Saint Vincent and the Grenadines

St. Vincent is among the youngest of the large volcanic islands in the Windward Group. The Soufrière volcano forms its northern end and last erupted in 1979, and volcanic activity through the late Pleistocene and Recent is thought to explain why its reefs are so poorly developed (Wells 1988; Spalding et al. 2001). The exposed north and east coasts have no reefs, and only small fringing reefs occur on the south, southeast, and west coasts. The roughly 28 rocky Grenadines to the south are a different setting. Three quarters of their platform lies 36–40 m deep without shallow reefs, but large bank barrier reef complexes have grown on the windward sides of Mayreau, Union, and nearby islands, their shallower parts often dominated by Acropora palmata above deeper head-coral zones (Wells 1988). In the Tobago Cays, each cay has its own fringing reef, Horseshoe Reef arcs around them to the east, and World's End Reef lies farther out (Spalding et al. 2001). Lewis recorded 30 coral species there in 1975, with Montastraea annularis dominating the fore-reef slope, but observations in 1980–84 found that most large A. palmata colonies had died at reefs such as Catholic Rock and Petit Tabac (Wells 1988). Spalding et al. (2001) report that the Tobago Cays had since deteriorated further through storm damage, white band disease, fishing gear, and anchors, and pollution from visiting yachts, and estimate about 140 km² of reef for St. Vincent and the Grenadines.

Souter et al. (2021) report recent trends only for the Eastern and Southern Caribbean as a whole, a subregion that groups the Lesser Antilles with Trinidad and Tobago, Aruba, Bonaire, Curaçao, and Venezuela. Across it, mean hard coral cover fell from about 26.2% in 1999 to 14.5% in 2019, the largest decline among the five Caribbean subregions, while algal cover nearly tripled between 1987 and 2019 to reach 48.3%. For the Caribbean as a whole, the same report links these losses to the 2005 mass bleaching, hurricanes, polluted, and nutrient-enriched coastal water, and overfishing of herbivores such as parrotfishes, and it names invasive lionfish, Sargassum influxes since 2011, and stony coral tissue loss disease (SCTLD) as newer threats; SCTLD, first seen in Florida in 2014, had been confirmed in 19 Caribbean countries and territories by October 2021. Earlier, Wilkinson (2006), drawing on the 2004 Caribbean Reefs at Risk assessment, placed the Lesser Antilles among the areas most threatened by coastal development and singled out its high islands for land-based pollution and sediment.

The four Saint Vincent and the Grenadines sites lie around Mayreau and the Tobago Cays in the southern Grenadines and date from a working trip between 26 September and 1 October 2019. The main focus was the natural volcanic CO2 seep off Mayreau, a shallow, silty patch reef dived 16 times to roughly 23-24 m. There, tagged colonies of Porites astreoides, Montastraea cavernosa, and Orbicella faveolata were sampled along a vent-to-control gradient for the Enochs et al. 2020 acidification study. The other sites are Mayreau Gardens nearby, the Horseshoe Reef arc that encloses the Tobago Cays lagoon, and a wreck off Mayreau dived at dusk on the final day. Logged depths ran from 11.3 m at the Mayreau Shipwreck to 24.1 m at Mayreau Gardens; Horseshoe Reef has no depth or underwater photographs, only topside views of the reef line.

The reefs photographed around Mayreau are sponge-and-gorgonian reefs rather than coral-framework reefs. Big brown tube and vase sponges, red and orange encrusting sponges, and a forest of sea rods, sea plumes, and sea fans rise off low, algal-crusted carbonate pavement, with crinoids in the crevices and white sand in the hollows. Stony corals are present but subordinate, as mounding, encrusting, and small brain-type heads. The dive notes for Mayreau Gardens also describe a lusher patch with boulder star coral, orange elkhorn-like colonies, and sea fans. Water was green and somewhat turbid in several frames.

The wreck, tentatively identified as the 1918 Puruni at about 11 m, carries only sparse coral colonization: small encrusting Siderastrea and Porites astreoides patches, a few brain corals, and sea fans and whip gorgonians. It holds a remarkable density of Caribbean spiny lobster in nearly every crevice. No bleaching, disease, or corallivores were observed there. Fish life was the standout at Mayreau Gardens, with clouds of small blue planktivores and a hundreds-strong school hanging over a thick carpet of green macroalgae. Blind computer-vision point counts of Anderson's edited photographs put coral cover at 23% (median 18%) at the CO2 vent site, 7.0% at Mayreau Gardens, where most frames are fish and open-water shots, 4.9% on the wreck, where low cover is expected, and 30% (median 25%) at a second vent found about 600 m to the south-southwest on the first dive of the trip; Horseshoe Reef has only topside photos. Enochs et al. (2020) describe high hard and soft coral cover around the vents but give no number.