Chagos Archipelago - Great Chagos Bank West (BIEA25)

Setting

BIEA25 is a site of two halves, and the split is almost exactly chronological.

General benthic features

For the first forty minutes (12:26 through 13:08) the camera is 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). Then, from 13:15, the dive arrives at 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).

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Assemblage composition. Across 20 analyzed frames the Coralscapes masks put live hard coral at 17.0% of image area, of which Acropora (non-tabular) is by far the largest component at 60.1%; the rest breaks down as other hard coral 19.8% and massive and meandroid 15.5% of live cover. The independent 30-point human annotation of the same frames (600 points) gives live hard coral as 21.0% of all points and apportions it Acropora 49.2%, Porites 40.5%, other hard coral 5.6%, Pocillopora 4.8% of live coral. Non-coral cover is dominated by crustose coralline algae 4.3%, turf 23.0% of all points.

Coral cover

The 15.7% transect number is the average of two worlds and describes neither: the last six frames, shot inside three minutes, carry roughly three times the coral of the first fourteen. These six carry most of the site's blue-water load, which is why the un-filtered photo estimates sit closer to the transect than the benthic-only ones do. Frame-to-frame live cover runs 0% to 60.0% by point count (0% to 72.5% by mask area). Coral cover: 16%. Live hard coral measured on the in situ benthic transects of the LOF BIOT-1 expedition (Mayfield & Dempsey 2026, Oceans, supplemental S1), against algal cover of 62% of the benthos. The transect figure is this site's authoritative cover value; no image-derived product overrides it. Across methods the same reef reads: transects 15.7% · CoralNet points 21% · Coralscapes masks 17% (20 frames). These products have different sampling footprints and denominators and should not be averaged.

Stressors

Two frames matter for condition. One is a macro of a cream-white knobby branching colony with prominent, well-separated circular corallites, tissue apparently intact but pigment absent (e.g., Image 15).

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It is the one frame where both methods flag bleaching in the same place — 2 human bleached points and Coralscapes' highest bleached reading at BIEA25, 5.53% of image area — but its corallite scale looks coarser than the Porites the annotation called it, so genus and bleaching both want checking at full resolution. 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).

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Structural loss registers as dead coral 1.4% of image area and rubble 5.2%. Bleaching stands at 2.5% of annotated points — 15 points in total — concentrated in Acropora 1.7%, other hard coral 0.8% of points. No COTS were recorded by either method or seen in review.

Fish and other fauna

The macrofauna is dominated by one animal. A large octopus, compatible with Octopus cyanea, occupies three consecutive frames — shot between 13:01:53 and 13:02:19 — wedged against a maroon- and pink-encrusted ledge, mantle, papillae, and red-rimmed eye filling the frame; the last is the best (e.g., Image 13, Image 14, Image 23).

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(The existing description calls this "four consecutive close-up shots" and then lists three; there are three.) Beyond it, a blue sea star compatible with Linckia laevigata appears on CCA-crusted pavement in one frame and on the sand under the thickets in two more — the latter two five seconds apart and almost certainly one animal, so two sightings, not three (e.g., Image 24).

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

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

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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).

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

Comparisons with other data

No independent survey was made at this exact site; the nearest ones are listed in the table below.

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.