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Recent trends (2009 to 2016) in warming, concentrations of oxygen and phosphorus, and cyanobacterial biomass in Lake Zurich (Switzerland). (a) Water temperature and two white isolines showing 4.5 °C and 5.0 °C, respectively. (b) Dissolved oxygen concentration and the 6 mg O 2 l −1 isoline which is a proxy for the depth of maximal water turnover during spring. Note the metalimnetic oxygen minima developing each autumn expanding between 15-40 m water depths. Concentrations of dissolved orthophosphate (c), total phosphorus (d) and particulate (i.e., mostly organismic bound) phosphorus (e). (f) Total biomass (chlorophyll a concentration) of the most dominant primary producer in Lake Zurich, the cyanobacterium Planktothrix rubescens. Data based on biweekly profiles (n = 192) of parameters measured in 1 m depth intervals (0 to 120 m depth) in a,b,f. Data based on monthly profiles (n = 96) of 17 sampling depths (n = 1,632) in c,d,e. 

Recent trends (2009 to 2016) in warming, concentrations of oxygen and phosphorus, and cyanobacterial biomass in Lake Zurich (Switzerland). (a) Water temperature and two white isolines showing 4.5 °C and 5.0 °C, respectively. (b) Dissolved oxygen concentration and the 6 mg O 2 l −1 isoline which is a proxy for the depth of maximal water turnover during spring. Note the metalimnetic oxygen minima developing each autumn expanding between 15-40 m water depths. Concentrations of dissolved orthophosphate (c), total phosphorus (d) and particulate (i.e., mostly organismic bound) phosphorus (e). (f) Total biomass (chlorophyll a concentration) of the most dominant primary producer in Lake Zurich, the cyanobacterium Planktothrix rubescens. Data based on biweekly profiles (n = 192) of parameters measured in 1 m depth intervals (0 to 120 m depth) in a,b,f. Data based on monthly profiles (n = 96) of 17 sampling depths (n = 1,632) in c,d,e. 

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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 speculat...

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... and mixis depth became first obvious from a biweekly monitoring conducted since 2009. The overall warming of Lake Zurich in the last four decades has been documented 6,18,19 and this trend continued dur- ing the last 8 years. Since 2013, warming steadily propagated in deeper water layers, and temperature increased to 5 °C even in 90 m depth (Fig. 1a). This extensive warming impeded vernal holomixis for four consecutive years (2013)(2014)(2015)(2016). Lake Zurich is classified as warm monomictic, and the intensity of spring water turnover depends on isothermal conditions in combination with strong wind actions (velocities of >20 km h −1 are essential for deep water circulation) 20 . ...
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... by mixis). The validity of this proxy to describe mixis intensity 18 was also shown for the continuation of the long-term dataset between years 1972-2011. Even if holomixis takes place, there is always a strong oxygen depletion starting from spring on in Lake Zurich. Oxygen depletion propagates from the ground upwards throughout the hypolimnion (Fig. 1b). Usually minimal oxygen concentration are reached in December, with an anaerobic zone above ground. Thus, vernal mixis in Lake Zurich always leads to an annually recurrent oxygen replenishment of deeper water strata (however, the spatial extension and quantity of replenishment depends on mixis intensity). Here we show that the isoline ...
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... Thus, vernal mixis in Lake Zurich always leads to an annually recurrent oxygen replenishment of deeper water strata (however, the spatial extension and quantity of replenishment depends on mixis intensity). Here we show that the isoline referring to 6 mg O 2 l −1 indicates a reduction of vernal mixis from 120 m in 2012 to only 69 m in 2016 ( Fig. 1b and Supplementary Fig. S1). In parallel, the last large import of PO 4 -P from the hypo-to the epilimnion was observed in 2012 (Fig. 1c). Since then, concentrations in the water column down to 40 m were close to the limit of detection (<1 µg l −1 ) throughout the years. However, levels of biologically available PO 4 -P were still high (>150 µg l −1 ) in layers ...
