USA - Florida

South Florida holds the only significant coral reefs of the continental United States, and together with Bermuda and the northern Bahamas it marks the northern limit of reef growth in the western Atlantic (Wells 1988). North of Miami, true reef building gives way to octocoral-dominated hard bottom and, farther north, Oculina banks. The Florida Reef Tract itself is a band of reefs averaging 6.5 km wide that arcs roughly 370 km southwest from Fowey Rocks off southern Miami all the way to the Dry Tortugas, separated from the low Pleistocene-limestone Keys by Hawk Channel. It contains more than 6,000 patch reefs and about 96 km of outer bank reefs at 5–10 m, many with shallow spur-and-groove built by Acropora palmata; 62 stony coral species had been recorded from the Keys and Dry Tortugas. Wells (1988) adds that at least half the tract is poorly suited to coral growth, especially opposite the large tidal passes between Islamorada and Big Pine Key, where silty Florida Bay water spills onto the reefs on each falling tide. Spalding et al. (2001) describe Florida's reefs as among the most intensively studied and most degraded in the region: Acropora that covered up to 96% of the benthos in places in 1981 had dropped to about 3% by 1986, largely through white band disease, and overall coral cover was typically about 14% in the late 1990s. They give about 1,250 km² of coral reef for the USA's Atlantic sector.

Souter et al. (2021) report recent trends only for their Gulf of Mexico and Florida subregion, where mean hard coral cover slid steadily from about 13% (1999) to 9.1% (2019), and algal cover, after dipping to 16.9% in 2005, rose about 2.3-fold to 39% by 2015. The same report records that stony coral tissue loss disease (SCTLD) emerged in Florida in 2014, during a severe bleaching event and beside a dredging project on an urban reef, and calls it the deadliest disturbance on record in the Caribbean, with population losses above 90% in the most susceptible species. Earlier, Spalding et al. (2001) noted that bleaching in 1997 and the more intense bleaching of 1998, followed by Hurricane Georges, caused considerable coral loss in shallow areas, and listed ship groundings, eutrophication, sewage, and heavy fishing pressure among chronic threats. Wilkinson (2006), drawing on the 2004 status report for the American Caribbean, noted continuing fishing pressure along the Florida coast.

The Florida unit runs from the northern Gulf to the Lower Keys and covers very different settings. Anderson's dated dives run from May 2019 to November 2025, and recorded depths run from 1.8 m at MacArthur Causeway North to 29.3 m at the Orion shipwreck. The four Panhandle sites (Destin to Carrabelle) sit on limestone ledges, sand, seagrass, and artificial reefs rather than coral reef. Around Miami and Key Biscayne, the Urban Corals sites at MacArthur Causeway and Star Island are flat, silt-draped hard-bottom where tagged Pseudodiploria strigosa colonies were sampled, while Emerald Reef, the clean-water comparison site, is a low pavement under a meadow of sea plumes, sea rods, and sea fans. Flamingo Reef nearby is low-relief hard-bottom whose largest mounds are draped in zoanthid (Palythoa) mat, with no branching Acropora.

Three members are artificial structures: the Orion wreck, the Belcher Barge scatter, and Neptune Memorial Reef. On all of them stony corals occur only as scattered recruits among sponges, octocorals, and coralline algae, and fish schools are the main feature.

In the Upper Keys, Anderson's NOAA study sites span inshore and offshore reefs. The inshore reef Cheeca Rocks, where Anderson resampled in 2019 colonies tagged and genotyped by the NOAA AOML team in July 2017, had 32% coral cover in August 2016 (NOAA NCRMP and AOML photo transects; Manzello et al. 2019) and 29% on permanent plots in 2014 (Gintert et al. 2018), and heat stress there peaked at 22.3 degree heating weeks in 2023 (NOAA Coral Reef Watch, Florida Keys virtual station). Anderson's own 2019 frames and logs record the condition of the four study reefs that year: stony coral tissue loss disease (SCTLD, the diver's field call), a diseased Orbicella faveolata, and August bleaching at Cheeca Rocks; two tagged colonies paling at The Rocks in August; 'some incidence of bleaching' logged at Little Conch; and two diseased colonies and an active tissue-loss front at Crocker Reef in December. The Rocks' massive star corals show obvious partial mortality, with 9.3% cover in 2016, and Little Conch (6.9%) and Crocker Reef (1.5%) offshore are low-relief reefs with isolated colonies. North Dry Rocks has large, intact elkhorn (Acropora palmata) and dense staghorn thickets alongside outplanted Orbicella faveolata fragments, although some elkhorn show pale or partly bare patches, and its Coral Health Index is 1.00 on the 0-5 scale (Key Largo Dry Rocks also scores 1.00, and Little Conch 0.50). Key Largo Dry Rocks is gorgonian-dominated spur-and-pavement around the Christ of the Abyss statue. Looe Key in the Lower Keys keeps its spur-and-groove architecture, but living coral is thin and octocorals do most of the growing.

Independent surveys near many sites average low: 1.2% near Brewster Reef and 5.9% near Looe Key. Several Keys sites carry only Reef Check records from 1997-1999, which predate stony coral tissue loss disease. On the natural reefs Anderson photographed, a blind Claude computer-vision point count of the photographs puts coral cover between 8.3% at Key Largo Dry Rocks and 40% at Cheeca Rocks, and the artificial structures between 1.4% (Belcher Barge) and 4.2% (Orion shipwreck). The Broward and Palm Beach sites have their own Southeast Florida summary.