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.