### BIBL86 — Blenheim
**CRD page:** [Blenheim Reef Coral Survey April 2015](https://coralreefdiagnostics.com/blenheim-201504)

**23 April 2015**
**GPS:** 5.17795°S, 72.45708°E
**Setting:** intermediate-exposure submerged forereef, 27.0 m
**Coral cover:** 47.2% (4 depth transects, D2–D5; **algal cover 44.2%**) · **published image bleaching:** 10.1%
**Live hard coral cover across methods:** formal transects 47.2% · CoralNet points 21.4% · Coralscapes masks 24.3% (27 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 29.7 °C · **Salinity:** 34.4
**Photo review:** all 27 stills examined
**Representative photos:** P4230850.jpg, P4230858.jpg, P4230859.jpg, P4230861.jpg, P4230868.jpg

BIBL86 is a submerged forereef at Blenheim under intermediate exposure, with coral and algal cover close to balanced — **47.2% coral against 44.2% algae, ranking 31st of 109 Chagos sites for coral cover and 37th of 109 for algal cover.** The algal budget is what sets the site apart: **fleshy turf (PTTS) reaches 22.6% of total benthos, the single highest turf value of all 109 Chagos sites**, against 13.4% crustose coralline algae (PCCATS, 56th of 109). Bare substrate is low at 3.0%, with PITS 5.6%, PMTS 5.3%, PETS 1.4% and PTSTS 1.5%. A reef that is simultaneously mid-pack for coral cover and first for turf is holding its cover on a turf-dominated rather than coralline-dominated ground.

Coral cover is spread across **29 nonzero genera (22nd of 109 for genus richness)**, and the 29 genus percentages sum to 47.2333% of the benthos — an exact match to the published cover figure. All values below are percent of total benthos, not percent of coral. **The leading taxon is *Millepora* at 10.63% — 23% of all the "coral" at the site — which is a fire coral, a hydrozoan rather than a scleractinian**, and its inclusion in the cover figure is a real caveat on comparing this site with scleractinian-dominated reefs (see Flags). *Porites* follows at 9.18% (19%), then *Pachyseris* 4.75% (10%), *Pocillopora* 4.65% (10%) and *Turbinaria* 4.20% (9%) — those five hold 71% of the coral recorded. *Echinopora* (3.32%) and *Acropora* (3.21%) come next — **note that *Acropora*, the dominant genus at most Chagos sites, is only seventh here** — then *Pavona* (1.20%), *Platygyra* (1.17%), *Goniastrea* (1.11%), *Heliopora* (0.67%, the blue coral, also non-scleractinian), *Leptastrea* (0.63%) and *Montipora* (0.57%), with a tail of 16 further genera each under 0.35%.

**The four depth transects swing violently — from 24.0% coral at the shallowest station to 66.0% at the deepest — and the leading genus changes three times down the profile.** D2, the shallowest station sampled, carries the profile's lowest coral cover at 24.0% against its highest algal cover, 66.6%, and is *Acropora*-led: *Acropora* 11.10% of the benthos, **46% of that transect's coral**, ahead of *Porites* 3.70%, *Montipora* 1.60%, *Pocillopora* 1.37%, *Pavona* 1.27% and *Favites* 1.00%. At D3 cover leaps to 61.2% and algae falls to 34.8%, and **the lead passes decisively to *Millepora* at 27.67% of the benthos — 45% of that transect's coral** — with *Echinopora* 11.27%, *Turbinaria* 7.73%, *Pocillopora* 5.03%, *Pavona* 2.07% and *Goniastrea* 2.00%. At D4 cover falls back to 37.7% against 46.5% algae while *Millepora* keeps the lead at 14.47% (38%), followed by *Turbinaria* 9.00%, *Pocillopora* 3.20%, *Echinopora* 2.00%, *Goniastrea* 1.93% and *Heliopora* 1.67%. Then **D5, one of only 18 D5 rows in the entire 336-row Chagos depth table, records the profile's highest cover at 66.0% against its lowest algae at 29.0%, and the lead changes once more — to *Porites* at 33.00%, exactly half of all the coral on that transect** — with *Pachyseris* 19.00% (29%), *Pocillopora* 9.00%, *Platygyra* 3.00%, *Acropora* 1.00% and *Leptastrea* 1.00%. **The site-level figures of 10.63% *Millepora* and 9.18% *Porites* describe no single transect**: *Millepora* is absent from the top ranks at both the shallowest and deepest stations while holding nearly half the coral in the middle of the profile, and *Porites* is a minor component until D5, where it dominates outright.

