Chagos Archipelago - Great Chagos Bank West (BIEA25)

Setting

Fringing reef, leeward from its position on the north side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (windward on Anderson's field sheet); the photographs show two habitats.

General benthic features

Porites was the most common coral genus on this station's transects (10% of the benthos), followed by Pocillopora (3.5%), Montipora (1.9%), Pavona (0.8%), and Acropora and Astreopora (0.5% each); bare substrate covered 17%, behind crustose coralline algae (26%), the largest single living non-coral group (LOF transects; all algae together 61%). Among the algal classes, crustose coralline algae led (26% of the benthos), followed by turf (20%) and sediment-laden turf (7.7%). The first frames are over a low-relief plain of pale consolidated carbonate, rubble, and coarse sand carrying small, well-spaced colonies — olive knobby and domal Porites-type heads, scattered small branching colonies, old tabular plates already given over to turf and coralline algae (four frames all show the same ground) (e.g., Image 2, Image 3, Image 9).

239

Later frames reach broad corymbose Acropora-type thickets standing clear of white sand — a run of six consecutive frames — and frame-level live cover jumps from single figures to 60% (e.g., Image 16, Image 18, Image 19).

161819

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 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 61% of the benthos.
• Transect mean 18 ± 2.1% SE (n = 19 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 16% (Mayfield & Dempsey 2026, Oceans, supplemental S1).
• 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

Two frames matter for condition. One is a macro of a cream-white knobby branching colony (e.g., Image 15): it is one of the four Pocillopora colonies sampled on this dive for a collaborator (ChJo73 on Anderson's sampling sheet, collected at 08:54), pale (about 10% of the colony) but likely healthy, per the sheet.

15

The other is a downward macro of pavement carrying a dense field of small dark boreholes or oscula — bioerosion or a boring sponge, not resolvable — ringed by pale coral recruits with blue-white polyp tips, which are healthy and should not be read as bleaching (e.g., Image 17).

17

No crown-of-thorns starfish was seen in review. Published bleaching is 18% (Mayfield & Dempsey 2026, suppl. S1); of the four colonies Anderson sampled here, one was 10% bleached or pale and one had 10% recent mortality (see Dive record below). The Living Oceans Foundation transects here recorded 1.1% cyanobacteria and no recently dead coral on the transect lines.

Fish and other fauna

The macrofauna is dominated by one animal. A large octopus occupies three consecutive frames wedged against a maroon- and pink-encrusted ledge, mantle, papillae, and red-rimmed eye filling the frame; the last is the clearest (e.g., Image 13, Image 14, Image 23).

131423

The thickets swarm with blue-green chromis, a blue-and-yellow damselfish in the branches of one frame, and a humbug-type damselfish in another (e.g., Image 10).

10

One frame is the site's best fish frame: three lined surgeonfish and two powder-blue surgeonfish over a broad mauve-pink corymbose table (e.g., Image 4).

4

The deepest and most algal frame — an eroded ridge falling into blue under pink CCA, maroon crusts, and yellow-green turf — carries a stream of fusiliers (Caesionidae) and a dark blotched mass that is probably an encrusting sponge but is not resolvable (e.g., Image 5).

5

Conspicuous fauna and sessile invertebrates appear as sponges in 7 frames, sea cucumbers in 3 frames, and anemones in 3 frames.

Comparisons with other data

Perry et al. (2015) surveyed a nearby station (Eagle Island middle site, Great Chagos Bank) in April 2015, 0.30 km from the pin; we could not confirm that it is the same reef. Cover values from that work are not among the survey records shown on this page. Within 5 km, one independent station, XL Catlin Seaview Survey at transect 36003, 2.35 km away, read 13% in 2015.

Dive record

Anderson sampled four pocilloporid colonies on this dive for a collaborator (the sampling sheet; Pocillopora sp.): ChJo71 (collected 08:41 at 29.9 m; small and healthy, too young to identify), ChJo72 (08:48, 20.3 m; 10% recent mortality, identity unsure; the healthy part was sampled), ChJo73 (08:54, 15.4 m; 10% bleached or pale, though likely healthy; the pale part was sampled), and ChJo74 (09:06, 10.9 m; large and healthy, either P. meandrina or P. eydouxi, very common at the site). The notes record calm, clear water, low coral cover at 10 m, and no Pocillopora 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

The opening sequence is a methodology record, not a benthic one. One frame shows a diver kneeling on the pavement working a black-and-white banded transect pole, camera housing and strobe beside them; another a second diver with slate and housing; a third two divers with the pole strung between them. One frame is the fullest apparatus frame at the site — five divers in open water, the banded pole, a camera tray, and at upper right a diver carrying a distinct white rectangular open frame that reads as a photoquadrat; another has a diver in the background blue holding the same frame. Anderson's field sheet records the aspect (LOF fellowship 2016 workbook: 'fringing reef', 'windward'); the expedition's 933-site table classes the station as 'intermediate' (fringing reef). The aspect shown, 'leeward', comes from the station's GPS position on the north side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (about 153° off the upwind direction; the reef-edge reading agrees), which takes precedence over slope facing, lagoon shelter, imagery, and field labels. 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 side of Eagle Island, facing northwest (about 300°): windward for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November.