Chagos Archipelago - Cauvin Banks (BICA01)

Setting

Submerged patch reef, leeward on Anderson's field sheet · station CTD 28.9°C · salinity 34 · 6.80368°S, 72.38544°E · marked Training in the supplement · the only site in the Cauvin region · dived 9 March 2015, the expedition's first survey, site #1 of 109.

General benthic features

Assemblage composition from transect data — the reef is nearly bare, and the little coral on it is evenly shared. The transect records eleven genera, summing exactly to the 4.6% cover: Porites 1.5%, Montipora 1.3%, Pocillopora 0.6%, Acropora 0.4%, Cyphastrea 0.2%, Favites 0.2%, Goniastrea 0.2%, Leptastrea 0.2%, Stylophora 0.2%, Fungia 0.1%, and the hydrocoral Millepora 0.1%. As a share of live coral that is Porites 32% and Montipora 28% — no genus holds even a third of the assemblage. H′ = 1.886, Simpson (1−D) = 0.794, Pielou's J′ = 0.787.

The benthos closes cleanly and is dominated by things that are not coral: coral 4.6%, algae 57%, bare substrate 37%, and non-coral invertebrates 1.2% (LOF transects). Algal cover breaks down as sediment-laden turf 22%, crustose coralline algae 16%, erect algae 9.1%, turf 7.4%, and macroalgae 2.5%; no cyanobacteria were recorded. Bare substrate is at 37% and sediment-laden turf at 22%. One-third bare, one-fifth sediment-bound turf, one-sixth CCA — mostly open substrate under a fine algal skin, not a collapsed framework. Image 1 is a dense low mound of bright-green strap-shaped leaves — a seagrass meadow, its leaves in terminal clusters on dark red-brown woody stems visible along its left margin — on open pale carbonate sand; no coral in frame at all.

1

It is the best-lit of the three. Image 2 is a wide, dim, deep-looking view of exactly the substrate the transect describes: sediment-veneered pale carbonate with scattered dark olive erect tufts, pink-mauve tufts compatible with articulated coralline algae, and no coral framework; it is low-contrast. Image 3 is the best habitat frame — low-relief pavement running away to a dark blue horizon under continuous dark turf stubble, with two dark surgeonfish-type acanthurids grazing over it and a few small smooth olive-gray discs compatible with encrusting or small massive colonies but not callable; it too is dim.

23

What the frames do support, qualitatively, is the character the transect describes: open substrate under a fine algal skin rather than a collapsed coral framework — a reef that is sparse rather than wrecked.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 4.6%, from the Khaled bin Sultan Living Oceans Foundation's BIOT-1 in situ transect survey (the mean of all of this station's transects); the photographs illustrate the site.
• Transect mean 4.6 ± 0.8% SE (n = 4 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 4.6% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• The survey comprised four transects × 500 points = 2,000 points, per-transect 6.2 / 4.0 / 5.6 / 2.6%, SD 1.6, and it describes a uniformly sparse bottom rather than a patchy one.
• The survey sits in a single depth stratum, D4 (dived to 22.6 m); there is no depth profile to read.
• Image-based estimates in the method table are listed for comparison only; the transect figure is this site's cover value.

Stressors

Anderson's published per-site figure for BICA01 is 0.0% bleached; nothing is estimated from the photographs. Qualitatively, no bleached tissue, crown-of-thorns, disease lesion, or predation scar is visible in any of the three frames — an absence in three photographs, not a survey result. The 0.0% belongs with the reading above rather than against it: the reef is not stressed, it is sparse. Anderson's sampling sheet records no bleaching on any of the six colonies sampled here (see Dive record below). The Living Oceans Foundation transects here recorded no 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. If you would like to share yours, please email us.

Dive record

Anderson sampled six pocilloporid colonies on this dive for a collaborator (species are field calls; the sampling sheet): Pocillopora verrucosa ChJo1 (collected 14:11 at 21.7 m; healthy), ChJo2 (14:17, 21.5 m; small and healthy), ChJo3 (14:19, 21.0 m; healthy, with very dark pigmentation), ChJo4 (14:24, 21.3 m; a very dense, stubby colony), and ChJo5 (14:30, 21.5 m; small, with light pigmentation), and Pocillopora sp. ChJo6 (14:35, 21.5 m; possibly P. elegans, with very poorly defined verrucae). No bleaching is recorded for any of them. The dive notes record extreme surge and low coral cover; P. verrucosa was the only pocilloporid found at the site, with no P. damicornis, P. acuta, or Seriatopora. 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

Image-by-image analysis — three frames, and they corroborate the transect and add texture, nothing more. The entire photographic record for BICA01 is three habitat frames; there are no sampled-colony photographs, and three frames cannot characterize a site. Not one of the eleven transect genera can be confirmed photographically. No frame carries a scale object, quadrat, or transect line, and none has a depth stamp, so nothing in them is measurable and none can be placed on the depth profile. Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'submerged patch reef', 'leeward'); the expedition's 933-site table classes the station as 'exposed' (patch 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 eastern edge of a small submerged bank (no bottom is visible at higher zoom), facing east (about 90°): leeward for the northwest monsoon winds of the March survey, though it would face into the southeast trades of April to November. The expedition's transect classes have no seagrass label, so any seagrass on the transects is counted as algae.