Mapping Sargassum on beaches and coastal waters of Bonaire using Sentinel-2 imagery

Sargassum is a genus of brown macroalgae or seaweed that can be found in shallow waters or free floating in the ocean. Sargassum patches on the open sea drift along sea current and can aggregate into larger Sargassum rafts or long slicks. Sargassum seaweeds that accumulate in the Sargasso Sea originate from The Gulf of Mexico where it blooms in the spring. This Sargassum bloom is induced by nutrient loadings from land that are discharged via the Mississippi River into the sea. Sargassum is not directly harmful on sea, in fact diverse biotic communities and animal species such as fishes, sea turtles and invertebrate depend on the seaweed for shelter and food source. However, Sargassum can potentially damage coastal ecosystems such as mangroves and seagrass beds if it accumulates on the coast. Therefore, monitoring of pelagic Sargassum is of great importance for managing coastal ecosystems.

An unprecedented amount of pelagic Sargassum invaded the Caribbean islands in the summer of 2011. Masses of Sargassum seaweed piled up on beaches trapping sea turtles and releasing high concentration of toxic hydrogen sulphide gas when it decomposes. Beside sea turtles, local tourism was also affected by the Sargassum beaching which led to temporarily closure of hotel resorts and high-cleaning costs of beaches. Climate change and increasing nitrification of seas might indicate that the amount of Sargassum in the Caribbean Sea might increase substantially within the near future.

The main objective of this research is to use multispectral data to map and classify Sargassum patches on the east coast of Bonaire. Accurate Sargassum maps will be useful for the coastal management to assess the location and coverage of Sargassum mats that have washed up along the shores and beaches. Consequently, the extent to which these Sargassum seaweeds affect nearshore benthic habitats and mangrove ecosystems in east Bonaire can be evaluated too.

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