Detecting climate driven changes in chlorophyll-a in deep subalpine lakes using long term satellite data

Lakes in the subalpine region of Northern Italy have experienced an increase in water temperature and mixing regime alteration due to climate change. Time-series of satellite chlorophyll-a concentration from four optical sensors between 2003-2018 allowed us to assess climate driven changes in phytoplankton phenology. Non-parametric multiplicative regression (NPMR) was used to analyse the changes occurred in lakes Garda, Como, Iseo and Maggiore. In all four deep lakes there has been a disruption from a traditional pattern of a spring chlorophyll-a peak followed by a clear water phase and summer/autumn peaks. This was replaced after 2010-2012, with lower spring peaks and a tendency for annual maxima to occur in summer. There was a tendency for this switch to be interspersed by a two-year period of low chlorophyll-a. Time, air temperature, total phosphorus, winter temperature and winter values for the North Atlantic Oscillation were found as significant factors. The change from spring to summer chlorophyll-a maxima, relatively sudden in an ecological context, could be interpreted as a regime shift. The cause is probably cascading effects from increased winter temperatures, reduced winter mixing and altered nutrient dynamics. Future trends will depend on climate change and inter-decadal climate drivers.