Chagos Archipelago - Blenheim (BIBL83)

Setting

The 16-frame set shows a dive that runs from a dense, structurally complex shallow reef down onto a hazy, lower-relief deeper slope.

General benthic features

The shallow ground is the best of it (e.g., Image 1, Image 2, Image 3, Image 4, Image 6, Image 7, Image 10, Image 15).

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It is a mixed, tightly packed reef of cream and tan plating and laminar colonies, honeycomb-textured massives with neat polygonal corallites consistent with Favites or Goniastrea, large smooth tan and olive massive Porites domes (e.g., Image 10, Image 15), meandering-valleyed brain colonies, small digitate and corymbose Acropora in cream with pale tips, mauve and pink Pocillopora, and a great deal of pink and magenta crustose coralline algae with bright red encrusting sponge threaded through it (e.g., Image 1, Image 2, Image 10). Image 7 is the most striking close frame: a very large tan leathery soft coral with thick digitate lobes filling the frame, a fine-textured pale gray tabular plate behind it in the water column, a green wire coral hanging beside it, and fusiliers streaming past overhead. Image 6 looks along the slope through a dense mid-water school of fusiliers over broad plating and tabular colonies. The deeper ground (e.g., Image 5, Image 9, Image 12, Image 13, Image 14, Image 16) is a different reef: hazier, lower in relief, with more sand and pavement between colonies, scattered mounding Porites and faviids, small branching colonies and rubble, and much less of the coralline-crusted framework.

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Image 5 looks across open sand with isolated massives; Image 16 is the haziest and shows an extensive low-relief field of small colonies and rubble. Porites was the most common coral genus on this station's transects (7.9% of the benthos), followed by Pocillopora (5.6%), Favia (3.1%), Stylophora (2.9%), Goniastrea (1.8%), and Platygyra (1.7%); bare substrate covered 3.9%, behind other invertebrates (24%), the largest living non-coral group (LOF transects). Among the algal classes, crustose coralline algae led (20% of the benthos), followed by turf (11%) and sediment-laden turf (5.2%).

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 33%, from LOF BIOT-1 in situ transects (the mean of all of this station's transects) — a diver transect measurement, not a computer vision estimate.
• Transect mean 33 ± 3.6% SE (n = 16 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 33% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The frames cannot corroborate or contradict that figure, and no cover was estimated from them.

Stressors

Condition, qualitatively: no obviously bleached, stark-white, or recently dead colonies appear in any of the 16 frames, and there are no disease lesions, no predation scars, no crown-of-thorns seastars, and nothing man-made. Some of the small digitate Acropora in Image 15 is cream with white tips, but at that working distance a photograph cannot separate naturally pale pigmentation from paling, and no bleaching figure is estimated here. Anderson's sampling sheet records two of the six colonies sampled here as 20–30% bleached, one with new disease or mortality, and one with some necrotic tissue (see Dive record below). The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and under 0.1% recently dead coral.

Fish and other fauna

A hawksbill-type turtle is the subject of four consecutive frames (e.g., Image 1, Image 2, Image 3, Image 4), resting on the reef with a pale, encrusted carapace as Anderson worked around it; those four are turtle portraits as much as habitat frames, though all four carry usable substrate. Fauna beyond the turtle: dense mid-water fusilier schools (e.g., Image 6, Image 7), dark surgeonfishes, damselfishes over the reef, and a dark, densely white-spotted fish resting on the substrate in Image 7 and Image 8 (puffer- or moray-type; the frames do not settle which).

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Comparisons with other data

The XL Catlin Seaview Survey covered a nearby station (transect 36020) in 2015, 0.90 km from the pin; we could not confirm that it is the same reef. Within 5 km, one independent station, XL Catlin Seaview Survey at transect 36020, 0.90 km away, read 17% in 2015.

Dive record

Blenheim, dived 22 April 2015. Anderson sampled six pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora sp. ChJo214 (collected 11:15 at 25.4 m; large and healthy, common at all depths, some old mortality), ChJo215 (11:21, 18.9 m; candelabrum morphology, 40% old mortality and 10% new disease or mortality, diseased part sampled), ChJo216 (11:27, 13.5 m; 30% bleached, very pronounced verrucae, bleached part sampled), ChJo217 (11:32, 10.3 m; an enormous colony, 20% bleached, mostly at the outer branch tips, bleached part sampled), and ChJo218 (11:38, 10.3 m; very darkly pigmented, recently toppled, some necrotic tissue), and Pocillopora verrucosa ChJo219 (11:45, 9.3 m; very pale, though likely healthy). The dive notes record murky water with some current along the wall, and no Seriatopora, P. acuta, or P. damicornis at the site. 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

Image 9 has two divers over that ground, Image 13 a distant turtle in the blue, and Image 11 is open blue water with a single fish and no substrate at all.

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What the frames cannot settle: depth is not recoverable frame by frame, so the shallow/deep contrast above is a habitat ordering rather than a measured profile; the haze in the deeper frames limits what can be identified there beyond growth form; and most colonies in this set cannot be taken past genus.

Nearby survey data. Independent survey data within 2 km: XL Catlin Seaview Survey transect 36020 (February 2015) recorded 17% coral cover, 0.90 km away. These are neighboring stations and different methods, not a survey of this site.