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Little is known on the dynamics of under-ice phytoplankton communities. We investigated phytoplankton communities in the upper (0–20 m) and lower (30–35 m) layer of oligotrophic Lake Tovel, Brenta Dolomites (Italy) over 6 years during summer and under ice. Winter conditions were different from one year to another with respect to ice thickness and snow cover. Proxies for light transmission (Secchi disc transparency, light attenuation) were similar between seasons, even though the incident solar radiation was lower in winter. Algal richness and chlorophyll-a were not different between seasons while biomass was higher during summer. In four of the 6 years, Bacillariophyta dominated during summer and Miozoa (class Dinophyceae) under ice while in 2 years Bacillariophyta also dominated under ice. Generally, a shift to larger size classes from summer to under ice was observed for Bacillariophyta, Chlorophyta, and Ochrophyta (class Chrysophyceae) while Dinophyceae showed the opposite pattern. No strong links between phytoplankton community composition and abiotic factors (under-ice convective mixing, snow on ice, under-ice light) were found. We suggest that inter-species relationships and more precise indicators of under-ice light should be considered to better understand under-ice processes.

Cyclopoid and calanoid copepods differ in how they move. Cyclopoid copepods use the thoracic legs for cruise and escape swimming while most calanoid copepods use the cephalic appendages for cruise swimming and the thoracic legs for escape reactions. Apart from this gross difference, little is known on the comparative aspects of the locomotor function of copepod appendages. This study investigated the main kinematic patterns of cruise and escape swimming of two small cyclopoid copepods, Thermocyclops oithonoides and Oithona davisae, and a small calanoid copepod, Paracalanus parvus, by video filming at a frame rate of up to 1200 frames/s. During escape swimming, O. davisae and the twice as large P. parvus showed similar movement, jumping at a frequency of 150 Hz and moving at 12 cm s

Under-ice community dynamics are barely understood. Temporal and spatial studies are needed to fully understand the consequences of a declining ice cover on microbial biodiversity. Here, bacterial communities of different years (2015, 2017–2021) and layers (upper and lower euphotic layer, euphotic layer, hypolimnion) were assessed by Illumina sequencing of the 16S rRNA gene. Alpha- and beta-diversity of summer and under-ice hypolimnetic communities were similar, and a seasonal difference was found only when excluding summer hypolimnetic communities. Similarly, in non-metric multidimensional scaling (NMDS), summer and under-ice communities were different even though hypolimnetic communities were similar. Investigating under-ice conditions, the year 2017 showed highest under-ice light and chlorophyll-a while 2021 showed no under-ice light and lowest chlorophyll-a. Under-ice communities were not linked to layer differences implying that a spatial distinction under ice was less important than in summer, especially in years with little or no under-ice light. Most under-ice bacterial classes and ASVs showed direct and indirect dependencies on light availability and primary production. Similarly in NMDS with only under-ice communities, light transparency and primary production were important. In the future, ice conditions with less snow cover might lead to bacterial communities similar to that of high-light years (2017, 2018, 2020).

Swimming is a fundamental trait of many protists and optimizes growth and survival. We present an easy to use set-up for filming algal cells using Apocalathium aciculiferum and Borghiella dodgei as case studies; both dinoflagellates are model organisms of psychrophilic dinoflagellates. We filmed at low temperature (4.5°C) using a digital camera directly connected to a microscope and used open-source software to track their movement. We encountered several technical issues that were solved by using specific software settings (scaling, grey scale of images, restrictive particle recognition), and thus we obtained unbiased speed estimates. Borghiella dodgei showed a faster (minimum = 80, maximum = 255, median = 141 µm s−1) swimming speed than A. aciculiferum (minimum = 29, maximum = 134, median = 85 µm s−1). We linked differences in swimming speed to the dinoflagellates’ environmental niche, and suggested that B. dodgei is more adapted to turbulent spring conditions than A. aciculiferum, occurring under ice. The use of a generic digital camera and open-source software makes filming and tracking of plankton movement very affordable. We provided code and detailed instructions to disseminate this type of movement analysis of plankton.

Little is known on the swimming activity and respiration rate of the cyclopoid copepod Cyclops vicinus. Here, the swimming speed and respiration rate of C. vicinus were measured at different temperatures using a high speed (up to 1200 frames per second) camera and a closed-system respirometry, respectively. For cruise and escape swimming, log-linear relationships were found between temperature (range 1-22 °C) and duration, speed, and frequency of locomotor acts, respectively. The respiration rate of immobilized and active individuals showed log-linear relationships with temperature (range of 2-20 °C) and a thermal coefficient Q

Community composition of freshwater prokaryotes and protists varies through time. Few studies contemporarily investigate temporal variation of these freshwater communities for more than 1 year. We compared the temporal patterns of prokaryotes and protists in three distinct habitats for 4 years (2014-2017) in Lake Tovel, a cold-water lake. This lake showed a marked temperature increase in 2017 linked to altered precipitation patterns. We investigated whether microbial communities reflected this change across habitats and whether changes occurred at the same time and to the same extent. Furthermore, we tested the concept of hydrological year emphasizing the ecological effect of water renewal on communities for its explanatory power of community changes. Microbe diversity was assessed by Illumina sequencing of the V3-V4 hypervariable region of the 16S rRNA gene and 18S rRNA gene, and we applied co-inertia analysis and asymmetric eigenvector maps modelling to infer synchrony and temporal patterns of prokaryotes and protists. When considering community composition, microbes were invariable in synchrony across habitats and indicated a temporal gradient linked to decreasing precipitation; however, when looking at temporal patterns, the extent of synchrony was reduced. Small-scale patterns were similar across habitats and microbes and linked to seasonally varying environmental variables, while large-scale patterns were different and partially linked to an ecosystem change as indicated by increasing water transparency and temperature and decreasing dissolved oxygen. Our advanced statistical approach outlined the multifaceted aspect of synchrony when linked to community composition and temporal patterns.

