[1] To make progress in understanding the distribution and genesis of coral mounds in cold and dark water, maps of morphology and oceanographic conditions resolving features at the 1 -10 m scale are needed. An autonomous underwater vehicle (AUV) cruising 40 m above the seafloor surveyed a 48 km 2 coral mound field in 600-800 m water depth at the base of slope of Great Bahama Bank. The AUV acquired 1 -3 meter resolution acoustic backscatter and bathymetry together with current vectors, salinity, and temperature. The multibeam bathymetry resolved more than 200 coral mounds reaching up to 90 m height. Mound morphology is surprisingly diverse and mound distribution follows E-W oriented off-bank ridges. Bottom currents reverse every 6 hours indicating tidal flow decoupled from the north flowing surface current. The AUV data fill the gap between low-resolution surface-based mapping and visual observations on the seafloor, revealing the dynamic environment and spatial relationships of an entire coral mound field.

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Cold-water coral mound morphology and development are thought to be controlled primarily by current regime. This study, however, reveals a general lack of correlation between prevailing bottom current direction and mound morphology (i.e. footprint shape and orientation), as well as current strength and mound size (i.e. footprint area and height). These findings are based on quantitative analyses of a high-resolution geophysical dataset collected with an Autonomous Underwater Vehicle from three cold-water coral mound sites at the toe of slope of Great Bahama Bank. The three sites (80 km 2 total) have an average of 14 mounds km )2 , indicating that the Great Bahama Bank slope is a major coral mound region. At all three sites living coral colonies are observed on the surface of the mounds, documenting active mound growth. Morphometric analysis shows that mounds at these sites vary significantly in height (1 to 83 m), area (81 to 6 00 000 m 2 ), shape (mound aspect ratio 0AE1 to 1) and orientation (mound longest axis 0 to 180°). The Autonomous Underwater Vehicle measured bottom current data depict a north-south flowing current that reverses approximately every six hours. The tidal nature of this current and its intermittent deviations during reversals are interpreted to contribute to the observed mound complexity. An additional factor contributing to the variability in mound morphometrics is the sediment deposition rate that varies among and within sites. At most locations sedimentation rate lags slightly behind mound growth rate, causing mounds to develop into large structures. Where sedimentation rates are higher than mound growth rates, sediment partially or completely buries mounds. The spatial distribution and alignment of mounds can also be related to gravity mass deposits, as indicated by geomorphological features (for example, slope failure and linear topographic highs) in the three-dimensional bathymetry. In summary, variability in sedimentation rates, current regime and underlying topography produce extraordinarily high variability in the distribution, development and morphology of coral mounds on the Great Bahama Bank slope.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe first AUV survey across five fields of deep-water coral mounds in the Straits of Florida reveals an unexpected high abundance and variability of mounds in water depths of 590 -875 m. A drop camera and a series of dives with the Johnson-Sea-Link submersible confirmed living corals on each of the five investigated sites. The morphology of the mounds is highly diverse, ranging from isolated mounds to welldeveloped ridges with more than 100 m of relief. Along the toe-of-slope of western Great Bahama Bank antecedent topography seems to be the controlling factor for mound location while further west currents appear to control the formation of ridges. The comprehensive suite of sensors on board the AUV allows correlation of geophysical parameters and oceanographic observations. Acoustic Doppler current meter data document three different bottom current regimes consisting of unidirectional or bi-directional tidal flow. The bidirectional current pattern is not visible on backscatter data and only vaguely reflected in the mound morphology. In areas of uniform current direction mounds face the currents and align perpendicular to the current to form long ridges and intervening troughs. The synoptic seabed and oceanographic data recorded by the AUV characterize the dynamic and complex environments of entire coral mound fields at a resolution of 1-3 m.

The deep-sea (200-1000 m) seafloor off the southeastern U.S. has a variety of extensive deep-sea coral ecosystem (DSCE) habitats including: deep-water coral mounds; various hard-bottom habitats off Florida including the Miami Terrace, Pourtalès Terrace, and deep-water canyons (Agassiz and Tortugas Valleys); and deep island slopes off western Bahamas and northern Cuba. The dominant structure-forming scleractinian corals are Lophelia pertusa and Enallopsammia profunda; other structure-forming taxa include stylasterid corals, gorgonians, black corals, and sponges. This biota is associated with hard-bottom seafloor of variable high-relief topography which can be remotely identified from bathymetric data. NOAA bathymetric contour maps and digital elevation models were used to identify and delineate the areal extent of potential DSCE habitat in the region from northeastern Florida through the Straits of Florida. These were ground-truthed with 241 dives with submersibles and remotely operated vehicles which confirmed deep-sea coral habitat. We estimate a total of 39,910 km 2 of DSCE habitat in this region. By comparison, the estimated areal extent of shallow-water coral habitat for all U.S. waters is 36,813 km 2. Bottom trawling remains the greatest threat to DSCEs worldwide, and as a result NOAA has established five deep-water Coral Habitat Areas of Particular Concern (CHAPCs), encompassing 62,714 km 2 from North Carolina to south Florida, which will protect much of the known deep-sea coral habitat in this region. High-resolution surveys are not only critical to define DSCE habitats but also to define areas devoid of coral and sponge habitats that may allow for potential bottom fisheries and energy development. KEYWORDS.-conservation, deep-sea coral ecosystem, habitat mapping, Lophelia 2005 b, Grasmueck et al. 2006, 2007, Reed 2008), and the percentage of seafloor explored visually with human occupied submersibles and remotely operated vehicles (ROVs) remains small. In the broad sense, DSCEs in this region occur at depths of 50 m to >1000 m and consist of structureforming, deep-water corals (including scleractinian corals, gorgonian octocorals, black corals, and stylasterid hydrozoan corals) and other associated structureforming species such as sponges, bryozoans, and hydroids, all of which may provide habitat to hundreds of species of invertebrates