Chagos Archipelago - Peros Banhos (BIPB72)

Setting

The setting is a lagoonal pinnacle reef (massive coral dominated on the LOF habitat map) inside the Peros Banhos lagoon (dived to 23.4 m), at a station CTD-cast midpoint of 30.3°C and a salinity of 34.4. Quality is generally good: clear water and wide oblique reef views, mostly natural light, with the deep-water frames going blue-green (e.g., Image 15, Image 39, and three other frames) and a handful running warm-yellow (e.g., Image 19 and two other frames).

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One frame shows severe whitening — a near-continuous field of white tabular Acropora running to the limit of visibility, with a diver crossing the upper frame — and a yellow survey tape lies on the bottom in Image 43 and Image 35, so a survey line was deployed on the 1 May dive.

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General benthic features

Non-coral invertebrates hold only 2.7% of the benthos and bare substrate a mere 3.6%; nearly everything that is not live coral is a thin algal fringe split between turf (5.9%), crustose coralline algae (4.3%), erect algae (4.2%), and macroalgae (2.5%), plus 0.1% turf-plus-sediment. The assemblage is overwhelmingly a single-genus reef. Acropora alone holds 67% of the entire benthos — 87% of all the coral recorded here. Twenty-one genera are present in total, but every other one is minor: Porites (1.6%), Montipora (1.4%), Pocillopora (1.3%), Pavona (1.2%), Goniastrea (1.0%), and Favia (0.8%) form a modest second tier, and the remaining fourteen — Seriatopora (0.4%), Astreopora and Millepora (0.3% each), Symphyllia, Lobophyllia, Stylophora, Gardineroseris, Leptastrea, Fungia, and Isopora (0.2% each), and Coscinaraea, Cyphastrea, Echinopora, and Montastrea (0.1% each) — each hold 0.4% or less. Genus richness is respectable on paper, but essentially all of it is a long tail beneath one dominant taxon. The depth profile is a clean two-part story: an Acropora monoculture in the shallow-to-mid zone giving way to a lower-cover, more even community at depth. D2 (79% coral, 18% algae) and D3 (78% coral, 12% algae) are both effectively Acropora reef — Acropora at 70% and 71% of the benthos respectively, 89% and 90% of all the coral on each transect — with only trace amounts of Porites, Favia, Pocillopora, Goniastrea, Pavona, and Montipora filling the rest. D4 breaks that pattern: cover falls to 60% and algae more than doubles to 38%, and while Acropora is still the leading genus at 36%, its share of the coral present drops to 61% as five other genera — Pavona (4.5%), Goniastrea (3.5%), Montipora (3.5%), Seriatopora (3.5%), and Porites (2.5%) — each cross the 2.5% mark for the first time. On the 18 April survey the wide frames confirm the shallow monoculture in dramatic fashion. Frame after frame — Image 13, Image 49, Image 36, and the wide panorama (e.g., Image 15), among others — shows a near-continuous canopy of broad, overlapping tabular and corymbose Acropora plates tiled edge to edge across a low-relief pavement and running out to the limit of visibility, with almost no bare substrate visible between them; exactly the structure implied by 70%-plus Acropora on D2 and D3.

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Tables are stacked in two and three tiers in places and shelter dense, near-opaque shoals of small silvery glassfish and cardinalfish directly beneath their overhangs (e.g., Image 23, Image 21, Image 42, and two other frames).

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Away from the tabular canopy the community broadens: mustard-colored massive and hemispherical colonies with digitate and branching Acropora on coarse rubble beside a large clam-like mantle or sponge in Image 26, a taller stack of pale, weathered massive colonies hosting anthias in another frame, and a mixed pavement of pink crustose coralline algae, green turf, small purple branching colonies, and low mounds in Image 42 — the visible counterpart of the site's 4.3% CCA.

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Those isolated massive boulder colonies are conspicuous, but transect Porites (1.6%) and Favia (0.8%) are both minor, so the imagery and the transect genus counts disagree on how much of the visible massive-coral cover these two genera actually represent; that disagreement is unresolved. The pale colonies sit among dark red and maroon fleshy macroalgae and turf that the April frames do not show as prominently (e.g., Image 37 and two other frames).

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The deeper, lower-relief zone photographed at the end of the dive (Image 14) is a different habitat again — a mixed carpet of small Pocillopora, digitate Acropora, encrusting and low massive colonies over coralline-crusted pavement and rubble, with a mushroom coral in Image 30 and a bare rubble-and-CCA close-up in another frame — and here whitening is patchy rather than continuous.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 77%, 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 17% of the benthos.
• That figure comes from three depth strata, D2–D4 (20 transects): 12 on the 18 April 2015 survey and 8 at the sub-site BIPB72b on 1 May 2015.
• Transect mean 77 ± 2.9% 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 72% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The site-level figure of 67% Acropora is the average of a near-total monoculture and a markedly more mixed, lower-cover deep zone; neither number describes the other, and the stratum means of 60–79% span the cover at this site.
• 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.
• Nothing photographic here is a measurement of either number.

