Climate change is increasing the amplitude of fluctuations in lake water levels, altering littoral habitats and affecting local biota. In Lake Kinneret, Israel, a 5-year drought (2013-2018) caused a 6 m drop in water level, 3 times the natural amplitude, resulting in the establishment of dense Tamarix jordanis forests on exposed shores. When the lake refilled in 2019 these forests were inundated, creating a novel littoral environment. We studied the dynamics of Tamarix and associated aquatic habitats under prolonged inundation, hypothesizing that physicochemical conditions would be adversely altered and impact the biota. Using remote sensing we tracked the spatial distribution and decay of live Tamarix. In a field study, we followed the temporal dynamics of physicochemical parameters, phytoplankton, and zooplankton. Using ecosystem modeling, we assessed potential impacts on the commercially valuable cichlid fish, Sarotherodon galilaeus. We found that, shortly after inundation, Tamarix habitats were hypoxic, shaded, poorly circulated, and rich in particulate nutrients and organic matter, with unique phytoplankton and zooplankton assemblages. Over time, canopy dieback improved light conditions and water exchange, alleviating the hypoxia. Timing of re-oxygenation varied among sites based on forest size, tree density, and wind exposure. Modeling predicted minimal effects on fish populations and commercial yields. Although Tamarix survived 3 years of flooding, it died in the fourth. Understanding the emergence and dynamics of novel habitats, such as the inundated Tamarix forests, is essential for managing freshwater ecosystems under climate change.

Extreme and compound events disrupt lake ecosystems worldwide, with their frequency, intensity and duration increasing in response to climate change. In this Review we outline evidence of the occurrence, drivers and impact of extreme and compound events in lakes. Univariate extremes, which include lake heatwaves, droughts and floods, underwater dimming episodes and hypoxia, can occur concurrently, sequentially or simultaneously at different locations to form multivariate, temporal or spatial compound events, respectively. The probability of extreme and compound events is increasing owing to climate warming, declining lake water levels in half of lakes globally, and basin-scale anthropogenic stressors, such as nutrient pollution. Most in-lake extreme events are inherently compound in nature owing to tightly coupled physical, chemical and biological underlying processes. The cascading effects of compound events propagate or dissipate through lakes. For example, a heatwave might trigger stratification and oxygen depletion, subsequently leading to fish mortality or the proliferation of harmful algal blooms. Interactions between extremes are increasingly observed and can trigger feedback loops that exacerbate harmful algal blooms and fishery declines, leading to severe ecological and socio-economic consequences. Managing the increasing risk of compound events requires integrated models, coordinated monitoring and proactive adaptation strategies tailored to the vulnerabilities of lake ecosystems. Extreme and compound events in lakes are increasing in severity and frequency in response to climate change and basin-scale anthropogenic stressors. This Review explores the occurrence, drivers and impact of such events, focusing on their physical and ecological drivers, impacts and management responses.

Lakeshores are ecotones encompassing diverse habitats that support high biodiversity, interact with pelagic ecosystems, and provide desirable ecosystem services. Hydrological regimes altered by climate change significantly impact lakeshore ecosystems. In this opinion paper, we highlight ecological consequences of distinct multiannual hydrological regimes (superimposed on natural seasonal ones) that affect the lakeshore environment of stratified lakes. Under natural hydrological conditions (Regime A), lakes maintain a stable annual mean water level, supporting diverse biota adapted to the seasonal fluctuations of water level. By contrast, under multiannual drought, the annual mean water level continuously declines (Regime B), and the resulting exposed shores become covered with dense vegetation, dominated by a few species. As the waterline recedes, structural complexity in the shallow water is reduced, limiting populations of native invertebrates and fish and facilitating establishment of invasive species. Storm-induced rapid rises in annual mean water levels during extremely rainy years (Regime C) are the least studied. While lake refilling is intuitively beneficial, undesirable short-term effects are common, including organic matter accumulation and decomposition, temporary hypoxia, pungent odors, loss of fish habitat, and formation of mosquito-infested shore pools. Littoral zone recovery and alleviation of nuisances occur only after the extreme conditions subside. Under climate change, the frequency and intensity of Regimes B and C are likely to increase. From a management perspective, maintaining water levels close to natural amplitudes can minimize the adverse conditions and damage caused by Regimes B and C.