Little is known about changes in microbial abundance and community composition during persistent ice cover of lakes. Here, the under-ice 16S rRNA diversity was assessed for different pelagic layers and compared between years (2015, 2017, 2018, 2019, 2020) at Lake Tovel (1177 m above sea level; Italy). Functional profiling of amplicon sequences variants (ASVs) was also done with Piphillin. Environmental parameters (chemistry, temperature, light climate, oxygen concentration) were linked to the observed diversity patterns. Despite relatively uniform temperature and chemistry profiles, the pelagic and hypolimnetic microbiome of different years were different as assessed by a Principal Coordinates Analysis. The under-ice light climate was a driving factor of the observed differences and related to different precipitations patterns. These results underline how a changing climate also influences life under ice.

The authors acknowledge COST Action ES 1105 "CYANOCOST Cyanobacterial blooms and toxins in water resources: Occurrence impacts and management" and COST Action Global Change Biology ES 1201 NETLAKE -Networking Lake Observatories in Europe" for contributing to this study through networking and knowledge sharing with European experts in the field. We acknowledge the members of the Global Lake Ecological Observatory Network (GLEON) for their collaborative spirit and enthusiasm that inspired the grassroots effort of the EMLS. E.M. was supported by a grant from the Swiss State Secretariat for Education, Research and Innovation to Bas Ibelings and by supplementary funding from University of Geneva. We thank Wendy Beekman for the nutrient analysis. We thank Pieter Slot for assisting with the pigment analysis. We thank Dr. Ian Jones for valuable feedback on an earlier version of the manuscript. We thank the Leibniz Institute of Freshwater Ecology and the Aquatic Microbial Ecology Group for logistic and technical support of J. Fonvielle and H.-P. Grossart, and the Leibniz Association for financial support. H.P. was supported by the US National Science Foundation (1840715, 1831096). A.C.'s work was funded by the Spanish Agencia Estatal de Investigacion and EU funds through the project CLIMAWET (CGL2015-69557-R). The collection of data for Lough Erne and Lough Neagh were funded by the Department of Agriculture, Environment and Rural Affairs, Northern Ireland. We are grateful to Kristiina Vuorio from the Freshwater Centre of the Finnish Environment institute for her help in organizing, collecting and analysing samples by the University of Jyvaskyla and to Gerald Dorflinger from the Water Development Department of Cyprus for his assistance with the sampling in Cyprus and for granting the CUT team permission to use WDD's equipment. Finally, we would like to thank the numerous other assistants that helped realizing each local survey. Open access funding provided by Universite de Geneve., To determine the drivers of phytoplankton biomass, we collected standardized morphometric, physical, and biological data in 230 lakes across the Mediterranean, Continental, and Boreal climatic zones of the European continent. Multilinear regression models tested on this snapshot of mostly eutrophic lakes (median total phosphorus [TP] = 0.06 and total nitrogen [TN] = 0.7 mg L-1), and its subsets (2 depth types and 3 climatic zones), show that light climate and stratification strength were the most significant explanatory variables for chlorophyll a (Chl a) variance. TN was a significant predictor for phytoplankton biomass for shallow and continental lakes, while TP never appeared as an explanatory variable, suggesting that under high TP, light, which partially controls stratification strength, becomes limiting for phytoplankton development. Mediterranean lakes were the warmest yet most weakly stratified and had significantly less Chl a than Boreal lakes, where the temperature anomaly from the long-term average, during a summer heatwave was the highest (+4 degrees C) and showed a significant, exponential relationship with stratification strength. This European survey represents a summer snapshot of phytoplankton biomass and its drivers, and lends support that light and stratification metrics, which are both affected by climate change, are better predictors for phytoplankton biomass in nutrient-rich lakes than nutrient concentrations and surface temperature., European Cooperation in Science and Technology (COST) ES 1105 ES 1201, Swiss State Secretariat for Education, Research and Innovation, University of Geneva, Leibniz Association, National Science Foundation (NSF) 1840715 1831096, Spanish Agencia Estatal de Investigacion, European Commission CGL2015-69557-R, Department of Agriculture, Environment and Rural Affairs, Northern Ireland

