Global lakes sentinel services: Evaluation of Chl-A trends in deep clear lakes

The aim of this study is the analysis of trend in the trophic level evolution in clear deep lakes which, being characterised by good quality state, are important socio-economic resources for their regions. The selected lakes are situated in Europe (Garda, Maggiore, Constance and Vättern), North America (Michigan) and Africa (Malawi and Tanganyika) and cover a range of eco-regions (continental, perialpine, boreal, rift valley) distributed globally. To evaluate trophic level tendency we mainly focused on chlorophyll-A concentrations (chl-a) which is a direct proxy of trophic status. The chl-A concentrations were obtained from 5216 cloud-free MERIS imagery from 2002 to 2012. The 'GLaSS RoIStats tool' available within the GLaSS project was used to extract chl-A in a number of region of interests (ROI) located in pelagic waters as well as some few other stations depending on lakes morphology. For producing the time-series trend, these extracted data were analysed with the Seasonal Kendall test. The results overall show almost stable conditions with a slight increase in concentration for lakes Maggiore, Constance, and the Green Bay of Lake Michigan; a slight decrease for lakes Garda and Tanganyika and absolutely stable conditions for lakes Vättern and Malawi. The results presented in this work show the great capability of MERIS to perform trend tests analysis on trophic status with focus on chl-A concentration. Being chl-A also a key parameter in water quality monitoring plans, this study also supports the managing practices implemented worldwide for using the water of the lakes.