Mesophotic Coral Ecosystems: A Geoacoustically Derived Proxy for Habitat and Relative Diversity for the Leeward Shelf of Bonaire, Dutch Caribbean

Abstract

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).

Why do mesophotic coral ecosystems have to be protected?

“…Some of the most remarkable examples of MCEs are found in Australia (Sih et al, 2017), Africa (Morais and Maia, 2017), the Gulf of Mexico (Muñoz et al, 2017), eastern Brazil (Pinheiro et al, 2017), Hawaii (Pyle et al, 2016), the Caribbean Sea (Trembanis et al, 2017), the Coral Triangle in the Indo-Pacific, the Red Sea (Shoham and Benayahu, 2017), and the Amazon continental shelf (Francini-Filho et al, 2018;Fig. 1).…”

Predicting the distribution of threatened orbicellid corals in shallow and mesophotic reef ecosystems

“…not created by recent reef building) creates ideal substrate (Jokiel et al 2004), and similarly, antecedent reef buttresses also create ideal habitat for MCE development along a portion of the Puerto Rico Shelf (Sherman et al 2010). In seafloor mapping studies, high rugosity values were indicative of hard coral occurrences in a shallow reef area of Puerto Rico (Prada et al 2008) and in shallow and mesophotic coral reefs in Bonaire (Trembanis et al 2017). Shallow coral occurrence was also predicted by both slope and rugosity in Hawaii (Franklin et al 2013).…”

Estimates of Sea Scallop (Placopecten magellanicus) Incidental Mortality from Photographic Multiple Before—After-Control—Impact Surveys

“…Autonomous underwater vehicle missions were of standard boustrophedon, or ''lawnmower,'' design, composed of 10 parallel north to south lines of 750 m in length spaced apart by 2 m in a manner consistent with previous AUV-based scallop studies (Walker et al 2016). The INS/DVL enabled precise underway navigation, with a total position drift rate of approximately 0.5 m/h (Patterson et al 2008), or 0.1% of distance traveled (Rankey & Doolittle 2012, Trembanis et al 2017. In this study, vehicle-estimated positioning between track lines of replicate missions varied at submeter precision based on digitization of repeatedly imaged acoustic targets registered in the side-scan sonar.…”

“…Recent examples include the use of integrated downward-facing cameras to measure the distribution and abundance of invasive benthic species (Forrest et al 2012) and monitor benthic assemblages (Bewley et al 2015). As a useful complement to digital imagery, high-resolution acoustic sonar imagery illuminates the texture of the seafloor, revealing sonar facies that elucidate bottom features and relative sediment types (Raineault et al 2012, Rankey & Doolittle 2012, Trembanis et al 2017. In the sea scallop fishery, AUV-derived imagery has been used to quantify scallop abundance and distribution (Singh et al 2014, Walker et al 2016, shell height distributions (Singh et al 2013), and visualize dredge scars (Walker et al 2016).…”