Stressors

No crown-of-thorns, disease lesions, or predation scars were noted in the 34 April frames. On the 1 May revisit the frames record the same reef in a different state, and whitening is no longer confined to scattered colonies. Whole table fields read pale or white in the wide views — Image 50 and nine other frames — and branching and corymbose Acropora stands are white or lilac-white across entire thickets in Image 37, Image 33, Image 24, and three other frames. Image 31 sets the contrast in a single frame: a bank of stark-white tables on the right, a fully pigmented yellow-green table on the left, olive-brown tables between them, and pale tables receding into the distance behind. Image 25 shows a large stark-white table above substrate carrying extensive white and lilac fine-branching colonies.

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The growth-form pattern legible in the imagery is the familiar one: acroporids — tabular, corymbose, and branching alike — carry most of the visible whitening, while the massive, meandroid, and encrusting colonies scattered through the same frames (e.g., Image 41, Image 18, Image 27, and one other frame) hold their color.

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Not everything is affected: fully pigmented yellow-green digitate and corymbose Acropora, brown Pocillopora, and gold tables appear in the same frames as bleached neighbors throughout (e.g., Image 31, Image 20, Image 48, Image 27, and three other frames).

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The Pocillopora-dominated slope frames show a different signature: not stark white but widespread paling — in Image 28 and one other frame the great majority of Pocillopora colonies are pale lilac or white-tipped rather than fully bleached, with a minority still carrying full dark-brown pigment and the occasional intact yellow-green Acropora table among them.

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Read together, the two signatures suggest the tables had progressed further into whitening by 1 May than the Pocillopora had; that is an ordering visible in the photographs, and no severity index is computed from them. Anderson's published per-site bleaching values are 10% on 18 April and 37% on the 1 May revisit, both CoralNet annotations of the informal survey images rather than diver estimates. What the photographs establish qualitatively is the shape of the event: on 18 April a moderate, patchy, colony-scale signal — a minority of tables stark white or pale lavender beside fully pigmented tan and olive neighbors throughout the tabular field (e.g., Image 29, Image 40, Image 15, and several other frames), diffuse paling of plate margins and branch tips in Image 49, Image 40, and two other frames, and one large massive colony in Image 34 bleached essentially in its entirety with tissue still clearly intact and no algal overgrowth.

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That is coherent, colony-scale pallor consistent with thermal stress rather than growth-tip whitening or a lighting artifact. By 1 May it is widespread and concentrated in the acroporids, as described above. Unlike 18 April, when bleaching stood at 10%, the 1 May revisit thirteen days later found 37% of the coral bleached. Both bleaching figures are CoralNet annotations of the informal survey images, not diver estimates (Mayfield & Dempsey 2026, suppl. S1). Anderson's sampling sheet records 19 colonies sampled here at 11.6–22.6 m: two Pocillopora damicornis and one P. acuta by RFLP; 12 Seriatopora hystrix by field call; four Pocillopora sp. sampled for a collaborator (field calls); the condition notes run from healthy to 100% 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 no cyanobacteria and 0.2% recently dead coral.

Fish and other fauna

Image 44 is a macro of a large anemone consistent with the magnificent sea anemone hosting anemonefish in a wall recess beside a bleached tabular colony, with encrusting sponge alongside.

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Fish are present throughout without dominating — butterflyfish, a Moorish idol, and damselfish over the flat in Image 13 and another frame, surgeonfish through the table field, and open-water schools of fusiliers and small jacks streaming over the reef crest in Image 47, Image 17, and several other frames, one of them a mid-water baitfish wall.

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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, five Spalding et al. (2025) stations list undated values of 10–80% hard coral cover (values above 70% are best patches, not site means); the nearest, Spalding et al. (2025) at Peros Banhos - West Ile Bois Mangue, lagoon (N, 2-6 m), 0.81 km away, read 30%.

Dive record

It was dived twice.

Other notes

The frames come from two dives: 34 frames (e.g., Image 1, Image 2, Image 3, Image 7, Image 26, Image 38, 18 April 2015; Anderson's field sheet: in 08:29, 65 minutes, 21.2 m) and 42 (e.g., Image 10, Image 14, 1 May 2015; in 10:29, 60 minutes, 23.4 m).

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They are treated as one site and described as two visits, because they are visibly different reefs thirteen days apart. A diver's transect tape is laid directly across the table field in Image 47 and three other frames — physical documentation of the formal survey passing through this same habitat — and a diver appears in Image 15. The photographs cannot quantify it and are not used to. Reef type follows the LOF habitat map: the pin sits inside 'lagoonal pinnacle reefs - massive coral dominated' ('lagoonal patch reefs' 22 m south); a lagoon pin has no rim facing. Anderson's field sheet (LOF fellowship 2016 workbook) records 'back reef pinnacles', 'leeward'; the expedition's 933-site table classes the station as 'protected' (pinnacle reef).