Chagos Archipelago - Great Chagos Bank West (BIEA23)

Setting

The setting is a leeward fringing reef (from its position on the northwest side of the Eagle Islands reef platform relative to the prevailing southeast trade winds; Anderson's field sheet: windward) at 29.2°C and 34.3 salinity (station CTD).

General benthic features

The transects give the genus breakdown. Twenty-eight scleractinian genera were recorded, and the reef is Porites-led: Porites holds 9.9% of the benthos — 37% of all the live coral here. Pocillopora follows at 3.9%, Favia at 2.7%, Acropora at 1.9%, and Montipora at 1.8%. Below those, Stylophora 0.8%, Platygyra and Gardineroseris 0.7% each, Favites 0.6%, Astreopora, Montastrea, Pavona, Goniastrea, and Ctenella 0.4% each, and Echinopora and Leptastrea 0.3% each, then a tail of Fungia (0.2%), Pachyseris, Turbinaria, Hydnophora, and Psammocora (0.1% each), and Galaxea, Cyphastrea, Cantharellus, Acanthastrea, Leptoseris, Madracis, and Symphyllia (<0.1% each). On the non-coral side the transect gives invertebrates 23% — the largest single non-coral category (all algae together 40%) — crustose coralline algae 12%, bare substrate 11%, erect algae 9.7%, turf 8.2%, turf-plus-sediment 6.8%, macroalgae 2.7%, and cyanobacteria <0.1%. In the 30-point photo annotation, non-coral cover is dominated by turf (17% of all points), with crustose coralline algae 4.5%. About 26 minutes in, the camera drops onto the substrate and stays there — Image 3, Image 4, Image 6, Image 7, Image 16, Image 20, Image 21, Image 22, Image 25, Image 28, and Image 29 are all benthic. Image 11 is the highest-cover wide frame: cream and tan massive Porites domes with pitted surfaces, lavender-purple digitate Acropora, laminar plates, and two Moorish idols working the tops. Image 3 holds a brown corymbose Acropora thicket against cream Porites plates; Image 22 sets three open Acropora tables and a maroon Pocillopora-type colony against a tan meandroid brain; Image 4 has two clean tables with blue-green damselfish over them. Image 15 is the busiest wide frame, packing olive massive colonies, foliose plates, a bright orange branching colony, and dark encrusting sheets together.

3467162021222528291115

Point-count live cover in the benthic frames runs 53–80% (e.g., Image 20 highest at 80%) against 0% in the blue-water frames — a 0.0–80% range that is itself the argument against averaging an unfiltered frame set. Set against those, Image 4, Image 14, Image 16, Image 21, and Image 29 are dominated by pale carbonate rock and rubble carrying turf film, pink coralline crusts, and green algal patches with only scattered small colonies; Image 21 is the extreme, a single open tan Acropora table standing alone on otherwise bare pavement.

14

The macro sequence holds strong coral portraiture. Image 6 is the finest frame — a near-circular plating colony, deep chocolate-brown with dense fine white-tipped corallites and a clean pale growing margin on bare white pavement, with a nodular meandroid colony above it and a large smooth olive dome to the left; the growth form is compatible with Echinopora or Montipora and the frame does not settle which. Image 7 and Image 28 are close studies of tan-and-cream nodular-columnar colonies with dark maroon tissue in the valleys and pink coralline algae running along their margins. Image 2 is a crowded benthic frame carrying a long slender cornetfish crossing the field beside a black antipatharian wire coral, with an orange-red encrusting sponge on the pavement below.

2

A gorgonian sea fan stands in Image 14 and antipatharian wire corals appear in Image 12, Image 14, and Image 24; olive algal tufts appear in Image 4 and Image 25.

1224

Qualitatively, structural loss is present but low-key. The pattern is broad pale carbonate pavement between coral patches: Image 4, Image 16, Image 25, and Image 29 are largely bare, turf-filmed rock, and turf is the largest non-coral category in the photo annotation (17% of all points, peaking at 47% in Image 4). Image 15 and Image 22 show bare pavement rather than freshly killed colonies.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• 26%, from the Khaled bin Sultan Living Oceans Foundation's BIOT-1 in situ transect survey (the mean of all of this station's transects; survey published in Mayfield & Dempsey 2026, Oceans, suppl. S1); the photographs illustrate the site. Algal cover is 40% of the benthos.
• Transect mean 26 ± 3.2% SE (n = 13 transects; Living Oceans Foundation per-transect benthic data); the headline above is this all-transect mean.
• Coral cover across methods, and why the agreement here is not what it looks like. Method comparison only.
• The image methods read above the transect here, and the excess grows as water-dominated frames are removed.
• Seven of the 26 frames are 40% water or more, and all seven fall in the first half of the dive.
• The full-set agreement with the transect is blue-water dilution canceling a genuine high bias.
• Do not cite BIEA23 as evidence that photo and transect methods agree.
• A further caution attaches to the image products: "Unknown" runs 20–37% of points in twelve frames, at or above 30% in seven, diluting live-cover-as-percentage-of-all-points.
• The independent 30-point human annotation of the same frames (780 points) gives live hard coral as 25% of all points and apportions it Acropora 46%, Porites 31%, other hard coral 21%, and Pocillopora 1.6% of live coral.
• Frame-to-frame live cover runs 0% to 80%.
• Nothing photographic here is a measurement of the transect cover.

