RED ALGAE BLOOM DETECTION IN OCCHITO LAKE COMBINING MERIS AND MODIS DATA

In winter 2008-2009 Lake Occhito, a strategic multipleuses (irrigation and drinking supply) reservoir located in South Italy, was affected by an extraordinary Planktothrix rubescens bloom. P. rubescens is a filamentous potentially toxic cyanobacterium which has recently colonized many environments in Europe and Italy. A number of studies is currently available on the use of remote sensing techniques to monitor different fresh water cyanobacteria species, using the common biomarker pigment phycocyanin. By contrast no specific applications are available on the remote sensing monitoring of P. rubescens, whose optical response is markedly influenced by the presence of the phycoerythrin accessory pigment. In this paper we present a simple band ratio algorithm based on Water Leaving Reflectances from MERIS data, atmospherically corrected using the Aerosol Optical Thickness retrieved by MODIS data, to detect P. rubescens blooms. The high accuracy in AOT data, provided by MOD09 surface reflectance product, at 1km spatial resolution, allowed obtaining a good correlation (R2=0.75) between the WLR and the P. rubescens chlorophyll-a concentrations measured in the field, through multiple stations fluorometric profiles. The combined use of MERIS and MODIS data resulted thus in a very useful tool to monitor such a phenomenon, even in areas where