Chagos Archipelago - Great Chagos Bank North (BINI78)

General benthic features

Porites was the most common coral genus on this station's transects (6.2% of the benthos), followed by Acropora (4.4%), Stylophora (3.3%), Pocillopora (3.2%), Montipora (1.2%), Favia (0.9%), and Platygyra (0.8%); bare substrate covered 6.6%, behind other invertebrates (23%), the largest single living non-coral group (LOF transects; all algae together 48%). Among the algal classes, sediment-laden turf led (17% of the benthos), followed by crustose coralline algae (14%) and turf (13%).

What the usable frames show is a low-relief, sediment-influenced reef. The wide views open on broad pale carbonate pavement and sand – Image 1 and Image 14 are largely sand and rubble with small colonies scattered singly across them, and bright open sand patches recur in Image 3, Image 4, and Image 9 – with the harder ground crusted pink and mauve by coralline algae and filmed with olive turf.

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Relief is low throughout; nothing in the set shows a wall, a spur, or a tall build-up. The corals present are mixed in form rather than dominated by one growth form: pale yellow-tan tabular plates lying almost in the plane of the substrate (a large intact one in Image 9, broad overlapping fields of duller plates in Image 4 and Image 15), overlapping olive-green foliose and plating colonies stacked into a low mound in Image 20, olive-yellow ruffled foliose colonies and pale green massive discs in Image 11, domal massives, and branching and digitate colonies throughout.

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Image 11 is the best structural frame in the photo set – close, downward-facing, no water column in it at all. It shows a large dark purple-brown digitate Acropora with white growing tips ringing its margin, an olive-yellow ruffled foliose colony at upper right, white-tipped corymbose Acropora at left and bottom, pale green massive discs, and blue chromis over the central colony. Image 6 is the other sharp close view: a large pigmented branching Acropora filling the frame, pale cream at the branch tips over olive and mauve bases. Image 19 is a single digitate colony, olive-yellow with cream-white tips.

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Two frames record activity rather than habitat. Image 10 is a sampling frame: a close view of a giant clam with a yellow-and-olive wavy mantle set into coralline-crusted pavement, with a gloved hand and a pair of stainless bone clippers in the frame – sampling was under way on this dive – and a small wholly white branching colony in the lower left corner.

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A diver appears upslope in Image 4 and Image 15. And a third pale thing in these frames is not coral at all: the pale blue-gray encrusting sponge sheeting over dead pavement in Image 8 and Image 20.

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 22%, 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 22 ± 2.9% SE (n = 9 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 19% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The photographs from this site are of limited quality.
• Image-based estimates in the method table are listed for comparison only; the transect figure is this site's cover value.
• Non-coral invertebrates cover 23% of the benthos on the transects, more than the live coral.

Stressors

The pale blue-gray encrusting sponge in Image 8 and Image 20 reads at a glance as coral tissue that has lost pigment. It is not bleaching. This is a site where the substrate's dominant secondary occupant is an organism that mimics paling, and any automated or rapid-visual assessment here will over-call for that reason alone. Published bleaching is 12% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1), and bleached coral is present in these frames. What the frames can be made to say, colony by colony, is this. Image 2 shows a massive dome bleached white over most of its surface, with a single retained tan pigmented sector. Image 12 shows a cluster of several stark-white plating and encrusting colonies just right of center, with pale plates scattered across the field, and Image 18 carries a bright white branching colony left of center and several very pale plates on the right.

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A small wholly white branching colony sits in the lower left corner of Image 10. Against all that, the pale cream branch tips in Image 6, Image 11, and Image 19 sit over fully pigmented bases and are the normal apical growth margin of a branching colony, not pigment loss.

Bleaching timing. BINI78 was surveyed 20 April 2015. Anderson's sampling sheet logs scattered bleached colonies from mid-March, more through late April, and bleaching was most widespread in early May. No crown-of-thorns starfish were seen here in 2015; the expedition's field report describes the back and patch reefs around Nelson's Island as damaged by an earlier outbreak, consistent with this site's low live coral cover (Bruckner 2015). Anderson's sampling sheet logs one of the four colonies sampled here as 15% bleached and two as very pale (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and no recently dead coral.

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, XL Catlin Seaview Survey at transect 36027, 1.76 km away, read 14% in 2015.

Dive record

Anderson sampled four pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora verrucosa ChJo201 (collected 10:34 at 19.6 m; large and healthy, very common at the site, below the thermocline at 29°C, with less evidence of bleaching there), Pocillopora sp. ChJo202 (10:40, 15.5 m; 15% bleached, an enormous colony of unknown identity, bleached part sampled) and ChJo203 (10:47, 14.5 m; very pale, likely still healthy, common at all depths), and P. verrucosa ChJo204 (11:00, 12.0 m; very pale, potentially still healthy, some old mortality; the shallowest coral at the site). The dive notes describe a very gradual slope from 20 m to 12 m (a 30-minute swim), low coral cover and high mortality, and no P. damicornis, P. acuta, or Seriatopora 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

Sixteen habitat frames survive from the dive: Image 1, Image 2, Image 3, Image 4, Image 5, Image 6, Image 8, Image 9, Image 10, Image 11, Image 12, Image 14, Image 15, Image 18, Image 19, and Image 20; photographs of the colonies sampled are not shown here.

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Six of the sixteen (e.g., Image 3, Image 4, Image 5, Image 12, Image 15, Image 18) are dim, blue-cast or outright hazy wide views in which condition cannot be read at range, and the two haziest are little more than a soft brown field of plates with a diver upslope. No scale object, quadrat, transect line, or depth stamp appears anywhere, so no colony dimension is measured and no depth zonation can be established from the imagery.