Chagos Archipelago - Great Chagos Bank West (BIEA26)

Setting

Reef type: fringing reef, leeward from its position on the north side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (windward on Anderson's field sheet).

General benthic features

The transect genus list makes this a Porites-led reef (LOF BIOT-1 survey, the mean of all of this station's transects): Porites 14% — more than half of all coral cover at the site — then Pocillopora 3.0%, Montipora 2.2%, Acropora 1.0%, Pavona 0.7%, Ctenella 0.5%, Favia and Goniastrea 0.3% each, Favites, Astreopora, and Cyphastrea 0.2% each, Lobophyllia, Symphyllia, Pachyseris, Leptastrea, Alveopora, Montastrea, Psammocora, Gardineroseris, and Galaxea 0.1% each, and Fungia, Cantharellus, Leptoseris, Goniopora, and Platygyra <0.1% each — 25 genera in all. The low Acropora figure (1.0%) is a transect result, not a framing accident of the photographs. The substrate and algal categories run crustose coralline algae 22%, bare substrate 19%, turf 14%, erect algae 5.9%, sediment-laden turf 9.2%, non-coral invertebrates 4.7%, macroalgae 1.3%, and cyanobacteria 0.3%. In Image 1, Image 2, Image 5, Image 7, Image 8, Image 9, Image 10, Image 12, Image 13, Image 17, Image 18, Image 19, Image 20, Image 23, Image 24, Image 29, Image 33, and Image 36 the camera works a dense, structurally complex reef top — a mosaic of small, tightly packed colonies on CCA-crusted pavement under clouds of anthias and fusiliers. Image 6, Image 12, Image 13, Image 18, Image 29, and Image 36 show dense benthic cover.

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Coral occurs as patches and clusters rather than a continuous carpet: rounded and lobed massive domes compatible with Porites, cream and tan digitate and nodular thickets, encrusting and small foliose forms, bushy Pocillopora-type colonies and scattered small branching colonies in mauve, purple, and olive, among broad expanses of pale exposed carbonate, rubble and low turf with pink and maroon coralline crusts on the harder surfaces. Tabular Acropora is scarce in these frames — consistent with the transect's 1.0% Acropora. Image 8 and Image 24 hold the most striking piece of structure in the set, a large gray-white foliose colony perched on a bommie with crinoids and red sponge beneath it, and Image 10 is an isolated foliose or leathery colony standing alone on white sand. Then the dive crosses onto a flat, pale, low-relief plain of consolidated pavement, rubble, and coarse sand carrying only scattered small colonies — Image 14, Image 15, Image 26, Image 27, Image 30, Image 32, Image 34, and Image 35 are all that same barren ground.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 24%, 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 52% of the benthos.
• Transect mean 24 ± 3.2% 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 21% (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.
• Three frames are diver portraits: Image 21 is a clean overhead of a diver carrying the white banded transect pole across a Porites-boulder pavement; Image 4 repeats it with three divers; Image 25 shows two more.
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Stressors

Survey conditions on 17 Mar 2015 at 6.17221°S, 71.33191°E: station CTD temperature 29.2°C, salinity 34.5. Anderson's published percentage bleached for this site is 8.61% (% bleaching (2nd training), Mayfield & Dempsey supplemental file S1). Two frames stand outside the survey. Image 19 shows a bright green St Patrick's Day hat, shamrock hatband and all, set down on top of a living Pocillopora-type colony against white sand. A prop rather than equipment, recorded because it is resting directly on live tissue. Colonies through these frames are normally pigmented in brown, olive, cream, tan, and mauve; the wide pale areas read as old exposed carbonate and turf-covered pavement rather than as recently bleached tissue, and no colony in the set is unambiguously white. No disease lesion or clear predation scar resolves at these ranges, and no crown-of-thorns is present in any frame. The photographs do not measure this site's bleached fraction and none is estimated from them: Anderson's published 8.61% (Mayfield & Dempsey supplemental file S1) is the authoritative number. Four of the 13 points the photo annotation scored as bleached sit where the frames do not support them (two bleached-Acropora points on Image 10, a frame holding one brown laminar colony on white sand with no Acropora in it, and two on Image 4, a 70%-water diver frame; Image 26 and Image 27 carry three more on turf pavement). Anderson's sampling sheet records two Pocillopora colonies sampled here, ChJo75 (26.2 m) and ChJo76 (9.5 m), both large and healthy. Anderson's sheet records no P. damicornis or P. acuta at this site; these colonies were collected for a collaborator. Pocillopora acuta colonies from elsewhere in the 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 0.3% cyanobacteria and no recently dead coral.

Fish and other fauna

Sponges appear in 21 frames, urchins in 1, and anemones in 1. The macrofauna is notable, and nearly all of it is in the second half. Four frames hold sharks. Image 15, Image 26, and Image 32 appear to show one animal: broad-headed, dorsals set well back over the pelvics, caudal long and low — compatible with a tawny nurse shark; Image 32 is the clearest. Image 27 shows a slimmer animal with a more forward first dorsal, plausibly a whitetip reef shark and probably a second individual, but neither call is safe from these frames, so both are left hedged. A large olive-yellow, densely speckled moray compatible with the giant moray appears in Image 17 and again in Image 2 and Image 20; whether that is one animal or two is not resolvable. Image 30 and Image 35 are portraits of a single mauve-pink, orange-eyed snapper. Also a jack or trevally (Carangidae) over the barren pavement in Image 14, a strongly magenta-cast frame, raccoon-type butterflyfish in Image 6 and Image 22, a powder-blue surgeonfish in Image 34, and dense anthias over almost every reef-top frame — thickest in Image 6, Image 13, and Image 24.

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Comparisons with other data

Perry et al. (2015) surveyed a nearby station (Eagle Island north, Great Chagos Bank) in April 2015, 0.24 km from the pin. Cover values from that work are not among the survey records shown on this page. Within 5 km, one independent station, XL Catlin Seaview Survey at transect 36003, 4.89 km away, read 13% in 2015.

Other notes

The transects were run in three depth strata (D2 29%, D3 23%, and D4 11% coral). All 32 habitat frames were reviewed (the second dive of 17 March). No quadrat and no scale bar appear anywhere in the set; the only measuring object is the white banded transect pole a diver carries in Image 21. Anderson's field sheet records the aspect (LOF fellowship 2016 workbook: 'fringing reef', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (fringing reef). The aspect shown, 'leeward', comes from the station's GPS position on the north side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (about 145° off the upwind direction; the reef-edge reading agrees), which takes precedence over slope facing, lagoon shelter, imagery, and field labels. Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the north side of Eagle Island near its northeastern end, facing north-northwest (about 330°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November.