AUV-Based Environmental Characterization of Deepwater Coral Mounds in the Straits of Florida

Abstract

Abstract The 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. Introduction Deep and cold-water coral ecosystems are less known but more widespread than their warm-water counterparts restricted to shallow tropical seas1,2. Cold-water corals and associated fauna flourish in oceanic waters of all latitudes at depths of several hundred to over one thousand meters with temperatures between 4° and 12°C and require no sunlight3. The limited availability and high cost of deep-water instrumentation has focused most research activity and related discoveries of deep-water coral habitats to the north and central Atlantic, the Gulf of Mexico and the north-east Pacific4,5. In the Straits of Florida, abundant mound-forming corals in water depths of 400-800 m have been documented in over 40 years of dredge sampling, submersible dives and seismic acquisition6–12 (Figure 1). This extensive collection of samples and observations however can not be put into a geomorphologic context as existing bathymetric charts do not resolve coral mounds. Such sparse information has proven inadequate to answer questions in regards to mound morphology, bottom current dynamics and nutrient sources supporting life at these depths. Furthermore the limited data set has so far prevented assessment of the biodiversity and the potential need for protection from over-fishing and underwater construction. High-resolution maps of morphology and oceanographic conditions resolving features at the 1-10 m scale are a basic requirement to make further progress in understanding deep-water coral mound distribution and genesis. Autonomous Underwater Vehicles (AUV) bring an integrated suite of mapping and oceanographic sensors close to the seabed for high-resolution data acquisition and give the opportunity to fill the scale gap of basic information in deepwater environments. Described here are the initial results of a 7 day cruise during which the AUV mapped five deep-water coral mound fields in the Straits of Florida covering a total area of 130 km2.