Using high-resolution satellite imagery to assess the impact of Sargassum inundation on coastal areas

Abstract

ABSTRACT A change detection analysis utilizing Very High-resolution (V.H.R.) satellite imagery was performed to evaluate the changes in coastal vegetation in La Parguera, southwestern Puerto Rico, attributable to the increased influx of pelagic Sargassum and its accumulations in cays, bays, inlets and near-shore environments. Imagery was co-registered, corrected for atmospheric effects and masked for water and land. A Normalized Difference Vegetation Index (N.D.V.I.) was used to evaluate the changes in coastal vegetation. N.D.V.I. was used to calculate the differences from 2010 baseline imagery to potential hurricane plus Sargassum impacts (2018 image) and potential Sargassum impacts (2020 image). Results show a total decrease in N.D.V.I. from 2010 to 2020 of 28%, or 3,687,493 m2, and these changes were also observed in true colour images. Total decrease in values in N.D.V.I. was higher from 2018 to 2020, especially for the Isla Cueva site (233,807 m2), and were consistent with the field observations and drone surveys conducted since 2018 in the area. Areas prone to Sargassum accumulation were also identified. This approach provides a quantifiable method to evaluate Sargassum impacts to the coastal vegetation and benthic composition using change detection of V.H.R. images and to separate these effects from other extreme events.