Mesophotic Coral Ecosystems

3.4. The United States Virgin Islands, USA Tyler B. Smith , University of the Virgin Islands, USA Daniel Holstein , University of the Virgin Islands, USA

The insular shelf of the United States Virgin Islands (USVI) supports diverse MCEs that form on steep walls around the island of St. Croix and on the extensive banks and steep walls in the northern USVI around the islands of St. John and St. Thomas (Figure 1). Seventy-five per cent of the total shelf area above 65 m depth (1918 km 2 ) is potentially MCE habitat (25–65 m), suggesting that MCEs could be more extensive than shallow reefs. This is certainly true around St. John and St. Thomas on the southeast Puerto Rican Shelf, where the identified hard bottom habitat below 30 m depth constitutes 60 per cent of the total hard bottom habitat (137 km 2 ). * The northern USVI presents one of the most spectacular known examples of bank reef MCEs in the Caribbean (Figure 2). Within the well-characterized MCE depths (30–45m) of the southeastern Puerto Rican Shelf, there is strong habitat heterogeneity, with shelf-edge reefs forming on a drowned barrier reef complex andmore inshore banks forming at similar depths (Smith et al. 2010). The most extensive area of reef development is on the southern entrance to the Virgin Passage, separating USVI from Puerto Rico. This area may represent one of the best developed MCEs within the U.S. Caribbean. The shelf-edge reefs of the Virgin Passage tend to be low in coral cover (< 10 per cent), most likely as the result of natural disturbances from storms (Smith pers. obs.), whereas the Atlantic Ocean

secondary and tertiary bank reefs have higher coral cover (25–50 per cent) — representing the highest in USVI and very high for the Caribbean (Smith et al. 2010). Importantly, the dominant coral genus that forms over 85 per cent of coral cover is Orbicella , which has recently been listed as threatened under the U.S. Endangered Species Act (NOAA 2014). This genus is very abundant in the upper mesophotic zone, with a conservative estimate of 50 million Orbicella colonies on the 23 km 2 of hard bottom habitats in the Hind Bank Marine Conservation District (Smith 2013). Other bank reef systems at similar depths in the Western Atlantic may be similarly dominated by Orbicella spp., while only 6 per cent of theMCEs of the south shelf are in the no- take or restricted-take fishery areas (Kadison pers. com.). MCE development around St. Croix is limited by a mostly narrow shelf that drops steeply into deeper water, which may typify many small island MCEs of the Caribbean. Only 13 per cent (48 km 2 ) of the St. Croix shelf is at mesophotic depths (25– 65 m), which is a much smaller area than that of the northern USVI shelf (1385 km 2 ). Most MCE development is on steep walls and slopes, the exception being some deeper linear reefs at the eastern extent of the Lang Bank (García-Sais et al. 2014, Smith et al. 2014). Since the 1970s, a few of the walls have been very well-studied, such as Salt River Canyon and Cane Bay walls on the northwest. These wall systems form dramatic precipices that extend from shallow depths to below 100 m. Mesophotic coral cover was historically above 25 per cent for Salt River Canyon (Aronson et al. 1994) and Cane Bay (Sadd 1984), but there has been degradation in recent years due to the combination of several large hurricanes and a thermal stress and bleaching event in 2005. MCE coral cover at these sites is now below 10 per cent (Smith et al. 2014). The coral communities are a typical mix of plating forms; predominantly lettuce corals ( Agaricia spp.) and star corals ( Orbicella spp.), which form on vertical buttresses surrounded by channels where sediment is transported off-shelf. The Salt River Canyon and areas at the eastern end of the Lang Bank are in Fisheries Protected Areas, covering about 25 per cent of the potential MCE shelf depths. Despite the moderate coverage of Marine Protected Areas, fishing intensity on the narrow shelf is quite high and many commercially-important fish, such as large-bodied snappers and groupers, are absent or rare relative to historical levels (Kadison pers. com.). The MCEs of USVI are not immune to anthropogenic disturbance. Local and global stressors have caused slow to precipitous declines in coral cover over the last 10 years or more. Potential climate change effects were noted between 2005–2014, with thermally induced coral bleaching occurring at least twice, and causing an approximately 28 per cent loss of coral cover in Orbicella . The nearshore MCEs of St. Croix are potentially vulnerable to sedimentation from natural reef processes (Hubbard 1989), whereas the offshore MCEs of the northern Virgin Islands are not influenced by terrestrial sediment (Smith et al. 2008).

British Virgin Islands

United States Virgin Islands

Tortola

St.Thomas

Virgin Passage

St. John

Culebra

Vieques

Caribbean Sea

Virgin IslandsTrough

Depth in metres

0

65-4000

St. Croix

0 5 10

20

30

Kilometres

Figure 1. MCEs are found on shelves, slopes, and walls in USVI. The northern islands of St. Thomas and St. John are surrounded by a shelf largely in mesophotic depths with well-developed MCEs. The St. Croix shelf has less mesophotic shelf area, but extensive mesophotic wall systems. (Map Tyler B. Smith using NOAA bathymetric data.)

* This calculation does not include any of the uncharacterized hard bottom MCE habitats on the deep and wide northern bank.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 26