Chagos Archipelago - Peros Banhos (BIPB70)

Setting

A lagoonal pinnacle reef in Peros Banhos (branching coral dominated on the LOF habitat map), dived to 24.2 m on 17 April 2015, 5.32515°S, 71.85739°E; the station's CTD-cast midpoint temperature was 30.3°C. On the 1 May revisit the frames show the reef in a visibly different state, and Anderson noted bleaching at the time of that dive.

General benthic features

Anderson's own records note that the second (1 May) dive was made to document bleaching, and the account below keeps the two visits separate. The wide frames show a low-relief submerged bank whose surface is paved, almost edge to edge, in overlapping tabular and plate Acropora. No scale bar appears in any frame, so no colony dimension is quoted. Image 35 and Image 48 are single ghost-white tables set among still-pigmented neighbors; another frame carries a stark chalk-white tabular Acropora mid-frame beside cream-white massives, gold tables, and pink-red colonies in the same view; Image 18 shows a white branching colony beside a pigmented mushroom coral; and Image 28 is an unambiguous, fully white Pocillopora colony photographed directly above a normally pigmented dark-brown Pocillopora — the within-frame contrast that rules out flash artifact or naturally pale tissue.

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The severe frames come late: Image 10 shows extensive stark-white tabular and plating Acropora along a reef edge; Image 22, Image 37, and two further frames are wide views in which a large share of the tables and plates in frame are white or bone-cream, and Image 44 and Image 40 are landscape-scale views in which large white tables occupy the mid-ground and pale tables recede across the whole visible flat.

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Not all of the reef reads as affected: Image 32, Image 38, and Image 42 are dense, colorful reef in which gold and tan tabular and corymbose Acropora still dominate, with several conspicuously white or pale tables among them rather than across them.

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A reef whose coral is more than four-fifths one thermally sensitive genus is maximally exposed to exactly this kind of event, and the photographs are consistent with that exposure.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 70%, 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 24% of the benthos. The figure comes from four depth strata, D2–D5 (20 transects), surveyed on 17 April 2015.
• Transect mean 70 ± 3.3% SE (n = 20 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 60% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Assemblage composition from transect data — overwhelmingly a single-genus story. Twenty-eight genera are non-zero, but Acropora alone holds 59% of the benthos — 84% of all coral recorded here, in dense tabular and plate stands. A long tail follows well below it: Porites 4.0%, Pocillopora 2.6%, Montipora 1.2%, Echinopora 0.7%, Lobophyllia and Favites 0.4% each, Fungia and Seriatopora 0.3% each, Ctenella 0.2% (the Chagos-endemic monotypic genus), and Leptastrea, Pavona, and Psammocora 0.2% each; a scatter at or below 0.1% includes Goniastrea, Favia, and Astreopora 0.1% each, Heliopora 0.1% (a blue coral, not the free-living mushroom Heliofungia), Stylophora, Herpolitha, Platygyra, Acanthastrea, Leptoseris, and Symphyllia 0.1% each, and Montastrea, Galaxea, Turbinaria, Millepora, and Oxypora <0.1% each.
• The depth structure is the most important feature of the dataset and it splits the site into two very different reefs. D2 carries 69% coral, of which Acropora is 56% — 81% of the coral on that transect. D3 is higher still at 75% coral, with Acropora at 68% — 91% of the coral, an effective monoculture and the site's single highest-cover, highest-Acropora stratum. Then the pattern breaks. D4 drops to 41% coral, the site's lowest, on the site's highest algal cover of 49%, and Acropora falls to 20% — only 48% of a much more diverse coral component that now includes Lobophyllia 4.4%, Fungia 3.0%, Favites 2.0%, Pocillopora 2.0%, and Porites 1.8%. D5 recovers somewhat to 55% coral (39% algae), with Acropora at 27% — still under half the coral present — and Echinopora 8.0%, Lobophyllia 4.0%, Montipora 3.0%, Seriatopora 3.0%, and Goniastrea 2.0% filling out a genuinely mixed assemblage. The site-level figures of 70% cover and 59% Acropora describe neither zone well: they average a near-monoculture tabular Acropora reef (D2–D3, 69–75% cover) against a lower-cover, algae-heavier, genus-diverse zone (D4–D5, 41–55% cover) where Acropora holds under half the coral present.
• 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 brown lobate and hemispherical massives among the tables are consistent with Porites or Favites-type growth but are not confidently assignable from the imagery, and they are structural exceptions in an Acropora landscape — which is exactly what the transect's genus breakdown says independently. One of Anderson's frames (Image 5) is especially colorful: a dense corymbose and digitate Acropora thicket in mixed yellow-green, brown-purple, and cream color morphs, a visual counterpart to the genus-rich D4/D5 transects even though the frame itself is still Acropora-dominated.
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• Survey apparatus is present and worth recording: a diver with a quadrat frame at the top of Image 45, a yellow transect tape laid across a table in another frame, a thin line across the pavement in Image 27, and camera-and-strobe rigs in Image 36 and Image 17, with a diver resting on a massive coral head with camera and strobe in Image 43, giving useful scale.
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• Formal transects recorded 70% live coral against 24% algae, and the reef is overwhelmingly Acropora, which alone makes up 59% of the benthos and 84% of all coral present, in dense tabular and plate stands.

