Chagos Archipelago - Speakers Bank (BISP90)

Setting

BISP90 is a submerged bank reef inside Speakers Bank, windward from its position on the southeast side of Speakers Bank relative to the prevailing southeast trade winds (leeward on Anderson's field sheet), the northernmost of the Chagos survey areas, surveyed 24 April 2015 on a dive to 23.1 m (e.g., Image 2, Image 4).

24

General benthic features

Across 27 genera, Acropora leads at 16% of the benthos, 35% of all coral, ahead of Porites 6.5% and Montipora 4.0%, and the Chagos-endemic brain coral Ctenella chagius is present at 0.4%. The depth finding is a clean handover: the shallower stratum carries 47% coral and is emphatically Acropora ground at 18% of the benthos, 38% of its coral, while the deeper falls to 32% and the ordering inverts entirely — Montipora takes the lead, Acropora collapses to 5.1%, and Goniastrea and Echinopora rise from near-absence.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 44% coral against 41% algae, with a fairly even split among its three main algal groups: macroalgae 9.2%, turf 12%, and coralline algae 11% (sediment-laden turf 6.8% and erect algae 1.6% make up the rest).
• Transect mean 44 ± 3.3% 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 40% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (the mean of all of this station's transects), against total algal cover (all algal groups) of 41% of the benthos.
• Image-based estimates (CoralNet, DINOv3, CoralSCOP, and Claude) are listed in the method table for comparison only; the transect figure is this site's cover value.

Stressors

Published bleaching is 2.1% (CoralNet analysis of the survey images; Mayfield & Dempsey 2026, suppl. S1). The photographs show at least five small stark-white fine-branching colonies among pigmented corymbose, plating, and massive colonies, separate from the cream-tipped corymbose colonies with pigmented bases (e.g., Image 1).

1

All four Seriatopora hystrix colonies Anderson sampled here were 95-100% bleached, and the notes describe the bleaching as pervasive, though not yet in every colony (see Dive record below). No crown-of-thorns, disease, or predation was seen in the eight frames. The Living Oceans Foundation transects here recorded no cyanobacteria and 0.5% 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. If you would like to share yours, please email us.

Dive record

Anderson's late-morning dive on 24 April 2015 entered the water at 11:17 and lasted 54 minutes, to a maximum depth of 23.1 m (the field sheet); the notes describe a lagoonal back reef with a coral assemblage very similar to that of the earlier dive that morning (BISP89) but with higher recruitment, smooth glassy seas letting very high light reach depth, likely contributing to bleaching that was pervasive but not yet in 100% of the colonies, and Seriatopora hystrix very common at 20-30 m, nearly all of it dying or dead; there was no P. acuta. Anderson sampled four Seriatopora hystrix colonies: ChSe128 (collected 11:22 at 23.2 m; 100% bleached, possibly dead, live tissue not evident), ChSe129 (11:26, 20.9 m; more than 95% bleached, some pigment at the branch bases), ChSe130 (11:33, 19.6 m; 100% bleached with a neon-green fluorescent hue, live tissue not evident), and ChSe131 (11:39, 16.3 m; 100% bleached, rare above 15 m, no live tissue evident). For a collaborator Anderson also sampled four other pocilloporids (species by field call): Pocillopora verrucosa ChJo229 (11:43, 14.9 m; large, healthy, common at all depths), ChJo230 (11:46, 14.3 m; healthy, very dark pigmentation), and ChJo232 (11:52, 14.0 m; healthy, the shallowest coral on the reef), and Pocillopora eydouxi ChJo231 (11:50, 14.2 m; a strange-looking colony, healthy, the only one seen). 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 records the aspect (LOF fellowship 2016 workbook: 'submerged bank', 'leeward'); the expedition's 933-site table classes the station as 'protected' (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 lagoon side of the south-east rim of Speakers Bank, consistent with the sheet's 'leeward' and Anderson's note 'a lagoonal backreef'. The aspect shown, 'windward', comes from the station's GPS position on the southeast side of Speakers Bank relative to the prevailing southeast trade winds (about 11° off the upwind direction; the reef-edge reading agrees), which takes precedence over slope facing, lagoon shelter, imagery, and field labels.