Mapping Corals, Algae, Seagrass and Substrata on Fringing Coral Reefs Using Synchronised Remotely Piloted Aircraft and Underwater Video at Vavanga Village, Kolombangara Island, Solomon Islands
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Stockley, Aidan
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Breen, Daniel
Hinchliffe, Graham
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Auckland University of Technology
Abstract
Fringing reefs are important ecological systems that support diverse marine life, fisheries and the livelihoods of coastal communities, particularly in large ocean states such as the Solomon Islands. Increasing disturbances, such as sedimentation, rising temperatures and coastal development and land use, have each heightened the vulnerability of fringing reefs to long-term degradation. To address these issues, accurate, repeatable assessments of reef condition and benthic community structure are necessary to monitor marine habitats and inform sustainable management.
This study assesses the effectiveness of georeferencing coral reef benthos and substrata in underwater video from synchronised Full Motion Video (FMV) acquired by a Remotely Piloted Aircraft (RPA). Nine-point overlays on still underwater video images were used to manually estimate percentage cover of benthic biota and substrata at 4,011 georeferenced point locations across and along the customary fringing reef of Vavanga, Kolombangara Island, Solomon Islands. Classification and regression trees (CART) were used to identify and map spatial predictors of benthic taxa and substrata in ArcGIS Pro and extracted underwater video frames were also analysed using the ReefCloud artificial intelligence classification software, which achieved moderate accuracy in broader classifications of coral and substrata.
Distinct patterns in benthic composition were observed across and along the reef and in relation to freshwater streams and lagoons. Macroalgae occurred across the shallow Vavanga reef flat, particularly Padina, with distinct bands of Sargassum and Turbinaria ornata on the mid to outer reef flat. South-east of the river, dense seagrass beds (Thalassia hemprichii) occurred on coarse sand across the inner to mid reef flat, while Enhalus acoroides occurred at the edge of the inner lagoon, further south. The inner reef flat north-west of the river at Vavanga was dominated by bare, weathered karst limestone, silt and fine sand, Padina and little or no seagrass or coral. This area may be influenced by the river, several small streams, mangroves, and another large inner lagoon. Small scattered, encrusting, and tightly branched hard corals (Pocillopora verrucosa and P. meandrina, Acropora verweyi, Montipora digitata, Porites spp.) increased from the mid to outer reef flat and crest, and coral cover rapidly increased with depth on the upper slope to approximately 30%. Finer-scale differences in coral cover and composition occurred along the coast to the north-west, at the river mouth at Vavanga, and in lagoons and creeks to the north and south of the village. The synchronised RPA-underwater video workflow demonstrates a repeatable, high-resolution method to map shallow marine environments. It provides a practical, spatially explicit tool to map fine-scale patterns in reef biota and morphology, and support community-based management in remote Pacific Island ecosystems.