Stressors

Substrate is led by turf at 8.8%, ahead of CCA (6.1%), erect algae (4.9%), bare substrate (3.4%), non-coral invertebrates and macroalgae (2.5% each), sediment-laden turf (1.8%), and cyanobacteria (0.1%) — a reef that is mostly coral or fleshy and erect algae, with very little bare space. The photographs come from two dives, fourteen days apart. The frames on file split roughly evenly between the 17 April survey dive (e.g., Image 1, Image 3, Image 7) and the 1 May revisit, on which Anderson noted bleaching (e.g., Image 10, Image 14, Image 26, Image 38). Image 45, Image 41, Image 47, Image 21, Image 33, Image 36, Image 43, and several other frames repeat the structure — dense, overlapping plate and table colonies stacked several deep over a hard pavement, cream, tan, and gold, laid over one another like roof tiles — with corymbose and staghorn thickets filling the gaps and only narrow strips of pale sand, rubble, and pink-mauve coralline pavement showing between them.

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Brown lobate and hemispherical massives punctuate the tables: a large dome in Image 41, another in the foreground of Image 47 with dark boring scars in its surface, more in Image 33 and another frame. Early in the sequence the reef still reads as largely pigmented: Image 38 is dominated by intact yellow-green Acropora tables, olive foliose colonies and brightly pigmented faviids, with only scattered pale branches, and two further frames show a gorgonian sea-fan garden on the upper slope whose fans are naturally pale and carry no bleaching signal. From mid-sequence the signal is unmistakable. One macro frame is informative: a staghorn thicket showing an eerie electric-blue-and-white coloration on otherwise bleached branch tips, with stark white tissue, pale peach and vivid pale-blue pigment side by side — almost certainly host-pigment (GFP-like) fluorescence exposed as zooxanthellae are lost, not a distinct taxon. Some table surfaces in Image 10 are brown-olive rather than white, which is consistent with either recently dead, turf-colonized skeleton or pre-existing dead framework; fourteen days is fast for that transition and the photographs cannot separate the two, so no mortality is claimed here. Anderson's published per-site bleaching figures for BIPB70: 3.7% of coral bleached on 17 April and 33% on 1 May (CoralNet annotations of the informal survey images, not diver estimates; % bleaching, 2nd training, Mayfield & Dempsey 2026, Oceans, suppl. S1). No percentage of the reef bleached can be or has been read off these photographs, and none is estimated here; the frames are qualitative and are not offered as a check on either number. What they do show, qualitatively, is set out above: on 17 April a dense, largely tan-and-gold tabular Acropora canopy with a small number of conspicuously paler colonies in it, noting that at depth the ambient blue-green cast makes pale-versus-bleached an unreliable distinction at range in that set; on 1 May, whitening that is widespread, patchy at colony and patch scale rather than uniform, and clearly present in the same frames as fully pigmented reef. The taxonomic pattern matters at this site in particular. The visibly bleached colonies in the May set are overwhelmingly tabular and branching Acropora, with Pocillopora the clearest close-range case, while massive Porites-type heads and meandroid colonies largely retain brown pigmentation in the same frames — and BIPB70's April transects record Acropora at 59% of the benthos, 84% of all coral at the site. This is an observed association between the assemblage recorded in April and the condition photographed in May; it is not a tested causal claim. The gorgonian sea fans photographed on the upper slope are not scored in the hard coral bleaching denominator. Unlike 17 April, when bleaching stood at 3.7%, the 1 May revisit two weeks later found 33% of the coral bleached, and the May photographs show the change unevenly: parts of the reef still fully pigmented, elsewhere landscape-scale fields of stark-white tables, and one unambiguous fully bleached Pocillopora photographed directly above a normally pigmented neighbor. The bleached colonies are overwhelmingly Acropora — the genus holding more than four-fifths of this reef's coral. Both bleaching figures are CoralNet annotations of the informal survey images, not diver estimates (Mayfield & Dempsey 2026, suppl. S1). Anderson's sampling sheet records 21 colonies sampled here at 8.0–24.3 m: five Pocillopora damicornis and four P. acuta by RFLP; three Seriatopora hystrix and two Seriatopora sp. by field call; five Pocillopora sp. and two P. verrucosa sampled for a collaborator (field calls); the condition notes run from healthy to 100% bleached, with some diseased or 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 0.5% recently dead coral.

Fish and other fauna

The opening frames (Image 1) are shot into dense clouds of silversides over a lower, more mixed pavement of encrusting and foliose colonies, orange and red sponges, and green fleshy algae, with jacks working the bait ball in Image 29; streams of blue-and-yellow fusiliers or anthias thread the coral canopy in Image 33, Image 36, and Image 27, and schooling fish pass over the reef in Image 12. Image 14 is a stand of pale knobby-columnar colonies; Image 46 shows a small pale branching colony tucked under a coralline-crusted ledge; and one frame is water column only — two divers deploying a white PVC quadrat frame in blue water, no benthos.

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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.

Dive record

The site was dived twice, on 17 April and 1 May 2015; times, depths, and bottom times for both dives are in the Dive history table.

Other notes

Reef type follows the LOF habitat map: the pin sits inside 'lagoonal pinnacle reefs - branching coral dominated' (with 'deep lagoonal water'); a lagoon pin has no rim facing. Anderson's field sheet (LOF fellowship 2016 workbook) records 'lagoonal patch reef', 'leeward'; the expedition's 933-site table classes the station as 'protected' (patch reef).