Imaging the twilight zone: the morphology and distribution of mesophotic zone features, a case study from Bonaire, Dutch Caribbean

Student thesis

Coral reefs are some of the most economically valuable and biologically diverse ecosystems on Earth. Current trends show that throughout the world, reef health is in decline. Some of the most pristine coral reefs in the Caribbean are located in the waters surrounding Bonaire, Dutch Caribbean (Steneck and McClanahan, 2004). These reefs have been shown to extend into deeper waters (up to 110 m). Through the use of Autonomous Underwater Vehicle Technology (AUVs) these deep reef structures may be studied much more efficiently than through the use of SCUBA divers. AUVs were used to collect geospatial reef data in January of 2008, by a collective group of Universities, Industry partners, and the Island Government of Bonaire. This geoacoustic data (swath bathymetry and backscatter) has been processed and analyzed to calculate depth, sediment type, seafloor slope, and rugosity. Backscatter data was processed using an acoustic ground discrimination system (QTC Swathview) to classify bottom types into five separate categories. These classification and bathymetry values were then used as a method to determine dominant mechanisms for enhanced reef structure at depth, locations of paleoshorelines, substrate types, and the effects that conservation efforts have had on the deep-water corals.
Processed data shows the existence of a second reef structure, located at a depth range of 75 to 105 meters, which surrounds much of the leeward side of the island. This reef is in addition to the more commonly known shallow reef, which exists from 10 to 30 meters. Many significant reef-like structures were also discovered at even greater depths in many locations, which may have been generated by significant wave events in the area. While this data provides a preliminary view of several specific locations surrounding Bonaire, additional surveys around the island, as well as neighboring islands, are necessary to gain a full understanding of the processes that affect this region.