AUV Based Environmental Characterization of Deep-Water

OTC 18510

AUV-Based Environmental Characterization of Deep-Water Coral Mounds in the Straits of Florida M. Grasmueck, G.P. Eberli, T.B.S. Correa, D.A. Viggiano, J.Luo, RSMAS University of Miami; G.J. Wyatt, Quester Tangent; J.K.Reed, A.E. Wright, and S.A. Pomponi, Harbor Branch Oceanographic Institution.

Copyright 2007, Offshore Technology Conference instrumentation has focused most research activity and related This paper was prepared for presentation at the 2007 Offshore Technology Conference held in discoveries of deep-water coral habitats to the north and Houston, Texas, U.S.A., 30 April–3 May 2007. central Atlantic, the Gulf of Mexico and the north-east This paper was selected for presentation by an OTC Program Committee following review of 4,5 information contained in an abstract submitted by the author(s). Contents of the paper, as Pacific . In the Straits of Florida, abundant mound-forming presented, have not been reviewed by the Offshore Technology Conference and are subject to corals in water depths of 400–800 m have been documented in correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at over 40 years of dredge sampling, submersible dives and OTC are subject to publication review by Sponsor Society Committees of the Offshore 6-12 Technology Conference. Electronic reproduction, distribution, or storage of any part of this seismic acquisition (Figure 1). This extensive collection of paper for commercial purposes without the written consent of the Offshore Technology samples and observations however can not be put into a Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous geomorphologic context as existing bathymetric charts do not acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. resolve coral mounds. Such sparse information has proven inadequate to answer questions in regards to mound Abstract morphology, bottom current dynamics and nutrient sources The first AUV survey across five fields of deep-water coral supporting life at these depths. Furthermore the limited data mounds in the Straits of Florida reveals an unexpected high set has so far prevented assessment of the biodiversity and the abundance and variability of mounds in water depths of 590 – potential need for protection from over-fishing and underwater 875 m. A drop camera and a series of dives with the Johnson- construction. High-resolution maps of morphology and Sea-Link submersible confirmed living corals on each of the oceanographic conditions resolving features at the 1-10 m five investigated sites. The morphology of the mounds is scale are a basic requirement to make further progress in highly diverse, ranging from isolated mounds to well- understanding deep-water coral mound distribution and developed ridges with more than 100 m of relief. Along the genesis. toe-of-slope of western Great Bahama Bank antecedent Autonomous Underwater Vehicles (AUV) bring an topography seems to be the controlling factor for mound integrated suite of mapping and oceanographic sensors close location while further west currents appear to control the to the seabed for high-resolution data acquisition and give the formation of ridges. The comprehensive suite of sensors on opportunity to fill the scale gap of basic information in deep- board the AUV allows correlation of geophysical parameters water environments. Described here are the initial results of a and oceanographic observations. Acoustic Doppler current 7 day cruise during which the AUV mapped five deep-water coral mound fields in the Straits of Florida covering a total meter data document three different bottom current regimes 2 consisting of unidirectional or bi-directional tidal flow. The bi- area of 130 km . Acquiring the same amount of data without directional current pattern is not visible on backscatter data an AUV would have taken multiple cruises and possibly and only vaguely reflected in the mound morphology. In areas several years with costs exceeding the single AUV mapping of uniform current direction mounds face the currents and cruise. By simultaneously acquiring a comprehensive suite of align perpendicular to the current to form long ridges and high-resolution seabed and oceanographic data the dynamic intervening troughs. The synoptic seabed and oceanographic and complex environment spanning entire coral mound fields data recorded by the AUV characterize the dynamic and can be assessed for the first time. We selected five mapping complex environments of entire coral mound fields at a sites off the Miami Terrace, in the center of the Straits of resolution of 1–3 m. Florida and at the toe-of-slope of Great Bahama Bank. Mound distribution, morphology and currents are different for each Introduction survey site. Subsequent groundtruthing with a drop camera Deep and cold-water coral ecosystems are less known but and a submersible found corals in all five areas. By comparing more widespread than their warm-water counterparts restricted the submersible observations with the AUV data the to shallow tropical seas1,2. Cold-water corals and associated distribution of deep-water coral habitats in each area can be fauna flourish in oceanic waters of all latitudes at depths of predicted. This comprehensive dataset allows us to assess the several hundred to over one thousand meters with relationship between mound morphology and current direction temperatures between 4° and 12° C and require no sunlight3. and the abundance and distribution of corals on the mounds. The limited availability and high cost of deep-water 2 M. Grasmueck et al. OTC 18510

