Chagos Archipelago - Blenheim (BIBL82)

General benthic features

Stylophora was the most common coral genus on this station's transects (15% of the benthos), followed by Favia (4.8%), Gardineroseris (3.8%), Pavona (3.1%), Millepora (2.6%), and Pocillopora and Acropora (1.7% each); bare substrate covered 0.7%, behind crustose coralline algae (33%), the largest single living non-coral group (LOF transects; all algae together 52%). Among the algal classes, crustose coralline algae led (33% of the benthos), followed by turf (7.0%) and macroalgae (5.5%).

The dominant impression across the wide frames is density and small colony size. Image 4, Image 10, Image 17, Image 19, Image 22, and Image 27 show low-relief pavement carpeted edge to edge with hundreds of small colonies – lilac, rose-pink, mauve, cream, and tan digitate and corymbose forms packed against one another with almost no open rock between them, over a substrate itself crusted pink and magenta with coralline algae.

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Larger and structurally different colonies punctuate that carpet rather than replace it: yellow-gold massive and lobed mounds in Image 9, Image 17, and Image 21, dark meandroid brains, and a coarse-corallited massive in Image 11 and Image 15 (the two macro frames, and the only frames in which individual corallites resolve), a yellow-green tabular colony on the upper slope in Image 13, branching yellow stands on the flank in Image 6 and Image 8, and one large brown lobate leather coral in Image 29.

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Image 12 is a distinct zone – broad gray-green ruffled foliose plates covering most of the frame – and Image 25 shows the reef breaking over an edge into open blue water.

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The sequence closes the way a real swim does: the reef thins to rubble-and-pavement with smaller, more scattered colonies (e.g., Image 23), rises over a shallow surge-lit bommie under bright surface ripple (e.g., Image 28), gives way to rippled open sand (e.g., Image 24), and ends with a diver working back along the reef edge toward the drop (e.g., Image 7).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 42%, 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 52% of the benthos.
• Transect mean 42 ± 2.5% SE (n = 18 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 38% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Image-based estimates in the method table are listed for comparison only; the transect figure is this site's cover value.

Stressors

The one striking invertebrate is the black, white-tubercled sea star sitting on the pavement in Image 21, immediately beside the bleached branching colonies – a chocolate-chip-type sea star. This is not a crown-of-thorns, and it is not counted as benthic cover. Bleaching is present at BIBL82 and, in these frames, is unmistakably partitioned by growth form. The overwhelming impression across the frames is of normally pigmented purple, gold, olive, and tan tissue; white appears as scattered individual colonies rather than as any site-wide whitening. The clearest cases are all branching: a stark bone-white cluster tucked among healthy faviids and orange encrusting tissue in the macro frame (e.g., Image 15); a bright white corymbose Acropora in the middle of an otherwise pigmented flat in Image 18; and Image 21, the most affected frame at the site, where a fully bleached staghorn-type Acropora and an adjacent white Pocillopora sit side by side while brown-orange digitate Acropora and healthy Porites immediately around them keep full color.

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More diffuse partial-colony pallor – blanched branch tips, whitened plate and colony margins – shows in Image 4, Image 6, Image 10, Image 12, Image 13, and Image 29. The growth-form partitioning is the notable signal: branching and corymbose acroporids and pocilloporids carry essentially all the visible whitening, while massive, meandroid, lobed, and encrusting colonies are unaffected in every frame. No bleaching percentage is estimated from these images, and the photographs are not used to test one. Anderson's published per-site percentage bleached is the authoritative figure for this reef; the frames record which colonies and which growth forms were affected, nothing more. Anderson's sampling sheet records eight colonies sampled here at 9.2–31.0 m: two Seriatopora hystrix and one Seriatopora sp. by field call; three Pocillopora verrucosa and two Pocillopora sp. sampled for a collaborator (field calls); the condition notes run from healthy to >90% bleached, with some partly dead colonies. 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 0.2% cyanobacteria and no recently dead coral.

Fish and other fauna

Fish are present without being conspicuous, and no large-bodied aggregation appears in any frame. Moorish idols and several butterflyfishes move through the coral in Image 4 and Image 5; bannerfish appear over the foliose stand in Image 12; a loose school of fusiliers streams across the upper slope in Image 13; damselfishes are dense over the wall in Image 8 and Image 25; surgeonfishes graze the flat in Image 18.

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Sponges register in 13 frames, giant clams in 1, and urchins in 1.

Comparisons with other data

The XL Catlin Seaview Survey covered a nearby station (transect 36019) in 2015, 0.42 km from the pin; we could not confirm that it is the same reef. Within 5 km, one independent station, XL Catlin Seaview Survey at transect 36019, 0.42 km away, read 29% in 2015.

Other notes

Twenty-six frames from this dive are on file (22 in Anderson's dive folder and four in the "Best of Chagos" folder), including Image 4, Image 5, Image 6, Image 7, Image 8, Image 9, Image 10, Image 11, Image 12, Image 13, Image 15, Image 17, Image 18, Image 19, Image 21, Image 22, Image 23, Image 24, Image 25, Image 27, Image 28, and Image 29. Two frames carry little or no benthic information (e.g., Image 24 is open rippled sand; Image 28 is a surge-lit bommie shot through heavy surface glitter), leaving 20 usable and about 16 genuinely informative on community composition. No quadrat, scale bar, or depth stamp appears anywhere in the set, so no colony dimension quoted here is measured.