Time Series Analysis of Landsat TM/OLI images to detect changes in macroalgae distribution and coverage in the coast of Rio Grande do Norte, Brazil.
macroalgae; spatio-temporal variation; remote sensing; Landsat.
Macroalgae are an essential component of the marine ecosystem as a whole, as they support a large biodiversity while providing for a number of ecosystem services. Nonwithstanding their ecological value, macroalgae are also a resource of increasing economic value. The goal of this study was to assess the evolution of algal cover, their distribution as well as their biomass fluctuation along two beach areas located at the coastline of the Rio Grande do Norte State (Rio do Fogo and Baía Formosa beaches), focused on a multitemporal approach. Landsat 5 TM and Landsat8 OLI satellite digital orbital images were used, covering approx. a 3 decade timespan. An image analysis based on hydrodynamics revealed 6 distinct algal cover environments present over the reef study areas. An isodata classification analysis was carried out to tell apart distinct algal cover environments according to their flooding degree. Concerning the total algal coverage, the Rio do Fogo beach showed an 8% increase in size (from 27.27 ha to 29.34 ha), whereas the Baía Formosa beach showed a 30% decrease in size (from 12.25 ha to 8.55 ha). As to the total biomass, the Rio do Fogo area presented a 64% biomass increase (from 8,063 Kg to 13,271 Kg), vis-à-vis just a slight 1% increase (from 3,538 Kg to 3.570 Kg) at the Baía Formosa beach. In conclusion, this study showed that
remote sensing may be an effective tool, when used in multitemporal studies, to track fluctuations in algal cover both in the temporal and in the spatial scales.