Figure 1. Seafloor morphology of the Straits of Florida based on National Ocean Service Hydrographic Survey Data. This major seaway connects the Gulf of Mexico with the Atlantic. It is bordered by the Florida peninsula in the NW, Cuba in the south, and the Bahamas in the east. The Florida Current, a warm surface current, flows through the Straits of Florida into the North Atlantic where it converges with the smaller Antilles Current to form the Gulf Stream. The Florida Current has produced stream line shaped drift deposits. Superimposed on the map are the locations of scientific dredge sampling in the 1960s and 70s at water depths deeper than 400 m 8,11,19. The dredge locations highlighted with red dots retrieved mound-forming deep-water corals Lophelia pertusa, Enallopsammia profunda and Madrepora oculata. The five areas mapped with the AUV are marked with black and white rectangles

Survey Methods

Sea Surface-Based Multibeam Reconnaissance Mapping. To make the best use of the AUV for mapping coral mound fields and avoiding featureless flat seabed areas we performed sea surface-based multibeam bathymetry surveys just before launching the AUV. The hull-mounted 12 KHz EM120 multibeam system on board the R/V NORTHERN RESOLUTION (Figure 2) produced 50 m gridded bathymetric maps resolving mound structures not visible on the best available bathymetric chart of the Straits of Florida (Figure 1). As the dredge location coordinates have large position uncertainties of where exactly the samples were retrieved from the seabed, the reconnaissance multibeam mapping was essential to locate the coral mound fields. The dredge coordinate locations served as a starting point for Figure2. The R/V NORTHERN RESOLUTION used for hull- scanning the seabed at 10 kts vessel speed. The multibeam mounted multibeam reconnaissance mapping and AUV swathwith was 3.5 km at 800 m water depth. The resulting deployment preliminary maps (Figure 3) served as the basis for designing and planning the tracks of the subsequent AUV surveys.

OTC 18510 AUV-Based Environmental Characterization 3

Positioning and navigation accuracy of the AUV at 800 m depth is better than 3 m 13. A Kalman filter combines the inputs from the AUV fiber optic gyro compass, acoustic Doppler profiler and high precision pressure (depth) sensor. Positioning drift is minimized with fixes from an ultra short baseline (USBL) acoustic and differential global positioning system installed on the mother vessel14,15. Communications between mother vessel and AUV by acoustic modem allow quality control on decimated data in real time during the mission. The high-resolution bathymetry including backscatter amplitudes were acquired using a 200 kHz EM2000 multibeam system installed on the AUV. The swath width is 300 m providing 100 m overlap between adjacent lines. The multibeam data were corrected for heave, pitch and roll monitored by precision accelerometers. The data were processed on board the R/V NORTHERN RESOLUTION into bathymetric maps with 3 m resolution and acoustic backscatter maps with 1 m resolution within a couple hours after the AUV had resurfaced. Average bottom current velocity and direction in the 40 m water column between AUV and seafloor were extracted from the acoustic doppler profiler installed on the AUV. Salinity and water temperature were recorded at 1 s intervals for later fusion with the position data and plotting along the AUV track lines, monitoring how oceanographic conditions evolved during the survey.

