Chagos Archipelago - Great Chagos Bank West (BITB38)

General benthic features

The assemblage is led by Echinopora (14%), ahead of Acropora (10%), Heliopora (8.8%), and Turbinaria (5.8%). D2 is the low-cover, algae-dominated interval: 34% coral against 57% algae, with Heliopora at 18%, turf at 36%, and Acropora down to only 2.4%. D3 rises to 69% coral, split among Echinopora 13%, Acropora 13%, Heliopora 8.6%, and Turbinaria 4.6%. D4 reaches 75% coral and is a different reef again – Echinopora 42% and Turbinaria 20%, with Acropora at only 4.7% and algae down to 12%. Two D4 transects are extraordinary: 88% and 94% coral, composed almost entirely of Echinopora (88% and 78%, with 16% Acropora in the latter). The site therefore runs a pronounced D1-to-D4 transition from Acropora dominance, through a Heliopora-and-algae interval, to an Echinopora/Turbinaria-dominated deep stratum.

Echinopora, Heliopora, and Turbinaria together make up 28 percentage points of the benthos, 49% of all coral cover on the transects.

The reef photographs as a plate reef, and the photographic plate fields are the visible expression of the deep Echinopora/Turbinaria-dominated stratum in the transect data. Frame after frame – Image 4, Image 11, Image 25, Image 33, Image 37, and Image 44 – shows the substrate carpeted edge to edge in large overlapping foliose and whorled plate colonies, gray-green to tan, imbricated like roof tiles down the slope with almost no bare rock between them; Image 25 and Image 33 are near-continuous plate fields.

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Separately and just as consistently, the shallower reef top is a table-Acropora reef: Image 5, Image 10, Image 20, Image 41, Image 45, and Image 48 show broad flat tables and plating sheets, singly and in overlapping ranks, over pale pavement and sand.

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Mixed through the set are branching and corymbose Acropora in mauve, cream, and gold (e.g., Image 2, Image 10, Image 28, Image 43), digitate and bushy colonies (e.g., Image 8, Image 26), tan and yellow massive domes including large smooth Porites-type heads (e.g., Image 14, Image 16, Image 18, Image 23, Image 28), yellow-green leathery soft corals (e.g., Image 4, Image 46), a large-corallite massive with regular pale calices in Image 12, and a Lobophyllia-type brain in Image 41.

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Pink and maroon coralline crusts, red and orange encrusting sponges, and dark boring-sponge-pitted pavement fill the gaps. Image 46 is the single densest benthic frame in the photo set – leathery soft corals, ruffled plates, cream corymbose Acropora, and small branching colonies packed together with essentially no open substrate visible.

The reef is not uniform, and the photographs show it. Image 15 is the counter-frame: camera equipment resting on its tray on pale rubble and sand among widely spaced coral heads. Image 31 and Image 39 are similarly open frames of pale rubble and sand carrying only scattered colonies, and Image 20 and Image 23 set their tables on broad sand.

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Structure includes a substantial overhang or ledge system – Image 3, Image 6, Image 29, and Image 49 work its dark face, with Image 29 placing a diver against it for scale.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 58%, 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 34% of the benthos.
• Twenty formal transects give the site estimate: 58% coral, 34% algae, 6.2% barren substrate, and 2.7% non-coral invertebrates.
• Transect mean 58 ± 5.6% SE (n = 20 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 56% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The 58% site mean hides a strong depth turnover, and the depth-stratum breakdown is the most important detail at this site. D1 averages 46% coral and is Acropora-led at 34%.
• Replication is sufficient to treat that turnover as real, and the same data show the reef is patchy within strata. There are 20 transects here, not a single low-effort snapshot.
• Individual D2 transects nevertheless range from 8.0% to 61% coral, and D3 from 33% to 92%; two D4 transects are at or above 88% coral while the other two are 57–60%.
• The single 58% mean legitimately describes the total survey, and the within-stratum range is what tells you the reef is a mosaic rather than a uniform plate carpet.
• Both the dense plate fields and the open rubble plain are real features of this reef.
• 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

