Monitoring primary producers in Lake Varese using multisensor satellite data

Due to the excessive eutrophication that has occurred in Lake Varese over the last few decades, the local management authorities have embarked on a rehabilitation plan that includes intensive monitoring activities (AQST Lake Varese). In addition to in situ sampling and a buoy equipped with instruments for continuous measurements, monitoring is carried out by satellite images (Sentinel-2) to study the spatial and temporal dynamics of primary producers (phytoplankton and macrophytes). Chlorophyll-a concentration maps of the euphotic zone, for the period 2019-2023, have been obtained by applying algorithms parameterised with the specific optical properties of the lake, and show high concentrations in autumn due to the surface accumulation of cyanobacteria, also mapped by the PRISMA hyperspectral sensor. The covered area, conditions and seasonal evolution of macrophytes - floating plants and helophytes – were quantified from WAVI (a spectral proxy for canopy density) maps in the period 2016-2023. The results show the overgrowth of invasive hydrophytes during the spring-summer of last years in the south-western part of the lake. In addition, high spatial resolution (2 m) IKONOS, WorldView-2/3 imagery was used for retrospective mapping (2001-2023) of helophyte. The results highlighted the retreat of the common reed in particular near to the Bardello tributary.