Figure 3. EM120 multibeam bathymetry acquired for Figure 4. The AUV resting on the retractable launch and retrieval reconnaissance of Site 1. The white stippled rectangle shows the area selected for high-resolution AUV mapping. The 50 m system at the stern of the R/V NORTHERN RESOLUTION. resolution map images numerous mound features associated with low relief ridges extending in a slightly divergent pattern in Groundtruthing with Drop Camera and JSL Submersible. E-W direction The 40 m cruising altitude of the AUV is too high to illuminate the seabed for still pictures or video to verify the High-Resolution Data Acquisition with the AUV cruising presence of corals on the mounds. Therefore, we developed a 40 m above the Seabed. low-cost drop camera to get a first glimpse of corals The Autonomous Underwater Vehicle (AUV) was immediately after the AUV mapping was completed. The drop deployed from the stern of R/V NORTHERN RESOLUTION. camera consisted of a compact digital camera within a 15 cm Over 7 days in December 2005, the Hugin 3000 AUV (Figure diameter evacuated glass sphere rated to 6700 m depth taking 2 4) mapped five sites ranging from 14–48 km in 590–875 m pictures every 1.5 seconds for 70 minutes (Figure 5). The water acquiring 1–3 m resolution multibeam data together camera is kept neutrally buoyant 1–2 meters above the with sidescan data, subbottom profiles, current vectors, seafloor by the variable weight of a loose steel chain. LED salinity, temperature and methane content. The AUV is lights illuminate the seafloor. An acoustic pinger attached 50 powered by an aluminum oxygen fuel cell providing a mission m above the camera enabled ± 5m accurate USBL tracking. endurance of up to 55 hours and cruises 40 m above the With the 50 m of line between pinger and camera the seabed seafloor at a speed of 1.8 m/s scanning the seafloor in parallel position of the photographs taken is known to within about 50 lines spaced 200 m in similar fashion to a lawn mower. m radius. The AUV bathymetric map was used to select an 4 M. Grasmueck et al. OTC 18510 off-mound camera landing site and to steer the camera trajectory during the deployment. Currents and the vessel pulling on the line moved the camera at an approximate speed of 0.5 m/s along the seabed. We only deployed the drop camera once during the AUV cruise in order to save valuable AUV time. The drop camera deployment was, however, a proof of concept for future inexpensive groundtruth missions from small boats before or after AUV surveys.

Figure 6. The JOHNSON-SEA-LINK (JSL) submersible carries four people: Two in the acrylic capsule up front and two in an aluminum pressure chamber in the back.

Results

Site 1: Mounds aligned along off-Bank Ridges at the Toe- of-Slope of Great Bahama Bank. This site in 590-710 m water depth is just north of the ODP Site 1007 drilling location at the base-of-slope along the western margin of Great Bahama Bank. Several mound features protruding from the seafloor had been noticed on

Figure 5. Compact drop camera developed for groundtruthing of some of the high-resolution 2D seismic lines acquired during coral mounds from small boats. the site survey for Ocean Drilling Program (ODP) Leg 16617. Although the seismic data detected mounds, the 600 m line Harbor Branch Oceanographic Institution’s (HBOI) spacing did not provide information about the abundance and JOHNSON-SEA-LINK (JSL) submersible dives launched distribution of the mounds. The new 3 m grid resolution AUV from R/V SEWARD JOHNSON some months after the AUV bathymetry map now shows 37 mounds, which are at least 25 cruise provided us with a more extensive and precise m high (Figure 7). The tallest mound reaches a height of 85 m groundtruthing of mound coverage at all 5 AUV survey sites and has a minimum base width of 350 m. The drop camera (Figure 6). The JSL submersible carries four persons to a deployment across this mound revealed living corals near the maximum depth of 1000 m. It has an acrylic sphere that peak and coral rubble with sponges and brittle stars on the provides > 180º visibility to the pilot and the observer in the north facing slope. The AUV map also images more than 180 front. Color videotapes were recorded along the dive tracks smaller mounds. The distribution of mounds is non-random with an external pan and tilt video camera. Submersible and follows low relief ridges (500-1500 m wide and 5 m high) navigation used an USBL positioning system and calculated extending in a slightly divergent pattern in an E-W direction the submersible’s real-time position throughout each dive. (Figure 3). The shapes of the mounds are diverse, including Analysis of USBL tracking accuracy for a worst-case tracking single conical, pyramid or wedge shaped peaks, twin peaks, scenario estimated a maximum statistical positioning error of and an almost perfect heart shape. Fields of 2-4 m tall ridges 9.6 m at a depth of 500 m (J. Kloske, Florida Institute of spaced by 15-25 m occur at the base of most large mounds. Oceanography, pers. comm.; and16). A Geographical Some mounds are completely or partly surrounded by 5-10 m Information System (GIS) was used to overlay the deep and 25-50 m wide moats. Smaller mounds generally lack submersible track positions and time stamps onto the AUV the moat but have a sediment wedge attached that thins acquired bathymetry, backscatter and sidescan sonar maps. towards the north. The timestamps on the video frames allowed a direct The bottom currents within the 40 m water column correlation of visual observations and related patterns on the between AUV and seafloor showed a remarkable variation in AUV maps. strength and direction18. The currents changed direction between north- and southward flow seven times over 45 hours. The average time interval between changes was approximately 6 hrs. Current strength never exceeded 0.5 m/s with sustained peak strengths of 0.2 m/s in north or south direction. The change of direction every six hours indicates tidal control on the currents at the toe-of-slope of Great Bahama Bank in 590- OTC 18510 AUV-Based Environmental Characterization 5

