Stima dei trend di stato trofico in laghi profondi da dati MERIS (2002-2012)

The aim of this study, developed in GLaSS project frame, is the analysis of trend in the trophic level evolution in some selected clear deep lakes, which, being characterised by good quality state, are important socio-economic resources for their territories. This work was conducted exploiting the ten years of remote observations from MERIS sensor, suitable for obtaining accurate estimation of water trophic level indicators. In particular, available validated time series of chlorophyll-a (chl-a) concentration and water transparency were exploited, obtained through ad hoc algorithms for each lake. For time trend analysis Seasonal Kendall test was exploited: this test allows taking into account the seasonal behaviour, typical of these lakes. For each year or season, an analysis on phenology was conducted, identifying the initial date of chl-a increase and the events of algal bloom. Trend analysis results showed a slight evolution towards a more oligotrophic level in lake Garda and Tanganyika, a slight decrease in water quality in lakes Maggiore and Constance and stable conditions for lakes Vättern, Michigan and Malawi. The analysis on seasonal dynamics and on algal bloom frequency highlighted that, in most cases, the worst years were 2005, 2007 and 2010.