Chagos Archipelago - Great Chagos Bank West (BIDI10)

Setting

The site sits on the western Great Chagos Bank off Danger Island and was worked once, on 12 March 2015.

The reef reads as two different bottoms.

General benthic features

One is structural loss: Image 12 is a broad field of standing dead branching skeleton — a collapsed staghorn framework, every branch turf-covered and gray-brown, running out of frame in every direction — with pale rubble-and-pavement patches recurring through (e.g., Image 13, Image 28, Image 34).

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The other is a dense, diverse, and healthy living mosaic. Image 3 and Image 35 are the best benthic frames — dense mosaics of lilac and purple digitate Acropora, brown Pocillopora, massive and plating Porites, dark foliose Echinopora or Pavona, yellow-green tabular Acropora and pink crustose coralline algae packed together at low relief, with almost no bare rock between them.

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Image 5 and Image 25 are comparable: low-relief pavement packed edge to edge with pink, lilac, and brown digitate colonies, small corymbose tables, mustard to olive massive and lobed mounds, dark ruffled foliose plates, and heavy pink CCA crusts.

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Two zones stand apart. Image 33 and Image 34 record a distinctly foliose-dominated zone — gray-brown ruffled leafy plates covering large areas.

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Image 2 and Image 31 show enormous yellow-gold plating Porites forming tiered ledges and overhangs several meters across, big enough to shelter fish beneath the plate margin; Image 14 belongs with them and adds a third frame to that gold-plating series.

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Macro detail is limited to two frames: Image 17, an extreme close-up of coral polyps with white-tipped nematocyst batteries clearly resolved, and Image 29, a large brown branching colony filling the frame. Image 24 is a large brown meandroid brain.

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Porites was the most common coral genus on this station's transects (7.1% of the benthos), followed by Pocillopora (3.1%), Favia (2.9%), the blue coral Heliopora (2.9%), Acropora (1.6%), and Pavona (1.1%); bare substrate covered 12%, behind crustose coralline algae (21%), the largest single living non-coral group (LOF transects; all algae together 57%). Among the algal classes, crustose coralline algae led (21% of the benthos), followed by sediment-laden turf (16%) and turf (12%).

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 23%, 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 57% of the benthos.
• The transects were run in three depth strata (D2 14%, D3 35%, and D4 20% coral).
• Transect mean 23 ± 4.8% SE (n = 16 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 23% (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.

Stressors

Bleaching is not a feature of this frame set: no colony in any of the 31 habitat frames resolves confidently as bleached. Anderson's published per-site % bleached for BIDI10 is 2.3%; it comes from CoralNet analysis of the expedition's survey images (Mayfield & Dempsey 2026, suppl. S1), not from a field estimate. The pale surfaces that do appear in Image 8, Image 13, and Image 34, and the large pale, tiered plating colony in Image 19, read as coralline-crusted pavement or older mortality rather than recent whitening, and the frames cannot settle that distinction at these ranges.

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Two other condition signals are logged. Image 24 shows a clean white lesion running across the surface of the brain coral — a band of exposed skeleton with live tissue either side, consistent with disease, predation scarring, or physical damage and not separable from a single frame. Given the dead standing framework in Image 12 and the rubble fields, the long-spined urchins on the open pavement (five or more together in Image 4) are worth recording: they are bioeroders and grazers, and their abundance is a variable in how this reef's framework is being reworked.

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Anderson's sampling sheet records one colony sampled here, a Pocillopora eydouxi by field call (ChJo31, 9.5 m) about 2 m across and rare 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). The Living Oceans Foundation transects here recorded 0.5% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

Faunally the site is dominated by planktivores and their predators. Image 21 is the dive's spectacle: a dense wall of tiny silver baitfish — cardinalfishes or silversides — streaming across the reef with dark snappers and a large jack cutting through them, with comparable swarms in Image 10 and Image 15.

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Blue fusiliers fill the water column in Image 6, Image 19, Image 20, Image 22, Image 23, and Image 27.

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Emperors and snappers school through (e.g., Image 7, Image 16, Image 18, Image 32).

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Reef fish include powder-blue surgeonfish (e.g., Image 32, Image 35), parrotfishes (e.g., Image 8), butterflyfishes, and red squirrelfishes and bigeyes. And long-spined urchins are conspicuous — five or more together on open pavement in Image 4, and again around the lesioned brain in Image 24.

Comparisons with other data

Perry et al. (2015) surveyed a nearby station (Danger Island, Great Chagos Bank) in April 2015, 0.22 km from the pin. Within 5 km, two XL Catlin Seaview Survey stations recorded station-year means of 6.3–25% hard coral cover in 2015; the nearest of these, XL Catlin Seaview Survey at transect 36002, 1.64 km away, read 25% in 2015.

Other notes

Thirty-one habitat frames exist for BIDI10, all from 12 March 2015; there are no sampled-colony frames. Three frames are not benthic accounts — Image 23 is essentially open water with fusiliers, Image 7 is mostly blue water over distant reef, and Image 17 is an extreme macro — leaving 28 with substrate, of which roughly 22 are genuinely usable for community description. No quadrat, tape, or scale object appears in any frame, so no colony dimension can be measured from them.