Extreme meteorological events such as storms are increasing in frequency and intensity, but our knowledge of their impacts on aquatic ecosystems and emergent system properties is limited. Understanding the ecological impacts of storms on the dynamics of primary producers remains a challenge that needs to be addressed to assess the vulnerability of freshwater ecosystems to extreme weather conditions and climate change. One promising approach to gain insights into storm impacts on phytoplankton community dynamics is to analyse long-term monitoring datasets. However, such an approach requires disentangling the impacts of short-term meteorological disturbances from the effects of the seasonal trajectories of meteorological conditions. To this end, we applied boosted regression tree models to phytoplankton time series from eight relatively large lakes on four continents, coupled with a procedure adapted to detect and quantify rare events. Overall, the patterns and potential drivers we identified provide important insights into the responses of lakes to short-term meteorological events and highlight differences in the response of phytoplankton communities according to lake morphological characteristics. Our results indicated that deepened thermoclines and lake-specific combinations of drivers describing altered thermal structures caused deviations from the typical trajectories of seasonal phytoplankton succession. For shallow polymictic lakes, shifts in phytoplankton succession also depended on changes in light availability. Overall, our study highlights the value of long-term monitoring to improve our understanding of phytoplankton sensitivity to short-term meteorological disturbances.

In Lake Kinneret (the biblical Sea of Galilee), Israel, internal waves of significant amplitude are induced by westerly winds. These waves give rise to upwelling into the surface mixed layer of colder, oxygen-depleted water from the hypolimnetic and metalimnetic layers. If upwelling occurs soon after the onset of annual thermal stratification, when surface mixed layer extends over a narrow depth range, but the hypolimnion is already anoxic, there is a potential for massive fish kills as fish cannot escape the anoxic water that intrudes into the surface mixed layer along the western shore. This study uses a coupled three-dimensional atmosphere-lake model to elucidate the mechanisms behind these infrequent major fish kills in Lake Kinneret. Remarkably, nowadays fish-kill events happen at the same location in the lake where the biblical Miracle of Loaves and Fishes and presumably the Miraculous Catch of Fish occurred two millennia before the present and may explain the appearance of large numbers of easy-to-collect fish close to the shore described in the biblical narratives.

On May 31, 2012, thousands of dead fish were found along the north-western shore of the Sea of Galilee. Analysis of fish gill tissue revealed no evidence of poisoning, and the fish looked healthy. This event adds to reports of similar fish kills at the same location, from the early 1990s, from May 2007, and a subsequent event on June 27, 2012, a month after the May 31 event. The common hypothesis for the massive kill suggests that a seiche induced by strong winds caused the upwelling of colder and anoxic hypolimnetic water along the western shores of the lake. Still, this hypothesis has not yet been tested.The WRF (The Weather Research and Forecasting) atmospheric model was recently coupled with the ocean model MITgcm (MIT general circulation model). The coupled model was named SKRIPS (Scripps–KAUST Regional Integrated Prediction System). The two SKRIPS model components (WRF and MITgcm) are well-tested at high resolution, allowing us to investigate the physical mechanism of the fish-kill event in an interactive system. To test the hypolimnetic water upwelling hypothesis for the massive fish-kill, we have set up and integrated the SKRIPS model for the May 31, 2012, event at a horizontal grid resolution of 400 m2, both for the atmospheric and lake component of the model.In this talk, I will present results from a high-resolution coupled atmosphere-lake regional simulation indicating an upwelling of cold anoxic hypolimnetic water into the surface during the event. The upwelling of cold water is increased close to the shore. The discussion will be supplemented by field data of temperature and oxygen concentrations, collected before, during, and after the fish-kill event. Our simulation results agree with the field observations, adding confidence to the anoxic hypolimnetic water upwelling hypothesis. Such fish-kill events may explain the biblical ‘miraculous catch of fish’ and the ‘miracle of the loaves and fish’. Also, it may provide a possible seasonal time frame (spring) for their occurrence in the past.

