Chagos Archipelago - Egmont (BIEG04)

Setting

Station CTD conditions were 29.2°C at 34.0 salinity. Nothing in the set resolves as high-relief spur or wall; the impression across the wide frames is of an open, low-relief acroporid reef top.

General benthic features

Twenty-five scleractinian and hydrocoral genera were recorded, and this is unambiguously an Acropora reef: Acropora holds 13% of the benthos — 38% of all the live coral at the site. Porites follows at 7.7% and Montipora at 3.5%, then Pocillopora 1.6%, Favia 1.4%, Cyphastrea 1.0%, Goniastrea 0.9%, Leptastrea, Platygyra, and Favites 0.7% each, Pavona 0.6%, Fungia and Psammocora 0.4% each, and Astreopora 0.3%. The tail runs Montastrea and Gardineroseris 0.2%, Galaxea 0.2%, Turbinaria 0.1%, Pachyseris 0.1%, Stylocoeniella and Symphyllia <0.1%, Heliopora <0.1%, Millepora <0.1%, and Goniopora and Echinopora <0.1% each. The Heliopora record carries a correction of Anderson's own: the HELI code is listed as Heliofungia in the LOF codebook, but the field data are Heliopora (blue coral) — determined by A. Mayfield, 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, the Heliopora code, has only four records across all 7,029 GRE transect rows. The non-coral benthos is led by crustose coralline algae at 23%, then bare substrate 15%, turf 12%, turf-plus-sediment 6.8%, macroalgae 3.9%, invertebrates 3.5%, erect algae 2.9%, and cyanobacteria 0.1%. Six frames (e.g., Image 8, Image 10, Image 12, Image 14, Image 21, Image 24) are open blue water with a shark or a jack and carry no benthic information, and Image 8 is hazy and very dim; Image 5 and Image 18 are close pavement macros.

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That leaves about fifteen usable benthic frames, most of them clear, well-lit mid-range reef-top views, several hazy with a strong blue cast at range. BIEG04 is a leeward Egmont fore reef (Anderson's field sheet) whose frames divide almost evenly between two states, and the division matters more than the site mean. Roughly half the set — Image 2, Image 4, Image 9, Image 15, and Image 22 — shows genuinely dense Acropora development: overlapping lilac, mauve, fawn, and brown corymbose and branching colonies packed edge to edge over a low-relief pavement, with orange anthias and purple-and-yellow fairy basslets clouding above them.

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The other half — Image 7, Image 19, Image 23, Image 25, and Image 28 — crosses broad pale carbonate pavement where turf-dusted hardground carries only isolated colonies and coral cover by eye is well under 10%.

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The two most striking benthic frames are both isolated structures standing on that pavement rather than parts of a continuous framework: Image 16, a large, perfectly formed tabular Acropora with a clean white growth margin, and Image 27, a broad yellow-green table on the upper slope.

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Further tables sit on a dark undercut pedestal in Image 28, range along the reef edge in Image 23 and front the frame as a bright yellow plate in Image 4. Massive domes compatible with Porites recur throughout as isolated bommies (e.g., Image 4, Image 22, Image 27, Image 28), joined by polygonal-corallite faviids compatible with Favites or Dipsastraea, fine-pored domes compatible with Astreopora, a lobed fleshy colony in Image 7, and a large yellow encrusting-to-plating sheet in Image 28. The two macro frames in Image 5 and Image 18 add the octocorals — brown finger-lobed colonies with white polyps, growing beside gray digitate and orange tube sponges on rubble crusted with red coralline algae; a conspicuous blue sponge appears in Image 23.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 33%, 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 48% of the benthos.
• Transect mean 33 ± 3.6% 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 29% (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.
• One frame documents the survey method directly: Image 2 shows a white PVC photoquadrat frame resting on a dense Acropora stand with the transect line running through it and a diver working in the upper right, the transect survey in progress.
• No photograph here is a measurement of coral cover.

Stressors

Image 20 shows a stainless steel bone-rongeur laid across a digitate Acropora colony, plainly a fragment-collection or measurement shot.

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Anderson's published per-site bleached fraction for BIEG04 is 0.5% (CoralNet analysis of the informal survey images; Mayfield & Dempsey 2026). Qualitatively, no colony in any of the 25 frames is bleached white. The only pallor worth noting is in Image 17, a club-branched Pocillopora whose branch tips are cream-white — but the pattern is confined to the apical few millimeters of each branch with fully pigmented tissue immediately behind, which reads as normal actively growing tips rather than thermal stress.

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This is a 10 March dive, the earliest leg of the expedition and well ahead of the 2015 event's development at Egmont; the value of BIEG04 in the bleaching record is as a clean pre-event photographic baseline for this atoll rather than as an impact site. Anderson's sampling sheet records two large Pocillopora colonies sampled here, ChJo15 (20.5 m, healthy) and ChJo16 (15.7 m, P. meandrina by field call). 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 0.1% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

Conspicuous fauna and sessile invertebrates appear as sponges in 6 frames, sea cucumbers in 4, urchins in 1, and anemones in 1. The dive is dominated by sharks, present in six frames — Image 8, Image 10, Image 11, Image 12, Image 14, and Image 24 — singly, in pairs, and in a loose group of three over the drop-off in Image 10.

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Most are gray reef sharks; Image 11 is an unambiguous close pass over the reef, showing the broadly dark-margined caudal fin. But the animals in Image 12 and Image 14 carry conspicuous white tips on the first dorsal, pectorals, and caudal lobe, which is the signature of the silvertip shark. Whitetip reef shark is a slender, blunt-headed, bottom-resting animal, and is not what these frames show; the remaining shark frames are hazy, distant, and blue-shifted and should not be pushed further. Image 1 is the dive's most spectacular frame: a bait ball of many thousands of small silver fish, compatible with cardinalfishes or sweepers, packed into a reef hollow and filling the entire field of view.

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A single jack patrols the blue in Image 21, and butterflyfishes, damselfishes, and dense damselfish clouds populate the reef frames throughout.

Comparisons with other data

Perry et al. (2015) surveyed a nearby station (Egmont north-east side) in April 2015, 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, one independent station, XL Catlin Seaview Survey at transect 36029, 4.06 km away, read 28% in 2015.

Other notes

The transects were run in four depth strata (D1 12%, D2 43%, D3 41%, and D4 21% coral), and there are no per-frame depth data; the two-zone structure described above is inferred from framing and light, not measured. The photographic record is 26 habitat frames (25 in Anderson's dive folder and one in the "Best of Chagos" folder); there are no sampled-colony frames. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'barrier forereef', 'leeward'); the expedition's 933-site table classes the station as 'exposed' (fringing reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the north-northeast rim of the Egmont atoll, facing north-northeast (about 20°): side-on to the northwest monsoon winds of the March survey and close to leeward for the southeast trades of April to November, so the imagery does not contradict the sheet's leeward.