Satellite Remote Sensing of Chlorophyll-a inSubalpine Italian Lakes in the last 15 Years

In this work a dataset of maps of Chl-a derived from different sensors (MERIS-OLI-MSI-OLCI) satellite images is used to evaluate water quality of the four most important Italian subalpine lakes (Garda, Maggiore, Iseo, Como) in the period 2003-2018. In order to produce Chl-a concentration maps, imagery needs to be processed along a processing chain, which includes radiometric correction (to convert digital numbers into at-satellite-atmospheric radiance), atmospheric correction to obtain Remote sensing reflectance (C2R for MERIS, 6SV for MSI and OLI, Polymer for OLCI), and finally the application of bio-optical models to retrieve chl-a concentration. The results presented in this study show: i) that the seasonal variability of chl-a concentration is particularly pronounced during spring and autumn; ii) Lake Iseo is the one that has a worse judgment than WFD; iii) 2005 and 2016 were quite critical years; iv) trends show a slight tendency to increase Chl-a concentrations during the period considered, particularly for Iseo and Maggiore lakes.