Chagos Archipelago - Speakers Bank (BISP95)

Setting

BISP95 is a high-cover lagoonal patch reef whose defining feature is not its coral but its heterogeneity — and a dense marine macrophyte meadow that no other site in this batch carries.

General benthic features

Bare substrate is correspondingly high at 15.2%, with coralline algae 9.6%, turf 8.7%, macroalgae 2.9%, non-coral invertebrates 1.2%, and erect algae 0.9%. The assemblage is strongly Acropora-led across 15 non-zero genera. Acropora holds 46.67% of the benthos — 82% of all coral recorded — then Porites 3.07%, Pocillopora 2.77%, Stylophora 0.93%, Heliopora 0.67%, Pavona 0.60%, and Cynarina and Platygyra both at 0.53%. Seven further genera fall below 0.5%. Algal cover barely moves (25.0%, 25.6%, 29.5%). The genus order is stable throughout — Acropora, then Porites and Pocillopora — so unlike most sites in this batch the site mean is a fair summary of composition, if not of cover. The strap-leaved meadow is the site's distinguishing habitat. One frame shows elongated ribbon-like blades rooted among coral and rubble, and a later run of frames shows a broad meadow occupying much of the bottom. Its form is compatible with seagrass or a seagrass-like marine macrophyte, but the photographs do not justify a species name and none is assigned. The meadow provides a habitat wholly different from the table-coral patches and is the main reason the site reads as heterogeneous. Coral architecture is most developed in the first and middle portions of the sequence (e.g., Image 1, Image 3).

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Two frames show overlapping tables with branching colonies in the gaps; two more retain table and corymbose forms but include more exposed substrate; another two show mixed coral patches reappearing after the open rubble views; and a final pair shows the rubble-dominated ground between them. The habitat is genuinely patchy — locally broad living canopies coexisting with low-relief areas where carbonate debris and old framework dominate (e.g., Image 2, Image 4).

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The photographs document a strongly patchy reef — table canopies, open rubble and pavement, and an extensive strap-leaved macrophyte meadow — plus localized partial bleaching in branching colonies.

Coral cover

Coral cover is 57%, 12th of the 109 Chagos sites, against 26.7% algae, but the photographic sequence turns over sharply from table-dominated coral habitat through open rubble and pavement into a broad vegetated meadow. Cover is flat across the two shallower bands and drops at depth. D2 carries 64.8% coral across 11 genera with Acropora at 53.40%; D3 is essentially identical at 65.4% across seven genera with Acropora at 56.60%; D4 falls to 40.5% across seven genera with Acropora at 30.00%. Assemblage composition. Formal surveys give 56.9% coral, 26.7% algae and 1.2% non-coral invertebrates, the coral 82% Acropora with a stable genus order across all three depth bands.

Stressors

Bleaching is localized but convincing, consistent with the published 6.5% (56th of 98). One frame shows several compact branching colonies with broad white tissue patches and white branch ends while retaining brown tissue elsewhere — a partial-bleaching pattern at high confidence. Another frame shows a table edge beside the vegetation with an extended white margin, but regular edge growth and close illumination are alternative explanations, so it is retained only at medium confidence. Other pale margins were excluded where whitening was narrow and regular. The vegetation and coral are in close contact in the table-edge frame, but the still does not establish competition, abrasion, or causation and no such inference is drawn. Scattered broken branches on the rubble fields likewise cannot be attributed to a storm, an animal, or the survey. No crown-of-thorns starfish (COTS) or other corallivores evident.

Comparisons with other data

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