Abstract

Since 2011, Caribbean islands are facing major events of pelagic Sargassum washing ashore, principally Sargassum natans (Børgesen) (Schell et al. 2015). A priority objective for local governments is to detect offshore floating algae and assess the risk of landings on the coastline. Early detection of Sargassum, risk assessment and alert releases are key elements in the process of helping the local populations to prepare for Sargassum landings. Satellite-based macroalgae detection and tracing can be derived from the Moderate Resolution Imaging Spectrometer (MODIS) (Hu 2009) in detecting macroalgae mats on the surface of the ocean based on the red-edge reflectance of vegetation. The algae-mats appear as slicks in MODIS imagery. In particular, a customised data product (AFAI: Alternative Floating Algae Index) has been generated and distributed in near real-time through a Virtual Antenna System (VAS, Hu et al., 2014) for the Eastern Caribbean (10 – 23oN, 75 – 60oW), the Central West Atlantic (22.0°N - 0.0°N, 38°W - 63°W), and other areas in the entire Intra-Americas Sea, since 2011 (http://optics.marine.usf.edu/projects/SaWS.html). Together with surface ocean currents also available in near real-time from a numerical model, the products effectively formed a prototype Sargassum Watch System (SaWS, Hu et al., 2016). Local groups used the SaWS on a routine basis to detect and trace Sargassum mats. Island authorities and managers expect reliable alerts to anticipate Sargassum washing ashore and mobilise technical teams, thus reducing timing and costs of cleaning operations. Here, the objective of this work is to demonstrate and evaluate a simple and fast method for release of early-warning alerts of Sargassum risk in the Lesser Antilles, based on the MODIS AFAI products and analyses of surface currents. We present a prediction method to use the SaWS system for near-real-time tracking of floating algae in the central Atlantic. These online products are integrated and made available to users in Keyhole Markup Language (KML) format and uploaded in Google Earth. Using GPS coordinates of Sargassum rafts and distance from coast, direction and speed of HYCOM current vectors, we have provided an effective time window of possible washing-ashore in the Guadeloupe archipelago. The method has been used to release daily report to the Guade-loupe local environment agency. We obtained over 60% prediction accuracy between forecast and real events of washing ashore in 2015. The method offers a simple framework for governmental agencies and environmental groups based on existing online resources.

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