Chagos Archipelago - Great Chagos Bank West (BIDI15)

General benthic features

Eighteen scleractinian genera were recorded, and this is a Porites reef, not an Acropora reef: Porites alone holds 13% of the benthos — 63% of all the live coral at the site. Pocillopora (2.4%) and Montipora (2.1%) form the only other meaningful components. Everything else is a long tail of half a percent or less: Stylophora 0.5%, Pavona 0.3%, Acropora only 0.3%, Goniastrea, Fungia, and Favites 0.3% each, Platygyra 0.2%, Astreopora, Leptastrea, Echinopora, Turbinaria, and Cantharellus 0.1% each, and Favia, Gardineroseris, and Leptoseris <0.1% each. The non-coral benthos is dominated by bare substrate at 19% — the largest single category — followed by invertebrates 18%, crustose coralline algae 14%, erect algae 12%, turf-plus-sediment 10%, turf 3.8%, and macroalgae 2.5%. BIDI15 is a windward (Anderson's field sheet), low-relief fore reef of eroded pavement and rubble, and the wide frames are consistent and unflattering. Image 1, Image 3, Image 7, Image 8, and Image 10 all show the same country: pale, largely bare carbonate hardground strewn with coarse rubble and open sand, crusted with turf and pink, mauve, and magenta coralline algae, across which small and well-spaced colonies are scattered — low tan and olive knobby heads, olive-green foliose and plating colonies compatible with Pavona or Echinopora, domal Porites, a broad pale tabular colony low in Image 8, a large porous olive-yellow dome compatible with Astreopora at the left edge of Image 3, and small meandroid massives.

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Image 7 in particular is a broad plain in which live coral is a clear minority of the surface, and almost nothing in these frames stands more than a colony's height above the substrate. Two features genuinely distinguish this site in the imagery. First, large gorgonian sea fans — yellow-brown and finely branched — stand clear of the pavement in Image 1, Image 6, and Image 8, a growth form invisible to a hard coral point or mask scheme.

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Second, and more striking, BIDI15 has large, well-photographed brain corals. Image 2 is a large, near-spherical meandroid colony filling most of the frame, its valleys tightly and evenly sinuous, flanked by a porous yellow-green dome compatible with Astreopora and by pale plating colonies over pink-crusted rock. Image 13 and Image 14 then go closer still — two extreme macros resolving individual polyps and extended tentacles along the valley walls.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 20%, 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 42% of the benthos.
• The transects were run in two depth strata (D3 20% and D4 20% coral), and there are no per-frame depth data.
• Transect mean 20 ± 1.6% SE (n = 11 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 20% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• It is worth registering that the transect ranks the meandroid brains — Platygyra at 0.2% — as a very minor component of cover even though they are the photographic highlight of the photo set; the frames and the transect are answering different questions.
• 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.

Stressors

Station CTD conditions on 14 March 2015 were 29.2°C at 34.6 salinity. Image 13 and Image 14 are high-magnification coral images. Their orange-to-teal gradient is a strobe and white-balance artifact, not natural pigmentation, so those two frames say nothing about color or condition. Anderson's published per-site bleached fraction for BIDI15 is 8.8% (CoralNet analysis of the informal survey images; Mayfield & Dempsey 2026). The imagery is not a measurement of it, and no bleaching percentage was read off these frames. Qualitatively: no colony in any of the ten frames is white or obviously paled — the brain corals, the Porites domes, the foliose plates, and the gorgonians all read as normally pigmented, with the two extreme macros excluded on color-artifact grounds. That is a statement about ten opportunistic frames, three of them macros of a single colony and one of them empty water. The pale tan mound and whitened surrounding substrate in Image 10 are the only ambiguous feature, and read more as old eroded pavement and coralline crust than as recent bleaching; that is reported as seen and diagnosed no further. Anderson's sampling sheet logs one colony sampled here as 25% bleached, one as 50% dead, and one as 5% dead or bleached (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

The dive is quiet — no schooling aggregations, only scattered damselfishes and small reef fish over the pavement and a scatter of planktivores in the blue above (e.g., Image 7, Image 10). The exception is a whitetip reef shark cruising low over the sand and pavement at mid-distance in Image 6.

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, 3.04 km away, read 25% in 2015.

Dive record

Anderson sampled five pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora sp. ChJo42 (collected 08:32 at 20.1 m; a large, sickly colony, 25% bleached, likely P. verrucosa, bleached part sampled), ChJo43 (08:39, 20.2 m; a large, healthy colony with P. eydouxi-like branches, likely P. verrucosa or P. meandrina), Pocillopora eydouxi ChJo44 (08:47, 18.2 m; small and healthy), and Pocillopora sp. ChJo45 (08:56, 16.0 m; 50% dead, likely P. meandrina or P. verrucosa) and ChJo46 (09:01, 15.8 m; 5% dead or bleached, healthy part sampled). The dive notes record clear water, a current, 29°C, a very gradual slope, 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

The photographic record is ten habitat frames; there are no sampled-colony frames. The effective reef-scale sample is six frames, since three are macros of one colony and one is open water. Image 4 is open blue water with no subject and no substrate.

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No quadrat, transect line, diver, or scale reference appears in any of the ten frames, so no colony dimension here is measured. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'submerged fore reef', 'windward'); the expedition's 933-site table classes the station as 'protected' (bank reef). Esri World Imagery (no LOF habitat map covers the Great Chagos Bank) puts the pin on the western, open-ocean half of the Danger Island platform north of the island, facing west (about 275°): windward for the northwest monsoon winds of the March survey (December to March), though it would be leeward for the southeast trades of April to November.