No crown-of-thorns starfish, disease lesion, or clear corallivore predation scar is visible in any of the 44 frames reviewed. The 3.9% mean dead-coral mask and 5.1% rubble estimate are consistent with old exposed framework and the open rubble and sand patches, not an acute mortality front. Published bleaching is 7.6% for this 21 March dive (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). Across 44 of the 45 frames on file, the overwhelming plate, branching and massive assemblage is pigmented brown, olive, green, blue-gray, cream, and mauve. Pale plate rims and pale branch tips recur throughout, but they follow colony margins and growing tips on otherwise pigmented colonies, which is what active growth looks like, and their regularity is also compatible with image lighting. Two frames deserve a narrower note without being generalized. Image 46 contains several pale or partly white branching colonies amid a dense, otherwise pigmented assemblage; recent branch mortality, local paling, natural pale color, and flash cannot be separated confidently. Image 30 is a close small branching colony whose branch ends are cream-white with visible polyps over tan-brown inner branches: partial paling. Image 31 and Image 39 include chalky pale patches on the rubble plain that read as old exposed framework rather than as an acute front.

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These are individual, local observations. No bleaching percentage is estimated from these frames, and none is derived from or tested against them – Anderson's published per-site percentage bleached is the authoritative figure for this reef. Anderson's sampling sheet records seven colonies sampled here: Pocillopora acuta (confirmed by RFLP) CHPd34 (15.6 m, 5% pale or bleached); Seriatopora hystrix ChSe17 (healthy), ChSe18 (sickly, smothered by macroalgae), ChSe19 (pale), ChSe20 (healthy), ChSe21 (fairly pale), and ChSe22 (large, healthy, dark pigmentation). 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). The Living Oceans Foundation transects here recorded under 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

About ten frames (e.g., Image 3, Image 9, Image 14, Image 16, Image 21, Image 29, Image 35, Image 42, Image 47, Image 49) are dominated by baitfish or fusilier schools and give only partial substrate; Image 6, Image 17, Image 27, and Image 32 are close ledge and fauna frames.

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The animal record. Image 17 is a fine close frame of a lionfish, fins fully spread against a crimson sponge- and coralline-crusted crevice beside a feather-duster worm crown; the species cannot be identified from the frame. Image 27 and Image 32 show a den containing at least six large spiny lobsters – striped legs, long pale antennae, and patterned carapaces, with orange sponges and glassfish around them – compatible with painted spiny lobster but retained as a morphological compatibility; Image 3, Image 6, and Image 22 establish the overhang and the approach to it.

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The water column repeatedly becomes a moving wall of small silversides or baitfish: Image 3, Image 9, Image 14, Image 16, Image 21, Image 29, Image 35, Image 42, Image 47, and Image 49, thick enough in several frames to obscure the reef behind them. Image 4, Image 18, and Image 40 add close, sharply lit schools of silver and blue-finned trevallies over the reef.

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A red-orange grouper sits in the dense reef top of Image 36, and butterflyfishes, damselfishes, anthias, and a banded damselfish appear in Image 8, Image 10, and Image 28.

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These are conspicuous-fauna observations, not abundance estimates.

Comparisons with other data

Thriving Oceans surveyed a nearby station (GCB South Brother Lagoon) in 2015, 2018, and 2021, 0.37 km from the pin; we could not confirm that it is the same reef. Cover values from that work are not among the survey records shown on this page. Within 5 km, two independent stations recorded station-year means of 21–41% hard coral cover between 2006 and 2015; the nearest of these, XL Catlin Seaview Survey at transect 36004, 2.45 km away, read 21% in 2015.

Other notes

All forty-four habitat frames were reviewed. Anderson's sampling sheet calls it a "stunningly beautiful site, calm, good viz", and the imagery bears that out – mostly well-lit, sharp mid-range, and wide benthic views with good color. Setting aside the fish-dominated and close ledge frames (see Fish and other fauna) still leaves roughly 28 genuinely informative benthic frames. No scale bar appears in any frame, so no colony dimension here is measured.

Survey apparatus is visible, tying the photographs to the transect survey. A yellow transect tape runs across three frames – Image 46 (across the top), Image 35 (along the bottom), and Image 9 (across mid-frame) – and Image 15 shows a camera housing on its tray standing on the rubble. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (bank 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 north side of South Brother, facing north (about 10°): windward (close to the side-on cutoff) for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November; the sheet's 'leeward' matches the trade-wind reading.