### BINI79 — Nelson Island
**CRD page:** [Blenheim, Nelson Island & Victory Bank Coral Surveys (Apr 2015)](https://coralreefdiagnostics.com/new-page-4)

**20 Apr 2015**
**GPS:** 5.68903°S, 72.32247°E
**Setting:** protected (originally logged leeward) emergent submerged bank reef, inside the Nelson Island lagoon, 23.4 m
**Coral cover:** 31.5% (3 depth transects, D2–D4; **algal cover 42.7%**) · **published image bleaching:** 5.0%
**Live hard coral cover across methods:** formal transects 31.5% · CoralNet points 10.8% · Coralscapes masks 11.3% (20 frames). These products have different sampling footprints and denominators and should not be averaged.
**Temperature:** 29.8 °C · **Salinity:** 34.2
**Photo review:** all 25 jpg stills examined (the folder holds 27 files; two are video clips — see Flags)
**Representative photos:** P4200858.jpg, P4200846.jpg, P4200861.jpg, P4200863.jpg, P4200837.jpg

BINI79 is a submerged bank reef inside the Nelson Island lagoon, surveyed at 23.4 m under protected exposure, and it sits below the archipelago median on almost every headline number: **31.5% coral against 42.7% algae, ranking 73rd of 109 Chagos sites for coral cover and 41st of 109 for algal cover.** The composition of that algal budget is what makes the site distinctive. Fleshy turf (PTTS) accounts for **19.8% of total benthos — the 5th-highest turf value of the 109 Chagos sites** — while crustose coralline algae (PCCATS) reaches only 9.4%, 85th of 109 and in the bottom quartile of the archipelago. Add 11.2% turf-and-sediment (PTSTS), 16.3% bare substrate (PB), 9.5% non-coral invertebrates (PITS), 2.0% macroalgae (PMTS) and 0.4% encrusting algae (PETS), and the picture is of a soft, turf-dominated lagoonal pavement rather than the hard coralline-cemented ground typical of the exposed forereef sites. The photographs bear this out directly: frame after frame shows low-relief pavement carrying an olive-brown turf mat with only patchy pink coralline crust, quite unlike the pink-and-mauve CCA pavement of the outer-atoll sites.

**Coral cover here is concentrated in an exceptionally small number of taxa: 12 nonzero genera, the 102nd of 109 Chagos sites for genus richness — in the bottom decile of the archipelago.** The 12 genus percentages sum to 31.5333% of the benthos, an exact match to the published cover figure. All values below are percent of total benthos, not percent of coral. *Acropora* dominates at **18.72% of the benthos — 59% of all coral present at the site** — followed by *Porites* 4.40% (14% of the coral), *Pocillopora* 3.83% (12%), *Favia* 1.95% (6%) and *Stylophora* 1.60% (5%). **Those five genera alone hold 97% of the coral recorded here.** The remaining seven genera are vanishingly small: *Galaxea* 0.38%, *Leptastrea* 0.17%, *Symphyllia* 0.17%, and then *Montipora*, *Pavona* and *Psammocora* at 0.08% each and *Echinopora* at 0.07%. There is no *Pachyseris*, no *Platygyra*, no *Goniastrea*, no *Millepora* and no faviid tail of the kind that carries 20–30 genera at the archipelago's more species-rich sites. This is a low-diversity, *Acropora*-driven lagoon assemblage.

**The three depth transects show coral cover peaking sharply at the middle station and collapsing at both ends of the profile, with bare substrate rising six-fold from shallow to deep as *Acropora* hands over to *Porites*.** D2, the shallowest station sampled, carries just 26.5% coral against the profile's highest algal cover, 55.5%; *Acropora* is already overwhelming at 18.25% of the benthos — **69% of that transect's coral** — with *Stylophora* 3.00%, *Favia* 1.75%, *Porites* 1.50%, *Galaxea* 0.75%, *Pocillopora* 0.50% and *Montipora*, *Pavona* and *Psammocora* at 0.25% each (9 genera). At D3 cover jumps to 43.6% and algae falls to 30.2%; *Acropora* climbs to 28.40% (65% of that transect's coral), *Pocillopora* rises to 8.00% — more than double its site-level value — and *Porites* sits at 4.20%, with *Favia* 1.60%, *Stylophora* 0.80%, *Galaxea* 0.40% and *Echinopora* 0.20% behind (only 7 genera). Then D4, the deepest station, **drops back to 24.5% coral and the assemblage changes character: *Acropora* falls to 9.50% — a third of its D3 value and now only 39% of that transect's coral — while *Porites* rises to 7.50% (31%)**, with *Pocillopora* 3.00%, *Favia* 2.50%, *Stylophora* 1.00%, and *Leptastrea* and *Symphyllia* at 0.50% each. **Bare substrate is the other axis of this profile: PB runs 5.0% at D2, 13.8% at D3 and 30.0% at D4**, so nearly a third of the deepest transect is open, uncolonised ground. Non-coral invertebrates run the other way (13.0%, 12.4%, 3.0%). Unusually for this dataset, all three depth rows account for 100% of the benthos with no shortfall. **The site-level figure of 18.72% *Acropora* describes no single transect**: it averages a shallow and a mid station where *Acropora* holds roughly two-thirds of the coral against a deep station where it has been more than halved and *Porites* has nearly caught it.

