Chagos Archipelago - Egmont (BIEG05)

Setting

Most frames are well-exposed wide reef views on a clear-water day.

General benthic features

The assemblage is evenly co-dominated by two genera: Porites 14% of the benthos and Acropora 13%, i.e. 33% and 29% of all live coral, together more than three-fifths of the reef's coral. Montipora follows at 3.1%, then Pocillopora 1.7%, Pavona 1.6%, Favia 1.3%, Leptastrea 1.2%, Favites and Platygyra 1.1% each, Cyphastrea 1.0%, Symphyllia 0.7%, Goniastrea 0.6%, Isopora 0.5%, Heliopora 0.4%, Psammocora, Pachyseris, and Montastrea 0.3% each, Gardineroseris, Galaxea, and Astreopora 0.2% each, and Echinopora, Fungia, Stylophora, Leptoseris, Acanthastrea, Hydnophora, and Turbinaria 0.1% each, then a tail of Goniopora, Podabacia, Lobophyllia, and Leptoria <0.1% each. The Heliopora record is Anderson's own determination: the LOF codebook lists code HELI as Heliofungia, but the field data are Heliopora, blue coral — determined by A. Mayfield, expedition coral-physiology lead; HELI reaches 93% cover on one transect, 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 substrate at BIEG05 is crustose-coralline-led: CCA 16%, bare substrate 14%, turf 13%, non-coral invertebrates 6.0%, erect algae 2.7%, macroalgae 2.1%, turf-with-sediment 1.5%, and cyanobacteria 0.2%. That is a well-consolidated, low-fleshy-algae bottom — erect algae and macroalgae together hold only 4.8% — and the transect recorded 31 genera.

The defining structure is a tabular Acropora plain. Across Image 5, Image 7, Image 15, Image 20, Image 22, Image 31, Image 35, Image 45, Image 50, and Image 54 the reef is carpeted in overlapping golden tables, some stacked plate-on-plate in tiers, running unbroken to the limit of visibility, with only narrow lanes of pale pavement and rubble between them — the photographic counterpart of the transect's 13% Acropora. Image 15 and Image 39 isolate single perfect tables with clean white growth margins; individual tables also fill the frame in Image 50, and Image 53 catches one growing in a whorled spiral.

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A frame not shown here records a startling lime-green corymbose colony — a natural fluorescent-pigment morph — beside a Moorish idol.

But the site is more than one habitat. The dive also crosses a deep wall or steep drop (e.g., Image 19, Image 23, Image 34, Image 37, Image 39) where the tabular corals give out and the community switches to large sea fans, big ridged massive Porites boulders, red and magenta encrusting sponge, wire corals, bannerfish, and red soldierfish. Image 37 shows a fan silhouetted against open blue on the drop, with further fans in Image 23, Image 34, and Image 39 — gorgonian fans are a structural feature here.

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A third facies — mixed low-relief pavement with domal Porites, digitate Acropora, and meandroid Platygyra — fills Image 47, Image 51, and Image 55.

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Beneath and between the tables the reef is genuinely mixed rather than monospecific: rounded massive Porites-type mounds in Image 22, Image 23, Image 35, and Image 39; whorled foliose plating colonies in Image 7; digitate and staghorn branching stands in Image 14, Image 17, and Image 47; Pocillopora-type bushes, faviid domes, encrusting Montipora, and thick pink and mauve crustose coralline crusts in Image 27 and Image 41 — the photographic face of the transect's 16% CCA.

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Fine-scale diversity comes from the macro frames. Image 24 is a smooth-surfaced branched colony; a frame not shown here packs domal Porites, yellow meandroid Platygyra, digitate Acropora, and encrusting colonies into one crowded field; and Image 26 resolves honeycomb-corallite Favites/Goniastrea over CCA-crusted pavement.

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The site is diverse. Schools of large silvery emperors dominate (e.g., Image 6, Image 10, Image 40, Image 43, Image 48, Image 52), sometimes twenty or more in frame hanging just above the table plain, and the same schools run on through (e.g., Image 30, Image 47).

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A frame not shown here puts diver and quadrat in the middle distance while blue fusiliers stream past. Frames not shown here catch the quadrat frame drifting at frame edge mid-deployment. Image 25 and Image 33 show the transect line itself running across the substrate, with line also visible over the bottom in Image 28.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 44%, 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 36% of the benthos. Surveyed 11 March 2015 at Egmont Atoll (6.64248°S, 71.31116°E; station CTD temperature 29.0°C, salinity 34.1).
• Transect mean 44 ± 4.0% SE (n = 19 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 47% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Live hard coral 44% against algal cover 36% of the benthos, across 30 scleractinian genera plus the blue coral Heliopora.
• Image-based estimates in the method table are listed for comparison only; the transect figure is this site's cover value.
• An oblique photograph of a horizontal table plain foreshortens the tables and gives away area to water column and fish, whereas the transect samples points on the bottom.
• Here the visual impression supports the high transect number: the tabular plain in Image 5 and Image 31 is close to continuous coral cover.

