Despite coral reef’s rich contributions to people and marine life, scientists estimate that the Caribbean has lost 60 percent of its healthy coral in the past three decades. The challenge to help conserve the region’s reefs has been due in part to the lack of a complete map of the reefs’ topography, their habitat communities and different coral types. Without the big picture of a comprehensive map of the basin’s entire ecosystem, it’s been difficult to dive under the surface to better understand the coral world, monitor it and develop protective strategies.
The Caribbean Division of The Nature Conservancy (TNC) has been focused on monitoring, protecting and restoring the region’s marine environments for more than 40 years. As the plight of coral reefs has become more urgent, so too have TNC’s efforts to tackle coral conservation—and meet the demands for better maps.
“Reef maps are an essential tool for coral resource managers, but historically these maps have had insufficient detail, been outdated or been produced for small areas,” said Dr. Steve Schill, TNC’s lead scientist and marine conservation specialist in the Caribbean Division. “Not having access to accurate, large-area reef maps has limited our understanding of these ecosystems and the benefits they provide.”
Having used Trimble’s eCognition® object-based image analysis (OBIA) software for automatically classifying and mapping small reef areas, Schill believed eCognition could be the enabling, scalable approach to map the hundreds of thousands of reefs across the region.
To begin, Schill worked with technical professionals at Earth observation company Planet and researchers at Arizona State University (ASU) to select more than 30,000 4-meter-resolution scenes from the Dove satellite constellation. The team then created a seamless mosaic of the whole Caribbean basin. He also partnered with eCognition specialists Tama Group to develop the OBIA method to automatically classify benthic habitats.
To map reefs, Tama Group experts integrated the Dove satellite surface reflectance and Dove-derived bathymetry into eCognition. The software first delineates land and sea areas deeper than 15 meters. Based on depth data and known topographic characteristics, it then categorizes the overall reef structure, distinguishing reef crest, fore reef, back reef, patch and fringing reef. Once it defines the five reef classes, eCognition determines seagrass classes (dense and sparse), sand types, dredged areas, and then finishes with mapping the deeper hard bottom algae classes. In total, the software automatically classifies 13 different benthic habitats.
Using this workflow, eCognition successfully classified the shallow water benthic habitats of the entire Caribbean Basin in four months—an area about the size of Texas and California combined. The software exported each classified area as vector shapefiles and Schill and his team downloaded them for analysis and quality control.
With each classification, Schill’s team has been assessing the data to map the benthic habitats. Moving from reef to reef, they’ve been using field data to methodically analyze the accuracy of the classifications, and making manual corrections where needed.
To date, a full suite of benthic habitat maps at a 4-meter resolution have been produced for 23 countries and territories across the insular Caribbean. These maps can be viewed and downloaded at http://caribbeanmarinemaps.tnc.org and will eventually be made available later in 2021 on the Allen Coral Atlas’ online atlas https://allencoralatlas.org/.