710 m water depth. This current behavior is rather surprising direction appears to be slightly prevailing. This is in as the surface currents flow northward. The bottom currents agreement with the consistent northward orientation of the obviously are decoupled from the surface currents and sediment wedges associated with small mounds. dominated by diurnal tides. The north-flowing current

Figure 7. Site 1: High-resolution bathymetry based on 200 kHz multibeam data acquired with the AUV at a cruising altitude of 40 m above the seafloor. The map images more than 200 mounds within an area of 48 km2 at the toe-of-slope of Great Bahama Bank. The shapes of the mounds are diverse, including single conical, pyramid or wedge shaped peaks, twin peaks, random shaped mounds and even an almost perfect heart shape. Insets are zoomed up views of the largest mounds with a height of 85 m and photographs of coral cover taken with the drop camera.

Site 2: Mounds related to Slump Features at Toe-of-Slope small-scale ridges lining the tip and northern edge of the of Great Bahama Bank. elongate plateau. The backscatter data show stronger seabed Following the toe-of-slope 40 km to the north from Site 1 reflection amplitudes from these ridges than the surrounding we encountered a section with a steep escarpment reaching seabed. Based on the submersible dive, the higher reflectivity heights of over 100 m (Figure 8). The escarpment follows two is interpreted to be caused by living- and/or dead coral on large scars and a 3.2 km long and up to 0.9 km wide plateau these ridges. The acoustically softer seabed between the ridges that prominently raises 40-150 m from the surrounding gently and in the southern portion of the plateau are likely composed sloping seabed. The JSL submersible dive I-4812 visiting this of muddy sediments. Numerous mounds features are also elongate plateau in 2005 encountered 1.0–1.2 m high living visible to the south-west of the plateau. Backscatter Lophelia coral thickets near the peaks of NE-SW oriented amplitudes suggest coral cover and soft sediment surrounding ridges in 690 m of water. The slopes of the coral ridges were the mounds. For some mounds the shapes of the darker covered with 100% coral rubble and some 1.0 m dead standing colored backscatter anomalies do not follow the contours of coral thickets. The AUV multibeam data show numerous the mounds but extend over the flat seabed around and 6 M. Grasmueck et al. OTC 18510 between the mounds, indicating biological growth and rubble into the current feature darker backscatter amplitudes than the production. These mounds have not yet been groundtruthed. It down current side and adjacent troughs. The JSL submersible appears that the elongate plateau is a remnant between two dives confirmed bio-accumulation in form of coral rubble and slump scars originating from basal slope instability of Great standing dead and living corals and associated fauna on the Bahama Bank. The foundations for deep-water corals on the slopes facing the current and muddy sediments in the lee-side group of mounds in front of the escarpment are likely the troughs. blocks from this mass gravity flow. The bottom current data from the AUV survey show a north-south reversal indicating a Site 5: Sinusoidal Ridges at the Base of the Miami Terrace. bi-directional tidal current regime as already encountered at Neumann and Ball (1970) reported from their 1969 the first site with maximum current strengths reaching 0.25 Aluminaut submersible dive “….at the base of the Miami m/s. Terrace, is an elongate trough. The bottom here is characterized by ridges and mounds of muddy sand capped by Site 3: Coalesced Mounds form off-Bank Ridges at the thickets of living deep-water branching coral.” The new AUV Toe-of-Slope of Great Bahama Bank. bathymetric map (3.8 x 7.1 km) shows how coral mounds at Site 3 is also on the base-of-slope of Great Bahama Bank the toe-of-slope of the Miami Terrace (875 to 660 m) are in 710 to 840 m water depth. The site is located 15 km west of developed as a series of sinusoidal, asymmetrical ridges that Bimini and 80 km north from the Site 2. In 1964 R/V GERDA are approximately 800 m long, less than 30 m high, and 100 to retrieved Enallopsammia profunda corals in one dredge 400 m wide (Figure 12). The AUV bottom current data sample (Sample g29719). The high-resolution bathymetric map confirm consistent southward bottom flow (opposite to the covering 3.8 x 3.6 km area shows 4 major ridges of coalesced surface current) as Neuman and Ball had already experienced mounds aligned in NE-SW direction (Figure 9). The largest during their dive. The ridges are steeper with stronger ridge is 0.5 km wide and 2 km long. The tallest peak on the (=darker) backscatter on their northern sides that face the ridge is 120 m high and has a pyramidal shape. The 2006 JSL currents. The 2006 JSL submersible dive encountered deep- submersible dive on this mound found dense living coral water coral rubble and thickets on the north