The photographs — all 27 stills in the folder — show a structurally complex forereef with extensive branching thickets, interleaved plates, massive colonies and dense fish activity. The opening frames show large schooling fishes moving over pale branching habitat. P4230850 contains a cluster of pale anemones, while P4230858 includes a diver and provides scale for the branching field. P4230861 and P4230868 show the most extensive thickets; the later frames retain mixed branching, plating and massive architecture rather than a single uniform growth form. Turf- and crust-covered framework is visible beneath and between the thickets, consistent with the archipelago-high turf value, with broader plate-dominated patches through the middle sequence.

**Condition: the photographs show widespread pale-to-white branching habitat, and the transect data suggest why identification is difficult.** The published image-based bleaching figure is **10.1%, ranking 27th of the 98 Chagos sites carrying a value.** P4230859 is a close view of a uniformly white compact branching colony; P4230861 shows extensive white branching thickets across much of the reef surface, with darker or yellow-brown colonies giving within-frame contrast; P4230868 retains numerous pale-to-white branches amid more pigmented colonies. The photo review flagged these as "white hydrocoral-like forms" without asserting a genus — and the transects independently make *Millepora* the leading taxon at this site, which is consistent with that read, though **a naturally pale hydrocoral and a bleached scleractinian cannot be separated from these photographs**, and no genus is assigned from imagery. The repeated within-frame contrast nonetheless supports genuine widespread paling in the photographed habitat. The published figure remains the sole site-level percentage. No crown-of-thorns starfish or diagnostic disease sign was identified in the 27 stills, and no tissue-loss pattern could be confidently separated from bleaching, old mortality or encrustation.

Fish are unusually conspicuous for a site-photo sequence. Dense schools fill the water column in the opening frames, and numerous dark reef fishes occur above the branching fields. These are qualitative records from non-standardized photographs rather than abundance estimates.

<b>Assemblage composition.</b> Formal transects give 47.2% coral against 44.2% algae on the most turf-dominated ground in the archipelago (PTTS 22.6%, 1st of 109), across 29 nonzero genera led by the hydrozoan *Millepora* (10.6%), then *Porites* (9.2%), *Pachyseris* (4.8%), *Pocillopora* (4.7%) and *Turbinaria* (4.2%), with *Acropora* an atypical seventh. The depth profile swings from 24.0% cover at D2 to 61.2% at D3, back to 37.7% at D4 and up to 66.0% at D5, the leading genus passing from *Acropora* to *Millepora* to *Porites* on the way down. The photographs document extensive branching thickets, plates and massive colonies with prominent fish schools, and record widespread pale-to-white branching habitat whose cause — bleaching or naturally pale hydrocoral — cannot be resolved from imagery.

**Flags:** (1) **The leading taxon, *Millepora* (10.63%), is a hydrozoan fire coral, not a scleractinian**, and *Heliopora* (0.67%) is an octocoral. Together they are 11.3 pp of the 47.2% "coral cover" figure — roughly a quarter of it. Any comparison of this site's cover with scleractinian-only figures, or with CV products trained on scleractinian classes, must account for this. (2) **This ambiguity bears directly on the bleaching read**: the white branching thickets in P4230859/P4230861/P4230868 may be pale *Millepora* rather than bleached scleractinian coral, and photographs alone cannot separate them. (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 (21.4%) and Coralscapes masks (24.3%) sit 23–26 percentage points below the formal-transect figure (47.2%, deviation range −25.8 to −22.9 pp). Do not average the three figures; the transect value is treated as ground truth for cover. (5) **The CV figures rest on a 27-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` = 4** for the site-level figures; the published `formal vs. informal comparison` sheet records **27** 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–D5 carry no metre boundaries** in `Chagos benthic data by site-depth (n=336 rows).csv`; they are ordered shallow-to-deep stations, and the 27.0 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) **The D5 row rests on visibly rounded values** (33.00, 19.00, 9.00, 3.00, 1.00, 1.00) unlike the fractional values at D2–D4, suggesting a single transect or a coarser count at that station — treat the D5 genus figures as lower-precision. (11) **No COTS, disease, or predation scarring was observed** across the full 27-frame review — an explicit absence-of-evidence finding. (12) **A diver appears in P4230858** — 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 BIBL86 benthic survey data (CSV)](../benthic_data/BIBL86_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).