Chagos Archipelago - Egmont (BIEG02)

General benthic features

Porites was the most common coral genus on this station's transects (10% of the benthos), followed by Acropora (8.3%), Montipora (6.7%), Pocillopora (2.6%), Favites (1.4%), and Cyphastrea (1.1%); bare substrate covered 5.8%, behind crustose coralline algae (17%), the largest single living non-coral group (LOF transects; all algae together 56%). Among the algal classes, crustose coralline algae led (17% of the benthos), followed by turf (16%) and erect algae (12%).

The site photographs as two habitats averaged together, and the split is obvious across the set. The poorer half is extensive low-relief pavement. Image 4, Image 7, Image 12, Image 15, Image 25, and Image 29 are wide views over consolidated, turf- and coralline-crusted hardground carrying only scattered small Porites knobs, small corymbose colonies, and rubble; Image 7 in particular has almost no live coral above the plane of the substrate. Image 13 is the same ground seen downward – a mosaic of rubble, encrusting sponges, small massives, and coralline crusts.

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The denser half is a mound-and-thicket ground. Image 11 is the best of them – dense corymbose and digitate Acropora packed over a mound with pink crustose coralline algae and a mustard-yellow lobed colony in the foreground – and Image 6, Image 10, and Image 24 are comparable close-packed fields of branching and corymbose Acropora over pavement. Image 16, Image 17, and Image 28 sit between the two: mixed assemblages of small mauve corymbose Acropora, cream and fawn massive Porites, dark foliose colonies with pale growing margins, and yellow encrusting sheets. Image 18 and Image 19 look along and over the drop, with planktivores in the blue above the slope. Image 8 is the one close macro of the sessile community – maroon encrusting sponge with open oscula, a gray perforated sponge, pale branching coral tips, and yellow-green calcified algae.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 37%, 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 56% of the benthos.
• Transect mean 37 ± 3.4% SE (n = 10 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 32% (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.

Stressors

Two coral-predator taxa are documented at this one site. A crown-of-thorns starfish is unambiguously present in Image 22 – a full-frame animal with long gray-green spines over a maroon disc, photographed alongside a stainless steel bone rongeur laid on the substrate beside it.

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Survey and collection gear appears in three frames – a stainless transect T-bar, camera rig, and yellow mesh collecting bag on the pavement in Image 4; the bag again in Image 25; the rongeur beside the crown-of-thorns in Image 22 – fixing these as working survey images rather than pure scenics. This is an early-March dive, before the 2015 bleaching had properly developed, and most of the reef in these frames is pigmented fawn, lilac, mauve, and olive; Anderson's sampling sheet nonetheless records two of the four Pocillopora colonies sampled here as pale (one "pale and possibly unhealthy") and a third as 5% bleached. Anderson's sheet records no P. damicornis or P. acuta at this site; these colonies were collected for a collaborator. Pocillopora acuta colonies from elsewhere in the archipelago were analyzed in Thomas et al. (2026, Global Ecology and Biogeography 35:e70305, https://doi.org/10.1111/geb.70305). Whitening is nevertheless present, it is not zero, and it is instructive. Image 21 is the key frame: two or three fully bone-white branching Acropora colonies stand out against pigmented dark neighbors in a single macro field, alongside several partially blanched colonies – a tight cluster of affected colonies rather than diffuse whitening across the frame.

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Image 26 adds a second, separable signal: a large massive Porites whose upper face is dead and crusted pink-gray with coralline algae and turf, with live lobes remaining at its margins, in the middle of an otherwise healthy staghorn thicket.

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That is older partial mortality rather than active bleaching, and the two should not be pooled. No bleaching percentage is estimated from these images and none is tested against them. Anderson's published per-site percentage bleached is the authoritative figure for this reef; the photographs record which colonies and which growth forms were affected, and that a distinct, older mortality signal is present alongside them. A crown-of-thorns starfish is visible in Image 22. The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

Separately, two pincushion (cushion) sea stars appear – the brown, black-netted cushion in Image 13 and a second, very cleanly photographed individual in Image 23, the latter resting within centimeters of a small white-tipped Acropora.

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Cushion stars are themselves coral predators, so with the crown-of-thorns starfish this dive documents two kinds of coral predator at one site. A gray reef shark patrols the blue water off the drop in Image 5; a second frame of a single shark is not shown here.

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Dense silver schools – bigeye scad or fusiliers – sweep the reef top in Image 25; unicornfish hang over the slope in Image 15; orange anthias cloud almost every coral-dominated frame (e.g., Image 12, Image 19, Image 28, Image 29); and two palette surgeonfish are clearly visible in Image 16, with small blue surgeonfish also in Image 28. Sponges register in 7 frames, urchins in 1, and sea cucumbers in 1.

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, two independent stations recorded station-year means of 11–28% hard coral cover in 2015; the nearest, XL Catlin Seaview Survey at transect 36028, 3.25 km away, read 11% in 2015.

Other notes

All of the habitat frames were reviewed (26 on file: 23 in Anderson's dive folder and three in the "Best of Chagos" folder). The frames from this dive cover a wide range of subjects: two coral-predator taxa, two sharks, and a dense coral mound all come from this one swim. There are no sampled-colony frames. Quality is generally good, with a few hazy blue-cast frames (e.g., Image 7, Image 18, Image 19) and two water-column-only shark frames carrying no benthic information. No quadrat or scale bar appears anywhere, so no colony dimension here should be treated as measured.