Chagos Archipelago - Great Chagos Bank West (BITB41)

Setting

BITB41 is an intermediate-exposure submerged bank reef outside the lagoon at Three Brothers, surveyed 22 March 2015 at 28 m.

General benthic features

Across 28 genera, 29th of 109, the assemblage is overwhelmingly Porites-led at 17.96% of the benthos, 43% of every coral point and four times the next genus, while Acropora is an unusually minor sixth at 2.03% (e.g., Image 2, Image 5, Image 8, Image 15, Image 16).

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One frame captures a coral-on-coral overgrowth margin (e.g., Image 3).

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Coral cover

Formal transects recorded 41.9% coral against 28.8% algae — 46th of 109 Chagos sites for cover, 90th for algae, the lowest fifth of the archipelago — but this is not the usual hard coralline pavement: coralline algae is only 8.6% of the benthos and turf 6.6%, both near the archipelago's bottom, and the space is taken by 15.4% bare substrate and 13.9% non-coral invertebrates (e.g., Image 1, Image 4, Image 6, Image 11, Image 12).

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The three depth strata are nearly identical in cover — 44.3%, 40.5%, 41.0% — yet individual transects range 28% to 73%, so patchiness is roughly five times the depth signal and no cover trend can be defended; composition does change, Acropora falling to zero at the deepest stratum while Pachyseris rises from absent to 7.00% and bare substrate climbs to 25.0%. Image methods read 12 to 17 points lower (CoralNet 25.4%, Coralscapes 29.5%) and should not be averaged with the transects. Coral cover: 42%. Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (Mayfield & Dempsey 2026, Oceans, supplemental S1), against algal cover of 28.8% of the benthos. The transect figure is this site's authoritative cover value; no image-derived product overrides it. Across methods the same reef reads: transects 41.9% · CoralNet points 25.4% · Coralscapes masks 29.5% (13 frames). These products have different sampling footprints and denominators and should not be averaged.

Stressors

Bleaching is 7.65% on the site table's Mean(%bleaching) column, 45th of the 98 sites carrying a value, and the two CoralNet runs agree, so the canonical % bleaching (2nd training) figure is the same. All 13 frames were reviewed: paling is real but patchy, a stark-white branching colony partly overgrown by algae and white branch tips on an otherwise pigmented Pocillopora, but no site-wide field, no crown-of-thorns, and no disease (e.g., Image 9, Image 13).

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Comparisons with other data

No prior survey data from this site. If you would like to share yours, please email us.