Chagos Archipelago - Great Chagos Bank West (BITB27)

General benthic features

Acropora was the most common coral genus on this station's transects (13% of the benthos), followed by Porites (8.4%), Pocillopora (4.1%), Montipora (2.0%), Favia and Astreopora (1.0% each), Goniastrea and Pachyseris (0.9% each), and Pavona (0.8%); bare substrate covered 12%, behind turf and crustose coralline algae (17% each), the largest single living non-coral groups (LOF transects; all algae together 48%). Among the algal classes, crustose coralline algae and turf led (17% each), followed by erect algae (5.7%) and sediment-laden turf (5.5%). Where the reef is not plated it is branched, and densely so (e.g., Image 14, Image 44).

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One frame is the highest-cover branching frame — golden-tan branching Acropora filling the foreground, with a single rounded Porites-type dome and tabular plates behind (e.g., Image 14). One frame holds arborescent staghorn thickets with bright pink encrusting sponges and dark gaping bivalves set into the framework; another carries the site's most distinctive colony, a large purple-mauve branching Acropora beside a tan tabular plate; a third is a mixed stand of at least four Acropora forms with knobby brown Pocillopora-type clumps at its base; and five frames all carry meandroid brains compatible with Platygyra tucked at the foot of the plates (e.g., Image 16).

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Massive Porites is constant but never dominant — domes in four frames, and a knobby-lobed sheet across the foreground of another (e.g., Image 39, Image 40).

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One taxonomic note needs recording. Anderson's field sampling record states that neither Seriatopora spp. nor P. acuta/P. damicornis were present here, and that other pocilloporids were collected instead for a collaborator. 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 frames are consistent with that — the pocilloporid material visible in three frames (e.g., Image 28, Image 39, Image 40) is blunt and knobby, compatible with a P. verrucosa-type habit rather than the fine compact damicornis/acuta form, and nothing reads as Seriatopora.

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This is a hedged photographic impression, not a survey result.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (the mean of all of this station's transects), against total algal cover (all algal groups) of 48% of the benthos.
• Transect mean 36 ± 4.9% 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 37% (Mayfield & Dempsey 2026, Oceans, supplemental 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.

Stressors

Most of the mortality present is old. The dominant structural loss is long-dead skeleton already resurfaced in pink and mauve CCA — maroon-crusted branching frameworks in two frames, a CCA-plated dead table in another, algal-toned plates in two more (e.g., Image 11).

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That is ordinary plate-reef turnover, not an event. Two frames need separate attention. Image 19 is a macro of a massive Porites-type surface with partial paling: pigmented brown polyps still stand out against a coenosarc gone stark white across the right two-thirds of the frame, with normal tan tissue at the left edge; the tissue is present, so this is paling rather than recent mortality.

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The other shows a cream-white massive dome among normally-pigmented neighbors. The dome cannot be resolved photographically as bleaching versus a pale morph versus recent partial mortality; the published 0.35% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026) remains this site's only quantitative bleaching statement. The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

The fauna is good but not the story. The water column is worked continuously by fusiliers (six frames) and by schools of dark surgeonfish and rudderfish (four frames) (e.g., Image 3, Image 9, Image 20, Image 21, Image 29, Image 30, Image 37, Image 48).

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On the reef itself: powder-blue surgeonfish in five frames; moorish idols in two frames; red soldierfish under plates in two frames; blue chromis, cleaner wrasse, and butterflyfish throughout (e.g., Image 28, Image 42).

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The best macrofauna frame holds a large dark grouper, white-spotted with pale saddle bars, resting on pavement beside a tabular plate, compatible with a dark-phase blacksaddled coralgrouper but not callable from this frame (e.g., Image 41).

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Conspicuous fauna and sessile invertebrates appear as sponges in 12 frames, giant clams in 1 frame, sea cucumbers in 1 frame, survey gear in 2 frames.

Comparisons with other data

Perry et al. (2015) surveyed a nearby station (Middle Brother, Great Chagos Bank) in April 2015, 0.13 km from the pin. Within 5 km, two independent stations recorded station-year means of 21–41% hard coral cover between 2006 and 2015; the nearest of these, Sheppard et al. (2008) at Great Chagos Bank, 0.42 km away, read a mean of 41% in 2006.

Other notes

Only six of the 45 frames are measurement frames. One carries a transect line running diagonally across a tabular plate; another a small white rectangular frame on a plate in the mid-background; a third a large white PVC quadrat frame in the immediate foreground over sparse pavement; a fourth is the collection frame — a diver with a punch/hammer and a yellow mesh collecting bag, a slate diver behind, and a vertical downline; and the last two are close-up "spot" frames, pavement patches of pink CCA and small recruits composed for settlement or size measurement rather than cover (e.g., Image 8, Image 15).

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The other 39 frames are ordinary wide-angle survey imagery. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'fringing reef', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (fringing reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the west side of Middle Brother, facing west (about 265°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November.