Chagos Archipelago - Blenheim (BIBL86)

Setting

BIBL86 is a submerged fore reef at Blenheim, leeward on Anderson's field sheet, surveyed 23 April 2015 on a dive to 30.0 m.

General benthic features

The leading taxon is Millepora, a fire coral rather than a stony coral, at 31% of the recorded cover, ahead of Turbinaria, Echinopora, Acropora, and Pocillopora (e.g., Image 5, Image 10, Image 13, Image 18, Image 20).

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Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• Formal transects recorded 43% coral cover against 49% algae, with 22% fleshy turf (e.g., Image 8, Image 14, Image 16, Image 25, Image 29).
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• Transect mean 43 ± 4.9% 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 47% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The four depth transects swing hard, from 24% cover at the shallowest station to 61% at mid-depth under heavy Millepora dominance, back to 38%, then up to 66% at the deepest station where Porites holds half the coral.

Stressors

Published bleaching is 10%, and the photographs show extensive white branching thickets (e.g., Image 1, Image 2, Image 12, Image 15, Image 17, Image 19, Image 27), including the fire coral that Anderson's dive notes record as bleaching (severe bleaching, "especially pocilloporids and fire coral"; "most of the corals were bleaching, particularly fire coral").

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No crown-of-thorns or disease signs were identified in the 27 stills reviewed (e.g., Image 3, Image 6, Image 7, Image 11, Image 22, Image 28).

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The bleaching percentage is a computer vision figure — CoralNet annotation of the informal survey images, not a diver's field estimate — and human-vs-model agreement is weaker for bleaching (r = 0.40) than for coral cover (r = 0.72). Anderson's sampling sheet records two colonies sampled here: Seriatopora hystrix ChSe111 (29.0 m, tiny, 90% bleached, live tissue evident); Seriatopora sp. ChSe112 (26.2 m, small, sickly, >90% bleached). It also records three other pocilloporids sampled for a collaborator (species by field call): Pocillopora verrucosa ChJo221 (16.0 m, >90% bleached, live tissue still evident) and ChJo223 (10.7 m, 80% bleached, live tissue still evident); Pocillopora sp. ChJo222 (15.2 m, healthy, among mostly bleached colonies). 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). The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and 0.1% recently dead coral.

Comparisons with other data

Perry et al. (2015) surveyed a nearby station (Blenheim west side) in March 2015, 2.82 km from the pin; we could not confirm that it is the same reef. If you have survey data from this site to share, please email us.

Other notes

Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'forereef', 'leeward'); the expedition's 933-site table classes the station as 'intermediate' (fringing reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin on the northwest corner of Blenheim Reef, facing west-northwest (about 290°): leeward for the southeast trades that set in during April, as on the sheet, though it would face into the northwest monsoon winds of December to March.