Chagos Archipelago - Great Chagos Bank West (BIDI12)

General benthic features

The transect genus list is the site's community record (LOF BIOT-1 survey, the mean of all of this station's transects): Pocillopora 1.9%, Porites 0.8%, Montipora 0.6%, Favites 0.2%, Goniastrea 0.2%, Pavona 0.2%, Favia 0.1%, Acropora 0.1%, Millepora 0.1%, Astreopora 0.1%, Caulastrea <0.1%, Leptastrea <0.1%, and Montastrea <0.1% — thirteen genera, none of them above 2%, which is the quantitative statement of a reef whose scleractinian community is present but structurally marginal. The substrate categories carry the rest of the story: turf 20%, bare substrate 19%, crustose coralline algae 14%, non-coral invertebrates 13%, erect algae 13%, macroalgae 8.6%, sediment-laden turf 7.9%, and cyanobacteria 0.1%. Twelve of the 27 — Image 6, Image 7, Image 9, Image 10, Image 18, Image 19, Image 24, Image 25, Image 28, Image 30, Image 31, and Image 32 — are wide views over an unbroken seagrass meadow, bright yellow-green strap-shaped leaves carpeting the bottom to the limit of visibility.

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The leaves grow in terminal clusters on dark woody stems, the habit of a seagrass rather than an alga, and Anderson's sampling-sheet note at neighboring BIDI13 later that day records seagrass meadows. Image 7 catches the margin cleanly, where the meadow terminates against pale sand and pavement patched with pink coralline crusts and scattered purple coralline nodules. Live hard coral is not a meaningful component of any of those frames. Where the bottom is not meadow it is low-relief pavement and rubble: Image 4, Image 11, Image 13, Image 17, Image 20, and Image 27 show consolidated, eroded hardground with algal turf, pink and mauve coralline crusts, scattered rubble and only occasional small colonies; the largest coral in Image 17 is a foliose or encrusting colony.

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Two frames hold a genuinely mixed coral community — Image 15 and Image 22, a coralline-crusted ridge with small branching and digitate colonies, encrusting sheets, small massives, and anthias — and even there the colonies are small and discrete rather than framework-forming. Image 5 and Image 22 hold the modest rubble ridge with small colonies compatible with Pocillopora and Acropora, orange anthias, and pink CCA, the closest thing to reef structure at the site. Image 8 is a dense cloud of small planktivores over that same low-relief ground.

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In the photographs, the bottom is, in order of extent: seagrass meadow > algal-turf pavement > coralline-crusted rubble >> live hard coral.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 4.2%, 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.
• Against it the transects put algal cover at 64% of the benthos.
• Transect mean 4.2 ± 1.4% SE (n = 12 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 4.2% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The transects were run in three depth strata (D2 1.3%, D3 9.3%, and D4 2.0% coral).
• 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.
• With live coral this sparse and the water this hazy, no bleaching figure was derived from this photo set: Anderson's published per-site percentage bleached for the Chagos sites is the authoritative number (Mayfield & Dempsey 2026, Oceans).
• The image-based products in the method table have different sampling footprints and denominators and should not be averaged.

Stressors

Survey conditions on 13 Mar 2015 at 6.4534°S, 71.2337°E: station CTD temperature 29.3°C, salinity 34.1. No bleached colony is identifiable in any frame, and no disease lesion or predation scar resolves. The meadow is seagrass, not macroalgae, so it is habitat, not a condition signal. Anderson's sampling sheet records three pocilloporid colonies sampled here for a collaborator (species by field call): Pocillopora sp. ChJo32 (25.4 m, 5% bleached), ChJo33 (19.8 m, 5–10% dead), and ChJo34 (16.5 m, 5–10% dead, healthy part sampled). 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 0.1% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

Faunally this dive is dominated by one thing: sharks. Animals compatible with the gray reef shark cruise low over the meadow in Image 4, Image 6, Image 9, Image 10, Image 11, Image 16, Image 19, Image 24, Image 25, Image 28, Image 31, and Image 32.

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Two of them are in frame together in Image 16, and possibly again in Image 24 or Image 32. One further animal sits in the haze of Image 14 beneath a large silver jack, and a jack of the same kind patrols above the meadow in Image 32.

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Image 11 and Image 27 show a school of large parrotfishes working the eroded pavement, one visibly expelling a sand plume — bioerosion photographed in the act, and directly relevant to the state of this substrate. A dark animal crosses the hazy blue of Image 13 at a range that puts identification beyond the frame; its outline is compatible with a turtle, and neither the group nor the species can be taken further. An emperor or bream forages over the rubble ridge in Image 5. An octopus occupies a coralline-crusted crevice in Image 21 and Image 33, the second frame a clean close view of the mantle and eye among red, pink, and magenta encrusting crusts and sponges — a clean macro study.

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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. If you would like to share yours, please email us.

Other notes

Image-by-image: the seagrass is the first thing in the frames. The 27 photographs are habitat frames, with no sampled-colony frames, so this is a habitat account throughout; visibility is poor here, with a blue-violet cast and a hazy water column through most of the set. Image 23 is a logistics frame rather than a survey one: staged dive gear — twin cylinders, a yellow mesh collecting bag and camera hardware — resting on the pavement with a diver above.

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No quadrat, transect line, or scale reference appears in any of the 27 frames. The expedition's transect classes have no seagrass label, so the transects count this meadow as algae (erect algae 13%, macroalgae 8.6%).