By simulating holomixis in experiments, we could induce significant vernal algal blooms, confirming that there would be sufficient hypolimnetic phosphorus which presently accumulates due to reduced export. We show that repeated lack (since 1977) and complete stop (since 2013) of holomixis caused drastic epilimnetic phosphorus depletions and an absence of phytoplankton spring blooms in Lake Zurich (Switzerland). However, nutrient inputs are essential for algal spring blooms acting as boost for annual food web successions. Reduced mixis impedes down-welling of oxygen rich epilimnetic (surface) and up-welling of phosphorus and nitrogen rich hypolimnetic (deep) water. Rising air temperatures strengthen thermal stabilization of water columns which prevents thorough turnover (holomixis). Here we show that recent re-oligotrophication processes indeed accelerated, however caused by lake warming. This caused speculations that restoration was overdone and intended fertilizations are needed to ensure ecological functionality. In line with reduced phosphorus and nitrogen loadings, total organismic productivity decreased and lakes have now historically low nutrient and biomass concentrations.
After strong fertilization in the 20 th century, many deep lakes in Central Europe are again nutrient poor due to long-lasting restoration (re-oligotrophication).
