Utilização de LANDSAT-TM no estudo de uma floração de macroalgas de deriva no estuário da Lagoa dos Patos (RS, Brasil)

Abstract

Green tides, formed by fast growing opportunist algae, may cause several negative impacts to coastal ecosystems. During late winter 2010, thick blooms of drift macroalgae were observed ocurring in several shallow areas of Patos Lagoo nestuary. Due to its ecological importance, a field survey and a satellite image (LANDSAT-TM) analysis were conducted inorder to investigate the occurrence, distribution and extension of the bloom. The field data was obtained in the same day (September 7th, 2010) and three days after the satellite imagery. Macroalgal percentage cover,abundance and digital images were obtained in 14 GPS located points within the shallow bay. Samples were also collected for species identification. During image processing and analyses, several supervised and no-supervised classification methods were trialled and the Normalized Digital Vegetation Index (NDVI) was applied. The results showed a bloom formed mainly by filaments of Rizoclonium spp and Cladophora spp, with biomasses up to 0.6 kg. m-2. In some shallow banks and margins, the green tide formed a continuous mat throughout the water column, with biomasses up to 30 kg. m-2. The Fisher supervised classifier achieved the best separation between classes of vegetated and no vegetated bottoms. The distribuition obtained by NDVI allowed detecting the emerged floating algae of those later dense accumulations. The field data and remote sensing based results of the present study showed the extension and distribution of one of the largest green tide that has occurred in Patos Lagoon estuary during last decade.