### BIBL87 — Blenheim
**CRD page:** [Blenheim Reef Coral Survey April 2015](https://coralreefdiagnostics.com/blenheim-201504)

**23 April 2015**
**GPS:** 5.23198°S, 72.44305°E
**Setting:** intermediate-exposure submerged forereef, 27.4 m
**Coral cover:** 40.6% (3 depth transects, D2–D4; **algal cover 47.1%**) · **published image bleaching:** 9.6%
**Live hard coral cover across methods:** formal transects 40.6% · CoralNet points 18.6% · Coralscapes masks 16.0% (35 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 29.7 °C · **Salinity:** 34.3
**Photo review:** all 35 stills examined
**Representative photos:** P4230891.jpg, P4230959.jpg, P4230960.jpg, P4230972.jpg, P4230973.jpg

BIBL87 is a submerged forereef at Blenheim under intermediate exposure, with algal cover slightly exceeding coral — **40.6% coral against 47.1% algae, ranking 48th of 109 Chagos sites for coral cover and 31st of 109 for algal cover.** Crustose coralline algae (PCCATS) accounts for 17.3% of total benthos (37th of 109) against 12.5% fleshy turf (PTTS, 34th of 109). Two substrate categories stand out: **bare substrate is comparatively high at 7.4% (PB) and sediment over framework higher still at 14.8% (PMTS)**, with PITS 4.9%, PTSTS 1.6% and PETS 0.6% — a reef carrying noticeably more open and sediment-covered ground than its neighbour BIBL86 (PB 3.0%, PMTS 5.3%), 6 km to the north.

**The defining feature of this site is diversity, not cover: 36 nonzero genera — the highest genus richness of all 109 Chagos sites.** The 36 genus percentages sum to 40.5529% of the benthos (see Flags for the 0.08 pp shortfall against the published 40.6290%). All values below are percent of total benthos, not percent of coral. **No taxon dominates**, which is the other half of the same finding: *Acropora* leads at only 8.20% of the benthos (20% of the coral present), followed by *Porites* 5.97% (15%), *Pocillopora* 4.13% (10%), *Pavona* 3.99% (10%) and *Montipora* 2.63% (6%) — those five hold just 61% of the coral, against 71–86% for the top five at every other site in this batch. The next tier is *Goniastrea* (2.30%), *Favia* (1.68%), *Isopora* (1.66%), *Favites* (1.38%) and *Pachyseris* (1.25%), then a long tail of 26 further genera — *Echinopora*, *Stylophora*, *Gardineroseris*, *Seriatopora*, *Leptastrea*, *Fungia*, *Leptoseris*, *Psammocora*, *Platygyra*, *Turbinaria*, *Herpolitha*, *Symphyllia*, *Montastrea*, *Astreopora*, *Heliopora*, *Mycedium*, *Cyphastrea*, *Echinophyllia*, *Stylocoeniella*, *Hydnophora*, *Podabacia*, *Goniopora*, *Millepora*, *Lobophyllia*, *Galaxea* and *Acanthastrea* — each under 0.85% of the benthos, and the smallest (*Acanthastrea*) at 0.0095%. This is a comparatively even, high-richness assemblage rather than a high-cover one.

**The three depth transects peak in the middle of the profile, and the mid-depth station is the most even of the three.** D2, the shallowest station sampled, carries 36.5% coral against 45.6% algae and is the one *Acropora*-dominated station: *Acropora* 16.48% of the benthos, **45% of that transect's coral**, ahead of *Isopora* 3.48%, *Porites* 3.44%, *Pocillopora* 2.68%, *Favites* 1.52% and *Pavona* 1.40%. At D3 cover peaks at 52.9% and algae eases to 41.2%, and **the assemblage flattens out completely — the leading genus, *Pocillopora*, holds only 8.57% of the benthos, 16% of that transect's coral**, with *Pavona* 7.31%, *Goniastrea* 5.77%, *Acropora* 5.03%, *Porites* 4.57% and *Favia* 3.23% close behind; no taxon comes near dominance. At D4 cover falls to its profile low of 32.6% against the profile's highest algal cover, 54.5%, and **the lead passes to *Porites* at 9.90%** (30% of that transect's coral), with *Montipora* 4.55%, *Pavona* 3.25%, *Acropora* 3.10%, *Pachyseris* 3.05% and *Seriatopora* 1.25%. **The site-level figure of 8.20% *Acropora* describes no single transect**: *Acropora* is twice that at the shallowest station and roughly a third of it at the deepest, while the mid-depth peak in cover coincides with the point at which no genus leads by more than a few percent.

The photographs — all 35 stills in the folder — show a heterogeneous forereef alternating between open crust-covered pavement and structurally complex branching-and-table habitat, consistent with a site carrying both above-average bare substrate and above-average sediment cover. Early frames include relatively open carbonate framework and a diver carrying yellow equipment in P4230891. The central and later sequence becomes more coral-abundant: P4230959 shows broad branching beds across the slope, while P4230972 contains large overlapping tables above a diverse field of massive, submassive and branching colonies — a growth-form spread that matches the archipelago-high genus count, though photographic genus calls remain tentative. The set also records small fish schools and a large jack-like fish in P4230973.

