Keller, B.

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.

Current trends demonstrate coral reef health in serious decline worldwide. Some of the most well-preserved coral reefs in the Caribbean basin are located in the waters surrounding Bonaire, in the Dutch Caribbean. In many places on the leeward side on islands dominated by trade winds, the shallow reef systems extend into deeper water where they are known as Mesophotic Coral Ecosystems (MCE). Autonomous Underwater Vehicles (AUVs) were used to collect geoacoustic data of these leeward reefs at multiple sites as part of an ocean exploration project. AUV swath bathymetry and side-scan sonar data were analyzed for depth, acoustic backscatter intensity, seafloor slope, and rugosity. These geomorphic metrics were then used as inputs to generate a composite synthetic index of bottom-type to delineate MCE features. A confusion matrix statistical analysis of the acoustic class map showed an overall accuracy of the acoustic classes at 66%, with accuracy of the hard coral class the highest at 83%, and the sandy-bottom class the lowest at 55. The hard coral class was also the statistically most reliable, at over 80%, with the noise class coming in as the least reliable. This morphologic habitat index is a potentially useful new tool in quantifying the extent of MCE located in proximity to Marine Protected Areas (MPAs).