Chagos Archipelago - Great Chagos Bank West (BIDI08)

Setting

Station CTD conditions on 12 March 2015 were 28.4°C at 34.5 salinity.

General benthic features

The transect's genus breakdown is the most detailed account of this reef. Twenty-eight scleractinian genera were recorded. Acropora alone holds 34% of the benthos — 56% of all the live coral on the site, and it is the form that builds the reef. Porites follows at 9.9%, then Favia and Montipora 2.9% each, and Pocillopora 2.5%. A well-populated third tier carries Platygyra 1.1%, Gardineroseris and Isopora 1.0% each, Montastrea 0.9%, Stylophora and Favites 0.8% each, and Goniastrea 0.7%, and a long tail of 0.4% or less adds Psammocora, Pavona, and Symphyllia (0.4% each), Leptastrea (0.3%), Cyphastrea (0.2%), Echinopora, Hydnophora, Mycedium, and Turbinaria (0.1% each), and Goniopora, Pachyseris, Podabacia, Lobophyllia, Leptoseris, Oxypora, and Astreopora (<0.1% each). On the non-coral side the transect gives crustose coralline algae 15%, turf 8.8%, invertebrates 4.6%, bare substrate 4.2%, erect algae 3.2%, turf-plus-sediment 1.6%, and macroalgae 1.5%. The reef the frames show is a tabular plain of exceptional development. Image 12, Image 19, Image 36, and Image 43 show dozens of large olive and gold plates overlapping edge to edge and stepping away to the limit of visibility, with massive Porites bommies wedged between them; in Image 12 the plates step downslope like roof tiles. Image 8 and Image 11 add big yellow-green tables in the shallows.

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Between the plates the substrate is a pink, mauve, and maroon crustose-coralline pavement (e.g., Image 4, Image 29) carrying corymbose and digitate Acropora in tan, gold and a distinctive purple-mauve (e.g., Image 23, Image 29, Image 33), knobby and lobed Porites, Pocillopora-type clumps, faviids, and encrusting Montipora.

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That bare-looking pink pavement is well consolidated and reads as ordinary architecture for this reef rather than as loss. The plates shade deep black undercuts (e.g., Image 20, Image 30) holding small cryptic colonies and coralline crusts.

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Two macro frames are outstanding: Image 25, a large green meandroid brain with tight sinuous valleys, flanked by pale-green knobby Porites; and Image 13, a plocoid colony with raised polygonal corallites among golden Porites and white-tipped Acropora.

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A second meandroid brain sits in Image 38.

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Brown foliose and encrusting sheets appear in Image 7 and Image 24, and green calcified algal tufts in Image 23 and Image 41.

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Image 29 records a distinct habitat — discrete corymbose Acropora colonies scattered like garden plantings across clean white sand.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 61%, 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 30% of the benthos.
• Transect mean 61 ± 3.6% SE (n = 19 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean.
• The transects were run in three depth strata (D2 45%, D3 76%, and D4 56% coral), and there are no per-frame depth data; the published site value, which weights each depth equally, is 59% (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.
• The image methods in the table read well below the transect here, and the frames suggest why: this reef's cover is carried by large horizontal tabular Acropora plates, which a downward or oblique photograph foreshortens badly and which mask-based segmentation under-calls wherever a plate edge shadows the substrate beneath it.

Stressors

Anderson's published per-site bleached fraction for BIDI08 is 1.7% (Mayfield & Dempsey 2026); nothing below is a measurement of it. Qualitatively, the frames show a structurally sound reef: no colony in the photo set reads as unambiguously bleached, and the three pale features worth logging are each better explained by something else; Anderson's sampling sheet does record one of the four Pocillopora colonies sampled here as 5% bleached (the other three healthy); all four were field-called P. verrucosa or P. meandrina and were collected for a collaborator, and the sheet records no P. damicornis or P. acuta at this site. 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). Image 19 shows a large table whose center has gone pale pink-white across a broad area while its margins remain pigmented, and Image 31 a pale gray-white knobby colony in the foreground; both read as localized mortality or partial paling rather than active thermal bleaching, which on a 12 March dive at a site of this cover is the more plausible interpretation.

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The chalky gray-white lobed structure prominent in Image 38, and again in Image 21, is a sponge, not a coral — its oscula are visible at full size — not whitened coral tissue.

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The extensive bare pink CCA pavement between the tables (e.g., Image 7, Image 21, Image 24) is likewise normal reef architecture at this site rather than loss: the tables are healthy and the pavement is well consolidated. The Living Oceans Foundation transects here recorded no cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

The dive is dominated by two things. First, fusiliers: Image 5, Image 8, Image 10, Image 14, Image 15, Image 17, Image 18, Image 26, Image 28, Image 32, and Image 34 are an almost unbroken run of blue-backed and blue-and-yellow fusilier schools streaming over the reef, several frames holding well over a hundred fish; Image 14 and Image 34 are essentially water-column frames carrying no benthic information at all.

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Second, whitetip reef sharks — Image 33 and Image 39 are unambiguous whitetip reef sharks, slender and blunt-headed with white-tipped dorsal and caudal lobes, one photographed at close range resting on sand beneath a tabular Acropora.

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Image 41 is a striking invertebrate image: a giant clam, mantle fully extended, deep purple-black shot through with iridescent blue-green flecking, wedged into pink coralline pavement. Reef fish include raccoon butterflyfish (e.g., Image 21, Image 30), powder-blue surgeonfish (e.g., Image 35, Image 43), a large jobfish or snapper (e.g., Image 6, Image 12), parrotfish, and dense damselfish clouds over the plates (e.g., Image 20, Image 21, Image 30, Image 31).

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Comparisons with other data

Apart from the Living Oceans Foundation survey shown here, we found no other surveys at this site as of 2026. Within 5 km, two XL Catlin Seaview Survey stations recorded station-year means of 6.3–25% hard coral cover in 2015; the nearest, XL Catlin Seaview Survey at transect 36002, 1.53 km away, read 25% in 2015.

Other notes

The photographic record is 39 habitat frames; there are no sampled-colony frames, so the photographic account is habitat throughout. Anderson's photo folder for this site is named "meter site". Three frames show the survey in progress — Image 16 has a diver holding the white PVC quadrat frame squarely on the reef with a second diver at right; Image 20 and Image 36 put divers and the frame over the tabular plain.

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But the important frame is Image 22: a gray cylindrical instrument housing, cable-tied to a mount with two upright black rods, sits deployed on the pavement beside a yellow mesh collecting bag and a blue-and-white sampling tube on a Porites bommie, with a vertical line running up out of frame.

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That is deployed instrumentation, not a hand-held tool — most consistent with a logger or meter left in place, which would account for this being designated the "meter site". Anyone reconstructing where in situ instrument records came from should start with this frame.