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# Temperature profiles, thermal gradients between 1 m depth layers and binary image of the metalimnion exemplified for the years 1981 (a–c) and 2000 (d–f). Details on the determination of metalimnion boundaries are described in the text

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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 assesse...

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... In contrast, constantly elevated biomass of P. rubescens is attributed to changes in the patterns of stratification and mixing due to climate change. During the last decades, years with incomplete mixing have become more frequent, for example, in Lake Zürich, allowing P. rubescens to increase its population as more cells survive as inoculum for the next season's population while holomixis would diminish cell density more strongly due to higher hydrostatic pressure in deep water layers [39,144]. On the other hand, reduced mixing can also result in nutrient depletion of the epilimnion through sedimentation and thus lead to lower abundance of P. rubescens, as observed in Lake Garda [40]. ...
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Concern is widely being published that the occurrence of toxic cyanobacteria is increasing in consequence of climate change and eutrophication, substantially threatening human health. Here, we review evidence and pertinent publications to explore in which types of waterbodies climate change is likely to exacerbate cyanobacterial blooms; whether controlling blooms and toxin concentrations requires a balanced approach of reducing not only the concentrations of phosphorus (P) but also those of nitrogen (N); how trophic and climatic changes affect health risks caused by toxic cyanobacteria. We propose the following for further discussion: (i) Climate change is likely to promote blooms in some waterbodies—not in those with low concentrations of P or N stringently limiting biomass, and more so in shallow than in stratified waterbodies. Particularly in the latter, it can work both ways—rendering conditions for cyanobacterial proliferation more favourable or less favourable. (ii) While N emissions to the environment need to be reduced for a number of reasons, controlling blooms can definitely be successful by reducing only P, provided concentrations of P can be brought down to levels sufficiently low to stringently limit biomass. Not the N:P ratio, but the absolute concentration of the limiting nutrient determines the maximum possible biomass of phytoplankton and thus of cyanobacteria. The absolute concentrations of N or P show which of the two nutrients is currently limiting biomass. N can be the nutrient of choice to reduce if achieving sufficiently low concentrations has chances of success. (iii) Where trophic and climate change cause longer, stronger and more frequent blooms, they increase risks of exposure, and health risks depend on the amount by which concentrations exceed those of current WHO cyanotoxin guideline values for the respective exposure situation. Where trophic change reduces phytoplankton biomass in the epilimnion, thus increasing transparency, cyanobacterial species composition may shift to those that reside on benthic surfaces or in the metalimnion, changing risks of exposure. We conclude that studying how environmental changes affect the genotype composition of cyanobacterial populations is a relatively new and exciting research field, holding promises for understanding the biological function of the wide range of metabolites found in cyanobacteria, of which only a small fraction is toxic to humans. Overall, management needs case-by-case assessments focusing on the impacts of environmental change on the respective waterbody, rather than generalisations.
... Due to the scarcity of turbulence measurements in the presence of deep chlorophyll maxima in lakes however, the behaviour and scales of lacustrine thin layers is often inferred only from stratification (Leach et al. 2018). In Lake Zurich, for example, long-term persistence of P. rubescens is attributed to the strengthening and deepening of the metalimnion and to the increased duration of seasonal stratification (Yankova et al. 2016). Both of these sets of factors arise from warming experienced over the last decades (Livingstone 2003;Schmid and Köster 2016). ...
... Early onset of the metalimnion is also thought to be critical for the formation of the P. rubescens thin layer (Yankova et al. 2016). This onset took place more than one month earlier in Lake Zurich than in Lake Geneva due to the faster spring warming in the former. ...
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Harmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline ( $$N \gtrsim 0.05$$ N ≳ 0.05 s $$^{-1}$$ - 1 ) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates ( $$\varepsilon \approx 10^{-8}$$ ε ≈ 10 - 8 W kg $$^{-1}$$ - 1 ). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale ( $$L_O \approx 1$$ L O ≈ 1 cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.
... Our time-series metabarcoding approach confirms previously reported patterns of distinct protistan plankton communities in the cold and warm season [3,4,75,76] and the absence of phytoplankton spring blooms from the epilimnion in Lake Zurich [77]. These observed succession patterns are largely the result of seasonally changing physicochemical parameters (Figure 1, Figure 4 and Figure S1, Table S1) that have been drastically altered by global warming during the last decades [45][46][47][48][49]. By inferring the protistan co-occurrence network of each season, we revealed consequences for succession patterns of a model lake ecosystem subjected to climate change. ...
