Chagos Archipelago - Speakers Bank (BISP96)

Setting

BISP96 is a rim reef on the west side of Speakers Bank, surveyed 26 April 2015 on a dive to 30.7 m (see Dive record below); Anderson's notes describe a very gradual slope from 30 m up to a reef crest at 13 m.

General benthic features

Almost nothing here is not coral: non-coral invertebrates hold 1.8% of the benthos, coralline algae 4.4%, turf 4.3%, bare substrate 3.5%, erect algae 2.0%, and macroalgae 1.3%. The assemblage is overwhelmingly one genus across 21 non-zero codes. Acropora holds 66% of the benthos — 80% of all coral recorded — with Porites a distant second at 6.3%, then Favia 1.6%, Montipora and Pocillopora 1.5% each, Astreopora 1.3%, Leptastrea 1.2%, Platygyra 0.8%, and Goniastrea 0.7%. Eleven further genera fall below 0.5%, among them Ctenella, Cantharellus, Echinopora, and Gardineroseris — so the reef is not literally monospecific, but it is close. Algal cover halves from 14% to 7.0%. Canopy structure is the defining habitat feature. Tables overlap at several levels, creating broad shaded refuges, narrow corridors, and a complex understory. One frame shows very large tables filling the foreground, the next run of frames shows how plate edges repeat across the field, and a later run shows contiguous tables broken only by small coral heads and encrusted substrate (e.g., Image 3, Image 9, Image 10, Image 13, Image 15).

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Beneath it, red-purple encrusting material, massive colonies, smaller corymbose branches, and occasional sand or rubble pockets add the structural and taxonomic diversity that the smaller published categories record. Formal surveys give 82% coral, 12% algae, and 1.8% non-coral invertebrates, the coral 80% Acropora and rising to 86% of coral at D4. The photographs document an extensive multi-layered table canopy, encrusted understory framework, abundant schooling fishes, and two white colonies.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• BISP96 coral cover is 82%, with 12% algal cover.
• Transect mean 82 ± 3.3% SE (n = 15 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 86% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Cover rises with depth and so does the dominance.
• D3 carries 80% coral across 21 genera with Acropora at 62%; D4 carries 92% — with Acropora alone at 79%, 86% of the coral in that stratum — across only nine genera.

Stressors

Colony margins are often intact and pale-tipped and most surfaces remain gold-brown or olive, indicating retained pigmentation across the photographed canopy (e.g., Image 2).

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Published bleaching is 4.2% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). One frame shows a small branching colony wholly white beside pigmented tables and darker neighboring branches (e.g., Image 12).

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Another shows a wholly white corymbose colony beneath pigmented tables (e.g., Image 5).

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Anderson's notes record badly bleached colonies within several meters of a healthy colony sampled, and the one Seriatopora hystrix found here was 50% bleached (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and 0.1% recently dead coral.

Fish and other fauna

Fish abundance is visually notable. Two frames show streams of yellow-striped fusilier-like fishes above the tables and a third records another dense school over the canopy, with darker surgeonfish-like silhouettes and small reef fishes throughout (e.g., Image 11, Image 16, Image 1).

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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. If you would like to share yours, please email us.

Dive record

Anderson's morning dive on 26 April 2015 entered the water at 10:26 and lasted 48 minutes, to a maximum depth of 30.7 m (the field sheet); the notes describe a gorgeous site dominated by healthy table Acropora, a very gradual slope from 30 to 13 m, where the reef crested, and no P. acuta or P. damicornis. Anderson sampled one Seriatopora hystrix, ChSe142 (collected 10:50 at 15.5 m; 50% bleached, pigmented at the base, likely still healthy; the only colony seen at the site). For a collaborator Anderson also sampled three pocilloporids (species by field call): Pocillopora verrucosa ChJo247 (10:37, 25.6 m; diseased; the diseased tissue was sampled), and Pocillopora sp. ChJo248 (10:50, 20.5 m; pale, likely still healthy) and ChJo249 (10:56, 14.7 m; healthy, with badly bleached colonies within several meters). 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

Anderson's sampling sheet calls it a "gorgeous site dominated by table Acropora"; the numbers bear it out. Anderson's field sheet (LOF fellowship 2016 workbook) labels this dive both 'rim reef', 'windward' and 'channel patch reef', 'leeward', so no aspect is shown; the expedition's 933-site table classes the station as 'intermediate' (bank reef). Esri World Imagery (no LOF habitat map covers Speakers Bank; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the outer west rim of Speakers Bank, facing west: leeward for the southeast trades (about April to November) and windward for the northwest monsoon winds (about November to March).