Chagos Archipelago - Great Chagos Bank West (BITB58)

Setting

BITB58 is a channel reef off the north side of Middle Brother, leeward on Anderson's field sheet, surveyed 28 March 2015 on a dive to 23.7 m. The reef photographs as a broad, low-relief channel bottom rather than a wall or steep slope, and its dominant architecture is tabular (e.g., Image 30).

30

General benthic features

The formal assemblage is carried almost entirely by two genera: Heliopora at 31% and Acropora at 28%, together 88% of all coral recorded. Goniastrea (3.0%), Pocillopora (2.0%), and Porites (2.0%) follow, with Pavona and Favia below 1%. Only seven coral codes are non-zero. Crustose coralline algae hold 21% of the benthos, turf 5.3%, bare substrate 4.0%, macroalgae 2.0%, and erect algae 1.0%. The depth structure is the finding here, and it is extreme. The three transects are not variations on a theme; they are three different reefs. D2 carries only 28% coral, spread thinly across Goniastrea (9.0%), Pocillopora (6.0%), Porites (6.0%), Acropora (3.0%), Pavona (2.0%), Heliopora (1.0%), and Favia (1.0%). D3 jumps to 93% coral — and every coral point on that transect is Heliopora. D4 holds 80% coral, every point Acropora. Two adjacent depth bands, each effectively a monoculture, of different genera. This is genuine zonation, not a scoring artifact: the surveyor recalls the dive as blue coral as far as the eye could see. The site mean of 31% Heliopora should still be read as the average of three very different transects rather than a uniform site value. One frame is filled by a single enormous table Acropora, meters across, its olive surface set off by pale growing margins and a brilliant orange-red encrusting sponge at its rim; another pulls back to show a continuous field of such tables — olive and yellow-green, tiered and overlapping, running to the limit of visibility with white sand only at the margins (e.g., Image 19, Image 24).

1924

Two frames show the branching zone: multiple large staghorn thickets standing on clean white sand among massive Porites-type domes, reticulate faviids, and foliose plates, with clouds of damselfish overhead (e.g., Image 10, Image 29).

1029

Together these are the photographic match to the 80%-cover Acropora transect. The site is not uniformly dense. One frame looks down on broad white sand flats carrying only isolated coral patches, and two more show pavement where massive Porites domes, reticulate faviids, and pedestalled plates sit widely spaced on open sand — the ground that produces the 28%-cover D2 transect (e.g., Image 13, Image 33).

1333

Closer frames show more genus-level variety than the seven-code transect list implies. One frame holds branching Acropora-type thickets alongside large ruffled foliose plates in cream-yellow and one deep maroon, massive meandroid brain coral heads, smooth Porites-type domes, and encrusting colonies (e.g., Image 18).

18

Another contains a solitary free-living fungiid mushroom coral, radiating septa clearly visible (e.g., Image 25).

25

A third is a macro frame: a small brown Acropora whose branch tips are bright white — these are growing tips, not bleaching — wedged between dense brown-green foliose algae and reticulate faviids (e.g., Image 3).

3

A fourth is pure substrate: turf, dark red encrusting algae, green calcified algal segments, small ascidians, pink CCA, and a single pale coral recruit — the matrix behind the 21% CCA and 29% algal figures (e.g., Image 4).

4

A fifth holds the leading candidate for the site's dominant taxon: a robust olive-brown colony with a smooth surface and thick, blunt, finger-like branches, growing beside a delicate fine-branched coral and beneath a large meandroid brain coral (e.g., Image 31).

31

That blunt digitate form is characteristic of Heliopora, and it is the only colony in the set whose growth form matches the 93%-cover D3 transect (e.g., Image 31). Formal surveys give 67% coral and 29% algae, the coral split 88% between Heliopora and Acropora. Photographs document more genus-level variety than that — foliose plates, meandroid faviids, and Porites domes are all conspicuous — which is the expected direction of disagreement when three transects sample a heterogeneous channel bottom.

