Assessment of the influence of climate change on the trophic status of Italian subalpine lakes using time series of chlorophyll-a derived from satellite products

Lakes are good sentinels of the effects of climate change due to their global distribution and the capability of their physical, chemical, and biological properties to respond rapidly to climate-related changes. Water quality status of inland waters can be detected at high spatial-temporal scales by means of Earth Observation (EO) data. In this study, EO satellite data acquired from optical sensors (Envisat-MERIS, Sentinel2-MSI and Sentinel3-OLCI) were used for acquiring timely, frequent synoptic information of the four most important Italian subalpine lakes (Maggiore, Como, Iseo, Garda) over 15 years. Maps of chlorophyll-a (Chl-a), a proxy of the lake trophic state, were retrieved and used to evaluate changes in the water quality status of these ecosystems from 2003 to 2018. Imagery was processed using appropriate atmospheric correction codes to obtain remote sensing reflectance (C2R for MERIS, 6SV for MSI, Polymer for OLCI), and bio-optical models were applied to retrieve Chl-a concentration. The chain products (Rrs and Chl-a) were validated against field data. The Chl-a time series were analyzed in relation to physical and meteorological (water and air temperature) data. The median value of the Chl-a time series range between 1.03 and 1.52 mg m-3, with a seasonal variability particularly evident during spring and autumn. Chl-a concentration showed a slight tendency to increase during the 15 years analyzed, in particular for Iseo and Maggiore lakes (median annual Sen slope 0.109 and 0.111 mg m-3, respectively). This increase in trophic status is related to an increase in water and air temperature in the lakes of the Alpine region. A recent study indicated an increased temperature for subalpine lakes as 0.032 °C yr-1 during summer and an annual average rate of 0.017 °C yr-1.