With climate change and re‐oligotrophication of lakes due to restoration efforts, the relative importance of benthic cyanobacteria is increasing, but they are much less studied than their planktonic counterparts. Following a major water level rise event that inundated massive reed stands in Lake Kinneret, Israel, we discovered the appearance of a vast abundance of Gloeotrichia pisum (cyanobacteria). This provided an opportunity to investigate the biology and ecology of a benthic epiphytic colonial cyanobacterium, proliferating under altered environmental conditions, with possible toxin production potential and as a model for an invasive epiphyte. The species was identified by its typical morphology, and by sequencing its 16S rRNA gene and the intragenic space. We report on the abundance and spatial distribution of the detected colonies, their morphological characteristics, and pigment composition. High phycoerythrin content provides a brownish color and supports growth at low light levels. Genomic community composition analysis revealed that G. pisum colonies host a diverse microbial community of microalgae, cyanobacteria, bacteria, and archaea with a conserved and characteristic taxonomic composition. The Synechococcales order showed high relative abundance in the colony, as well as other prokaryotes producing secondary metabolites, such as the rhodopsin producer Pseudorhodobacter . The microbial consortium in the colonies performed nitrogen fixation. The diazotroph's phylogenetic relations were demonstrated. Tests for the presence of cyanotoxins (microcystin and cylindrospermopsin) proved negative. This study is the first documentation of this genus in Israel, providing insights into the invasive nature of G. pisum and the ecological implications of its appearance in a lake ecosystem.

Semi-quantitative microscope counts of phytoplankton are often a compromise between time-consuming cell biomass analyses and no phytoplankton data. We demonstrate how semiquantitative data from a monitoring program can be used to study phytoplankton community composition, its annual cycle, and aspects of the ecosystem it inhabits. Semi-quantitative counts from Agmon Wetlands, Israel, collected monthly from 2008 to 2021, were generated by allocating a score from 1 (rare) to 6 (extremely abundant) to each taxon observed in a sample. Five samples could be analyzed at the time it takes to count one sample by the conventional Utermöhl method. Using an exponential regression equation, the scores were transformed to estimated concentrations (algal units/ml), then summed into taxonomic or other groups of species. A strong annual pattern of the sum of scores for each taxonomic group was observed. The method was useful for assessing ecosystem features based on indicator species, and for presenting the contribution of morpho-functional groups to the phytoplankton community. If making a species list is planned, we recommend assigning scores, creating calibration curves, converting the scores to concentration estimates, and using those estimates to achieve higher resolution and better conclusions than possible with a species list alone.

The Lake Suwa (Japan) has a history of non-N-fixing Microcystis blooms. Lake Kinneret (Israel) experienced multiannual periods of sole domination by the dinoflagellate Peridinium gatunense and periods dominated seasonally by P. gatunense or cyanobacteria. Extensive studies have been carried out in both lakes regarding the role of dissolved inorganic nitrogen and phosphorus as drivers of primary productivity. There is growing evi-dence that dissolved organic nitrogen (DON) compounds also influence not only biomass and structure of phytoplankton communities but also microcystin production. This study focuses on relationships of DON with: (1) population dynamics of Microcystis spp. and concentrations of microcystins in Lake Suwa, and (2) population dynamics of P. gatunense as well as N-and non-N-fixing cyanobacteria in Lake Kinneret. Modelling results for historical data of Lake Suwa by means of the hybrid evolutionary algorithm HEA revealed that the prediction of abundances of four Microcystis species and concentrations of cyanotoxins achieved higher coefficients of corre-lation when DON/DIN-ratios were included as drivers. Population dynamics of P. gatunense in Lake Kinneret appeared to have a strong inverse relationships with DON/DIN-ratios reflected by inferential models of HEA with higher coefficients of correlation when driven by DON/DIN-ratios. When DON/DIN-ratios were included as drivers, models of Microcystis spp. in Lake Kinneret performed higher coefficients of determination compared to models of N-fixing cyanobacteria. The study highlights the need to consider DON for improved understanding and management of population dynamics of cyanobacteria and dinoflagellates in freshwater lakes.