A decrease in hypolimnetic dissolved oxygen (DO) is a commonly seen effect of climate change. However, in oligotrophic Lake Tovel (Italy), a deep mountain lake, annual mean DO (% saturation) has increased from near anoxia to >20% in the bottom layer (35–39 m). We analyzed long‐term patterns of DO (1937–2019) using different methods (correlation and trend analysis, identification of extreme events) to link DO to drivers and indices of mixing. While spring mixing remained temporally limited, later ice‐in (5.1 days decade−1) and the positive relationship between ice‐in and DO the following year evidenced autumn mixing as the main driver for hypolimnetic DO increase. Extreme meteorological events also replenished hypolimnetic DO. Using DO and conductivity (1995–2019), we identified 14 deep mixing events with hypolimnetic DO > 40%. Density‐based indices (Schmidt stability, relative thermal resistance, Lake Number, and Wedderburn Number) only partially captured these events that were related to snowmelt, flooding, and cold spells during spring and autumn, with a carryover effect sometimes lasting >1 year. Recently, annual mean DO in the upper layer decreased beyond temperature‐dependent solubility. This decrease was not comprehensively confirmed by statistical tests but was possibly linked to atmospheric stilling. We suggest that Lake Tovel's shift from meromixis to dimixis was driven by climate warming (i.e., increasing air temperature 0.6°C decade−1) that delayed ice‐in and increased autumn mixing. Our work underlines the vulnerability of mountain lakes and their different response to climate change with respect to more studied lowland lakes.

Many inland waters exhibit complete or partial desiccation, or have vanished due to global change, exposing sediments to the atmosphere. Yet, data on carbon dioxide (CO2) emissions from these sediments are too scarce to upscale emissions for global estimates or to understand their fundamental drivers. Here, we present the results of a global survey covering 196 dry inland waters across diverse ecosystem types and climate zones. We show that their CO2 emissions share fundamental drivers and constitute a substantial fraction of the carbon cycled by inland waters. CO2 emissions were consistent across ecosystem types and climate zones, with local characteristics explaining much of the variability. Accounting for such emissions increases global estimates of carbon emissions from inland waters by 6% (~0.12 Pg C y−1). Our results indicate that emissions from dry inland waters represent a significant and likely increasing component of the inland waters carbon cycle., Many inland waters seasonally or permanently dry up, thus exposing sediments to the atmosphere. Here the authors show that a substantial amount of CO2 is emitted from these dry sediments, increasing current inland water carbon flux estimates by 6%.

Ultraviolet radiation (UVR) is an environmental stressor in several ecosystems and can affect organisms’ survival and reproduction, and community structure. Rotifers cope with UVR stress adopting preventive behavioral and metabolic mechanisms. However, the demonstration of an immediate behavioral response in rotifers is missing. We investigated the short-term response of rotifers to UVR, by combining video analysis and movement ecology methods, in three common species: Brachionus calyciflorus, Keratella cochlearis, and Keratella quadrata. We recorded the behavior of B. calyciflorus (both sexes), K. quadrata, and K. cochlearis (females) exposed to white light, and to intermittent cycles of UVR (30:30 s). Individual trajectories were extracted from videos with open-source software. We found that B. calyciflorus females exposed to UVR exhibited strong negative phototaxis with increased swimming speed, and a weak positive phototaxis in males. Keratella cochlearis and K. quadrata showed a weaker response. Our study reveals a species-specific behavioral response to UVR in rotifers. Furthermore, we highlight how sexual dimorphism in B. calyciflorus does not only occur in morphology and movement, but also in behavioral traits. Our results help to understand zooplankton community dynamics by providing a mechanistic explanation of UVR response in one major zooplankton taxonomic group.

Community assembly depends on dispersal, environmental parameters and species interactions. Here, we compared rotifer communities from two environmentally distinct, but connected basins of Lake Tovel for 5 years, with biweekly (2002) and monthly (2003–2006) sampling. We applied taxonomic and functional diversity-based multivariate analyses (principal components analysis, redundancy analysis) to compare rotifer communities in both basins. For functional diversity, we focused separately on defence and feeding traits that were inferred from rotifer morphology. Taxonomic and functional diversity were generally higher in the deep compared to the shallow basin. In multivariate analyses, water residence time and surface water temperature were important environmental predictors for both taxonomic and functional diversity while the importance of competitors and predators varied according to defence or feeding traits used. The difference between basins showed a gradient from taxonomic (marked difference) to functional diversity, indicating that the processes that regulate feeding traits were similar in both basins while those regulating defence traits were different. The distinction of different aspects of rotifer life (i.e. feeding and predation) outlined different important drivers that would have been neglected by focusing solely on species abundance. Furthermore, we outlined the status quo of trait research in rotifer ecology including its challenges and difficulties.

Mougeotia (Zygnematales, Charophyta) first appeared in the plankton of Lake Kinneret in 1998. While initially rare, from 2004 onwards it was present in the plankton continuously, forming massive blooms in spring (2005, 2006, 2012) or in winter (2010), occasionally appearing in different morphological and life cycle forms. Mougeotia maintained its population under a wide range of water temperatures, nutrient concentrations, solar radiation, pH levels and stratification patterns, making it a highly versatile alga. In multiple regression, year and month as the only predictors explained 36% of the pattern of Mougeotia biomass. However, Mougeotia biomass could not be explained by any of the environmental parameters considered. Modeling the temporal dynamics of Mougeotia biomass using an autoregressive integrated moving average (ARIMA(1,0,0)) explained 56% of variability indicating that intraspecific factors (e.g., competition for nutrients or self-shading) may determine the dynamics of Mougeotia biomass. To explain the lack of relationships with the environmental parameters, we hypothesize that (1) Mougeotia possesses exceptional physiological plasticity and/or (2) Lake Kinneret may host two or more genetically distinct cryptic species of Mougeotia with different environmental niches. Both explanations may hinder any inference on Mougeotia–environment relationships and require confirmation by experimental work.