facing slopes and thickets on the flank and at the peak. In addition to the crests alternating with a muddy seabed in the leeward troughs. coalesced mound ridges, more than 150 individual mounds are At the base of the slope the coral coverage ends abruptly as it distributed throughout the site. Some of the mounds have transitions sharply into a barren sand dune field devoid of pronounced elliptic scour depressions on the north side while corals. In this sand dune field, the steeper slopes of the sand others have elongate sediment wedges attached. The acoustic waves face to the south as expected from loose sand backscatter map shows linear dark striations originating at the transported by south flowing bottom currents. mounds with some extending to over 1 km in a NNW direction (Figure 10). These striations are nearly Discussion perpendicular to the orientation of the mound ridges. The currents measured during the AUV survey indicate tidal AUV provides Synoptic View of entire Coral Mound reversals every 6 hours in a north or south direction with the Fields. north flowing currents slightly prevailing and a sustained Previous studies of the deep-water coral mounds in the strength of 0.3 m/s. Straits of Florida relied primarily on data acquired from a single mound perspective with a limited number of sensors. Site 4: Chevron Pattern Mound Clusters in the Center of The 130 km2 of bathymetric data acquired by the AUV Straits. provides high-resolution maps of entire mound fields. This site is located 30 km offshore from Bimini in the Furthermore, together with the multibeam backscatter and center of the Straits of Florida. Three R/V GERDA dredges side-scan swaths bottom type and coverage can be assessed (Samples g311, g317, g35419) from this area retrieved field wide. There is a close association between high Lophelia pertusa and Enallopsammia profunda. The AUV backscatter patches (dark shades on backscatter plots) with mapped an area of approximately 3.2 x 4.2 km. Nearly 70% of biologic accumulations composed of coral rubble, standing the area is covered with small mounds, most less than 5 m dead and live corals. On the basis of this association verified high, producing a knobby seafloor topography (Figure 11). In at 5 sites we can now predict with a high degree of certainty the center of the study area the mounds are coalesced to form where such deep water coral habitats occur on multibeam hills and ridges of approximately 40 m height. Individual maps. The new 1-3 m grid resolution maps now allow precise mounds have a horseshoe shape with the convex side facing planning of submersible or ROV tracks for efficient towards the S and SSE. The backscatter map shows lineations groundtruthing at critical locations. The resolution of the attached to the mounds which trend in a NNW direction. The seabed maps has proven adequate to georeference bottom currents recorded over the 13 hour AUV survey time observations made within the illuminated area of the JSL were consistently flowing to the north with an average submersible. velocity of 0.23 m/s (maximum 0.43 m/s) with azimuths varying between 330° and 50°. Many knobs are aligned and Surprising Diversity and Abundance of Deep-Water form chains oriented at an angle of 70° – 90° with the current Corals in the Straits of Florida. direction. The knob chains form a characteristic chevron Mound morphology is not uniform, even for mounds in pattern very different from the ridge morphology observed at close proximity to each other. Mound morphologies range Site 3 just 15 km to the east. The slopes of the mounds facing from predominantly isolated structures (Site 1) to enormous, OTC 18510 AUV-Based Environmental Characterization 7 well-developed ridges with more than 100 m of relief (Site 3). 5) The majority of chains and ridges of coral mounds are At these two sites on the toe-of-slope of western Great aligned perpendicular to the prevailing current. Bahama Bank the mounds are aligned with low-relief These observations partly corroborate but also contradict sediment ridges and lows, characteristic of the lower slope earlier interpretations. For example, the fact that mounds face morphology surrounding carbonate platforms20. The mound into the current has already been noted in early submersible distribution seems to follow the sediment ridges that trend observations21. We found no good examples of the “tear drop” approximately perpendicular to the platform margin. This morphology with the long mound axis oriented parallel to the configuration is also perpendicular to the tidal-controlled prevailing current direction10. Our data show no correlation current that flows either north or south. Similarly mound between mound height and current strength. The tallest morphology seems not to be controlled by the current mounds are situated in areas where the current is tidally direction as mounds display a wide variability of shapes. dominated and switches directions every six hours. The style of coral mound architecture