Untangling causal links and feedbacks among biodiversity, ecosystem functioning, and environmental factors is challenging due to their complex and context-dependent interactions (e.g., a nutrient-dependent relationship between diversity and biomass). Consequently, studies that only consider separable, unidirectional effects can produce divergent conclusions and equivocal ecological implications. To address this complexity, we use empirical dynamic modeling to assemble causal networks for 19 natural aquatic ecosystems (N24 ◦ ~N58 ◦ ) and quantified strengths of feedbacks among phytoplankton diversity, phytoplankton biomass, and environmental factors. Through a cross-system comparison, we identify macroecological patterns; in more diverse, oligotrophic ecosystems, biodiversity effects are more important than environmental effects (nutrients and temperature) as drivers of biomass. Furthermore, feedback strengths vary with productivity. In warm, productive systems, strong nitrate-mediated feedbacks usually prevail, whereas there are strong, phosphate-mediated feedbacks in cold, less productive systems. Our findings, based on recovered feedbacks, highlight the importance of a network view in future ecosystem management.

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.

The biogeochemistry of dissolved selenium (Se) was investigated over 3 years (2015–2017) in the subtropical, warm monomictic and meso-eutrophic Lake Kinneret ( Sea of Galilee , Israel). We monitored seasonal variation and vertical distribution of dissolved total Se (T.Se), inorganic oxyanions (Se(IV) & Se(VI)), reduced Se fraction (Red.Se), organic (Org.Se) and volatile Se compounds. T.Se varied between ~ 100 and 160 ng L −1 with Red.Se comprising 40–80% of the Se inventory, and Se(VI) dominating over Se(IV) most of the time. The variation in T.Se and species correlated with winter holomixis vs. summer fall stratification periods. The annual cycle includes: (a) increase of T.Se from fall/winter to spring, representing increased allochthonous Se input flux, along with Se recycling via holomixis; (b) decrease of T.Se from spring to the end of the year, representing the diminishing Se inputs and the evolving output fluxes to the lake’s bottom and to the atmosphere. Org.Se variations are directly associated with Chlorophyll-a and primary production attesting for the significant role of phytoplankton activity in the Se cycle. An important Se output flux comprises spring to summer Se uptake by phytoplankton and further volatile compounds production and volatilization accounting for ~ 10% of estimated Se input. The similarity of total dissolved Se concentrations in this work and from mid 1990s attests for long-term stability of the Se inventory. The TVSe concentrations in lacustrine systems being similar to that of estuary systems, the biological role of phytoplankton and eventually the degradation of organic material may produce similar fluxes of volatile Se to the atmosphere.

The Andalusian International University held a workshop entitled Temporary wetlands' future in drylands under the projected global change scenario in March 2020 in Baeza, Spain, with 26 participants from 10 countries. The workshop objectives were to promote international cooperation and scientific exchange on the conservation and protection of temporary wetlands. The participants highlighted the extreme conditions that temporary and permanent wetlands, ponds, and shallow lakes are currently facing and predicted a dismal future for these systems due to climate change. To foster a holistic view of these ecosystems, the workshop included wetland watersheds. It was concluded that the main threats are those affecting water quality and quantity as well as egg-seed banks, species population dynamics, and food webs. The inherent characteristics of waterbodies in drylands, including high resilience and resistance to harsh conditions, are already negatively impacted by direct human actions and climate change. Another threat is the time lag between scientific warnings about threats and the social and political concern leading to mitigating actions. Thus, more effective actions to protect and conserve temporary wetlands are essential. Research networks could help stimulate the necessary conservation actions, but the global recession due to the COVID-19 pandemic will pose a challenge as economies are burdened with urgent expenditure. This special issue of the journal Inland Waters is the outcome of the workshop presentations and is composed of the ensuing papers on wetlands in drylands.

The intensity and frequency of storms are projected to increase in many regions of the world because of climate change. Storms can alter environmental conditions in many ecosystems. In lakes and reservoirs, storms can reduce epilimnetic temperatures from wind-induced mixing with colder hypolimnetic waters, direct precipitation to the lake's surface, and watershed runoff. We analyzed 18 long-term and high-frequency lake datasets from 11 countries to assess the magnitude of wind- vs. rainstorm-induced changes in epilimnetic temperature. We found small day-to-day epilimnetic temperature decreases in response to strong wind and heavy rain during stratified conditions. Day-to-day epilimnetic temperature decreased, on average, by 0.28 degrees C during the strongest windstorms (storm mean daily wind speed among lakes: 6.7 +/- 2.7 m s(-1), 1 SD) and by 0.15 degrees C after the heaviest rainstorms (storm mean daily rainfall: 21.3 +/- 9.0 mm). The largest decreases in epilimnetic temperature were observed >= 2 d after sustained strong wind or heavy rain (top 5(th) percentile of wind and rain events for each lake) in shallow and medium-depth lakes. The smallest decreases occurred in deep lakes. Epilimnetic temperature change from windstorms, but not rainstorms, was negatively correlated with maximum lake depth. However, even the largest storm-induced mean epilimnetic temperature decreases were typically <2 degrees C. Day-to-day temperature change, in the absence of storms, often exceeded storm-induced temperature changes. Because storm-induced temperature changes to lake surface waters were minimal, changes in other limnological variables (e.g., nutrient concentrations or light) from storms may have larger impacts on biological communities than temperature changes.

