Chagos Archipelago - Great Chagos Bank West (BITB37)

Setting

Reef type: fringing reef, windward on Anderson's field sheet; outside the lagoon.

General benthic features

Opposite to Acropora, which falls to 0.7% in D4, Porites rises from 5.4% in D1 and 6.3% in D2 to 12% in D4. The deepest stratum is also more open: barren substrate rises to 15% and sediment-laden turf to 11%, against 1–2.4% barren in D1–D3. Thus “40% coral” averages an Acropora-dominated shallow/mid-depth terrace and a more Porites-led, sediment-retaining deeper surface. D1 and D4 rest on one and three transects respectively, so the direction is strong but the endpoints should not be over-generalized, and the four-stratum structure should be retained whenever the single 40% mean is interpreted. The site-level genus list numbers 29 non-zero genera (LOF BIOT-1 survey, the mean of all of this station's transects): Acropora 14%, Porites 7.8%, Pocillopora 4.3%, Montipora 3.4%, Leptastrea 1.5%, Favites 1.1%, Pavona 1.0%, Favia 0.9%, Goniastrea 0.8%, Astreopora 0.7%, Psammocora 0.6%, Symphyllia 0.6%, Gardineroseris 0.5%, Leptoseris 0.5%, Millepora 0.5%, Echinopora 0.4%, Heliopora 0.3%, Montastrea 0.2%, Cyphastrea 0.2%, Platygyra 0.2%, Coscinaraea 0.2%, Stylophora 0.1%, Pachyseris 0.1%, Hydnophora 0.1%, Cantharellus 0.1%, Galaxea 0.1%, and Oulophyllia, Lobophyllia, and Plesiastrea at <0.1% each. The blue coral is Anderson's own determination: the field data are Heliopora, determined by A. Mayfield (expedition coral-physiology lead). Algae is mostly crustose coralline algae 16%, turf 10%, and sediment-laden turf 4.3%, with macroalgae 1.3% and erect algae 1.3%; barren substrate is only 4.4%, and non-coral invertebrate cover (PITS) is 22%. The set moves through two distinct settings. Part of the set (e.g., Image 1, Image 5, Image 6, Image 9, Image 10, Image 12, Image 15, Image 16, Image 20, Image 21, Image 22, Image 23, Image 25, Image 26, Image 28, Image 31, Image 33, Image 34, Image 35, Image 36, Image 38, Image 39, Image 46, Image 48, Image 50, Image 51, Image 54) works a steep outer wall and its upper flank: near-vertical rock stepping down into blue water, shaded overhangs and recesses, and a veneer community of encrusting and small plating stony corals, mauve and lavender branching colonies, pink and maroon crustose coralline algae, red and orange encrusting sponges, cup corals along an undercut lip (e.g., Image 46), and slender pale-yellow gorgonian sea fans standing off the wall (e.g., Image 6, Image 23, Image 25, Image 51).

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Image 31 is the cleanest wall view: overlapping tan and green plates, small digitate and corymbose colonies, massive and meandroid heads and vivid pink CCA descending along an exposed face. On the reef top and terrace (e.g., Image 43) the character changes completely: a near-continuous, structurally complex carpet of stony coral, dominated frame after frame by Acropora-compatible forms — broad flat tables, stacked and overlapping plates, dense corymbose and digitate thickets in olive, tan, mauve-pink, and a distinctive chartreuse green — interspersed with rounded massive Porites-compatible domes, a large yellow-green massive with coarse separated corallites (e.g., Image 2), bushy Pocillopora-type colonies, foliose and encrusting forms, and open patches of bare carbonate and turf-covered pavement between colony clusters.

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Image 52 is the defining wide view, with Image 3, Image 8, and Image 11 the strongest tabular-Acropora frames and Image 30 the strongest thicket frames.

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Photographic genus calls remain morphological compatibilities: plates compatible with Acropora/Montipora/Pavona, massive and lobate colonies compatible with Porites, verrucose bushes compatible with Pocillopora, and meandroid or honeycomb colonies compatible with several faviid lineages. The photographs are illustrative rather than a fish census. Image 4 and Image 24 are hazy or heavily color-cast distance views.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 40%, 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.
• The survey effort behind it is 17 transects across four depth strata (D1 n=1, D2 n=7, D3 n=6, D4 n=3), and the formal benthic breakdown reads 40% coral, 33% algae, 4.4% barren substrate, and 22% non-coral invertebrates.
• Transect mean 40 ± 4.0% SE (n = 17 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 39% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The formal transects resolve a real depth turnover hidden by the site mean. D2 is the coral maximum at 48%, with Acropora alone at 27%; one 100-point D2 transect reaches 79% coral and 64% Acropora.
• D3 falls to 36% coral, and D4 to 31%, while Acropora declines to just 0.7% in D4.
• 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

Image 39 shows a collector urchin heavily covered with algae and debris — not a crown-of-thorns starfish, and long-spined urchins appear on the pavement in Image 49.