changes dramatically across the Straits of Florida from Bimini to Miami. The Survey Strategy for efficient Characterization of the entire knobby mounds in the middle of the Straits of Florida at Site 4 Florida Straits and other Deep-water Habitat Regions. are densely clustered and form a chevron pattern. At the base The new AUV surveys cover only a minuscule portion of of the Miami Terrace the mounds form sinusoidal ridges the Straits of Florida. Time and cost would be prohibitive for trending perpendicular to the currents. The boundaries mapping the entire Straits with an AUV. As an outcome from between coral mound fields and featureless muddy or sandy this first application of AUV technology to mapping of deep- seafloor are sharp. water coral environments we propose the following survey Deep-water corals and related fauna exhibit a remarkable strategy, which is also applicable to deep-water habitats adaptability to different environmental conditions by changes elsewhere: 1) Regional surface bathymetric mapping and in mound morphology and configuration. It also appears that backscatter classification for identification of mound fields. 2) previous estimates of number of mounds and extent of deep- Drop camera deployment from small boats on selected water coral habitats will have to increase by orders of mounds to determine presence of live or dead coral cover. 3) magnitude. The five areas mapped with the AUV contain High-resolution AUV surveying of key mound fields. 4) hundreds of coral mounds with heights of 1–120 m covering Based on the AUV maps perform further drop camera 30% of the 130 km2 total area mapped. The abundance of deployments for more detailed groundtruth. 5) Use of deep-water corals was confirmed by subsequent visual submersibles and ROV for sample collection and deployment groundtruthing with the drop camera and the human-occupied of permanent monitoring equipment. This scaled approach submersible. The previously underestimated abundance and including modern oceanographic mapping tools requires a new diversity of modern deep-water coral mounds is likely to hold level of interdisciplinary deep-water research, unifying local true for the geologic record as well. As the AUV mapping observations with their regional environments. Such areas were specifically selected for potential coral coverage collaboration together with an extensive mapping strategy will the mound and area coverage numbers derived from this study be needed to resolve questions related to nutrient supplies and do not apply to the entire Straits of Florida. the seemingly high mortality of the deep-water coral mounds in the Straits of Florida. New Insights on Influence of Currents on Coral Mound Morphology and Distribution. Conclusion Mound distribution, morphology and currents are different In just a few days the AUV acquired the high-resolution for each of the five mapped sites. Taken together the AUV morphologic and oceanographic maps essential for the data reveal some general trends: comprehensive environmental assessment of five different 1) Bottom currents are not unidirectional in the Straits of deep-water coral mound fields. The AUV data provide for the Florida. Tidal dominated currents along the western slope of first time a field-wide perspective not obtainable by dredge Great Bahama Bank flow both north and southward. In the sampling or visual observation from remotely operated center of the Straits flow is northward while along the slope of vehicles or submersibles. Each of the five sites revealed a the Miami Terrace the flow is southward. Sediment wedges distinct character in terms of mound morphology, that are attached to mounds as well as the striation patterns configuration and bottom currents. Most surprising are the visible on backscatter maps do not show the tidal reversing as high abundance of coral mounds at all sites and the variability they are pointing in one direction only. Furthermore, the AUV of both the distribution and the shape of individual mounds. current data display directional variability, which is also not Many coral mounds align or coalesce along linear ridges seen in the backscatter striation patterns. approximately perpendicular to the prevailing current 2) Coral mounds grow taller in bi-directional tidal bottom direction. Tear drop shape of individual mounds is the current regimes. exception rather than the rule. Similar variability and local 3) Chains of closely spaced smaller coral mounds develop abundance of deep-water corals is expected at other locations. as a response to uniform bottom current direction. AUVs are efficient high-resolution mapping platforms which 4) Corals grow into the currents creating steeper mound can guide future sampling and monitoring to critical locations slopes facing the current, indicating that coral growth and not and provide the base maps for making inventories and sediment accumulation from traction current control the shape management plans of deep-water habitats. of the mounds.