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... spatial extension and quantity of replenishment depends on mixis intensity). Here we show that the isoline referring to 6 mg O 2 l −1 indicates a reduction of vernal mixis from 120 m in 2012 to only 69 m in 2016 ( Fig. 1b and Supplementary Fig. S1). In parallel, the last large import of PO 4 -P from the hypo-to the epilimnion was observed in 2012 (Fig. 1c). Since then, concentrations in the water column down to 40 m were close to the limit of detection (<1 µg l −1 ) throughout the years. However, levels of biologically available PO 4 -P were still high (>150 µg l −1 ) in layers near to the sediment in late winter (Fig. 1c). Notably, con- centrations of total phosphorus (TP = roughly the ...
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... large import of PO 4 -P from the hypo-to the epilimnion was observed in 2012 (Fig. 1c). Since then, concentrations in the water column down to 40 m were close to the limit of detection (<1 µg l −1 ) throughout the years. However, levels of biologically available PO 4 -P were still high (>150 µg l −1 ) in layers near to the sediment in late winter (Fig. 1c). Notably, con- centrations of total phosphorus (TP = roughly the sum of dissolved PO 4 -P and particulate phosphorus) did not mirror the lack of holomixis at all (Fig. 1d). Most of TP in surface layers of lakes is made up by organismic bound phosphorus (particulate fraction, Fig. 1e), which in case of Lake Zurich highly corresponds to ...
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... of detection (<1 µg l −1 ) throughout the years. However, levels of biologically available PO 4 -P were still high (>150 µg l −1 ) in layers near to the sediment in late winter (Fig. 1c). Notably, con- centrations of total phosphorus (TP = roughly the sum of dissolved PO 4 -P and particulate phosphorus) did not mirror the lack of holomixis at all (Fig. 1d). Most of TP in surface layers of lakes is made up by organismic bound phosphorus (particulate fraction, Fig. 1e), which in case of Lake Zurich highly corresponds to the dominant pri- mary producer, the harmful cyanobacterium Planktothrix rubescens (Fig. 1f). This filamentous cyanobacterium regulates its buoyancy by gas vesicles within ...
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... (>150 µg l −1 ) in layers near to the sediment in late winter (Fig. 1c). Notably, con- centrations of total phosphorus (TP = roughly the sum of dissolved PO 4 -P and particulate phosphorus) did not mirror the lack of holomixis at all (Fig. 1d). Most of TP in surface layers of lakes is made up by organismic bound phosphorus (particulate fraction, Fig. 1e), which in case of Lake Zurich highly corresponds to the dominant pri- mary producer, the harmful cyanobacterium Planktothrix rubescens (Fig. 1f). This filamentous cyanobacterium regulates its buoyancy by gas vesicles within cells. Gas vesicles have to withstand the combination of cell turgor pressure and the hydrostatic pressure, ...
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... dissolved PO 4 -P and particulate phosphorus) did not mirror the lack of holomixis at all (Fig. 1d). Most of TP in surface layers of lakes is made up by organismic bound phosphorus (particulate fraction, Fig. 1e), which in case of Lake Zurich highly corresponds to the dominant pri- mary producer, the harmful cyanobacterium Planktothrix rubescens (Fig. 1f). This filamentous cyanobacterium regulates its buoyancy by gas vesicles within cells. Gas vesicles have to withstand the combination of cell turgor pressure and the hydrostatic pressure, which increases linearly with depth when filaments are entrained into the hypolimnion during vernal water turnover events. Genetic markers for their ...
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... vesicles have a critical collapse pressure of 1.17 MPa, thus filaments could retain buoyancy 24 down to depths of 99 m. Hence, holomixis events result in the destruction 18,25 of large fractions of cyanobacterial pop- ulations during winter/early spring ( Supplementary Fig. S1). Thus, incomplete water turnovers allowed for high survival ratios, causing their permanent presence since autumn 2012 (Fig. 1f). ...