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No crown-of-thorns starfish, disease lesion, or clear corallivore predation scar is visible in any of the 78 frames. Anderson's published percentage bleached for this site is 5.82% (% bleaching (2nd training), Mayfield & Dempsey 2026 supplement); no bleached fraction is estimated from these photographs. Anderson's sampling sheet records the three pocilloporid colonies sampled here for a collaborator (Pocillopora sp.): ChJo102 (collected 14:55 at 25 m; large and healthy, a flattened morphology with a green hue), ChJo103 (15:01, 20 m; 10% bleached or dead, likely P. verrucosa; the healthy part was sampled), and ChJo104 (15:14, 15.2 m; healthy, with some pale branch tips). 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). What the frames themselves show is this: across the terrace the great majority of colonies are normally pigmented through brown, olive, chartreuse, cream, tan, mauve, and blue-gray. Pale branch tips are common on branching colonies and, at most framings, cannot be separated from ordinary growth tips, flash response, and natural pigment; but Image 18 and Image 42 show two small branching colonies with brown interiors and cream-white upper branches and tips, polyps still visible on the white parts: partial paling.

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The Living Oceans Foundation transects here recorded no cyanobacteria and no recently dead coral.

Fish and other fauna

Mobile fauna. The headline in these photographs is a large, sustained aggregation of yellow-spotted trevally-type carangids working the reef edge — they fill Image 9, Image 20, Image 33, Image 34, Image 36, and Image 50, at times a hundred or more animals in a single frame. Several show dark flank blotches and yellow-green spotting; the frames are good enough to say Carangidae confidently, but no species call is made from these angles. Two separate octopus encounters are recorded: Image 54 is a close, well-lit animal resting in a wall crevice among encrusting sponges, ascidians and small corals, and Image 46 is a second out on open rock in full mottled camouflage. Reef fish are steady rather than dense: powder-blue surgeonfish, Moorish idols (e.g., Image 21, Image 26, Image 29), orange anthias in the wall shadows (e.g., Image 35), blue damselfishes over the terrace, and butterflyfishes, wrasses, and parrotfishes scattered through the reef-top frames.

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Comparisons with other data

Perry et al. (2018) surveyed a nearby station (South Brother - east, GCB), 0.25 km from the pin. Cover values from that work are not among the survey records shown on this page. Within 5 km, two independent sources recorded hard coral cover: a mean of 41% across 5 Great Chagos Bank records from 2006 (Sheppard et al. 2008; 4.2 km away) and 21% on an XL Catlin Seaview Survey photo-transect in 2015 (3.5 km). None lies within 2 km of this station, so none is compared with it here.

Dive record

Anderson's afternoon dive on 20 March 2015 entered the water at 14:43 and lasted 59 minutes, to a maximum depth of 30.2 m (the field sheet); station CTD temperature 29.6°C, salinity 34.3 (6.17699°S, 71.54402°E). The notes describe a gentle current along the wall, more than 80% coral cover at 6–10 m, sharks, jacks, octopus, and lobsters. The transects give a lower figure: the D2 depth stratum averages 48% coral, more than either other stratum, and one of its transects reached 79% (see Coral cover).

Other notes

Image 9 is a close trevally school. All 78 habitat frames were reviewed. There are no sampled-colony frames, and no quadrat, scale bar, or transect line appears in any frame, so nothing in the photographic set is a measurement. Divers appear at the wall edge for scale in Image 22, Image 28, Image 29, and Image 41.

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Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'windward'); the expedition's 933-site table classes the station as 'exposed' (fringing reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin off the south side of South Brother, facing south-southwest (about 200°): side-on (close to the leeward cutoff) for the northwest monsoon winds of the March survey and side-on (close to the windward cutoff) for the southeast trades of April to November, so the imagery reads this station as side-on rather than windward; the aspect shown is the sheet's label.