Stressors

Condition — a detailed photographic record of bleaching. A frame not shown here is a particularly informative bleaching image: a full-frame macro of one large tabular Acropora in which broad patches of bone-white branches are interleaved with fully pigmented brown branches on the same colony — an unmistakable record of partial-colony bleaching caught mid-progression. Image 8 shows a small branching Pocillopora bleached completely white, stark against pigmented Porites, faviids, and Montipora neighbors. Image 11, Image 27, and Image 56 add a subtler and arguably more important signal: broad areas of tabular Acropora whose surfaces are pale gray-white and dulled with algal film rather than bright bleach-white, which reads as recent post-bleaching mortality rather than active bleaching — and the same dulled cast shows on the table and plating surfaces in Image 5, Image 7, Image 35, and Image 55.

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The two states are logged separately so they are not pooled. Image 32 shows the end state: a branching colony now dead and overgrown by crustose coralline algae.

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Read together, the frames record the whole sequence on one reef — healthy, partially bleached, wholly bleached, freshly dead, CCA-overgrown. No percentage bleached is read from these photographs and none is offered here. This site's published per-site bleached fraction is a separate, authoritative number from CoralNet analysis of the expedition's survey images, trained on Anderson's point annotations (Mayfield & Dempsey 2026, suppl. S1); the images illustrate what bleaching looked like at BIEG05, they do not measure how much of it there was, and they neither confirm nor revise the published figure. Elsewhere the reef looks structurally intact — no storm-scoured rubble fields, no cyanobacterial mats, no crown-of-thorns scars in any of the frames. The small round white objects scattered across one table's surface in Image 11 are unresolved — possible early bleaching foci, predation scars, or settled organisms. Anderson's sampling sheet records three Pocillopora colonies sampled here: two P. verrucosa by field call (ChJo17, 25.0 m, and ChJo18, 29.5 m), both large and healthy, and ChJo19 (14.8 m), 40% bleached, with its bleached part sampled. 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 0.2% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

Four frames carry no benthic information at all — Image 38 is a severely backlit silhouette, Image 44 and Image 46 are water-column fish schools, Image 29 is open blue — and Image 5, Image 7, Image 10, and Image 52 run hazy or blue-cast.

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Parrotfishes, red-brown with green-yellow tails, work the reef in frames not shown here. A silvertip shark patrols the drop in Image 36; a bluefin trevally hunts in Image 57, with a further trevally working the bottom in Image 19; jack schools cross the blue in a frame not shown here.

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An octopus is wedged into rubble beside a Pocillopora, mantle and eye clearly visible, in Image 9 and Image 13, with Image 26 logged as a probable third.

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Whether these are one animal revisited or separate individuals cannot be determined. A large giant clam, fluted scaly shell and dark maroon-and-green mantle showing, occupies the frame in Image 42.

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Reef fish include powder-blue surgeonfish, schooling unicornfish in Image 12 and Image 33, Moorish idols, bannerfish, and butterflyfishes in Image 39 and Image 40, soldierfish, and a large spotted grouper in Image 47 and Image 55.

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Dense clouds of small planktivores fill the water column in Image 44 and Image 46 and the upper water in Image 29 and Image 38, and a tight file of electric-blue fusiliers streams across the tables in a frame not shown here. An elongate mottled animal resting on a dead plate in a frame not shown here could not be settled as moray or holothurian at that range.

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 Egmont, 1.01 km away, read a mean of 19% in 2006.

Other notes

Seventy frames from this dive are on file, and they bear out Anderson's note that it was a great dive. There are no sampled-colony frames, so this is a habitat account throughout. Depth zonation is not resolvable from these frames — the set clearly spans a shallower table plain, a steeper wall, and a rubblier slope, but no frame carries a depth stamp or scale bar and colony dimensions cannot be measured from any of them; the only scale objects are the divers and the PVC quadrat frame.

How the numbers were made — a detailed photographic record of the survey methodology. Frames not shown here show a diver in yellow fins handling a large white PVC quadrat frame, a frame not shown here catching the frame laid directly onto a tabular Acropora.