A recent study based on DNA taxonomy indicated that the widespread rotifer Keratella cochlearis comprises several evolutionarily significant units (ESUs). Identification of ESUs based on DNA taxonomy alone is problematic and usually requires morphological, demographic, and/or ecological evidence. We isolated three haplotypes belonging to two ESUs of K. cochlearis and conducted life table experiments to investigate if this genetic diversity is reflected in demography. We found significant differences between haplotypes in life history traits (average lifespan, number of offspring, and percent of rejected eggs) and in demographic parameters (instantaneous growth rate, generation time, and net reproductive rate of the populations). During the experiments, all the haplotypes produced abnormal females with a deformed lorica, which was never reported before in K. cochlearis. We also report the first case of an amphoteric female (producing both females and males) in K. cochlearis. We hypothesize that K. cochlearis haplotypes and thus ESUs may exhibit niche differentiation through their different life histories. The link between demographic parameters of K. cochlearis and niche utilization requires further research.

Discordance between mitochondrial and nuclear phylogenies is being increasingly recognized in animals and may confound DNA-based taxonomy. This is especially relevant for taxa whose microscopic size often challenges any effort to distinguish between cryptic species without the assistance of molecular data. Regarding mitonuclear discordance, two strikingly contrasting scenarios have been recently demonstrated in the monogonont rotifers of the genus Brachionus. While strict mitonuclear concordance was observed in the marine B. plicatilis species complex, widespread hybridization-driven mitonuclear discordance was revealed in the freshwater B. calyciflorus species complex. Here, we investigated the frequency of occurrence and the potential drivers of mitonuclear discordance in three additional freshwater monogonont rotifer taxa, and assessed its potential impact on the reliability of DNA taxonomy results based on commonly used single markers. We studied the cryptic species complexes of Keratella cochlearis, Polyarthra dolichoptera and Synchaeta pectinata. Phylogenetic reconstructions were based on the mitochondrial barcoding marker cytochrome c oxidase subunit I gene and the nuclear internal transcribed spacer 1 locus, which currently represent the two most typical genetic markers used in rotifer DNA taxonomy. Species were delimited according to each marker separately using a combination of tree-based coalescent, distance-based and allele-sharing-based approaches. Mitonuclear discordance was observed in all species complexes with incomplete lineage sorting and unresolved phylogenetic reconstructions recognized as the likely drivers. Evidence from additional sources, such as morphology and ecology, is thus advisable for deciding between often contrasting mitochondrial and nuclear species scenarios in these organisms.

Under-ice dissolved oxygen (DO) metabolism and DO depletion are poorly understood, limiting our ability to predict how changing winter conditions will affect lake ecosystems. We analyzed under-ice DO dynamics based on high frequency (HF) data at two depths (5 and 25 m) for three winters (January to March 2014, 2015, and 2016) in oligotrophic Lake Tovel (1178 m above sea level; maximum depth 39 m). Specifically, we assessed diel metabolic rates based on HF data of DO, temperature, and light for winter 2016 and seasonal DO depletion rates based on HF data of DO for all three winters. For 2016, calculations of metabolic rates were possible only for 34% and 3% of days at 5 m and 25 m, respectively; these metabolic rates generally indicated net heterotrophy at both depths. Low success in modelling metabolic rates was attributed to low diel DO variability and anomalous diel DO patterns, probably linked to under-ice physical processes. Seasonal DO patterns for the three winters showed increasing, decreasing, or stable DO trends at 5 m while at 25 m patterns always showed decreasing DO trends but with different rates. Our multi-year study permitted us to hypothesise that the observed intra-and inter-annual differences in DO depletion can be attributed to variable snow cover determining the penetration of radiation and thus photosynthesis. This study brings new insights to DO dynamics in ice-covered systems, highlights the challenges linked to under-ice lake metabolism and advocates for a modelling approach that includes physical processes.

Global warming is affecting ecosystems worldwide, with mountainous regions particularly vulnerable. At mid-altitudes, changes in winter precipitation will have immediate effects on lake fun...

Most monogonont rotifers usually reproduce by cyclical parthenogenesis. For many rotifer species the factor inducing sexual reproduction is high population density (crowding). While Keratella cochlearis is one of the most ubiquitous monogonont freshwater rotifers, little is known about its reproduction. We provide experimental evidence that the switch from parthenogenesis to production of males in K. cochlearis is density-dependent. Single females (F1 generation) were kept in containers of varying volume representing various levels of crowding. The percentage of sexual daughters (F2 generation) producing males was higher in smaller volume treatments (higher crowding) than in larger volume treatments (lower crowding). The lowest population density at which males appeared was 38.5 females L−1 of K. cochlearis as estimated by beta regression. We also present a detailed photograph of a K. cochlearis male which has previously been known only from drawings. Surprisingly, we never observed mating and resting egg production.