... Previous studies [45,47] demonstrated that while environmental stress in Lake Zurich often originates in autumn and winter (corroborating with the cold season dataset of our study), the consequences for plankton communities are most severe in spring and summer (corroborating with the warm season dataset of our study). Especially lake water surface temperatures significantly increased in the wake of climate change [46]. This process of lake warming is further accelerated by significantly increased air temperatures in spring [46] and has led to drastic changes in the stratification regime that prevents complete water turnovers in Lake Zurich. ...
... Especially lake water surface temperatures significantly increased in the wake of climate change [46]. This process of lake warming is further accelerated by significantly increased air temperatures in spring [46] and has led to drastic changes in the stratification regime that prevents complete water turnovers in Lake Zurich. The strong impact of water temperature as a major stressor in Lake Zurich can be derived from its position in both networks, from which it emerged as the environmental parameter with most correlations to protists (Table S2). ...
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Network analyses of biological communities allow for identifying potential consequences of climate change on the resilience of ecosystems and their robustness to resist stressors. Using DNA metabarcoding datasets from a three-year-sampling (73 samples), we constructed the protistan plankton co-occurrence network of Lake Zurich, a model lake ecosystem subjected to climate change. Despite several documentations of dramatic lake warming in Lake Zurich, our study provides an unprecedented perspective by linking changes in biotic association patterns to climate stress. Water temperature belonged to the strongest environmental parameters splitting the data into two distinct seasonal networks (October–April; May–September). The expected ecological niche of phytoplankton, weakened through nutrient depletion because of permanent thermal stratification and through parasitic fungi, was occupied by the cyanobacterium Planktothrix rubescens and mixotrophic nanoflagellates. Instead of phytoplankton, bacteria and nanoflagellates were the main prey organisms associated with key predators (ciliates), which contrasts traditional views of biological associations in lake plankton. In a species extinction scenario, the warm season network emerged as more vulnerable than the cold season network, indicating a time-lagged effect of warmer winter temperatures on the communities. We conclude that climate stressors compromise lake ecosystem robustness and resilience through species replacement, richness differences, and succession as indicated by key network properties.
... Numerous studies already documented the impact of climate change on Lake Zurich and the resulting dramatic lake effects, including economic consequences (Livingstone, 2003;North et al., 2014;Posch et al., 2012;Yankova et al., 2016Yankova et al., , 2017. This impact is highlighted by the thermal stratification of the water column, which has led to a stop of complete water turnover in spring (vernal holomixis) for several years (Yankova et al., 2017). ...
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Microbial planktonic communities are the basis of food webs in aquatic ecosystems since they contribute substantially to primary production and nutrient recycling. Network analyses of DNA metabarcoding data sets emerged as a powerful tool to untangle the complex ecological relationships among the key players in food webs. In this study, we evaluated co‐occurrence networks constructed from time‐series metabarcoding data sets (12 months, biweekly sampling) of protistan plankton communities in surface layers (epilimnion) and bottom waters (hypolimnion) of two temperate deep lakes, Lake Mondsee (Austria) and Lake Zurich (Switzerland). Lake Zurich plankton communities were less tightly connected, more fragmented and had a higher susceptibility to a species extinction scenario compared to Lake Mondsee communities. We interpret these results as a lower robustness of Lake Zurich protistan plankton to environmental stressors, especially stressors resulting from climate change. In all networks, the phylum Ciliophora contributed the highest number of nodes, among them several in key positions of the networks. Associations in ciliate‐specific subnetworks resembled autecological species‐specific traits that indicate adaptions to specific environmental conditions. We demonstrate the strength of co‐occurrence network analyses to deepen our understanding of plankton community dynamics in lakes and indicate biotic relationships, which resulted in new hypotheses that may guide future research in climate‐stressed ecosystems.
