Detecting Climate Driven Changes in Chlorophyll-a in Deep Subalpine Lakes Using Long Term Satellite Data

Climate change has increased the temperature and altered the mixing regime of highvaluelakes in the subalpine region of Northern Italy. Remote sensing of chlorophyll-a can helpprovide a time series to allow an assessment of the ecological implications of this. Non-parametricmultiplicative regression (NPMR) was used to visualize and understand the changes that haveoccurred between 2003-2018 in Lakes Garda, Como, Iseo, and Maggiore. In all four deep subalpinelakes, there has been a disruption from a traditional pattern of a significant spring chlorophyll-a peakfollowed by a clear water phase and summer/autumn peaks. This was replaced after 2010-2012, withlower spring peaks and a tendency for annual maxima to occur in summer. There was a tendencyfor this switch to be interspersed by a two-year period of low chlorophyll-a. Variables that weresignificant in NPMR included time, air temperature, total phosphorus, winter temperature, andwinter values for the North Atlantic Oscillation. The change from spring to summer chlorophyll-amaxima, relatively sudden in an ecological context, could be interpreted as a regime shift. Thecause was probably cascading effects from increased winter temperatures, reduced winter mixing,and altered nutrient dynamics. Future trends will depend on climate change and inter-decadalclimate drivers.