Coral cover

• Reference: Mayfield & Dempsey (2026)
• CoralNet submission package
• BITB58 has 67% transect coral cover, with a short genus list (7 genera).
• Transect mean 67 ± 20% SE (n = 3 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 67% (Mayfield & Dempsey 2026, Oceans, suppl. S1).
• Coral cover at BITB58 is therefore a mosaic of near-continuous tabular fields and near-bare sand, not a uniform 67% carpet.

Stressors

The published image-based bleaching figure is 0.8%, and one of the colonies Anderson sampled here was a large, healthy colony, though 5% bleached (see Dive record below). The single clear bleaching subject is a corymbose Acropora with its outer branches stark white and the colony center still fully pigmented brown — a partial, colony-scale event on an otherwise healthy reef, and exactly the kind of isolated case a sub-1% site figure describes (e.g., Image 8).

8

Two frames record partial mortality of older colonies rather than bleaching: one shows patchy dark red algal overgrowth across parts of the branching thickets with pale, algae-bound branches, and the other holds a large standing table whose whole upper surface is dead and algae-filmed, bare skeleton showing through, immediately beside healthy branching thickets (e.g., Image 17, Image 22).

1722

Both read as older loss on large colonies, not an active event. No COTS, no disease fronts, and no corallivore scars were seen in the frames reviewed. The Living Oceans Foundation transects here recorded no cyanobacteria and no recently dead coral.

Fish and other fauna

The channel carries substantial fish biomass, and this is the site's most consistent visual feature: eight of the 28 frames reviewed are dominated by a very large yellowback fusilier aggregation (e.g., Image 1, Image 11, Image 12).

11112

Two frames show it extending to the limit of visibility; six more catch it working over the pavement, with survey gear — a transect tape, a slate — visible at the frame edges (e.g., Image 2, Image 16, Image 20).

21620

Other fauna is sparse but notable: a large golden gorgonian sea fan and a barred grouper in one frame, a blue sea star on sand in another, black finger sponges, a vivid orange-red encrusting sponge on the rim of the great table, and dense damselfish clouds over the thickets throughout (e.g., Image 23).

23

Comparisons with other data

The XL Catlin Seaview Survey covered a nearby station (transect 36004) in 2015, 0.47 km from the pin; we could not confirm that it is the same reef. Within 5 km, two independent stations recorded station-year means of 21–41% hard coral cover between 2006 and 2015; the nearest, XL Catlin Seaview Survey at transect 36004, 0.47 km away, read 21% in 2015.

Dive record

Anderson's morning dive on 28 March 2015 entered the water at 10:34 and lasted 34 minutes, to a maximum depth of 23.7 m (the field sheet); the notes record a fairly strong current, reef only at 20–25 m (so the dive ran out of bottom time), and spectacular Acropora gardens. The sampling sheet lists eight colonies from this dive. Pocillopora acuta (field calls; CHPd57, CHPd58, and CHPd61 confirmed by RFLP): CHPd57 (collected 10:41 at 22.5 m; a large, healthy colony, though 5% bleached; a mix of healthy and unhealthy tissue was sampled), CHPd58 (10:52, 23.5 m; healthy, living under an Acropora table, dark pigmentation), CHPd59 (about 10:45; sampled by Andy), CHPd60 (about 10:45, 17 m; sampled by Alex), and CHPd61 (about 11:15, 25 m; sampled by Sam, appears healthy). Seriatopora hystrix: ChSe59 (10:54, 23.5 m; relatively healthy, among live and dead Acropora), ChSe60 (about 11:15, 23 m; collected by Alex; fairly common at the site), and ChSe61 (about 11:15, 23 m; collected by Sam). 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

One frame catches a surveyor working a slate above a large cream-tan foliose colony growing as a ruffled vase (e.g., Image 21).

21

Aspect follows Anderson's field sheet (LOF fellowship 2016 workbook: 'channel reef', 'leeward'); the expedition's 933-site table classes the station as 'protected' (channel reef). Esri World Imagery (no LOF habitat map covers this station; the LOF Chagos habitat map covers only Peros Banhos) puts the pin off the north side of Middle Brother, at the edge of its reef flat, facing north (about 10°): windward (close to the side-on cutoff) for the northwest monsoon winds of the March survey and leeward for the southeast trades of April to November; the sheet's 'leeward' matches the trade-wind reading.