A first synoptic and trans-domain overview of plankton dynamics was conducted across the aquatic sites belonging to the Italian Long-Term Ecological Research Network (LTER-Italy). Based on published studies, checked and complemented with unpublished information, we investigated phytoplankton and zooplankton annual dynamics and long-term changes across domains: from the large subalpine lakes to mountain lakes and artificial lakes, from lagoons to marine coastal ecosystems. This study permitted identifying common and unique environmental drivers and ecological functional processes controlling seasonal and long-term temporal course. The most relevant patterns of plankton seasonal succession were revealed, showing that the driving factors were nutrient availability, stratification regime, and freshwater inflow. Phytoplankton and mesozooplankton displayed a wide interannual variability at most sites. Unidirectional or linear long-term trends were rarely detected but all sites were impacted across the years by at least one, but in many case several major stressor(s): nutrient inputs, meteo-climatic variability at the local and regional scale, and direct human activities at specific sites. Different climatic and anthropic forcings frequently co-occurred, whereby the responses of plankton communities were the result of this environmental complexity. Overall, the LTER investigations are providing an unparalleled framework of knowledge to evaluate changes in the aquatic pelagic systems and management options.

Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains. © 2018 by the authors. Licensee MDPI, Basel, Switzerland., The authors acknowledge COST Action ES 1105 “CYANOCOST—Cyanobacterial blooms and toxins in water resources: Occurrence impacts and management” and COST Action ES 1201 “NETLAKE—Networking Lake Observatories in Europe” for contributing to this study through networking and knowledge sharing with European experts in the field. Evanthia Mantzouki was supported by a grant from the Swiss State Secretariat for Education, Research and Innovation (SERI) to Bas Ibelings and by supplementary funding from the University of Geneva. We thank Clare Ahnlund, Ena Suarez and Irene Gallego for helping out with the Swiss survey. We thank Wendy Beekman and Els J. Faassen for the nutrient and toxin analysis.

Temporal dynamics of bacterioplankton are rarely investigated for multiple habitats and years within individual lakes, limiting our understanding of the variability of bacterioplankton community (BC) composition with respect to environmental factors. We assessed the BC composition of a littoral and two pelagic habitats (euphotic zone and hypolimnion) of Lake Tovel monthly from April 2014 to May 2017 by high-throughput sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. The three habitats differed in temperature, light, oxygen and hydrology. In particular, the littoral was the most hydrologically unstable because it receives most of the lake inflow, the hypolimnion was the most stable because of its hydrologically sheltered position, and the pelagic euphotic habitat was intermediate. Consequently, we hypothesized different temporal patterns of BC composition for all three habitats according to their environmental differences. We applied PERMANOVA, nonmetric multidimensional scaling and source-sink analysis to characterize BC composition. Overall, BCs were different among habitats with the littoral showing the highest variability and the hypolimnion the highest stability. The BC of rainy 2014 was distinct from the BCs of other years irrespective of the habitats considered. Seasonal differences in BCs were limited to spring, probably linked to meltwater inflow and mixing. Thus, temporal effects related to year and season were linked to the hydrological gradient of habitats. We suggest that despite potential within-lake dispersal of bacterioplankton by water flow and mixing, local environmental conditions played a major role in Lake Tovel, fostering distinct BCs in the three habitats.

Swimming is of vital importance for aquatic organisms because it determines several aspects of fitness, such as encounter rates with food, predators, and mates. Generally, rotifer swimming speed is measured by manual tracking of the swimming paths filmed in videos. Recently, an open-source package has been developed that integrates different open-source software and allows direct processing and analysis of the swimming paths of moving organisms. Here, we filmed groups of females and males of Keratella cochlearis separately and in a mixed experimental setup. We extracted movement trajectories and swimming speeds and applied the classification method random forest to assign sex to individuals of the mixed setup. Finally, we used advanced statistical methods of movement ecology, namely a hidden Markov model, to investigate swimming states of females and males. When not discriminating swimming states, females swam faster than males, while when discriminating states males swam faster. Specifically, females and males showed two main states of movement with many individuals switching between states resulting in four modes of swimming. We suggest that switching between states is related to predator avoidance. Males of K. cochlearis especially exhibited switching between turning in a restricted area and swimming over longer distances. No mating or other male-female interactions were observed. Our study elucidates the steps necessary for automatic analysis of rotifer trajectories with open-source software. Application of sophisticated software and analytical models will broaden our understanding of zooplankton ecology from the individual to the population level.

Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includes in-situ lake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment., The authors acknowledge COST Action ES 1105 “CYANOCOST – Cyanobacterial blooms and toxins in water resources: Occurrence impacts and management” and COST Action ES 1201 “NETLAKE – Networking Lake Observatories in Europe” for contributing to this study through networking and knowledge sharing with European experts in the field. Evanthia Mantzouki was supported by a grant from the Swiss State Secretariat for Education, Research and Innovation (SERI) to Bas Ibelings and by supplementary funding from University of Geneva. We thank Wendy Beekman for the nutrient analysis and the University of Wageningen for covering the costs of this analysis from the personal funding of dr. Miquel Lürling. We thank Pieter Slot for assisting with the pigment analysis and the University of Amsterdam for covering the costs of the analysis through funding from the group of Prof. Jef Huisman and dr. Petra Visser (IBED). We would like to thank the Environmental Data Initiative for covering the cost of archiving the EMLS dataset. We would like to thank the Leibniz Institute of Freshwater Ecology and Inland Fisheries, Dept. of Experimental Limnology and the Aquatic Microbial Ecology Group for logistic and technical support of J. Fonvielle and H.-P. Grossart; and the Leibniz Association for financial support. The collection of data for Lough Erne and Lough Neagh were funded by the Department of Agriculture, Environment and Rural Affairs, Northern Ireland.

