Deep and Cold-Water Corals Have Been Known for More Than a Century

AUV-based analysis of the deep-water coral ecosystem

in the Strait of Florida

Thiago B.S. Correa

Deep-water corals live in temperaturesranging from 4 to 12°C and at depths of 50 to over 1000m. Unlike their shallow water zooxanthellate counterparts, deep-water azooxanthellatecorals do not require sunlight. However,deep-water reefs can still be incredibly large structuresup to 120m in relief; this is almost six times higher than shallow-water reefs.The distribution, morphology, growth and food source of deep-water corals has been studied in the Florida-Bahamas-Hatteras regionfor more than half of a century using echo sounders, dredges, sub-bottom profilers, and submersibles. Although these spatially limited surveys generated the first paradigm of deep-water coral systems in the Straits, the fine-scale spatial distribution and morphology of coral mounds remains largely undocumented.

In December of 2005, an Autonomous Underwater Vehicle (AUV) was deployed in the Strait of Florida (total area coverage 130 km2), providing 1-3m high-resolution topographic and backscatter-strength maps, side-scan sonar imagery, sub-bottom profiles and physical parameter measurements at five sites. In 2006, a man-occupied submersible was used to ground-truth these sites, recording video transects and sampling the ocean bottom. Preliminary results revealedan unexpectedmorphological variability and abundance of deep-water coral mounds.Mound morphologies range from isolated structures to enormous well-developed ridges. At the toe-of-slope of western Great Bahama Bank (GBB), in an area of 48km2,more than 500 individual mounds were found, varying from a few meters to 90m in height (Figure 1).Our multibeam maps also show that mounds are growing atop ridges, in canyons, and on plateaus and boulders at the toe-of-slope of GBB (Figure 2).These and other antecedent topographies maybe importantpotential sites of mound development.

Mounds are spatially heterogeneous structures, including stands of both live and dead coral, as well as lithified and soft substrates. Backscatter and Side Scan Sonar (SSS) maps (groundtruthed by video transects and superficial bottom samples) will be used to classify the abundance and distribution of epibenthic fauna on and between individual mounds, as well as the substrate types that these animals associate with. Total benthic coral coverage will be estimated and, using an acoustic texture analysis, the relative abundances and distribution of live and dead corals will be determined. This distinction is necessary in order to understand depositional processes on mounds, including mapping areas of likely net mound accumulation (dominated by live coral) and likely erosion (dominated by dead coral). By examining both antecedent topography and the distribution of epibenthic fauna and substrate types, we can understand the optimal conditions for mound formation and development.

Figure 1: (A) AUV multi-beam bathymetric map of site at the toe-of-slope of GBB with the 503 coral mounds identified within the 48km2area in blue. A box outlines the Twin Peak (B) “Twin Peak” is the highest mound in the area; submersible dive reveals an abundance of living corals on the mound. (C) Histogram displaying the frequency of mounds at GBB7.

Figure 3: (A) 3-D view of the Escarpment site at the toe-of-slope of GBB. Mounds are atop plateau-like promontory and randomly distributed in a triangular shaped area outside of the escarpment thatmay be a slump debris topography.