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USGS Scientists Use the SeaBOSS to Explore What Could Be the Deepest Coral Reef in the Continental United States
U.S. Geological Survey (USGS) scientists from Woods Hole, MA, and St. Petersburg, FL, departed at the end of April aboard the Florida Institute of Oceanography's research vessel Suncoaster to continue a multiyear investigation of what could be the deepest coral reef in the United States, at the southwest edge of the continental shelf off West Florida.
Their main goal was to characterize the geologic underpinnings and biological architecture of the Pulley Ridge coral reef. Pulley Ridge is a north-south-trending drowned barrier island, more than 100 km long, approximately 70 km west of Dry Tortugas National Park.
The ridge is a subtle feature, about 5 km across, with less than 10 m of relief. The shallowest parts of the ridge are in about 60 m of water. Surprisingly, at that depth, the southern part of the ridge hosts a variety of zooxanthellate scleractinian corals, macroalgae, and tropical fish.
The focus of this cruise was to use the SeaBOSS to collect video transects, still photographs, and coral or algae samples from along the ridge. The SeaBOSS (Sea Bottom Observation and Sampling System) is a modified van Veen grab sampler in a stainless-steel frame with integrated still photography and video systems.
The device is lowered straight down from a ship with a winch and conducting cable. It can be deployed quickly and allows many sites to be investigated efficiently. Real-time video allows the selection of still-photograph subjects and the placement of the grab sampler for retrieval of coral samples. In addition to the SeaBOSS data, the scientists collected geophysical map data consisting of boomer seismic and multibeam bathymetry.
The corals Agaricia sp. and Leptoseris cucullata are most abundant and are deeply pigmented in shades of tan-brown and blue-purple, respectively. These corals, which form plates as large as 50 cm in diameter, account for as much as 60 percent of the live coral cover at some sites.
Less common species include Montastrea cavernosa, Madracis formosa, Madracis decactis, Porites divaricata, and Oculina tellena. Sponges, calcareous and fleshy algae, soft corals, gorgonians, and sediment occupy surfaces between the stony corals, and coralline-algal nodule-and-cobble zones surround much of the ridge in deeper water (deeper than 80 m).
The fish at Pulley Ridge comprise a mixture of shallow- and deep-water species sharing this unusual habitat; more than 60 species have been identified.
Several factors help to account for the existence of this community. First, the underlying drowned barrier island provides both elevated topography and lithified substrate for hardbottom community establishment. Second, the region is dominated by the west edge of the Loop Current, which brings relatively clear and warm water to the southern region. Third, the ridge's position on the continental shelf places it within the thermocline, a water mass that is known to provide nutrients to shallow reefs in Florida during upwelling.
This largely photosynthetic community appears to be thriving on 1 to 2 percent (5-30 microEinsteins/m2 per second) of the available surface light (photosynthetically available radiation [PAR]) and about 5 percent of the light typically available to shallow-water reefs. This community is clearly one that has adapted to low-light conditions, and so the variety and extent of photosynthetic organisms between 60- and 70-m water depth is impressive.
USGS cruise participants included Bob Halley and Kate Ciembronowicz from St. Petersburg, FL, and Dave Twichell and Dann Blackwood from Woods Hole, MA. University of South Florida faculty members Al Hine, Stan Locker, and Brian Donahue and graduate students Bret Jarrett, Beau Suthard, Steve Obrochta, and Monica Wolfson also participated.
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