We explored phytoplankton communities from a functional perspective in two connected but hydro-morphologically distinct basins during the ice-free period in the cold and oligotrophic Lake Tovel, Italy. Despite the absence of dispersal barriers, we expected a clear separation of the phytoplankton communities between basins based on their physical distinctions: a shallow and a deep basin with low and high water residence time, respectively. To investigate seasonal succession and spatiality, taxa were classified according to their functional groups (FGs) and traits (FTs). Relationships between functional classifications and environmental parameters were assessed by non-metric multidimensional scaling integrated with cluster analysis. Clustering of FGs and FTs was complementary and reflected different hydrological conditions of each basin: (i) more stable conditions and higher functional variation across seasons in the deep basin and (ii) less stable conditions and lower functional variation across seasons in the shallow basin. Phytoplankton functional composition evidenced how local conditions selected for corresponding functional attributes adapted to each basin’s environment. These results, together with the presence of rare, cold-tolerant taxa such as Pseudotetraedriella kamillae and Stephanocostis chantaicus, highlighted the need for long-term phytoplankton studies, and the creation of a FG that includes cold-adapted, oligotrophic taxa.

Summary Cryptic species (species that are difficult or impossible to distinguish morphologically) are found worldwide and neglecting them may hamper our understanding of species-environment relationships. The rotifer Polyarthra dolichoptera occurs across wide environmental gradients, indicating the potential for a cryptic species complex. We used P. dolichoptera as a model to investigate spatial patterns and relationships of cryptic species with environmental variables along a broad altitudinal gradient. We sequenced 575 specimens of Polyarthra from 42 waterbodies from Germany, Hungary and Italy sampled in summer. Thirty-five lakes were distributed along an altitudinal gradient (250–2564 m a.s.l.) in a geographically restricted area, the Trentino–South Tyrol region, Italy, and one lake (Lake Tovel) was sampled in two basins and at different depths year-round to investigate temporal and spatial patterns of cryptic species. We used the Generalised Mixed Yule Coalescent model to delimit cryptic species (GMYC entities) based on a phylogenetic tree and related their presence/absence to space and the environment by applying Mantel tests and variance partitioning. We found twelve GMYC entities of P. dolichoptera. In variance partitioning, most explained variability was attributed to the interaction between space and altitude and by environmental variables and geographical space alone. However, by splitting the data into sets of low- and high-altitude lakes, the strong effect of altitude was removed; conductivity and silica played a major role in high-altitude lakes, whereas neither environment nor space were related to entity occurrence in low-altitude lakes. In Lake Tovel, three cryptic entities were found that did not show spatial segregation; however, two entities showed temporal segregation while the third one occurred throughout the year. Our study showed that P. dolichoptera is a complex of species which distribution is driven by geography and environmental parameters whose importance is different in low- and high-altitude lakes. Considerable unexplained variability points to a major contribution of other variables, potentially in-lake properties such as competition, predation and species behaviour.

Altitude encompasses broad environmental gradients that influence the isotopic composition of lake water. We selected 55 lakes in the Eastern Alps along an altitudinal gradient [214–2,532 m above sea level (a.s.l.)] to model the isotopic signal of surface water dependent on intrinsic (lake geomorphometry) and extrinsic (air temperature, precipitation) factors. Ordinary and generalised least squared regression were used for statistical analysis. The isotope signal of lake water was lower in spring than in summer and decreased with altitude (−0.21 δ18O ‰/100 m; −1.5 δ2H ‰/100 m). This pattern largely depended on temperature and a pseudo-latitude effect. The isotopic signal in monthly precipitation (12 stations; altitudinal gradient 90–2,730 m a.s.l.) generally showed the expected pattern of less enriched values with altitude; however, unusual values were related to weather anomalies. The local meteoric water line was similar to the global meteoric water line as shown by overlapping confidence intervals. By discriminating different elevational bands, we could show that high elevation lakes (>1,500 m a.s.l.) experience different patterns of evaporation with respect to low elevation lakes (

Little is known about the UV photobiology of psychrophilic dinoflagellates, particularly in freshwater systems. We addressed the life strategies of Borghiella dodgei Moestrup, Gert. Hansen et Daugbjerg to cope with ambient levels of ultraviolet radiation (UVR) under cold conditions. Several physiological parameters related to growth, metabolism, and UVR protection were determined for 4 d in UVR-exposed and control cells by applying stable isotope analysis, spectrophotometry, and liquid chromatography-mass spectrometry (LC/MS). In UVR-exposed cells, assimilation of (15) N and (13) C and content of chl a and carotenoids, specifically diatoxanthin with respect to dinoxanthin and diadinoxanthin, were increased; furthermore, catalase activity showed a cyclic pattern with a strong increase after UVR exposure but a rapid return to preexposure levels. Both in UVR-exposed and control cells, no lipid peroxidation of galactolipids was observed. However, in UVR-exposed cells, content of galactolipids was higher and linked to an increase in monogalactosyldiacylglycerols (MGDGs). We concluded that Borghiella's adaptation to UVR depended on a general metabolic enhancement and efficient scavenging of oxygen radicals to mitigate and counteract damage. While Borghiella seemed to be well adapted to ambient UVR, the interactive effects of higher temperature and UVR on psychrophilic species in front of climate change merit further investigation.

