DETECTION OF ALGAL BLOOM IN THE COASTAL WATERS OF BORACAY, PHILIPPINES USING NORMALIZED DIFFERENCE VEGETATION INDEX (NDVI) AND FLOATING ALGAE INDEX (FAI)

Abstract. Boracay is a top tourist island in the Philippines known for its 4-km beach with powdery white sand. Recently, abundance of green algae, an indicator of high nutrient discharge, along the coastal waters of the island had led to concerns on its water quality and prompted its closure to allow ample time for rehabilitation. This study examined the algal bloom along the coastal waters of Boracay through the determination of Normalized Difference Vegetation Index (NDVI) and Floating Algae Index (FAI). ENVI and SNAP software were used to process the satellite images of Boracay obtained from its pre-closure to its reopening. Necessary corrections such as scaling to Top of Atmospheric radiance were applied. NDVI was calculated using orthotile images to determine vegetation and FAI was calculated using Sentinel-2A images. Secondary data showed that the coliform level decreased from 1 million to 18.1 MPN per 100 mL after the rehabilitation of Boracay. Lower NDVI and FAI index values were observed during the reopening of the island. The NDVI value decreased while the FAI value slightly increased on the 1st month of closure which was late dry season. Both NDVI and FAI values increased to their maximum during early wet season and eventually decreased during the reopening of Boracay. Results showed that the abundance of algae had lowered after the rehabilitation of Boracay. However, fluctuating NDVI and FAI values showed the possible seasonal effects on the algal bloom in the island. Further studies considering the other factors on algal blooms may be done.