... In the first instance, there is no universal method for estimating the mixed depth with a range of temperature and density thresholds and gradients currently being used in the literature. Examples of these include: 0.2 o C (Yang et al., 2018), 1 o C (Mackay et al., 2011), or 0.8 o C (Zhao et al., 2018) change in temperature from the surface; a 0.1 kg m −3 (Andersen et al., 2017) change in density from the surface; a 1 °C m −1 (Kasprzak et al., 2017), 0.5 o C m -1 (Yankova et al., 2016) or 0.25 o C m -1 (Özkundakci et al., 2011) temperature gradient; or a 0.07 kg m −3 m −1 , 0.03 kg m −3 m −1 (Tonetta et al., 2016) or 0.5 kg m −3 m −1 (Lamont et al., 2004) density gradient. Studies have also used the depth of the maximum temperature or density gradient to define the depth of the mixed layer (e.g. ...
... Among cyanobacteria, Planktothrix species are known to produce the highest amount of cyanotoxins per gram of dry weight (Fastner et al., 1999). Planktothrix rubescens (Gomont) Anagnostidis and Komarek, 1988 is a planktonic cyanobacterium that frequently causes blooms in temperate (mostly mesotrophic) lakes in Europe and, unlike other cyanobacteria, it is usually associated with ecosystem re-oligotrophication (Yankova et al., 2016;Ernst et al., 2009;Jacquet et al., 2005). A characteristic of this red filamentous cyanobacterium is that it initially occurs as a compact layer in the metalimnion, forming a so-called metalimnetic bloom (Ernst et al., 2009). ...
Article
Cyanobacterial blooms have become common all over the world, negatively affecting various ecosystem components. Planktothrix rubescens is a planktonic cyanobacterium that frequently causes blooms in temperate (mostly mesotrophic) lakes in Europe and, unlike other cyanobacteria, it is usually associated with ecosystem re-oligotrophication. The potential effects of P. rubescens on periphyton development have not been estimated in any recently published studies. The objective of the present paper was to assess how a Planktohrix rubescens bloom affects periphyton development in a temperate freshwater reservoir. A field study was performed in the Vrutci reservoir (western Serbia), where P. rubescens blooms have occurred since December of 2013. Periphyton was grown on glass slides that were incubated following lake stratification and sampled monthly from July to October of 2015. Dry mass, ash free dry mass and content of chlorophyll a, as well as composition of the community of developed periphyton, were estimated, together with P. rubescens biomass in the water column at the particular depths where substrates for periphyton were incubated. The metalimnetic bloom of P. rubescens induced calcite precipitation on the incubated substrates, which was reflected in obstructed colonization. The periphyton community composition was mainly determined by the depth gradient, which was potentially related to reduced light availability driven by shading effects caused by the P.rubescens bloom. Our results provide a significant update on periphyton ecology in relation to cyanobacterial blooms, elucidating complex interactions in the ecosystem. They also represent a stimulus for further studies on the capacity of P. rubescens for induction of calcium carbonate precipitation in lakes.
... Nutrient inputs are monitored, and regulated by the 25 sewage plants along its borders, water levels are modulated, and levels of cyanobacteria (Planktothrix rubescens) overseen. Over the last 40 years, the lake has experienced warming of the water column, with cyanobacteria becoming a consistent element in the microbial community (Kurmayer, 1999;Yankova et al., 2016). ...
Article
Ecosystem services is a young field that has been expanding exponentially for the last 20 years. This expansion has meant many questions and gaps continue to be revealed, not least how individuals perceive ecosystems, the dependencies that humans have on them, and the value of the contributions that nature makes to our their lives. Understanding perceptions is essential for facilitating the adoption of ecosystem services research outputs into policy mechanisms, and to better inform research directions. In the following study, I explore perceptions of experts from research and management across eight globally distributed research sites across four continents, to uncover how they perceive those ecosystems and human dependencies on them. I first give a General Introduction, which outlines key themes related to global change, biodiversity and ecosystem services. I also give detail about the research locations included in this study and the questions I seek to answer. In Chapter 1, I establish baselines of ecosystem services through 103 interviews with stakeholders within each location. Using presence/absence and importance scale responses from interviewees I test whether these baselines are unique to each site, and assess whether there are similarities between sites and with how services are perceived across the sites. In Chapter 2, I explore services that elicit uncertainty and disagreement from interviewees from Chapter 1. I use the service itself and attributes of the interviewees to determine whether these can predict uncertain answers, and areas of disagreement. As this chapter compares individual responses at sites, I restrict my data to two sites with higher numbers of interviewees. In Chapter 3 I use text analysis techniques to explore the interview notes and transcripts to uncover values that interviewees express about the research sites. The responses are framed around questions about ecosystem services the sites provide, offering greater detail and insight into how interviewees relate to each location. Finally, in Chapter 4 I take a narrative form, using the words of the interviewees to describe how people live and interact with the research locations, and how they are adapting to changing conditions. The chapter was written as part of a public outreach project, Klimagarten 2085, which explored human adaptations to climate change. In the General Discussion I draw broader conclusions about the key findings and wider relevance of this work, as well as outlining future directions.