Summary 1. The occurrence of unresolved complexes of cryptic species may hinder the identification of the main ecological drivers of biodiversity when different cryptic taxa have different ecological requirements. 2. We assessed factors influencing the occurrence of Synchaeta species (monogonont rotifers) in 17 waterbodies of the Trentino-South Tyrol region in the Eastern Alps. To do so, we compared the results of using unresolved complexes of cryptic species, as is common practice in limnological studies based on morphological taxonomy, and having resolved cryptic complexes, made possible by DNA taxonomy. 3. To identify cryptic species, we used the generalised mixed Yule coalescent (GMYC) model. We investigated the relationship between the environment and the occurrence of Synchaeta spp. by multivariate ordination using two definitions of the units of diversity, namely (i) unresolved species complexes (morphospecies) and (ii) putative cryptic species (GMYC entities). Our expectation was that resolving complexes of cryptic species could provide more information than using morphospecies. 4. As expected, DNA taxonomy provided greater taxonomic resolution than morphological taxonomy. Further, environmental-based multivariate ordination on cryptic species explained a significantly higher proportion of variance than that based on morphospecies. Occurrence of GMYC entities was related to total phosphorus (TP), whereas no relationship could be found between morphospecies and the environment. Moreover, different cryptic species within the same morphospecies showed different, and even opposite, preferences for TP. In addition, the wide geographical distribution of haplotypes and cryptic species indicated the absence of barriers to dispersal in Synchaeta.

Patterns of diversity reflect the balance between speciation and extinction over time. Here we estimate net diversification rates for samples of sexual and asexual rotifers using phylogenetic reconstructions from sequence data of one mtDNA locus, cytochrome oxidase c subunit I. All four clades of bdelloid rotifers, obligate asexuals, had higher number of species per clade and significantly higher accumulation of diversification events towards the root of the trees than the four clades of their sexual relatives, the monogonont rotifers. Such differences were robust to confounding effects of number of analysed sequences, haplotype diversity, overall genetic divergence, age of the clades or geographic coverage. Our results support the idea that differences in diversification rates could thus be ascribed to different mechanisms of speciation, with ecological speciation as the most plausible mechanism for asexual organisms.

Ecological research is moving from a species-based to a functional-based approach to better understand the underlying principles that govern community dynamics. Studies of functional-based ecology, however, have been limited for zooplankton and particularly for rotifers. While rotifers show a variety of trophi types and coronal shapes, suggesting the importance of niche differentiation in their feeding strategy, relatively little is known of how this relates to rotifer dynamics. We used the guild ratio (GR′, a ratio of raptorial to microphagous species), an index based on a functional trait (i.e. feeding strategy), as a novel approach to rotifer dynamics. We extracted the seasonal GR′ by using seasonal trend decomposition and investigated similarities between study sites (Lake Washington, USA and Lake Caldonazzo, Italy) and its relation to cladocerans by cross-correlation analysis. Our study indicated that (i) raptorial and microphagous rotifers showed alternating dominance, and that raptorial rotifers and cladocerans had a synchronous pattern, (ii) the seasonal pattern of the GR′ was consistent across different sampling frequencies, and (iii) the GR′ was similar in both lakes. We interpreted these patterns as the general strength of the GR′: discernment of species–environment relationships and robustness across sampling regimes. The limitations of the GR′ (i.e. species identity is neglected, simplification of food preferences) can also be seen as its strong point: synthesis of multi-species patterns. In addition, the independence of GR′ from species-level identification and its potential to make use of datasets with infrequent sampling intervals and low taxon resolution could further support its innovative aspect.

Aim Biodiversity patterns along altitudinal gradients are less studied in aquatic than terrestrial systems, even though aquatic sites provide a more homogeneous environment independent of moisture constraints. We studied the altitudinal species richness pattern for planktonic rotifers in freshwater lakes and identified the environmental predictors for which altitude is a proxy. Location Two hundred and eighteen lakes of Trentino–South Tyrol (Italy) in the eastern Alps; lakes covered 98% (range 65–2960 m above sea level) of the altitudinal gradient in the Alps. Methods We performed: (1) linear regression between species richness and altitude to evaluate the general pattern, (2) multiple linear regression between species richness and environmental predictors excluding altitude to identify the most important predictors, and (3) linear regression between the residuals of the best model of step (2) and altitude to investigate any additional explanatory power of altitude. Selection of environmental predictors was based on limnological importance and non-parametric Spearman correlations. We applied ordinary least squares regression, generalized linear, and generalized least squares modelling to select the most statistically appropriate model. Results Rotifer species richness showed a monotonic decrease with altitude independent of scale effects. Species richness could be explained (R2= 51%) by lake area as a proxy for habitat diversity, reactive silica and total phosphorus as proxies for productivity, water temperature as a proxy for energy, nitrate as a proxy for human influence and north–south and east–west directions as covariates. These predictors completely accounted for the species richness–altitude pattern, and altitude had no additional effect on species richness. Main conclusions The linear decrease of species richness along the altitudinal gradient was related to the interplay of habitat diversity, productivity, heat content and human influence. These factors are the same in terrestrial and aquatic habitats, but the greater environmental stability of aquatic systems seems to favour a linear pattern.

