Chagos Archipelago - Peros Banhos (BIPB61)

Setting

BIPB61 is a low-cover, algae-dominated windward channel reef on the west rim of Peros Banhos, dived to 30.2 m on 14 April 2015 (station CTD-cast midpoint 29.9°C).

General benthic features

Turf takes 24% of the benthos. Non-coral invertebrates take another 22%. Crustose coralline algae hold 15%, bare substrate 7.5%, turf-with-sediment 6.2%, erect algae 3.9%, and macroalgae 1.5%. This is a reef whose surface is largely turf, sessile invertebrates, and coralline pavement, with coral as the minority occupant. The coral that is present is a Porites assemblage with a thin tail. Porites alone holds 8.9% of the benthos — 45% of all coral recorded — followed by Acropora 4.5%, Pocillopora 2.3%, and Montipora 1.6%. Below that the list falls away fast: Leptastrea 0.5%, Astreopora, Favites, Favia, and Pavona 0.3% each, Montastrea 0.2%, and Gardineroseris, Symphyllia, Acanthastrea, and Millepora 0.1% each, then Goniastrea at <0.1% and a seven-genus floor at <0.1% each — Coscinaraea, Fungia, Galaxea, Heliopora, Oxypora, Scolymia, and Stylophora. Twenty-two genera are non-zero, but sixteen of them together account for under 2.5% of the benthos. The depth pattern is a shallow-to-deep genus handover, and the middle band is the poorest. D2 carries 24% coral and is the only band Acropora leads — 8.0% against Porites' 7.4%, with Pocillopora at 4.8%. D3 drops to 15%, the site's minimum, and the order inverts hard: Porites 9.0% against Acropora 2.6%. D4 recovers to 21% with Porites at its site maximum of 11% and Acropora at 3.1%. Genus richness runs the other way from cover — 9 genera at D2, 11 at D3, 18 at D4 — so the deepest band is the most genus-diverse, while Acropora is scarcest at D3 (2.6% of the benthos, against 3.1% at D4 and 8.0% at D2). Algal cover is high in all three bands (50%, 53%, 47%) and barely varies, which is the clearest signal that the algal dominance here is a site property rather than a depth effect. Gorgonian sea fans are the most conspicuous non-scleractinian element of this reef. One frame shows a slope falling into blue water carrying a dense stand of large yellow-green gorgonians and whip corals, with a haze of small planktivores above and only pale, sparsely colonized pavement beneath (e.g., Image 29).

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Two frames show the same fans and dark whip corals along the slope, another catches them beside a deployed transect tape with a diver working above, and a fourth places them across a broad, low-relief mixed bottom of small massive domes, digitate colonies, foliose plates, and black finger sponges (e.g., Image 10, Image 15, Image 18).

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None of these octocorals appear anywhere in the transect data, which records only scleractinians plus Millepora and Heliopora — so the 22% non-coral invertebrate figure is the only place they are counted, and their contribution is not resolved to taxon. Where coral does concentrate, it is Acropora in the shallows. Two frames show near-continuous corymbose and tabular Acropora thickets working with sergeant majors, black triggerfish, humbug damsels, and dense orange anthias — matching the D2 stratum, the only one Acropora leads (e.g., Image 19, Image 27).

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Away from those patches the ground is exactly what the cover figure implies: two frames show wide pale pavement and rubble carrying widely spaced small massive domes and ruffled foliose colonies, and a third shows the same at depth with a scrawled filefish over largely bare substrate (e.g., Image 5, Image 9, Image 14).

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Genus variety can be high within a single patch: a large massive Porites dome, encrusting and foliose plates, a clearly meandroid brain coral, small branching Acropora, orange sponges, red encrusting algae, and a pink sea whip occur together in one downward view. The site's other habitat is cryptic. Three frames are a sequence under a ledge or overhang holding a large aggregation of golden sweepers, with the ceiling carrying red and maroon encrusting algae and sponges, yellow encrusting sponges, ascidians, and green calcified algae; orange anthias and a reddish soldierfish or bigeye share the shelter, and two nurse sharks rest on the sand beneath the ledge (e.g., Image 2, Image 3, Image 6, Image 23).

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Another frame covers the same habitat with a yellow-green whip coral and a scarred massive Porites dome on the sand at the overhang mouth (e.g., Image 20).

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Formal surveys give 20% coral, 50% algae, and 22% non-coral invertebrates, the coral 45% Porites and led by Acropora only in the shallowest stratum.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Transect coral cover is 20%, against 50% algal cover.
• Transect mean 20 ± 2.0% SE (n = 15 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 20% (Mayfield & Dempsey 2026, Oceans, suppl. S1).

Stressors

Published bleaching is 12% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). One image is unambiguous: a top-down frame of a single large, entirely white massive colony, its lobed surface fully pigment-free, sitting on dark turf-covered pavement among small unbleached colonies (e.g., Image 28).

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One frame holds a white digitate colony on the pavement beside a pale cream soft coral, and two others each contain broad pale-to-white massive and plating colonies on the slope. The Living Oceans Foundation transects here recorded under 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

Two nurse sharks rest under a ledge (e.g., Image 3). One frame adds a large emperor or snapper at close range. Anderson's field notes also record a manta ray on this dive, filmed ('MANTA RAY! Got great footage').

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, one independent station, Sheppard et al. (2008) at Peros Banhos, 3.20 km away, read a mean of 44% in 2006.

Dive record

Anderson's dive on 14 April 2015 entered the water at 10:47 and lasted 47 minutes, to a maximum depth of 30.2 m (the field sheet); the notes describe calm, slightly murky water, low coral cover, and a high-flow environment, and record a manta ray, filmed on the dive. Anderson sampled two pocilloporid colonies for a collaborator: ChJo162 (likely Pocillopora verrucosa; collected 11:04 at 24.6 m; large and healthy) and ChJo163 (Pocillopora sp.; 11:28, 12 m, as shallow as sampling was possible because the site has no 10 m depth; fairly healthy, candelabrum morphology). The notes record no Pocillopora damicornis, P. acuta, or Seriatopora at the site. 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).

Other notes

Reef type and aspect follow Anderson's field sheet (LOF fellowship 2016 workbook: 'channel reef', 'windward'): the LOF habitat map has no reef polygon within 65 m of the pin (nearest 'deep fore reef slope' 130 m); the expedition's 933-site table classes the station as 'intermediate' (channel reef). The rim here faces west to west-northwest: windward of the northwest monsoon (December–March), leeward of the April–November southeast trades; the printed 'windward' is Anderson's sheet label.