Chagos Archipelago - Egmont (BIEG03)

Setting

Egmont Atoll, 6.69394°S, 71.37862°E · station CTD temperature 29.1°C · salinity 34. This is a strongly zoned reef.

General benthic features

Assemblage composition from transect data — an Acropora reef with a genuinely deep tail. Twenty-four coral genera are non-zero, counting the hydrocoral Millepora and the blue coral Heliopora. Acropora leads at 16% of the benthos, followed by Porites 9.7%, Montipora 4.0%, and Pocillopora 3.4%. Behind them: Pavona 2.0%, Favia 1.4%, Favites 0.8%, Montastrea 0.7%, Leptastrea and Psammocora 0.5% each, Goniastrea and Platygyra 0.3% each, the hydrocoral Millepora 0.2%, Echinopora 0.2%, Heliopora 0.1%, Cyphastrea, Hydnophora, Galaxea, Coscinaraea, and Stylophora 0.1% each, and Astreopora, Gardineroseris, Goniopora, and Leptoseris <0.1% each. A note on the HELI code: it is recorded as Heliofungia in the LOF codebook, but the field data are Heliopora (blue coral) — determined by Anderson as expedition coral-physiology lead, on the evidence that HELI reaches 93% cover on one transect elsewhere in the dataset, impossible for a solitary free-living mushroom coral, while HEFU has only four records across all 7,029 GRE transect rows. It is labeled Heliopora here. On the substrate side the transects give crustose coralline algae 20% — the largest single non-coral category (all algae together 43%) — bare substrate 12%, turf 10%, sediment-laden turf 9.2%, non-coral invertebrates 4.3%, erect algae 2.0%, macroalgae 1.2%, and cyanobacteria 0.4%. Low fleshy-algal cover on a heavily coralline-bound hardground, with more than a third of the benthos living coral.

The dense end is unambiguous. Image 4, Image 7, Image 30, and Image 38 are wall-to-wall lilac, fawn, and yellow-green corymbose and tabular Acropora with massive Porites domes wedged between them, and Image 13 adds a classic flat-topped table with a powder-blue surgeonfish working across it. Image 13 and Image 48 each hold a broad tan tabular Acropora plate well over a meter across, sitting above branching colonies.

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Image 7 is the best single mixed-community frame — corymbose and digitate Acropora, tan lobed Porites, encrusting faviids on maroon-crusted rock, with a cloud of damselfish in the blue above it. The impoverished end is equally clear: Image 3 is pale carbonate pavement carrying only scattered small faviid domes and a knobby Porites across most of the frame, and the long trevally sequence at the dive's end (e.g., Image 27, Image 35, Image 40, Image 44) is shot over low-relief rubble and turf-dusted hardground with coral confined to isolated clumps.

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The transect's 40% is the site's measurement across both of these states; the photo methods return what a frame set weighted toward the open half and the water column returns.

Structural variety within the coral-dense zone is real. Image 12, Image 15, and Image 46 all feature dark, whorled, pale-margined foliose stacks — a growth form the frames cannot separate to genus. Image 22 records a distinctive golden blunt-fingered colony compatible with digitate Porites. Image 16 resolves green meandroid brains compatible with Platygyra behind a club-branched Pocillopora. Image 11 packs honeycomb-corallite Favites/Goniastrea, cream Porites, corymbose Acropora, and Pocillopora into one macro field. Image 45 is the fine-detail frame: a thick-branched Pocillopora with prominent verrucae, red encrusting sponge and coralline algae, white hydroid plumes, a wire coral, and a cleaner wrasse, with pale feather stars and small ascidians on the pink and red coralline-crusted rock. Image 12 also looks in under a ledge at overlapping foliose plates with pale growing margins and maroon Pocillopora-type branching among them.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 40%, 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 43% of the benthos.
• Transect mean 40 ± 3.2% SE (n = 17 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 37% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The four leading genera (Acropora, Porites, Montipora, and Pocillopora) carry 33 of the 40 percentage points of live coral (all-transect means).
• Image-based estimates in the method table are listed for comparison only; the transect figure is this site's cover value.
• The image methods read below the transect, consistent with the two-habitat structure described above: the transect samples the reef, while roughly a third of the photographs show trevally in open blue and much of the rest the impoverished half of the dive.
• Nothing below is converted into a percentage from frames, and the photographic account is not offered as a check on that figure.