Zooplankton abundance was related to hydrological and environmental variables in a hydrologically dynamic lake fed by a pseudokarstic aquifer. The study period (2002–2006) in Lake Tovel covered different hydrological situations with water residence time (WRT) having the lowest values in 2002 and the highest values in 2003. WRT was negatively correlated with silica concentrations and algal biovolume. Furthermore, the biovolume of small algae was highest in spring and summer, while large algae did not show any pattern. In multivariate analysis, high abundance of crustacean species in autumn and winter was positively related to WRT and negatively to algal biovolume, while high abundance of rotifer species in spring and summer was negatively related to WRT and positively to algal biovolume. With the exception of Keratella cochlearis and Gastropus stylifer, rotifers showed a pattern of crustacean avoidance, and three groups were distinguished: (i) Ascomorpha ecaudis and Polyarthra dolichoptera, (ii) Asplanchna priodonta and Synchaeta spp., and (iii) Filinia terminalis and Keratella quadrata. These groups were associated with different food sources and depths. We suggest that WRT influenced the rotifer–crustacean relationship by wash-out effects and competition for food resources. The dynamics of single rotifer species were attributable to specific feeding requirements and adaptations. In summary, WRT determined the platform for abiotic and biotic interactions that influenced population dynamics of crustaceans and rotifers.

We studied zooplankton dynamics in a groundwater-fed, montane lake during four consecutive years and assessed the importance of water residence time for zooplankton dynamics. Crustacean abundance and biomass were significantly correlated with water residence time and temperature, but showed no significant correlation with phytoplankton biovolume. We hypothesised that temperature depended on water residence time (τ), and therefore we further investigated the functional relationship of crustacean dominance with the latter by logistic regression analysis. Water residence time values above a threshold value (τ = 193 days) determined crustacean biomass dominance while values below determined rotiferan dominance. Our results indicated that water residence time was an important factor structuring zooplankton succession in this lake that showed large fluctuations of τ values (median 263 days; range 23 – 786 days for the four year period) compared to other lakes. We suggest that crustacean biomass was directly controlled through water residence time as found for riverine systems, whereas rotifer biomass was controlled through exploitative competition with crustaceans for phytoplankton. The importance of water residence time may have been underestimated in lakes when explaining zooplankton community structure and succession, because studies usually focus on other factors such as temperature, predation, or food limitation.

Filming live organisms can give new insights into the hidden life of plankton. Accessibly priced digital cameras are now available for a large range of users. Here, we demonstrate the technical setup and workflow of using a single-lens reflex (DSLR) camera to film the behaviour of males of two rotifer species, Brachionus angularis Gosse (1851) and Keratella cochlearis Gosse (1851), and of the cladoceran Daphnia magna Straus (1820). Rotifers are cyclical parthenogens that produce males only under certain environmental conditions. Thus, knowledge on rotifer males is still limited because of their ephemeral nature and because they are often smaller than females. We filmed males of B. angularis and K. cochlearis with a DSLR camera connected to a compound microscope to better understand their morphology and behaviour in comparison to conspecific females. While written descriptions have their scientific value, seeing is complementary because everyone can verify what has been described. We made our videos publicly accessible through links connected to the paper. Our videos are, to our best knowledge, the first on males of B. angularis and K. cochlearis. Furthermore, we filmed the behavioural response of D. magna to ultraviolet (UV) radiation with a macro lens attached to the DSLR camera. Approaches like this are valuable tools in environmental teaching. To see live organisms with one’s own eyes may contribute to raising public awareness about the value of water resources and their hidden communities. In summary, filming can be a valuable tool to ignite scientific discussion, but the videos need an open-access platform where they can be referenced in a topic-related order.

Trait patterns can give insights into how communities assemble under a functional perspective. We constructed a rotifer trait matrix related to food acquisition and predator defence and calculated Rao’s quadratic entropy (Q) as an index of functional diversity to investigate trait patterns in different layers (0–2, 5–35, 0–35 m) for a 5-year dataset of Lake Tovel, Italy. Trait patterns were determined by comparing Q observed to Q from random communities. While trait patterns can be determined by species traits, richness, and abundance, in most samples, irrespective of layer, trait patterns could be solely attributed to traits indicating their importance for community assembly. Trait convergence dominated in the upper layer, while trait divergence dominated in the lower layer. Using logistic regression, we related trait patterns to environmental parameters. In the lower layer, trait divergence was linked to competition for food while trait convergence was linked to copepod predation. However, in the upper layer neither competitors nor predators influenced trait patterns, and we suggest that ultraviolet radiation and temperature were the main drivers of trait convergence. Our study indicated that environmental filtering drove rotifer trait patterns in the upper layer, whereas species interactions drove trait patterns in the lower layer.