digitate and corymbose Acropora, small Pocillopora, domal Porites, meandroid brains, encrusting Montipora, and occasional smooth yellow colonies.

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Nothing here forms continuous framework; the reef reads as a mosaic of individual colonies on consolidated pavement, relief a matter of tens of centimeters, with no spur, wall, or coral-built ridge anywhere in the set. Image 13 is the most diverse single frame — pink-purple knobby and encrusting colonies, massive Porites, tan encrusting sheets, and dense coralline, a large mauve-brown corymbose colony in the foreground, and a regal angelfish in the corner. Image 2 and Image 8 each center on a substantial branching Acropora-type colony with cream-pale branch tips over darker olive-brown to red-brown bases. Image 3 is a close, sediment-dusted view of coralline-crusted pavement carrying one small bushy colony among fully pigmented yellow-green and pink encrusting neighbors.

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Image 15 is the site's methodology frame: a transect line runs clearly across the reef with a diver working in the background, and the same frame happens to hold a regal angelfish, a red-orange coral hind, a meandroid brain, smooth yellow colonies, and a large domal Porites — the survey and the assemblage in one image.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 17%, 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 59% of the benthos.
• Transect mean 17 ± 3.0% 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 15% (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.
• The small white cup-shaped objects on the pavement in Image 9 are unresolved (bleached recruits, ascidians, or molluscs) and are not scored as coral.
• Anderson's published per-site figure — 9.8% of coral bleached (CoralNet analysis of informal survey images; Mayfield & Dempsey 2026, Oceans) — is the authoritative statement of how much of this reef was affected.
• Nothing below is converted into a percentage, and no frame count stands in for that figure.

Stressors

Qualitatively, three frames carry the signal clearly. Image 3 is unambiguous — a small Pocillopora-type colony, partly bleached cream-white, sitting on CCA-crusted pavement among fully pigmented yellow-green and pink encrusting neighbors. Image 2 and Image 8 both show larger branching Acropora colonies with cream-white branch tips over dark red-brown bases — an extensive apical paling on colonies whose lower branches are simultaneously darkening. That reads as genuine early thermal paling rather than growth tips, though a single-visit photograph cannot fully separate the two.

A second condition signal is independent of bleaching: Image 6 shows a gray-purple meandroid brain coral with old partial mortality on its upper third, the dead surface crusted by pink coralline and red turf algae beside healthy tissue.

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No crown-of-thorns and no predation scar resolves anywhere in the set. Anderson's sampling sheet logs the two colonies sampled here as 5% and 30% bleached (see Dive record below). The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

A second diver appears in Image 9.

Faunally the dive is dominated by schooling planktivores and mid-water predators. Emperors or breams — silvery with the characteristic large yellow-rimmed eye — school over the reef in Image 1 and Image 13. Blue-and-yellow fusiliers fill the water column in Image 11 and Image 12, and dark-bodied snappers or rudderfish run along the deeper slope in Image 11. A gray reef shark crosses the blue in Image 4. Reef fish include the two regal angelfish noted above, Moorish idols, surgeonfishes, and the coral hind.

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, 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 36001, 1.05 km away, read 6.3% in 2015.

Dive record

Anderson sampled two pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora verrucosa ChJo40 (collected 13:57 at 20.4 m; 5% bleached, the most common coral at the site, bleached part sampled) and Pocillopora eydouxi ChJo41 (14:22, 10.5 m; a huge, unhealthy colony, 30% bleached, bleached part sampled). The dive notes record surge with decent visibility 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 survive from this dive. There are no sampled-colony photographs. Image 1, Image 2, Image 3, Image 6, Image 8, Image 13, and Image 15 are sharp close or mid-range views; Image 5, Image 9, and Image 11 are dim and blue-cast, with Image 11 hazy enough at range that condition cannot be read on its substrate. No scale object or quadrat appears in any frame — the transect line in Image 15 is not a scale bar — so colony dimensions are not measurable, and with no depth stamps the frames cannot establish the site's depth zonation. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'lagoonal patch reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (patch reef). Esri World Imagery (no LOF habitat map covers the Great Chagos Bank) puts the pin on the eastern margin of the Danger Island platform, facing the Great Chagos Bank lagoon, which agrees with the sheet's leeward lagoonal patch reef.