Alon Rimmer, an outstanding tenacious hydrologist and a devoted editor and mentor passed away on April 22, 2018, at the age of 62, after fighting cancer. Alon was an eloquent writer and speaker who could express difficult concepts in simple terms with well-illustrated drawings. He applied a system approach with mathematical modeling to provide simple and practical solutions to complex problems. He expressed and publicized his scientific opinion even if it opposed the consensus. We will remember Alon's resolute personality as well as his strong physique. His sense of wisdom, wit, sensitivity, humor, modesty and incredible courage, and continue to apply the computational tools he left us.

The amazing morphological diversity of phytoplankton has to be considered an evolutionarily driven compendium of strategies to cope with the strong variability and unpredictability of the pelagic environment. Phytoplankton collects unicellular and colonial photosynthetic organisms adapted to live in apparent suspension in turbulent water masses. Turbulence represents a key driver of phytoplankton dynamics in all aquatic ecosystems and phytoplankton morphological variability is the evolutionary response of this group of photosynthetic organisms to the temporal and spatial scales of variability of turbulence. This paper reviews the existing literature on the effects exerted by turbulence on phytoplankton populations and is aimed at showing how deeply turbulence contributes to the shape and size structure of phytoplankton assemblages. Our aim is to explore how turbulence governs phytoplankton access to resources and, at the same time, how the shape and size structure of phytoplankton represent the evolutionary way in which this group of organisms has optimised its survival in the highly dynamic aquatic environment. The paper is intended to serve as an homage to the (phytoplankton) ecologist Colin S. Reynolds. His life-long work highlighted how profoundly the ecology of phytoplankton depends on the physical constraints governing the movements of the water masses in which phytoplankton evolved and lives.

We review the literature on the relationship between water temperature and size of freshwater phytoplankton, to examine the hypothesis that freshwater phytoplankton, like marine phytoplankton and many other groups of organisms, conform to Bergmann’s Rule and become smaller with warming. We provide both experimental and field evidence in support of the above hypothesis, much of this evidence was hidden in studies focused on other issues, but presenting temperature and phytoplankton size data. Freshwater phytoplankton size shrinks with increasing temperature at both the species level (by cells or colonies becoming smaller) and at the community level (shift to smaller species). Exceptions to the Rule do occur but in most cases those exceptions can be explained by indirect effects of temperature on phytoplankton size, via processes such as grazing or nutrient availability. With global warming, freshwater phytoplankton are likely to be of smaller size. This article is dedicated to Colin S. Reynolds, who has had a leading role in our personal education and understanding of phytoplankton ecology.

In many regions across the globe, extreme weather events such as storms have increased in frequency, intensity, and duration due to climate change. Ecological theory predicts that such extreme events should have large impacts on ecosystem structure and function. High winds and precipitation associated with storms can affect lakes via short-term runoff events from watersheds and physical mixing of the water column. In addition, lakes connected to rivers and streams will also experience flushing due to high flow rates. Although we have a well-developed understanding of how wind and precipitation events can alter lake physical processes and some aspects of biogeochemical cycling, our mechanistic understanding of the emergent responses of phytoplankton communities is poor. Here we provide a comprehensive synthesis that identifies how storms interact with lake and watershed attributes and their antecedent conditions to generate changes in lake physical and chemical environments. Such changes can restructure phytoplankton communities and their dynamics, as well as result in altered ecological function (e.g., carbon, nutrient and energy cycling) in the short- and long-term. We summarize the current understanding of storm-induced phytoplankton dynamics, identify knowledge gaps with a systematic review of the literature, and suggest future research directions across a gradient of lake types and environmental conditions.