After strong fertilization in the 20th century, many deep lakes in Central Europe are again nutrient poor due to long-lasting restoration (re-oligotrophication). In line with reduced phosphorus and nitrogen loadings, total organismic productivity decreased and lakes have now historically low nutrient and biomass concentrations. This caused speculations that restoration was overdone and intended fertilizations are needed to ensure ecological functionality. Here we show that recent re-oligotrophication processes indeed accelerated, however caused by lake warming. Rising air temperatures strengthen thermal stabilization of water columns which prevents thorough turnover (holomixis). Reduced mixis impedes down-welling of oxygen rich epilimnetic (surface) and up-welling of phosphorus and nitrogen rich hypolimnetic (deep) water. However, nutrient inputs are essential for algal spring blooms acting as boost for annual food web successions. 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). 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. Thus, intended fertilizations are highly questionable, as hypolimnetic nutrients will become available during future natural or artificial turnovers.

Gravity-driven membrane (GDM) filtration is a promising tool for low-cost decentralized drinking water production. The biofilms in GDM systems are able of removing harmful chemical components, particularly toxic cyanobacterial metabolites such as microcystins (MCs). This is relevant for the application of GDM filtration because anthropogenic nutrient input and climate change have led to an increase of toxic cyanobacterial blooms. However, removal of MCs in newly developing GDM biofilms is only established after a prolonged period of time. Since cyanobacterial blooms are transient phenomena, it is important to understand MC removal in mature biofilms with or without prior toxin exposure. In this study, the microbial community composition of GDM biofilms was investigated in systems fed with water from a lake with periodic blooms of MC-producing cyanobacteria. Two out of three experimental treatments were supplemented with dead biomass of a MC-containing cyanobacterial strain, or of a non-toxic mutant, respectively. Analysis of bacterial rRNA genes revealed that both biomass-amended treatments were significantly more similar to each other than to a non-supplemented control. Therefore, it was hypothesized that biofilms could potentially be 'primed' for rapid MC removal by prior addition of non-toxic biomass. A subsequent experiment showed that MC removal developed significantly faster in mature biofilms that were pre-fed with biomass from the mutant strain than in unamended controls, indicating that MC degradation was a facultative trait of bacterial populations in GDM biofilms. The significant enrichment of bacteria related to both aerobic and anaerobic MC degraders suggested that this process might have occurred in parallel in different microniches.

The most prominent responses of Lake Zurich to climate warming include the increase of surface water temperatures, a reduced depth of spring mixing, and the persistent thriving of the harmful cyanobacterium Planktothrix rubescens, a low-light adapted species concentrating in the metalimnion during summer. To study changes of its habitat, we assessed the spatio-temporal metalimnetic boundaries by applying low-pass filtering and binary thresholding to temperature profiles of long-term data (1978-2013, weekly measurements). Due to increasing temperatures over the last 3.5 decades, the onset and duration of metalimnion formation changed significantly (stratification increased by 33 days). Moreover, the upper metalimnetic boundary has undergone a significant drawdown of 2.3 m, accounting for an overall decrease in metalimnion thickness. Being the most abundant species in this zone, P. rubescens was not negatively affected by altered temporal or spatial stratification patterns during its phase of net growth (July-September), as the zone of its optimal light-dependent buoyancy was still located within the metalimnion. The biomass of P. rubescens in August was even significantly related to increasing temperatures. Nevertheless, a further depression of the metalimnetic top boundary may eventually restrict P. rubescens by forcing it into layers of unfavourable light conditions or into the turbulent epilimnetic zone. The most prominent responses of Lake Zurich to climate warming include the increase of 13 surface water temperatures, a reduced depth of spring mixing, and the persistent thriving of 14 the harmful cyanobacterium Planktothrix rubescens, a low-light adapted species 15 concentrating in the metalimnion during summer. To study changes of its habitat, we assessed 16 the spatio-temporal metalimnetic boundaries by applying low-pass filtering and binary 17 thresholding to temperature profiles of long-term data (1978-2013, weekly measurements).

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