Mapping phycocyanin cells density of Planktothrix rubescens in freshwater reservoirs

Abstract

In infested lakes, during periods of low solar irradiance when light weakly penetrates water and it cools, P. rubescens filaments float up to water surface forming red blooms. P. rubescens produces microcystins, which are harmful hepatotoxins. Water supply for civil purposes needs to be limited or suspended, as well as all recreational activities (e.g., boating, fishing etc.) and cattle watering. Thus, mapping techniques are required to routinely, promptly and cheaply quantify microcystins density. Since, in recent years a toxic bloom of P. rubescens has occurred in several Sicilian reservoirs (Ancipa, Pozzillo, Nicoletti, Raia di Prizzi, Trinità di Delia), water samples were collected for counting phytoplankton cells and for analysing microcystins occurrence. In this framework, this manuscript setup empirical relationships, based on reflectance products, to evaluate phycocyanin cells density, C(106 cells l-1), from MODIS Terra (at a spatial resolution 250 m) and Landsat ETM+ images (at 30 m) using both Red and NIR ground spectral reflectances.