Chagos Archipelago - Speakers Bank (BISP100)

Setting

BISP100 is a high-cover but visibly patchy bank reef at the north tip of Speakers Bank, surveyed 28 April 2015 on a dive to 15.7 m (see Dive record below), with 41% transect coral cover against only 34% algae.

General benthic features

Bare substrate is the notable substrate figure at 21%, with turf 12%, coralline algae 7.0%, macroalgae 4.3%, non-coral invertebrates 4.2%, and erect algae 0.4%. The assemblage is strongly Acropora-led across 16 non-zero genera. Acropora holds 23% of the benthos — 56% of all coral recorded — then Porites and Montipora 4.1% each, Astreopora 2.4%, Leptastrea 1.7%, Pocillopora 1.5%, Psammocora 0.9%, and Goniastrea 0.7%. Pavona and Montastrea hold 0.6% each, and six further genera sit below 0.5%: Favia, Favites, Hydnophora, Platygyra, Acanthastrea, and Cyphastrea. The site mean of 23% Acropora therefore averages a moderately diverse main stratum with two single deeper transects that are nearly all Acropora. The reef architecture alternates between locally dense coral patches and open framework. One frame shows several overlapping tables and branched colonies set at different heights, producing shaded cavities and small fish refuges (e.g., Image 4); two others show the same arrangement at wider scale, with live plates spaced by pale pavement and older irregular substrate (e.g., Image 3).

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Formal surveys give 41% coral, 34% algae, and 4.2% non-coral invertebrates, the coral 56% Acropora and reaching 64% of the benthos on the single deepest transect. The photographs document a patchy canopy of overlapping tables and branching colonies separated by pale pavement, one partly bleached colony, and a large holothurian.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Cover rises with depth and the assemblage narrows sharply as it does.
• Transect mean 41 ± 4.7% SE (n = 13 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 54% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• D3 carries 37% coral across 15 genera, with Acropora at 17% and a mixed runner-up field — Montipora 4.4%, Porites 4.3%, Astreopora 2.9%.
• D4, a single transect, rises to 53% across only six genera with Acropora at 43%.
• D5, also a single transect, reaches 72% coral across just three genera — Acropora 64%, Montipora 4.0%, Porites 4.0% — on 7.0% algal cover.
• One frame is notably more open: the photographs show a patchy rather than continuous canopy, as the 21% bare-substrate figure records (e.g., Image 1).
  1

Stressors

Published bleaching is 4.8% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1), and Anderson's notes record most pocilloporids at the site bleached, diseased, or partly dead (see Dive record below). In the photographs, a compact branching colony is mostly brown with many white branch ends and several larger white patches (e.g., Image 2).

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Much of the pale ground in the open frames is old eroded framework or sediment rather than bleached tissue. The Living Oceans Foundation transects here recorded 0.2% cyanobacteria and 0.4% recently dead coral.

Fish and other fauna

The same frame holds a large dark reticulated sea cucumber beneath the colony (e.g., Image 2). Small black-and-white fishes shelter among the branches.

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 28 April 2015 entered the water at 08:15 and lasted 48 minutes, to a maximum depth of 15.7 m (the field sheet); the notes record a mild current that picked up through the dive, only one depth within swimmable distance, no Seriatopora hystrix or P. acuta, and most pocilloporids bleached, diseased, or partly dead. For a collaborator Anderson sampled six pocilloporids (species by field call): Pocillopora sp. ChJo256 (collected 08:23 at 15.2 m; healthy, dark pigmentation and green fluorescence, common at the site), ChJo258 (08:39, 15.3 m; many pigment anomalies, 10% bleached or dead), ChJo260 (08:50, 14.9 m; large, 10% diseased), and ChJo261 (08:56, 14.6 m; 10% bleached, diseased, or dead; the healthy part was sampled), and Pocillopora verrucosa ChJo257 (08:30, 15.3 m; 15% bleached or diseased) and ChJo259 (08:44, 15.4 m; mostly diseased). 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 field sheet (LOF fellowship 2016 workbook) leaves the reef type and aspect blank, so no aspect is printed; 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 northern tip of the Speakers Bank rim (open bank water; aspect not testable at this scale).