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... MPa, thus filaments could retain buoyancy 24 down to depths of 99 m. Hence, holomixis events result in the destruction 18,25 of large fractions of cyanobacterial pop- ulations during winter/early spring ( Supplementary Fig. S1). Thus, incomplete water turnovers allowed for high survival ratios, causing their permanent presence since autumn 2012 (Fig. 1f). This may have severe consequences for the entire food web as P. rubescens stores various toxic secondary metabolites making this harmful cyanobac- terium an adverse food item for most consumers 26 . Meanwhile the vernal population dynamic of P. rubescens, i.e. the entrainment of filaments into deeper water layers, can be regarded as ...
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... may have severe consequences for the entire food web as P. rubescens stores various toxic secondary metabolites making this harmful cyanobac- terium an adverse food item for most consumers 26 . Meanwhile the vernal population dynamic of P. rubescens, i.e. the entrainment of filaments into deeper water layers, can be regarded as an additional proxy to determine the depth of water turnover in spring ( Supplementary Fig. S1). ...

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Technical Report
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Der Klimawandel führt auch in Seen und Talsperren zu bedeutenden Veränderungen, deren Auswirkungen durch kontinuierliches Monitoring erfasst werden müssen. Im Rahmen dieses Projektes sollte insbesondere der Einfluss des Klimawandels auf die physikalischen Prozesse in Seen systematisch untersucht werden. Dafür wurden 14 Seen mit Messketten ausgestattet, durch die Temperatur, Sauerstoff und Wasserstand in hoher zeitlicher Auflösung gemessen wurden. Ergänzend wurden Daten der drei Messstationen des Leibniz-Instituts für Gewässerökologie und Binnenfischerei (IGB) in die Auswertung einbezogen. Die Messketten lieferten einen breiten Datensatz, der für erste statistische Auswertungen und zur Kalibrierung numerischer Modelle verwendet werden konnte. Darüber hinaus bilden die im Projekt gemachten Erfahrungen eine Grundlage für die Ausarbeitung eines möglichen langfristigen Monitorprojekts. Die hochaufgelösten Messdaten wurden durch langjährige Messreihen von mehrmals pro Jahr stattfindenden Terminmessungen in mehr als 100 Seen Deutschlands ergänzt, um langfristige Trends identifizieren zu können. Dieser Datensatz zeigte zwischen 1970 und 2020 einen Trend von um ca. 0.4°C pro Dekade steigenden Oberflächentemperaturen bei sich wesentlich weniger ändernden Tiefenwassertemperaturen, was auf eine stärkere und länger anhaltende Schichtung in dimiktischen Seen hindeutet. Darüber hinaus konnten tendenziell stärkere Algenblüten im Frühling und sinkende Sauerstoffkonzentrationen im Tiefenwasser am Ende der sommerlichen Schichtung festgestellt werden. Nur wenige Male pro Jahr stattfindende Terminmessungen sind jedoch nicht ausreichend, um diese Veränderungen zuverlässig zu erfassen. Die Daten der Messketten bildeten die Grundlage, um für 12 der untersuchten Seen numerische Modelle zu kalibrieren, welche die Wassertemperaturen und Schichtungsverhältnisse der Seen reproduzieren können. Mit Hilfe dieser Modelle sind Prognosen für die zukünftige Entwicklung der Seen basierend auf zwei IPCC-Klimaszenarien erstellt worden. Während sich für das optimistische Szenario, gemäß dem das 2°C-Ziel der Klimakonferenz von Paris eingehalten würde, nur noch geringe Änderungen in Schichtungsdauer und Wassertemperaturen ergeben, sagen die Prognosen für das pessimistische Szenario deutliche Veränderung der physikalischen Eigenschaften voraus: Die durchschnittliche Wassertemperatur erhöht sich von 11.6 auf 13.8°C bis zum Ende des Jahrhunderts, was einem Anstieg von ca. 0.3°C pro Dekade entspräche. Die Anzahl der Tage, an denen die Wassertemperaturen an der Oberfläche 15°C überschreiten, erhöht sich bis zum Ende des Jahrhunderts um ca. 20 Tage pro Jahr, Wassertemperaturen von über 25°C werden im Sommer regelmäßig vorkommen, während sie heutzutage eine Ausnahme darstellen. Parallel dazu verkürzt sich die Dauer der inversen Schichtung im Winter um die Hälfte, Seen mit Maximaltiefen von mehr als 40 Metern ändern sehr wahrscheinlich ihr Mischungsverhalten von einem dimiktischen zu einem warm-monomiktischen Typus. Die Antwort des Ökosystems der Seen auf diese prognostizierten physikalischen Änderungen ist komplex, mit Sicherheit lässt sich aber sagen, dass die um durchschnittlich ca. 3 Wochen verlängerte Sommerschichtung die Gefahr von anoxischen Verhältnissen im Tiefenwasser vergrößert. Ein früherer Schichtungsbeginn, stabilere Schichtungsverhältnisse im Sommer und höhere Temperaturen im Frühling werden die Gefahr von Cyanobakterien-Blüten erhöhen.
