Mapping the timing, distribution, and scale of Sargassum influx events in the coastal zone of Bonaire, Caribbean Netherlands

Samenvatting

Since 2015, the island of Bonaire (Caribbean Netherlands) has periodically received massive influxes of the holopelagic brown algae Sargassum. These influxes have negative impact on the coastal environment, tourism industry and public health, while the costs and efforts related to Sargassum clean-up activities pose severe socioeconomical impacts on the local society. To offset these negative impacts, there is a growing interest towards potential opportunities for Sargassum reuse and valorization on Bonaire. Unfortunately, insufficient knowledge about the approximate volumes of Sargassum arriving at specific locations over time, makes it difficult to determine suitable uses of Sargassum and the necessary scale of potential enterprises on Bonaire. To fill this knowledge gap, we have used Sentinel-2 (S2) satellite imagery, to reconstruct the timing, distribution and scale of Sargassum influx events in the coastal zone of Bonaire in the years 2017 to 2022, with the ultimate goal to determine the mean annual harvestable biomass of Sargassum within the coastal waters of Bonaire. Ground truth data for training and validation purposes was determined for 6 S2-images by manually annotating floating Sargassum patches. For the detection of floating Sargassum we have used a Random Forest (RF) model for which we used the 10 m resolution S2-bands (Blue, Green, Red and NIR) and Normalized Difference Vegetation Index (NDVI) as predictors. Results showed that floating Sargassum can be well detected with the RF-model (overall accuracy 99.9%). However, to have the best results, good cloud masking is essential. Since the cloud masking using AI-techniques was unsatisfactory, the masking was done manually. Sargassum influxes were primarily observed between February and July. - The cumulative area coverage (ha) of Sargassum ranged from 20.3 ha in 2017 to 405.5 ha in 2018, and was on average 111.9 ha per year. Most of the floating Sargassum was detected near the eastern coast (i.e. 10-200 m from the shore) and in the western part of two coastal bays (i.e. Lagun and Lac Bay), which is mainly the result of prevailing easterly trade winds pushing Sargassum from open sea towards these locations. - Moreover, on average 950.9 ton dry weight of Sargassum (range 50.8 − 1751.8) washed up annually between 2017 and 2022 in Lac Bay and Lagun. These are the only 2 sites where Sargassum can potentially be harvested due to relatively calm waters in these bays. This study provides the first assessment of the approximate volumes of Sargassum biomass arriving at specific locations over time, which will aid identification of suitable uses of Sargassum and the scale of potential enterprises on Bonaire, while it also allows assessments of economic feasibility and sustainability for proposed ventures. In addition, the obtained information will help identify priority areas for coastal management of these Sargassum influxes