Stressors

Anderson's published per-site bleached fraction for BIEA23 is 5.8% (CoralNet analysis of the informal survey images; Mayfield & Dempsey 2026). Image 20 is the frame that needs care: an olive knobby colony on the left sits against a large fine-corallite plate whose surface grades from tan through to a broad pale gray-white area. The pallor is real, but at this range and lighting the photograph cannot separate a pale growth margin, a naturally pale morph, strobe overexposure, sediment or algal film, bleached tissue, and recently dead surface, and the annotator placed sixteen live Acropora points and only one bleached point on that colony. The frame carrying most of the annotated bleaching signal is instead Image 7, with seven of the site's eleven bleached points. Color balance in the shallower wide frames (e.g., Image 8, Image 14) is washed out enough that pale-looking tissue is not a reliable condition signal there either.

8

Anderson's sampling sheet records four pocilloporid colonies sampled here for a collaborator (species by field call): Pocillopora sp. ChJo65 (20.5 m, large, healthy), ChJo66 (15.5 m, 10% sick or bleached), ChJo67 (14.5 m, healthy), and ChJo68 (11.0 m, large, healthy, polyps extended). 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). No crown-of-thorns starfish is in Image 16, where an automated image detection flagged one (0.01% of the frame, a few pixels); the frame shows a rubble plain with corymbose, digitate, and massive colonies and small dark algal tufts, and Anderson's sampling sheet mentions none. The Living Oceans Foundation transects here recorded 0.1% cyanobacteria and no recently dead coral.

Fish and other fauna

Conspicuous fauna and sessile invertebrates appear as sponges in 13 frames, sea cucumbers in 3, giant clams in 2, anemones in 1, and seagrass in 1 — but the human points record zero sponge in all 26 frames, and the "seagrass" is most likely a calcified green alga. The first fifteen minutes (e.g., Image 1, Image 5, Image 8, Image 9, Image 10, Image 12, Image 18, Image 19, Image 27) are largely water column: schools of dark unicornfish and surgeonfish banked over the reef in Image 1 and Image 8, then wave after wave of yellowtail and yellow-striped fusiliers in Image 5, Image 9, Image 10, Image 12, Image 14, Image 18, Image 19, and Image 27.

15910181927

Macrofauna is modest but includes one clean record. Image 24 is the shark frame — a whitetip reef shark cruising a rubble-strewn sand plain, the white first-dorsal tip plainly visible and the animal unambiguous; a dark parrotfish forages on rubble in the same field. Image 17 holds a large dark triggerfish in mid-blue, with a diver in the background, and Image 9 a giant clam, fluted mantle open beneath the fusilier school.

17

Beyond those: a yellow tang in Image 25, bream- or snapper-type fish in Image 2 and Image 16, blue-green chromis in Image 4 and Image 22, a black-and-white striped reef fish over the domes in Image 11, and Moorish idols in three frames.

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, XL Catlin Seaview Survey at transect 36003, 4.16 km away, read 13% in 2015.

Dive record

Anderson's dive here began at 10:30 and lasted 57 minutes, to a maximum depth of 29.4 m (the LOF fellowship sheet); the photographs form one continuous sequence, and it is a dive of two halves.

Other notes

The transects were run in three depth strata (D2 22%, D3 31%, and D4 22% coral), and there are no per-frame depth data; the published site value, which weights each depth equally, is 25% (Mayfield & Dempsey 2026, Oceans, suppl. S1). Antipatharians and a gorgonian are present in the frames and are invisible to a hard coral point scheme — one reason the image products cannot describe this benthos completely. All 26 habitat frames were reviewed; there are no sampled-colony frames. Divers with housings appear in Image 15, Image 17, Image 18, and Image 19. 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 northwest side of the Eagle Islands reef platform relative to the prevailing southeast trade winds (about 178° 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 the Great Chagos Bank) puts the pin on the platform just south-southwest of Sea Cow Island, on its open-ocean (west-northwest) side, facing west-northwest (about 300°): windward for the northwest monsoon winds of the March survey (December to March), though it would be leeward for the southeast trades of April to November.