Chagos Archipelago - Great Chagos Bank West (BIEA19)

Setting

Submerged bank reef, leeward (Anderson's field sheet).

General benthic features

The transect genus list is the community record (LOF BIOT-1 survey, the mean of all of this station's transects) and it is a broad one — 26 scleractinian genera: Porites 7.1%, Acropora 6.2%, Pocillopora 4.6%, Favia 3.2%, Montipora 1.3%, Platygyra 1.4%, Favites 0.6%, Echinopora 0.3%, Stylophora 0.4%, Gardineroseris 0.6%, Pavona 0.4%, Montastrea 0.4%, Astreopora 0.4%, Cyphastrea 0.3%, Leptastrea 0.1%, Goniastrea 0.2%, Galaxea 0.1%, Symphyllia 0.0%, Pachyseris 0.1%, and Acanthastrea, Goniopora, Psammocora, Hydnophora, Leptoseris, Echinophyllia, and Fungia each at less than 0.1%. The substrate and algal categories run crustose coralline algae 18%, erect algae 16%, non-coral invertebrates 16%, turf 11%, bare substrate 5.8%, macroalgae 3.1%, and sediment-laden turf 1.7%. The site's defining character in the photographs is colony density without relief. Image 4, Image 8, and Image 20 show pavement packed almost edge to edge with massive and submassive Porites domes, encrusting sheets, columnar knobs, and small plates, none of them rising far off the substrate; Image 4 is the densest view in the set.

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Against that low-relief matrix sit two features that carry most of the structure. The first is the lime-green to yellow-green carpet of Image 6 and Image 14, densely packed short blunt branchlets spreading across meters of bottom — the colony whose genus is discussed under Coral cover below.

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The second is large tabular Acropora: Image 6 and Image 12 show plates up to a meter across overlapping in tiers, and Image 19 pairs a big brown table with a cream domal Porites while a checkerboard wrasse works between them. Image 19 also carries upright blade-like pale blue-white plates. Image 2 looks under the lip of a low spur or ledge, framework above and a sand-floored recess beneath, with tabular colonies and gray-brown massives ranged along it.

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At the sparse end, Image 3 and Image 13 are low, rubbly, largely open bottom with widely spaced small colonies, and Image 12 looks out over a field of overlapping tables in the middle distance that is too blue-attenuated to interpret; their gray is not evidence of death.

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That range across the frames is itself the reason the transect, not any photograph, is the site's cover value.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 28%, 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 51% of the benthos.
• Transect mean 28 ± 2.8% SE (n = 15 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 28% (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 are image products with their own footprints, denominators, and class schemes; they are reported here as a method comparison and do not qualify the transect. One genus-level disagreement inside the image products matters and is unresolved: the 30-point annotation of Image 14 scored eight of its points Acropora, while the morphology of that same cover-forming carpet colony — short, blunt-tipped, uniformly textured branchlets with no visible axial corallites — reads instead as Porites (P. rus/cylindrica-type) or Montipora. Both cannot be right, and this is a large and repeated taxon here, so the genus of that colony is left unnamed.
• Image 8 carries a thin, straight, pale line running horizontally across the upper background that is consistent with a transect tape, with a blue elongate object at the right that could be the reel; it is not confirmable at the resolution reviewed.

Stressors

Mortality is present and structural. Image 6 is the key frame: a pale mauve-white table that has lost its polyp texture and a uniformly gray, algal-filmed table beside it, both still standing in growth position, alongside a living green table. Living and dead tabular Acropora in one frame makes this a useful image for the mortality-and-persistence question. Image 9 is the site's condition frame and it is unambiguous: a frame-filling macro of a thick-branched Pocillopora-type colony that is strongly mottled, bone-white branch tips and patches interleaved with fully pigmented brown tissue across the same colony — textbook patchy partial bleaching.

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No bleached fraction is estimated from these frames, and Anderson's published per-site percentage bleached for the Chagos sites (Mayfield & Dempsey 2026, Oceans) is the authoritative number. The point annotation of Image 9 records six bleached points, six of the site's seven — an image-annotation product. The seventh point is on Image 5, a partly bleached colony: more than a dozen branch ends of a large tan digitate colony are cream-white with live tissue, paler than the rest of the colony and its neighbors.

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One cleanly snapped white branch stub in the same frame is breakage. Beyond those, colonies through the set are normally pigmented in brown, tan, green, and gray, and no disease lesion or predation scar resolves. Anderson's sampling sheet logs one colony sampled here (ChJo58, 9.6 m) as a dying colony, 60% bleached or recently dead, and another as 5% bleached (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

Sponges appear in 11 frames, sea cucumbers in 3, urchins in 1. Faunally the site is quiet but distinctive: blue-and-yellow fusiliers stream past in loose schools (e.g., Image 12, Image 13, Image 14, Image 17), a powder-blue surgeonfish — a characteristically Indian Ocean species — sits at the center of Image 6, and Image 17 holds a Moorish idol, damselfishes, dark surgeonfish, and a cloud of small silvery juveniles.

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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

Survey conditions on 15 Mar 2015 at 6.202°S, 71.3588°E: station CTD temperature 28.8°C, salinity 34.4. Anderson sampled six colonies on this dive (the sampling sheet; species are field calls): three Seriatopora hystrix, ChSe3 (collected 10:30 at 25.3 m; healthy, with very thin, delicate branches covered in brittle stars), ChSe4 (10:33, 25.3 m; healthy though pale, covered in brittle stars), and ChSe5 (10:37, 25.5 m; healthy, darkly pigmented; all three in a Halimeda meadow at 25 m, with none shallower), and, for a collaborator, three Pocillopora sp., ChJo56 (10:48, 10.5 m; healthy, with a green fluorescent hue), ChJo57 (10:54, 9.8 m; a huge, mostly healthy colony, 5% bleached, healthy part sampled), and ChJo58 (11:03, 9.6 m; a dying colony, 60% bleached or recently dead, bleached part sampled). The dive notes record a gentle current at 10 m, a well-defined slope, and no P. damicornis or P. acuta 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 transects were run in three depth strata (D2 31%, D3 26%, and D4 26% coral). All 15 habitat frames (e.g., Image 2, Image 3, Image 4, Image 5, Image 6, Image 7, Image 8, Image 9, Image 10, Image 12, Image 13, Image 14, Image 17, Image 19, Image 20) were reviewed; there are no sampled-colony frames, and these are publication-edited, post-processed frames rather than raw survey imagery.

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Two frames record the survey rather than the reef. Image 7 shows a diver at work, head-down at the substrate in black wetsuit and white gloves, bubbles streaming, with a second silver cylinder lying on the reef beside them — a spare tank or a sample container. A diver's fin is visible in the top corner of Image 20. No scale object, quadrat, or depth cue appears in any frame, so no colony dimension or zonation can be read from them. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'submerged bank reef', 'leeward'); 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 in deeper water off the southwest side of a submerged bank east of the Eagle Islands platform, in the lee of that platform for the northwest monsoon winds of the March survey, which agrees with the sheet's leeward submerged bank reef.