Abstract
Seagrass habitats are ecologically valuable and play an important role in sequestering and storing carbon. There is, thus, a need to estimate seagrass percentage cover in diverse environments in support of climate change mitigation, marine spatial planning and coastal zone management. In situ approaches are accurate but time-consuming, expensive and may not represent the larger spatial units collected by satellite imaging. Hence, there is a need for a consistent methodology that uses accurate point-based field surveys to deliver high-quality mapping of percentage seagrass cover at large spatial scales. Here, we develop a three-step approach that combines in situ (quadrats), aerial (unoccupied aerial vehicle—UAV) and satellite data to map percentage seagrass cover at Turneffe Atoll, Belize, the largest atoll in the northern hemisphere. First, the optical bands of four UAV images were used to calculate seagrass cover, in combination with in situ data. The seagrass cover calculated from the UAV was then used to develop training and validation datasets to estimate seagrass cover in Sentinel-2 pixels. Next, non-seagrass areas were identified in the Sentinel-2 data and removed by object-based classification, followed by a pixel-based regression to calculate seagrass percentage cover. Using this approach, percentage seagrass cover was mapped using UAVs (R2 = 0.91 between observed and mapped distributions) and using Sentinel-2 data (R2 = 0.73). This work provides the first openly available and explorable map of seagrass percentage cover across Turneffe Atoll, where we estimate approximately 242 km2 of seagrass above 10% cover is located. We estimate that this approach offers 30 times more data for training satellite data than traditional methods, therefore presenting a substantial reduction in cost-per-point for data. Furthermore, the increase in data helps deliver a high-quality seagrass cover map, suitable for resolving trends of deteriorating, stable or recovering seagrass environments at 10 m2 resolution to underpin evidence-based management and conservation of seagrass.
Highlights
Seagrasses are marine angiosperms that form meadows in shallow inter- and sub-tidal areas [1]
The random forest classification for the unoccupied aerial vehicles (UAVs) sites, between the green band and seagrass cover, had an adjusted R2 of 0.91 (p-value < 0.05), and 94.2% of the variation was explained by the model.for
Once percentage cover within the footprint seagrass cover, had an adjusted R of 0.91 (p-value < 0.05), and 94.2% of the variation was of the Sentinel-2 pixels was summarised and a stratified sample had thinned the data, a
Summary
Seagrasses are marine angiosperms that form meadows in shallow inter- and sub-tidal areas [1]. Seagrasses provide a range of provisioning, regulating and cultural ecosystem services [4] that contribute to human welfare, coastal populations [5] They offer nursery habitats [6], food sources and shelter to various marine organisms, supporting biodiversity, endangered marine species and an estimated 20% of global fisheries production [7]. Known as habitat builders, seagrass meadows help compensate for sea-level rise and reduce the flow, turbulence and wave action in their immediate vicinity, which results in increased sedimentation rates and stabilises the sediment [9]. Monitoring seagrass extent and cover, and their changes over time, is vital to identify threats and underpin evidence-based management strategies to conserve and restore them
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