10. Deep Acoustic Applications

Because cold-water coral ecosystems exist at relatively inaccessible depths of 500–1,000 m, only a limited number of accurate maps have been produced for this resource. This chapter describes a combined acoustic survey approach used to acquire high-spatial resolution (up to 0.5 m) maps from two cold-water coral sites in the Straits of Florida. The approach consists of reconnaissance surveys using hull-mounted multi-beam systems, followed by deployment of multi-beam and side-scan sonar systems on an autonomous underwater vehicle (AUV). The wide swath of the reconnaissance survey tools permitted coverage of large areas, producing coarse (20 and 50 m) resolution maps that identified coral-building mounds larger than 2,600 m². Areas of interest identified using the reconnaissance tools were then surveyed with the AUV platform, which resolved cold-water coral fields at 0.5–3 m resolution. The AUV maps detected mounds as small as 81 m² and revealed fine-scale coral ridges up to 20 m high that were not resolved by the reconnaissance maps. The AUV maps, and other remotely acquired data, were ground-truthed with submersible dives to produce an integrated, geo-referenced dataset. Spatial and quantitative analyses were applied to this dataset in order to characterize the morphology and distribution of coral-building features in each surveyed site. In the Miami Terrace site, where corals build low-relief ridges, a habitat classification map and spatial analyses show that coral patches preferentially grow on and along the northern sides of the ridges. A southward flowing bottom current, measured by the AUV, dictates the observed asymmetrical coral distribution. In the site on the lower slope of Great Bahama Bank, where corals form individual mounds, morphometric analyses show a lack of correlation between bottom current regime and mound morphology. Results from these analyses indicate that the two cold-water coral sites in the Straits of Florida are highly variable in terms of coral distribution, spatial parameters, and current regime. Given its high-resolution, the approach presented here is ideal for determining the biophysical processes that underlie these and other remote, fragile ecosystems. Assessment and monitoring of coral distribution and mound abundance based on geophysical data is crucial for managing cold-water coral habitats and is an important research priority.