**Condition reads as localized bleaching against a moderate published figure.** The published image-based bleaching value is **9.6%, ranking 31st of the 98 Chagos sites carrying a value.** P4230960 contains a pale-white table colony among darker neighbours, and P4230972 shows large, nearly white table surfaces with pigmented margins and surrounding colonies — both strong within-frame contrasts. P4230959 contains widespread pale branching beds, but distance and blue-water illumination reduce certainty, so it was not used as a stand-alone high-confidence record. The photographs corroborate individual bleaching observations without replacing the published site percentage. No crown-of-thorns starfish or diagnostic disease sign was identified in the 35 stills; some broken and encrusted branches are present, but their cause is indeterminate from imagery.

Fish are common across the sequence, including small schools over the slope and a large jack-like fish in P4230973. These are qualitative observations from a non-standardized photographic set and are not abundance estimates.

<b>Assemblage composition.</b> Formal transects give 40.6% coral against 47.1% algae across **36 nonzero genera — the highest genus richness of any of the 109 Chagos sites** — with no dominant taxon: *Acropora* (8.2%), *Porites* (6.0%), *Pocillopora* (4.1%), *Pavona* (4.0%) and *Montipora* (2.6%) together hold only 61% of the coral. The three-transect profile peaks at 52.9% cover at D3, precisely where the assemblage is most even, between an *Acropora*-dominated shallow station (45% of that transect's coral) and a *Porites*-led deep station under the profile's highest algal cover. The photographs document the matching alternation of open crusted pavement with branching-and-table habitat, prominent fishes, and localized pale-to-white table colonies consistent with the published 9.6%.

**Flags:** (1) **The genus percentages sum to 40.5529%, against a published site cover of 40.6290% — a 0.08 pp shortfall.** This is within rounding of the individual columns but is recorded rather than silently reconciled; BISA88 in this batch shows the same effect at 0.13 pp. (2) **The archipelago-high genus count (36, 1st of 109) is a count of nonzero genus columns in `Chagos benthic data by site (n=109).csv`**, and is sensitive to how near-zero values were recorded — the smallest entry here is *Acanthastrea* at 0.0095% of the benthos. The ranking is robust against BITB54's 34 and BISA88's 32, but the exact count should not be quoted as a species inventory. (3) **Exposure is blank in the `exposure-original` column** for this site; the header's "intermediate" comes from `exposure-revised`. No exposure class was invented. (4) **Multi-method deviation is large.** CoralNet points (18.6%) and Coralscapes masks (16.0%) sit 22–25 percentage points below the formal-transect figure (40.6%, deviation range −24.6 to −22.0 pp). Do not average the three figures; the transect value is treated as ground truth for cover. (5) **The CV figures rest on a 35-frame scored subset** which coincidentally equals the folder count here; the denominators still differ and the figures are not interchangeable. (6) **The CoralNet points figure is a CV product, not verified ground truth**; point-annotation data corpus-wide is roughly 76% unconfirmed machine output. (7) **Formal replication is thin: `N Rows` = 3** for the site-level figures; the published `formal vs. informal comparison` sheet records **35** rows for the bleaching comparison. (8) **Any rank off the `Mean(%bleaching)` column is approximate** — that column carries at least one known bad value (BITB58 listed at 22.22% against a corrected 0.77%). (9) **The depth codes D2–D4 carry no metre boundaries** in `Chagos benthic data by site-depth (n=336 rows).csv`; they are ordered shallow-to-deep stations, and the 27.4 m in the header is the site-level survey depth, not the depth of any one transect. **No D1 transect exists**, so the shallowest reef zone is unsampled. (10) **P4230959's pale branching beds were deliberately not counted as a bleaching record** — distance and blue-water illumination make the read unreliable; only the close-range P4230960 and P4230972 observations were logged. (11) **No COTS, disease, or predation scarring was observed** across the full 35-frame review — an explicit absence-of-evidence finding. (12) **A diver with yellow equipment appears in P4230891** — standard field documentation, not a data issue. (13) **The published bleaching figure is a COMPUTER-VISION product, not an in-situ observation.** It comes from CoralNet annotation of the informal survey images (Mayfield & Dempsey 2026, *Oceans*). Per-image human-vs-model agreement is substantially weaker for bleaching than for cover: r = 0.40 for bleached against r = 0.72 for live hard coral (`CHAGOS_MULTIMETHOD_README.md`). Treat the site bleaching percentage as the weaker of the two published numbers, and never read it as a diver's field estimate.

**Benthic data:** [Download BIBL87 benthic survey data (CSV)](../benthic_data/BIBL87_benthic.csv) — scleractinian cover by genus with full names, plus substrate and algal categories, from the LOF BIOT-1 survey (Mayfield & Dempsey 2026, *Oceans*, supplemental file S1).