Could the extreme meteorological events in Lake Maggiore watershed determine a climate-driven eutrophication process?

Lake Maggiore has reached a stable oligotrophic status after a recovery process from mesotrophy that began in the early 1980s. However, the most recent phytoplankton and water chemistry data seem to indicate that various changes are taking place, including a slight increase in TP values, and almost regular blooms have occurred since 2005. Meteoclimatic data collected over the last few decades in the Lake Maggiore watershed highlight an increase in the frequency of extreme rainfall events. Here, for the period 2000-2013, we analyse the relationships between the phytoplankton dynamics and selected extreme precipitation events, in order to evaluate the possible role of rainfall in affecting the nutrient availability and phytoplankton dynamics. Among the algal groups, cyanobacteria showed the strongest relationship with the precipitation pattern, particularly on a short-term temporal scale. Our results support the hypothesis that rainfall may lead to a short-term increase in nutrients that stimulate the growth of phytoplankton and the development of blooms in summer, when epilimnetic waters are usually nutrient depleted. The almost regular cyanobacterial blooms recorded in Lake Maggiore since 2005 can be interpreted as a response to changing precipitation patterns and therefore as a sign of climate-induced eutrophication.