Chagos Archipelago - Egmont (BIEG06)

Setting

Visibility is fair to hazy and several frames (e.g., Image 3, Image 12, Image 17, Image 19, Image 26, Image 27, Image 34) are soft or backscattered.

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General benthic features

Quadrat apparatus is visible in one frame; no scale bar appears anywhere.

This is a branching-Acropora reef, and the frames say so unambiguously. Many of the wide frames look out over near-continuous plains of fine-branched staghorn, digitate, and corymbose Acropora running away from the camera to the limit of visibility. Some frames show dense gold-brown thickets with almost no bare substrate, while Image 2 and Image 33 show the same kind of thicket broken by white sand channels, loose dead branch rubble, and pale pavement. Image 7 adds mixed brown and mauve branching colonies with mustard encrusting sheets over a sandy floor.

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Acropora was the most common coral genus on this station's transects (39% of the benthos), followed by Goniopora (4.0%), Goniastrea (2.4%), Favia (1.6%), Psammocora (0.5%), and Lobophyllia, Montipora, and Pavona (0.4% each); turf (15%) was the largest single living non-coral group (all algae together 37%), and bare substrate covered 13% (LOF transects). Among the algal classes, turf led (15% of the benthos), followed by sediment-laden turf (11%) and crustose coralline algae (7.3%).

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 49%, 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 37% of the benthos.
• Transect mean 49 ± 5.4% SE (n = 14 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 44% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The near-continuous branching thickets in the wide frames are exactly what a reef measured at 49% looks like.
• The transects were run in four depth strata (D1 17%, D2 55%, D3 59%, and D4 44% coral).
• The frames show extensive fine-branched staghorn thickets, their branches interleaving with shadow and dead skeleton.
• Image-based estimates in the method table are listed for comparison only; the transect figure is this site's cover value.
• This description is qualitative: it records what is visible in the frames.
• Anderson's published per-site % bleached for BIEG06 (10%) is the authoritative statement of this site's bleaching status; nothing in this photographic account is converted into a percentage, and nothing here is offered as a corroboration or a test of it.

Stressors

The stress signal here is the site's most important photographic content, and it takes three distinct forms which should not be pooled. First, widespread paling: Image 6 and Image 17 show entire Acropora plains in a washed-out gray-white rather than the golden brown of a healthy stand, and Image 32 shows the same over a broad area beneath an emperor school — they contrast with the gold-brown thickets in other frames, though the paled frames are hazy enough that light and water-column effects cannot be fully separated from tissue paling.

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Second, partial-colony bleaching on individual colonies: Image 5 is a clean macro of a Pocillopora with numerous bone-white branch tips against pigmented bases, another frame shows a branching Acropora with extensively whitened tips, and Image 11 records a small cream-white colony that appears wholly bleached.

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Third — and most striking — standing dead framework: Image 4 shows a large expanse of intact branching coral skeleton, gray-brown and algal-fuzzed with no live tissue, still standing in growth position among live colonies, with two butterflyfish working over it, and Image 33 shows piles of dead branch rubble across open pavement.

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Set against this, three frames (e.g., Image 8, Image 20, Image 24) document the opposite process — small pale Acropora recruits settled directly on dead, CCA-crusted substrate, including one ring of recruits surrounding an algal-covered dead mass.

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A single dive therefore records mortality and recruitment on the same reef. Anderson's sampling sheet records 0% bleaching for all six colonies sampled here; one was small and pale, and part of another had died (see Dive record below). The Living Oceans Foundation transects here recorded 0.5% cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

The fish community is dominated by large-bodied predators and grazers. Trevally schools fill four consecutive frames (Image 9, Image 16, Image 29, and Image 36) — sometimes thirty or more animals in one shot, hanging directly over the staghorn; a looser aggregation appears in Image 12, which also carries schooling pinkish-silver emperors or breams.

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Snappers appear repeatedly and at close range in Image 3, Image 13, Image 19, Image 26, Image 27, and Image 28, and emperors or breams school over the coral in Image 32 and Image 38.

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A gray reef shark appears in Image 2, and a reef shark passes beneath a diver in another frame. Blue-green chromis cloud the staghorn thickets in Image 18 and Image 37, and butterflyfishes work the dead framework in Image 4.

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Among the dive's macro subjects is Image 23 — a large chromodorid nudibranch, white to lilac with orange-yellow spots, a purple mantle margin and gill plume raised, not callable to genus or species from a photograph.

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

Dive record

Anderson sampled six colonies on this dive (the sampling sheet). Four are Pocillopora acuta, confirmed by RFLP: CHPd1 (collected 10:38 at 18.5 m; small and pale, living in isolation in the sand under a high sediment load), CHPd2 (10:50, 18.5 m; small, part of the colony dead, the rest apparently healthy), CHPd3 (11:12, 9.4 m; small and healthy, only 2–3 years old), and CHPd4 (11:24, 5.7 m; a small, healthy colony in a shallow, high-light setting). Two are Seriatopora by field call: S. hystrix ChSe1 (10:30, 18.3 m; a large, healthy colony with very thin branches) and Seriatopora sp. ChSe2 (10:44, 18.7 m; tiny, with thin, short branches, a morphology Anderson had not seen before). The dive notes describe an incredibly calm, slightly murky lagoonal back reef with jack schools; P. verrucosa and P. meandrina were present, but Anderson focused on P. acuta and Seriatopora. 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

Thirty-four habitat frames exist for BIEG06 (33 in Anderson's dive folder and one in the "Best of Chagos" folder); close-ups of the colonies sampled are on file but are not shown here. Image 13 is the clearest methodology frame at this site — a diver carrying the white PVC quadrat frame, with a large snapper crossing directly in front of the lens as the frame is carried. Another frame also records the survey in progress, but what the diver there is carrying is a camera rig with twin strobes, not the quadrat.