The photographs — all 25 stills in the folder — document a broad, low-relief bank-reef top rather than a steep wall, and they match the transect signal closely. The dominant habitat is a wide plain of pavement and consolidated rubble carrying turf, with coral present as scattered discrete colonies rather than continuous framework: P4200860, P4200864 and P4200861 show fields of small corymbose and digitate colonies consistent with *Acropora* and *Pocillopora* spaced across turf-covered ground; P4200862 shows the same growth forms sitting on open sand; and P4200843, P4200844 and P4200845 record extensive low-relief ground where turf and encrusting cover, not coral, hold most of the surface. Larger tabular *Acropora*-type colonies recur throughout (P4200837, P4200841, P4200846, P4200850, P4200858, P4200862) and are the site's most conspicuous structural element. P4200837 is the best single frame for reef architecture — a raised pavement ridge carrying tables, corymbose colonies and pink coralline crust, with a sand-and-rubble channel below it. Open sand and rubble margins appear in P4200842 and P4200862, and P4200840 records an undercut crevice in the pavement. Water clarity is moderate rather than excellent, and roughly a third of the set (P4200838, P4200843–845, P4200847–849, P4200853) is dominated by water column rather than substrate.

**Condition is the point on which the imagery and the published figure do not comfortably agree, and this is the main finding to carry forward.** The published image-based bleaching value is **5.0%, ranking 70th of 98 Chagos sites carrying a value — below the archipelago median.** The 25-frame review, however, found repeated and unambiguous pale-to-white colonies. **P4200858 is the strongest frame: two large tabular *Acropora*-type colonies in one view, both stark white across their outer margins with discoloured brown centres, against fully pigmented neighbours in the same frame** — a within-frame contrast that exposure or strobe artefact cannot explain. **P4200846** is a single large table, roughly a metre across, uniformly white over its whole surface with a second pale table behind it. **P4200861** shows a compact branching colony (growth form consistent with *Pocillopora* or *Stylophora*) that is completely white beside olive-brown colonies of similar size. **P4200863** is a close view of a branching colony mottled white and brown — the patchy pattern of a partially bleached, not a dead, colony. Bright white margins also appear on tables in P4200850 and P4200852. These records are logged as **ambiguous rather than confirmed bleaching**: white tabular *Acropora* with a fouled brown centre is equally consistent with a recently dead colony whose exposed skeleton has not yet been overgrown, and the distinction cannot be made from a single still. Either reading — active bleaching or recent mortality — sits awkwardly beside a published site figure of 5.0%, and the discrepancy is flagged below rather than reconciled. **No disease lesions, tissue-loss bands, *Drupella* aggregations or crown-of-thorns starfish were seen in any frame.**

Fauna is a strong element of this site. **A whitetip reef shark is documented resting under a pavement ledge on sand in P4200839.** Large pelagics appear twice in blue water off the reef — a solitary jack or trevally in P4200842 and a dogtooth-tuna-type predator in P4200847 and P4200848, the latter with a baitfish cloud behind it. Dense fusilier schools are near-constant over the reef top (P4200838, P4200841, P4200843–845, P4200849, P4200852, P4200854), at times obscuring the substrate entirely, alongside yellowfin goatfish, surgeonfish, damsels and anthias. **One corallivore was recorded: a cushion star (*Culcita*-type) sitting on pavement in P4200865** — logged for the predation record, not read as an outbreak. A diver deploying a survey frame is visible in P4200854, standard field documentation rather than a data concern.

<b>Assemblage composition.</b> Formal transects give 31.5% coral against 42.7% algae across only 12 nonzero genera (102nd of 109 for genus richness), with *Acropora* alone holding 59% of the coral and the top five genera holding 97%. The algal budget is turf-dominated — 19.8% PTTS, the 5th-highest turf value in the archipelago, against just 9.4% crustose coralline algae (85th of 109). The depth profile peaks at D3 (43.6% coral, *Acropora* 28.4%) and falls away at both D2 (26.5%) and D4 (24.5%), where bare substrate reaches 30% and *Porites* nearly matches *Acropora*. The photographs corroborate a low-relief, turf-covered lagoonal bank top carrying scattered tabular and corymbose *Acropora*, and add a whitetip reef shark, dogtooth tuna, dense fusilier schools, one cushion star, and five frames of conspicuous white or mottled colonies whose severity is not obviously compatible with the 5.0% published bleaching figure.

