Chagos Archipelago - Great Chagos Bank West (BIEA52)

Setting

BIEA52 is a submerged bank reef inside the lagoon, leeward on Anderson's field sheet, surveyed 25 March 2015 on a dive to 19.8 m; the field sheet and photo folder also label it BiTb52 (Three Brothers).

General benthic features

The striking figure is neither the coral nor the algae: non-coral invertebrates hold 29% of the benthos, more than the entire algal budget, unresolved to taxon. Across 25 genera no taxon holds much more than a third of the coral (Porites 34%) — Porites 13% of the benthos, Acropora 7.4%, Pocillopora 5.5% (e.g., Image 1, Image 4).

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The profile runs from an invertebrate-dominated shallow band at 14% coral and 55% invertebrates, through two coral-abundant mid-depth bands at 46% and 43% carrying 17 and 24 genera, to a single deep transect at 34% where Acropora drops out and Pachyseris takes 18% of the coral. One frame shows a dead table colonized by a dozen live Acropora recruits.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 37% coral against 25% algae, with fleshy turf at just 3.6%.
• Image-based estimates (CoralNet, Claude, DINOv3, and CoralSCOP on 22 photographs) are listed in the method table for comparison only; the transect figure is this site's cover value.
• 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 25% of the benthos.
• Transect mean 37 ± 4.8% SE (n = 14 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 34% (Mayfield & Dempsey 2026, Oceans, supplemental S1).

Stressors

Published bleaching at this site is 0% (Mayfield & Dempsey 2026, suppl. S1), yet Image 8 shows a small intact bone-white colony among pigmented neighbors, and one of the two colonies Anderson sampled here was 10% bleached (see Dive record below), so the reef should not be called unbleached.

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Another frame shows a crown-of-thorns starfish beside a stainless-steel bone-cutting forceps laid on the substrate; one individual, no aggregation (e.g., Image 3).

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The Living Oceans Foundation transects here recorded 0.6% 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, Sheppard et al. (2008) at Great Chagos Bank, 4.70 km away, read a mean of 41% in 2006.

Dive record

Anderson's afternoon dive on 25 March 2015 entered the water at 15:05 and lasted 58 minutes, to a maximum depth of 19.8 m (the field sheet). Anderson sampled two pocilloporid colonies on this dive for a collaborator (Pocillopora sp.): ChJo140 (collected 15:16 at 14.2 m; large and healthy, its morphology intermediate between the typical form and the candelabrum form, common at the site) and ChJo141 (15:24, 19.6 m; 10% bleached, common at the site; the bleached part was sampled). The notes describe the “meter site”: calm with surge, good Acropora patches, a reef sloping from 12 m down to 19 m, and no Pocillopora damicornis, P. acuta, or Seriatopora hystrix. 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

Aspect follows Anderson's field sheet (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 this station; the LOF Chagos habitat map covers only Peros Banhos) shows open water at the pin about 4 km southwest of Middle Brother, inside the Great Chagos Bank; no bottom or reef edge is visible, so the imagery cannot test the aspect.