USA - Dry Tortugas

The Dry Tortugas, about 116 km west of Key West, form a pseudo-atoll of seven keys on three main banks, Pulaski, Loggerhead, and Long Key, which ring a lagoon 12–23 m deep; roughly 100 km of open water separates them from the main Florida reef tract (Wells 1988). When the reefs were mapped in 1976, living reef occupied under 4% of the seafloor shallower than 18 m. Bank reef, confined to a narrow band on the southeastern edges of Long Key and Pulaski Banks, accounted for a mere 0.6% of the complex and had almost no Acropora palmata. Staghorn (A. cervicornis) reef covered 478 ha and supplied 55% of stony coral cover, mostly in one 220 ha reef northwest of Loggerhead Shoal, and coral-head buttresses of Montastraea annularis and Siderastrea siderea lined Pulaski Shoal. Research here is long-standing: the Carnegie Institution's laboratory on Loggerhead Key operated from 1904 until fire destroyed it in 1937. Wells (1988) ranks the Tortugas, far from mainland runoff, among the western Atlantic's least-disturbed reefs, but winter cold fronts periodically chill the shallows to lethal levels: a "black water" event killed nearly all staghorn in 1878, and a front in 1976–77 killed about 90% of its colonies, while the 44 ha elkhorn stand mapped on the Long Key reef crest in 1881 had been reduced to rubble by 1976. Spalding et al. (2001) give about 1,250 km² of coral reef for the USA's Atlantic sector as a whole, a figure that includes the Tortugas.

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 Florida's shallow reefs. Wilkinson (2006), drawing on the 2004 status report for the American Caribbean, described heavy losses from disease, bleaching, and human damage in the 1980s and 1990s that seemed to have leveled off at much lower cover.

The five Dry Tortugas sites are NOAA monitoring locations visited during the AOML climate-monitoring leg of late June 2021, with logged dives on 25-28 June. Logged maximum depths were 14.3 m at White Shoal (6.1 m average), 22.6 m at Bird Key Reef, 17.1 m at Texas Reef, and 25.6 m at Black Coral Rock, with Pulaski Shoal, near the park's northeastern boundary, a long-term USGS and NOAA location for tracking staghorn coral.

The stations differ sharply in character. White Shoal is carpeted in continuous thickets of staghorn, Acropora cervicornis, on a shallow rubble bottom, with mounding heads, brain corals, and gorgonians filling the gaps. Bird Key Reef is a gorgonian-draped hard-bottom instrumented with pH, light, and current sensors, where well-spaced massive heads, including Montastraea cavernosa and Orbicella faveolata, rise from stands of orange gorgonians. Black Coral Rock (logged to 25.6 m, 28.3°C dive average) is a low-relief rubble-and-turf bottom with scattered brain corals and mounding heads and far more fish than coral. Texas Reef, not one of the fixed stations, is a low rocky hard-bottom threaded with small swim-throughs, with scattered coral domes and tube sponges. Pulaski Shoal's surroundings are described as rubble, carbonate sand, seagrass, and remnant reef.

Hard coral is sparse across the group. The FWC-FWRI Dry Tortugas CREMP point counts of 2021, the year of these visits, read 3.3% at White Shoal, 6.8% at Texas Reef, 7.8% at Bird Key Reef, and 12% at Black Coral Rock, where the four stations ranged from 9.5% to 17%; the NOAA NCRMP station 0.58 km from Pulaski Shoal recorded no stony coral in September 2021. Shallow-station temperatures ran from the low 20s to nearly 32°C. Black Coral Rock has a Coral Health Index of 0.75 (0-5 scale). At Bird Key Reef one Siderastrea head showed white patches of tissue loss, and on 27 June at least two more massive colonies carried tissue-loss fronts consistent with stony coral tissue loss disease (SCTLD), beside a recently dead colony; Black Coral Rock had one more SCTLD-consistent front. The Pulaski Shoal site record also notes published USGS work documenting severe heat stress and loss of elkhorn coral, Acropora palmata, across the Dry Tortugas in the 2023 marine heatwave, two years after these visits.