**Flags:** (1) **Photographic bleaching signal conflicts with the published figure.** Six of the 25 stills (P4200846, P4200850, P4200852, P4200858, P4200861, and the mottled colony in P4200863) show stark white or strongly pale colonies with pigmented neighbours in the same frame, yet the published `Mean(%bleaching)` for this site is only 5.0% (70th of 98, below median). The photographs are not scored and do not generate a replacement percentage; the conflict is recorded, not resolved. (2) **The white tabular colonies are ambiguous between bleaching and recent mortality.** P4200858's two tables are white at the margins with fouled brown centres — a pattern equally consistent with partial bleaching or with an exposed dead skeleton beginning to be overgrown. Not resolvable from a single still and not called either way here. (3) **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. Any rank taken off that column is approximate: it carries at least one known bad value (BITB58 listed at 22.22% against a corrected 0.77%). (4) **Multi-method deviation is large.** CoralNet points (10.8%) and Coralscapes masks (11.3%) sit 20–21 percentage points below the formal-transect figure (31.5%; published deviation range −20.7 to −20.2 pp), i.e. the CV products recover roughly a third of the transect cover. Do not average the three figures; the transect value is ground truth for cover. (5) **Frame-count mismatch:** the photo folder holds 25 jpg stills, but `chagos_multimethod_cover_by_site.csv` records `n_photos = 20` for the CV comparison. The CoralNet and Coralscapes figures above rest on that 20-frame scored subset, not on the 25 frames reviewed here. (6) **The photo folder is named `BiNi79-did not collect-finished`.** The annotation is unexplained and it does **not** mean the benthic survey was skipped: three depth transects (D2–D4) and a full site row exist in the published tables. It most plausibly refers to something else not collected on this dive (e.g. samples), but the meaning is unverified and should be confirmed with the field notes before the phrase is repeated anywhere public. (7) **Genus richness is exceptionally low: 12 nonzero genera, 102nd of 109 Chagos sites.** Several genera that are near-ubiquitous elsewhere in the archipelago — *Platygyra*, *Goniastrea*, *Millepora*, *Pachyseris*, *Favites* — are entirely absent from the transects here. (8) **Turf cover is near the archipelago maximum (PTTS 19.8%, 5th of 109) while crustose coralline algae is in the bottom quartile (PCCATS 9.4%, 85th of 109)** — an unusual inversion of the typical Chagos algal budget and worth carrying into any grazing- or nutrient-related analysis. (9) **Bare substrate triples with depth (PB 5.0% at D2, 13.8% at D3, 30.0% at D4)**, so nearly a third of the deepest transect is open uncolonised ground. (10) **One corallivore recorded:** a cushion star (*Culcita*-type) in P4200865. No COTS, no disease lesions, no *Drupella* and no predation scarring were seen anywhere in the 25-frame review — an explicit absence-of-evidence finding. (11) **Two video clips in the folder were not reviewed** (P4200855.MOV, P4200857.MOV); this is a still-photo review only. (12) **A diver and a survey frame are visible in P4200854** — standard field documentation, not a data issue. (13) **Formal replication is thin: `N Rows` = 3**, and there is no D1 transect, so the shallowest reef zone is unsampled. 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 23.4 m in the header is the site-level survey depth, not the depth of any one transect. (14) **Reef-class fields read oddly together:** `reef emergence` = emergent while `reef type` = *submerged* bank reef, and exposure is logged as leeward in the original column but revised to protected. Both values are reported as published; neither was altered. (15) **NON-CANONICAL BLEACHING RANK — added 2026-08-26.** Any "of 98" bleaching rank above was computed off the site table's `Mean(%bleaching)` column, which equals the **1st** CoralNet training run. Anderson's decision of 2026-08-26 (HOW_TO rule 4e) makes **`% bleaching (2nd training)` canonical**, The file carries TWO pools for it, and they are not interchangeable: a **110-row pool** ranking every dive (revisit dives included, so BIPB70B/72B/69C compete) and a **99-site pool** ranking one value per site. The rank quoted below is on the 110-row dive pool. For this site the canonical value is **5.05%, rank 77 of 110** (1st training: 5.05%, rank 77 of 110). The two runs agree to <0.05 pp on 100 of 110 rows and diverge only on the bleaching-onset dives, so for most sites the distinction changes the rank but not the number.

**Benthic data:** [Download BINI79 benthic survey data (CSV)](../benthic_data/BINI79_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).