... The depth of the pycnocline was calculated as the mid-depth between the bottom of the epilimnion and the top of the hypolimnion. It was assumed that the lake was stratified when its stability (Idso 1973) exceeded a threshold value of 100 J m −2 (e.g., Engelhardt and Kirillin 2014;Yankova et al. 2016). Density profiles and the calculated epilimnion and pycnocline depths from Hafursá were used, together with wind speed records, to calculate the time series of Lake L N and Wedderburn W numbers (see Stevens and Imberger 1996 for definitions). ...
Article
The goal of the research was to identify the mixing features that distinguish wind forced, medium‐size fjord‐type lakes near the Arctic Circle from systems of similar geometry, but in a temperate climate. Experimental data and the results of 3D numerical hydrodynamic simulations were analyzed for Lake Lagarfljót (27 km long; 2 km wide; 110 m maximum depth; 65°N) during the 5‐month ice‐free period in 2009. The results showed that probably the most distinctive feature of arctic lakes is their low water column stability, one order of magnitude lower than that of mid‐latitude lakes, even in mid‐summer. The second characteristic is the high rate of energy imparted by wind in the Arctic, estimated as one order of magnitude larger than in a temperate climate. Frequent wind events with Lake and Wedderburn numbers below 1 occurred during the ice‐free period, leading to a strong shear at the base of the surface mixed‐layer and the upwelling of deep metalimnetic layers. As a result, nearly continuous density stratification developed in the shallowest 70 m of the water column, and longitudinal temperature gradients may therefore be very significant. On average, 15% of the wind forcing drove large‐scale internal motions. This energy was rapidly dissipated, partly as a result of stronger bottom velocities, nonlinear surges, and hydraulic jumps that repeatedly formed in the lake. The initial V1H1 setup rapidly evolved into spatially complex oscillatory modes, which, given the high latitude, are affected by the Earth's rotation, even in narrow basins.
... The presence of gas vesicles in P. rubescens allowed this species to move along the water column and grow in the layer with the best irradiance conditions, which were detected in Lake Iseo at a depth of around 20 m. This species is also well adapted to low light values, and has been reported to be an efficient light harvester [65,81,88]. It was detected growing in the radiation conditions between 40 and 60 m, where it developed with lesser biovolume values than in the upper layers. ...
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In Lake Iseo (Lombardia, Italy), the predominant species in the cyanobacterial taxa was Planktothrix rubescens. However, since 2014, the presence of an allochthonous Cyanobacteria, Tychonema bourrellyi, able to produce consistent biomasses and harmful toxins, was detected. The causes of this expansion are poorly understood. Many studies have linked the development of allochthonous Cyanobacteria populations with climate change. This study shows the spatio-temporal dynamics, the ecological requirements, and the interspecific relationship of P. rubescens and T. bourrellyi. Samples were collected monthly in 2016 at six different depths in the water column; 20 chemico-physical characteristics were measured; and Cyanobacteria density, morphology, and biovolume were evaluated. The results allowed a comparison of the spatial pattern of the two species, which showed a greater distribution at a depth of 10–20 m, and their seasonal dynamics. Both Cyanobacteria were present throughout the year, with the greatest abundance during the period from March to May. A temporal shift was observed in their development, linked to different capacities for overcoming winter and mixing periods. Principal Component Analysis, performed on 20 observations (4 months × 5 depths), highlighted the important role of the stability of the water column in determining T. bourrellyi settlement in Lake Iseo and the role of solar radiation in spring population development.
... The documented average warming per annum is mainly attributed to striking temperature increases during cold seasons (winter) across the Northern Hemisphere 4,5 . However, in a recent study for a pre-alpine region, the strongest warming was observed during spring periods 6 . This causes an earlier onset and prolongation of thermal stabilization in even deep large lakes. ...
... A strong relation between vernal epilimnetic PO 4 -P deficits 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 during 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). ...
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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 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.