Long-term trends and fine year-to-year tuning of phytoplankton in large lakes are ruled by eutrophication and atmospheric modes of variability

This study demonstrated how the impact of eutrophication in a deep lake at the southern border of the Alps (Lake Garda) was regulated by specific modes of atmospheric circulation relevant for the Mediterranean area. At the decadal scale, nutrients and phytoplankton increased concurrently since the 1970s. At the annual scale, year-to-year fluctuations in nutrients and phytoplankton were controlled through a chain of causal factors centred on deeply penetrative mixing events determining an upward transport of phosphorus from the hypolimnion to the trophogenic layers. The extent of mixing was in turn controlled by lake and air winter temperature, which were ultimately regulated by the winter fluctuations of the East Atlantic pattern (EA). In its negative state, the EA shows an intense high pressure over the West Atlantic, causing a north-easterly air flow bringing cold air from continental Europe to Mediterranean, thus favouring greater lake mixing and nutrient fertilisation. Cyanobacteria (mostly Planktothrix rubescens) were the organisms which greatly benefitted from the long-term increase in phosphorus concentrations and the year-to-year fluctuations in surface phosphorus availability controlled by the EA. Given the same availability of phosphorus in the water column, positive winter EA phases weakened the eutrophication effects and phytoplankton development.