Nutrients in combination with higher irradiance, rising water temperature and reduced turbulence 10 enable photoautotrophic organisms (algae) to develop massive populations, so called spring blooms 11. Thus vernal holomixis is not only fundamental for the replenishment of deep layers with oxygen but also for the transfer of nutrients from the hypo- to the epilimnion 7, 8, 9. Anoxia leads to a release of orthophosphate (PO 4-P) otherwise bound in sediments at aerobic conditions. Thus, chances of complete water turnover (holomixis), happening once or twice a year in the temperate zone, become smaller, owing to temperature-dependent density differences between warmer surface (epilimnion) and cold deep water layers (hypolimnion).ĭepending on the morphometry and productivity of thermally stratified lakes, hypolimnetic oxygen is gradually depleted due to microbial respiration. This causes an earlier onset and prolongation of thermal stabilization in even deep large lakes. However, in a recent study for a pre-alpine region, the strongest warming was observed during spring periods 6. ![]() The documented average warming per annum is mainly attributed to striking temperature increases during cold seasons (winter) across the Northern Hemisphere 4, 5. Inland surface waters are immediately affected by warming due to the strong correlation between air and surface water temperatures 2 and the impact of increased solar radiation 3. Global warming changes physical and chemical properties of lakes and catchments 1, as well as their biota 2. Thus, intended fertilizations are highly questionable, as hypolimnetic nutrients will become available during future natural or artificial turnovers. 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).
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