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Abstract
Coral reefs are declining worldwide as a result of the effects of multiple natural and anthropogenic stressors, including regional-scale temperature-induced coral bleaching. Such events have caused significant coral mortality, leading to an evident structural collapse of reefs and shifts in associated benthic communities. In this scenario, reasonable mapping techniques and best practices are critical to improving data collection to describe spatial and temporal patterns of coral reefs after a significant bleaching impact. Our study employed the potential of a consumer-grade drone, coupled with structure from motion and object-based image analysis to investigate for the first time a tool to monitor changes in substrate composition and the associated deterioration in reef environments in a Maldivian shallow-water coral reef. Three key substrate types (hard coral, coral rubble and sand) were detected with high accuracy on high-resolution orthomosaics collected from four sub-areas. Multi-temporal acquisition of UAV data allowed us to compare the classified maps over time (February 2017, November 2018) and obtain evidence of the relevant deterioration in structural complexity of flat reef environments that occurred after the 2016 mass bleaching event. We believe that our proposed methodology offers a cost-effective procedure that is well suited to generate maps for the long-term monitoring of changes in substrate type and reef complexity in shallow water.
Highlights
Coral reefs are declining worldwide as a result of the effects of multiple natural and anthropogenic stressors, such as increased water temperature, ocean acidification, overfishing and land reclamation [1,2,3]
In our work we aimed to develop a new approach for analyzing point clouds, obtained by applying photogrammetric techniques on shallow water unmanned aerial vehicles (UAV) imagery collected in the southern part of Faafu Atoll (Maldives)
Because of the increase in the frequency of extreme heating events driven by climate change, the future of the coral reefs in the Anthropocene is unknown
Summary
Coral reefs are declining worldwide as a result of the effects of multiple natural and anthropogenic stressors, such as increased water temperature, ocean acidification, overfishing and land reclamation [1,2,3]. Among these factors, temperature-induced bleaching events have been reported to cause significant coral mortality in the Indo-Pacific Ocean, with two main destructive events in 1998 and 2016 [4,5,6,7,8]. A significant loss in structural complexity was reported worldwide as a result of the two major coral bleaching events that took place in 1998 and 2016 [9,15]
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