Kenya - MMP snorkel site (Mombasa) snorkel

Setting

Surface light, wave shimmer, and a snorkeler's fins at the surface appear in the wide frames, confirming very shallow water throughout; visibility is moderate, green-tinged, and plankton-laden, with the far bommies fading into haze.

General benthic features

The site is a shallow back-reef bommie field: coalescing knobby, lobate, and columnar mounds of Porites type, a meter or so of relief, separated by floors of coarse sand, rubble, and low pavement. Frame after frame is the same architecture — dome beside dome, sand between. Algae are a major part of this site. Roughly a third of the set shows a dense fleshy canopy, brown, rust-red, and olive, draping whole bommie flanks and crests and in places burying the substratum between colonies (the frame Anderson named algal-choked is representative, not exceptional) (e.g., Image 1).

1

In several frames the macroalgae are overgrowing coral colonies, overtopping a bommie with a corymbose colony at its base and draping a mound flank around bleached branching colonies. This is a defining feature of the site, not incidental turf. Massive and knobby Porites dominate overwhelmingly, in every size from cobble-scale lobes to domes two meters across, along with columnar and finger-form Porites. Tabular and corymbose Acropora recur throughout — a meter-wide table growing off a bommie flank, several dinner-plate corymbose colonies wedged between Porites lobes, and two large bushy, finely branched Acropora-type colonies standing free on sand. Digitate and columnar corals in pale lilac, small encrusting and plating colonies (Montipora type, one distinctly green), and a scatter of cerioid massives fill in. Branching coral is subordinate to Porites but it is genuinely present and structurally conspicuous. A pink encrusting sponge or coralline and a tan barrel-form sponge occupy interstitial rock; a red whip-like organism rises from an algae-covered mound.

Coral cover

• Reference: Mayfield (2025)
• Data file: (Mayfield, 2025-Supplemental File S1)
• CoralNet submission package (accession #4963)
• Coral cover 40% — and 90% of hard coral tissue bleached (March-April 2024).
• Measured from 54 white-balanced photographs taken at this site during the 2024 Western Indian Ocean mass bleaching event and point-annotated in CoralNet, published in Mayfield, A.B. (2025) A Citizen Science Approach for Documenting Mass Coral Bleaching in the Western Indian Ocean. Environments 12(8):276. doi:10.3390/environments12080276.
• This is a published diver-photograph point-annotation value, not a computer vision estimate, and it is not recomputed here.
• Across the 20 Kenyan sites the mean was 30% coral cover and 77% of tissue bleached.

Stressors

Bleaching is severe and site-wide, entirely consistent with the published 90%. Whole wide frames read as fields of chalk-white, cream, and wan mint domes with only occasional pigmented colonies among them (e.g., Image 4).

4

Because these frames are color-adjusted, the strongest evidence is not absolute color but within-frame contrast and structure: one cream Porites dome retains a single discrete brown pigment patch on an otherwise uniformly whitened surface; several frames set fully pigmented ochre, tan, and orange-brown colonies immediately beside stark white ones under the same exposure; and the bleached surfaces show skeletal corallite texture reading straight through the tissue. Two large bushy branching colonies are bleached to a uniform white-lilac from base to branch tip. The pattern is a colony-by-colony mosaic rather than a total whiteout. Fully pigmented holdouts recur across the set. At least four separate deep violet-purple corymbose Acropora colonies appear across the set, each sitting unbleached in an otherwise chalk-white Porites pavement — so the vivid purple colony is a recurring feature of this reef, not a one-off (e.g., Image 2).

2

Several large Porites domes hold full orange-brown or ochre pigment, and a fully pigmented tan tabular Acropora sheltering a cloud of black-and-white humbug damselfish appears in two separate frames. One frame is a close-up of a large pale, mottled tabular colony with two deep purple branching colonies at its edge, one partly hidden under the table (Anderson: purple branching corals, not crown-of-thorns). No crown-of-thorns appear anywhere in the set. The near-total whitening across the 52 reviewed frames (of 56 on file) is entirely consistent with that 90%. Typical recent thermal stress: mean annual maximum of 4.9 degree heating weeks (2021-2025, NOAA CRW; Kenya virtual station). Seawater here was an estimated 32.5°C during the survey (read from the TG7 underwater camera and not validated, per Mayfield 2025, Environments 12:276, Table 1; the Kenya survey-wide mean of 32.0°C matched the 1998 event), and NOAA recorded a peak of 13.5 degree-heating weeks on 18 April 2024, the highest for Kenya since records began in 1985. For the 2024 Kenya survey as a whole, Mayfield (2025, Environments 12:276) estimated that in situ temperatures ran about 1.3°C above NOAA's, so that "nearly 22 DHWs may actually have transpired"; those in situ readings came from the TG7 underwater camera and have not been validated.

Fish and other fauna

Sergeant majors are everywhere over the bleached mounds. Humbug damselfish in the pigmented Acropora tables, a blackspotted sweetlips along a bommie flank, a dark surgeonfish over the crest, a small orange wrasse in the bushy branching colonies, and two frames of a large school of yellow-striped, yellow-tailed fish (goatfish- or snapper-type) drifting over open sand off the reef edge.

Comparisons with other data

We found no other surveys at this site as of 2026. Within 5 km, nineteen independent stations recorded hard coral cover between 1994 and 2022: two by benthic survey, station-year means of 22–27%, and seventeen by MERMAID bleaching-protocol quadrats, station-year means of 8.0–54%; the nearest, WCS Kenya via MERMAID (quadrats) at Mombasa 3, 0.52 km away, read a mean of 15% in 2016.

Dive record

Snorkeled 14 Mar 2024 at 1-3 m inside Mombasa Marine Park, during the 2024 Western Indian Ocean mass bleaching event. Surveys were run with Tropical Diving Kenya.

Other notes

This account is based on 52 of the 56 frames on file.