Mesophotic Coral Ecosystems

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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 26
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 km2) 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 km2). *
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
* This calculation does not include any of the uncharacterized hard bottom
MCE habitats on the deep and wide northern bank.
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 km2 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 km2). 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).
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
St.Thomas
St. Croix
St. John
Tortola
British Virgin Islands
Culebra
Vieques
Virgin IslandsTrough
Virgin
Passage
Caribbean Sea
Atlantic Ocean
United States Virgin Islands
Depth
in
metres
0
65-4000
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.)