Stressors

Several deeper frames (e.g., Image 15, Image 17, Image 24, Image 29) carry a strong blue-gray cast that makes pale tissue an unreliable condition signal at range, and no scale reference or quadrat appears in any of the benthic frames, so colony dimensions cannot be read off them.

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There are no per-frame depth data; the zonation above is inferred from framing and light. Anderson's published per-site figure — 4.0% of coral bleached (CoralNet analysis of informal survey images; Mayfield & Dempsey 2026, Oceans) — is the authoritative statement of condition at BIEG03. Bleaching is present, unevenly, and it partitions by growth form. Image 14 is the clearest frame: four to five fully bone-white branching colonies, interleaved with normally pigmented brown Acropora on the same substrate.

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Four further, independent signals appear elsewhere in the set: a white Pocillopora beside pigmented neighbors in Image 11; a partially bleached, partially dead Pocillopora in Image 28 whose branch tips are bone-white while its basal branches are dark and algal-fuzzed — a single colony caught mid-transition from bleached to dead; a pale-to-white branching colony in Image 46; and a conspicuous white colony on open pavement in Image 44.

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Massive and encrusting colonies in the same frames stay pigmented. The pattern is growth-form partitioning: branching acroporids and pocilloporids whiten first, massives do not. For an early-March dive, ahead of the peak of the 2015 event, that reads as an early-warning signature rather than an impact. No crown-of-thorns, disease lesion, or predation scar resolves in any of the frames. Anderson's sampling sheet records four Pocillopora colonies sampled here at 9.1–25.1 m (ChJo11–ChJo14: two P. verrucosa, one P. eydouxi, and one not settled, by field call), none recorded as bleached. 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 0.4% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

Faunally this is a trevally site. Blue-spotted bluefin trevally appear across roughly fourteen frames (e.g., Image 5, Image 6, Image 19, Image 23, Image 27, Image 35, Image 36, Image 39, Image 40, Image 42, Image 44, Image 47), in loose hunting groups of five to ten and repeatedly at close range — Image 39 is a textbook pair, purple-blue and black-speckled, and Image 36 and Image 47 show full schools moving low over the reef.

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Some individuals in the same frames are brassier and yellow-spotted, so the aggregation may well be mixed-species; the frames cannot settle it. Beneath the pelagics the reef holds dense baitfish and silverside clouds around bommies (e.g., Image 9, Image 25, Image 31, Image 43), fusiliers streaming over the tables (e.g., Image 26, Image 48), unicornfish, Moorish idols, and assorted butterflyfishes (e.g., Image 17, Image 24), a surgeonfish on the plate in Image 13 and a yellow tang-type fish at the edge of Image 11.

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Two moray eels occupy the macro frames (e.g., Image 20, Image 34) — black with fine pale-yellow speckling, and best left at family level rather than assigned.

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A large scalloped bivalve shell compatible with a giant clam sits on the pavement in Image 41, mantle not visible.

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Comparisons with other data

Sannassy Pilly et al. (2024) surveyed a nearby station (Tattamucca), 0.26 km from the pin; we could not confirm that it is the same reef. Cover values from that work are not among the survey records shown on this page. Within 5 km, two XL Catlin Seaview Survey stations recorded station-year means of 11–28% hard coral cover in 2015; the nearest of these, XL Catlin Seaview Survey at transect 36028, 2.17 km away, read 11% in 2015.

Other notes

Image-by-image analysis — the dive crosses two very different bottoms, and that is the site's structure. Forty-four frames from this Egmont dive are on file (42 in Anderson's dive folder and two in the "Best of Chagos" folder), and the photographic account rests on this habitat set. Roughly 15 frames are benthic-dominant and usable for community description, about 15 are pelagic frames whose main content is trevally in blue, and 3 record divers and survey apparatus.

Three frames record the survey itself. Image 33 catches a diver holding a clear rectangular photoquadrat frame in the background while two bluefin trevally cross the foreground — direct evidence of the quadrat methodology behind this site's numbers — and Image 10 and Image 42 place a diver, one of them with a transect pole, in the same water as the jacks, confirming these are survey-time observations.

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