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Acknowledgments hardgrounds in the northeastern Straits of Florida, Palaios, 5, We received ship time for the AUV survey from the NOAA 15–33. Ocean Explorer program. Justin Manley (OE program 11. Cairns, S., (2000), A revision of the shallow-water manager), Heather Langill (C&C Geophysical Operations & azooxanthellate scleractinia of the Western Atlantic, Studies of the natural history of the Caribbean Region, 75, 240. Bid Coordinator) and Chas Honea (C&C AUV Field Project 12. Reed, J.K., Weaver, D., and Pomponi, S.A., (2006), Habitat and Manager) were instrumental for facilitating this project. We fauna of deep-water Lophelia pertusa coral reefs off the express our gratitude to the officers and crew of the R/V Southeastern USA: Blake Plateau, Straits of Florida, and Gulf of NORTHERN RESOLUTION and the scientific and technical Mexico, Bulletin of Marine Science, 78, 343–375. staff of C&C Technologies. Their efficiency and competence 13. Jalving, B., Gade, K., Hagen, O.K., and Vestgård, K., (2003), A enabled data acquisition and processing in a very short time. Toolbox of Aiding Techniques for the HUGIN AUV Integrated We especially appreciated how they accommodated all our Inertial Navigation System, Proceedings from Oceans 2003, 8. special requests in their workflow. 14. Chance, T.S., and Northcutt, J.G., (2001), Deep water AUV The crews of the R/V SEWARD JOHNSON, and the experiences, Proceedings of the U.S. Hydrographic Conferences, 8. JOHNSON-SEA-LINK submersibles are gratefully thanked 15. George, R.A. Advances in AUV remote-sensing technology for for their logistical support. A grant provided by the State of imaging deepwater geohazards The Leading Edge, Volume 25, Florida, Medicines from Florida's Oceans Project (HBOI Issue 12, pp. 1478-1483 doi:10.1190/1.2405333 Project Nos. S2156 and S2168) provided funding for these 16. Opderbecke, J. 1997. At-sea calibration of a USBL underwater cruises. We are especially grateful to all the personnel of the vehicle positioning system, Oceans 1997, MTS/IEEE Conf. Division of Biomedical Marine Research at HBOI who are Proc. 1: 721–726. responsible for various aspects of research during these 17. Anselmetti, F.S., Eberli, G.P., and Ding, Z.-D., (2000), From the cruises. Great Bahama Bank into the Straits of Florida: A margin Pre- and post-cruise research was made possible by the architecture controlled by sea level fluctuations and ocean currents, GSA Bulletin, 112, 829–844. Industrial Associates of the Comparative Sedimentology 18. Grasmueck, M. Eberli, G., Viggiano, D.A. Correa, T. Rathwell, Laboratory. Angela D. Rosenberg helped with analyzing the G. and Luo, J., 2006, Autonomous Underwater Vehicle (AUV) submersible videos. IVS 3D Fledermaus™ software was used mapping reveals coral mound distribution, morphology and for rendering and visualization of the bathymetric maps. oceanography in deep water of the Straits of Florida, Geophys. Res. Lett., 33, L23616, 6p, doi:10.1029/2006GL027734. References 19. Cairns, S., 1976, Review of the deep-water ahermatypic corals 1. Paull, C.K., Neumann, A.C., Ende, B.A., Ussler, W., and (Scleractinia) of the tropical western Atlantic, Dissertation Rodriguez, N.M., (2000), Lithoherms on the Florida-Hatteras University of Miami.316p. slope, Marine Geology, 136, 83-101. 20. Mullins, H. T., Heath, K.C., Van Buren, H.M., and Newton, 2. Freiwald, A., Fosså, J.H., Grehan, A. Koslow, T. and Roberts, C.R., (1984), Anatomy of a modern open-ocean carbonate J.M., (2004), Cold-water Coral Reefs, 84 pp., UNEP-WCMC, slope: northern Little Bahama Bank: Sedimentology, 31, 141- Cambridge, UK. 168. 3. Roberts, J.M., Wheeler, A.J., and Freiwald, A., (2006), Reefs of 21. Neumann, A.C., Kofoed, J.W., and Keller, G.H., (1977), the Deep: The Biology and Geology of Cold-Water Coral Lithoherms in the Straits of Florida, Geology, 5(1), 4–10. Ecosystems, Science 28(312), 543–547, doi:10.1126/science.1119861. 4. De Mol, B., Van Rensbergen, P., Pillen, S., Van Herreweghe, K., Van Rooij, D., McDonnell, A., Huvenne, V., Ivanov, M., Swennen, R., and Henriet, J. P., (2002), Large deep-water coral banks in the Porcupine Basin, southwest of Ireland, Marine Geology, 188, 193–231. 5. Ferdelman, T.G., Kano, A., Williams, T., and the IODP

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