... The Urotricha species were collected in three oligo-mesotrophic lakes (Lake Mondsee, Lake Piburg, and Lake Zurich) located in Austria and Switzerland (Table 1). Details on these lakes can be found elsewhere (e.g., Bossard et al., 2001;Dokulil et al., 2006;Sonntag et al., 2011;Ficker et al., 2017;Yankova et al., 2017;Niedrist et al., 2018). Sampling was conducted by boat at the deepest point of each lake (68 m in Lake Mondsee, 25 m in Lake Piburg, and 136 m in Lake Zurich) and the water samples were collected along depth gradients from the surface to above lake bottom by means of a 5-L Schindler-Patalas sampler (Uwitec, Mondsee, Austria) and a plankton net (10-µm net gauze; Uwitec, Mondsee, Austria). ...
... Successfully grown clones were subsequently transferred from the small volumes of 96-well plates into 24-, 12-, 1 http://sagdb.uni-goettingen.de/detailedList.php?str_number=26.80 TABLE 1 | Characteristics of the study sites (e.g., Bossard et al., 2001;Dokulil et al., 2006;Sonntag et al., 2011;Ficker et al., 2017;Yankova et al., 2017;Niedrist et al., 2018). ...
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Species of the ciliate genus Urotricha are key players in freshwater plankton communities. In the pelagial of lakes, about 20 urotrich species occur throughout an annual cycle, some of which play a pivotal role in aquatic food webs. For example, during the phytoplankton spring bloom, they consume a remarkable proportion of the algal production. In ecological studies, urotrich ciliates are usually merely identified to genus rank and grouped into size classes. This is unsatisfying considering the distinct autecological properties of individual species and their specific spatial and temporal distribution patterns. As a basis for future research, we characterized in detail four common urotrich morphotypes, i.e., specimens identified as U. furcata and tentatively as U. agilis , U. pseudofurcata , and U. castalia , using state-of-the-art methods. We used an integrative polyphasic approach, in which morphological studies ( in vivo observation, silver staining methods, scanning electron microscopy) were linked with a molecular approach exploiting four different gene fragments as taxonomic DNA barcodes with different resolution potential (SSU rDNA, ITS-1, ITS-2, hypervariable V4 and V9 regions of the SSU rDNA). We shed light on the diversity of urotrich ciliates as well as on their global distribution patterns, and annual cycles. Additionally, we coupled individual species occurrences and environmental parameters, and subsequently modeled the distribution and occurrence, using logistic regressions. Furthermore, for one strain putatively identified as U. castalia , we ascertained the optimal cultivation media and food preferences. Thereby, our comprehensive view on these important freshwater ciliates that frequently occur in environmental high throughput sequencing datasets worldwide will allow future studies to better exploit protistan plankton data from lakes.