Chagos Archipelago - Speakers Bank (BISP101)

Setting

BISP101 sits just inside the north tip of the Speakers Bank rim, on the lagoon side ('lagoonal' on Anderson's field sheet), surveyed 28 April 2015 on a dive to 25 m (see Dive record below).

General benthic features

BISP101 is compositionally lopsided and Acropora-dominated. Every other genus is small beside it: Montipora 3.6%, Pocillopora 2.7%, Leptastrea 0.5%, Psammocora 0.8%, Porites 0.6%, Astreopora 0.6%, and Favia 0.5%, with Pavona, Favites, Millepora, Cyphastrea, and Montastrea below that. Thirteen coral genera were recorded on the transects. Crustose coralline algae hold 15% of the benthos, turf 10%, bare substrate 8.5%, macroalgae 2.5%, and non-coral invertebrates only 2.5%. The genus order is the same in both depth strata: Acropora, then Montipora. What changes is richness and algal cover, which rises from 34% to 45%. Reef structure is broad and laterally continuous rather than vertically towering. Large golden-brown plates recur from the foreground to the limit of visibility, often overlapping or standing on short central stalks, creating repeated horizontal shelves a short distance above the substrate and leaving open corridors between colony bases (e.g., Image 2).

2

Two frames show the field extending across the frame; a later run of frames demonstrates that the pattern persists along the photographed route; another gives a close view of living table surfaces (e.g., Image 5, Image 12, Image 6, Image 9, Image 2).

51269

Some plates appear thick and old, with irregular margins or localized dead patches; others retain fine radial branch texture and sharply defined edges. The spaces between the tables matter ecologically. Low rubble, consolidated pavement, and patches of brown or greenish growth form the understory, with occasional small branching and massive colonies (e.g., Image 1, Image 3).

13

The photographs document an extensive field of overlapping golden-brown tables above low pavement, rubble, and vegetated patches, shaded refuge beneath the plates, and localized bleaching in one pale branching thicket.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 52%, and Acropora alone holds 42% of the benthos — 80% of all coral recorded.
• Transect mean 52 ± 4.5% 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 50% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Both depth bands are the same reef.
• D3 carries 55% coral with Acropora at 42% across 13 genera; D4 carries 45% with Acropora at 41% across seven.
• Formal surveys give 52% coral, 37% algae, and 2.5% non-coral invertebrates, the coral 80% Acropora (78% in D3, 89% in D4).

Stressors

Published bleaching is 7.1% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). The clearest example is a frame where a compact field of branching colonies is pale lavender to white while surrounding tables retain gold-brown pigmentation (e.g., Image 4).

4

Anderson's notes record many dead tables where the reef was scoured at about 15 m, and three of the four pocilloporids sampled were sickly or partly dead, possibly from disease (see Dive record below). The Living Oceans Foundation transects here recorded no cyanobacteria and 0.4% recently dead coral.

Fish and other fauna

Several plate surfaces carry small dark fishes while others pass through the shaded gaps, so the table canopy provides habitat at multiple horizontal layers. A shark with the form of a nurse shark swims in the open over dead table plates (e.g., Image 11).

11

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 mid-morning dive on 28 April 2015 entered the water at 10:17 and lasted 47 minutes, to a maximum depth of 25 m (the field sheet); the notes describe more beautiful coral than on the earlier dive that morning (BISP100), with large Acropora tables from 25 to 20 m, a reef scoured at about 15 m with many dead tables, no P. acuta or Seriatopora hystrix, and odd thermoclines at around 15 m where the temperature dropped by 1°C or more. For a collaborator Anderson sampled four Pocillopora sp. colonies: ChJo262 (collected 10:35 at 15.7 m; healthy, green fluorescence in the interior, common at the site), ChJo263 (10:42, 15.6 m; enormous, 5% sickly, with some new mortality possibly from disease), ChJo264 (10:49, 14.8 m; 40% dead, possibly from disease), and ChJo265 (10:56, 14.3 m; huge, sickly, 20% diseased or dying); the notes add that these pocilloporids looked healthier than those on the bank fore reef. 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

Esri World Imagery (no LOF habitat map covers Speakers Bank; the LOF Chagos habitat map covers only Peros Banhos) puts the pin just inside the north tip of the Speakers Bank rim, on the lagoon side; Anderson's field sheet records only 'lagoonal', so no aspect is printed; the expedition's 933-site table classes the station as 'protected' (bank reef).