The pace of shifting seasons in lakes

Convergence and divergence of microbial communities in river- Qinghai lake sediment continuum on Tibetan Plateau

Microbiota within interconnected river-lake systems define cycles of carbon and nutrients, yet the mechanisms underlying the assembly of microbial communities during their transition from tributaries to lake remains to be deciphered. This study examined the communities of protists, fungi and bacteria in sediments of Qinghai Lake - a saline lake on the Tibetan Plateau, China - and its connected upstream freshwater tributaries, using high-throughput amplicon sequencing targeting the 18S, ITS2 and 16S rDNA regions. Our findings reveal divergent assembly mechanisms across microbial groups: communities of microeukaryotes (protists and fungi) in tributaries were predominantly shaped by stochastic processes (∼85% contribution), shifting to environmental selection dominance in the lake (∼55%). In contrast, bacterial community assembly in tributaries was primarily deterministic (∼60% environmental selection), shifting to stochastic dominance (∼70%) in the lake. Despite the differences, all groups exhibited congruent biogeographic patterns in terms of diversity and network complexity. The tributary-to-lake transition enhanced the complexity of microbial co-occurrence network but resulted in significant species loss, with α-diversity reduced by 56%-62%. β-diversity increased from tributaries to the estuary but decreased within the lake. Microbial α- and β-diversity correlated positively with sediment C: N ratio but negatively with total sediment C content. Notably, only 1% to 13% of microbial taxa in lake sediments originated from tributaries, suggesting alternative pathways that warrant further geological investigation. This study provides new insights into the convergent biogeographical patterns of diversity and network complexity, coupled with divergent assembly mechanisms, among protist, fungal, and bacterial communities along the river-Qinghai Lake sediment continuum.

Enhanced heating effect of lakes under global warming

Lakes play a crucial role in shaping both local and regional climates through heat exchange with the atmosphere. Amid global climate change, these interactions have undergone significant shifts. However, our understanding of the global heat release from lakes to the atmosphere, and its future trajectory, remains limited. In this study, we investigate changes in global lake heat release patterns and identify an amplified increase in heat release, particularly in mid to high latitudes (>45°N). This amplification is linked with a feedback mechanism, where the reduction in lake ice cover not only reduces the insulating effect between the warmer lake water and the colder atmosphere but also leads to increased heat absorption by lakes. As a result, lakes in mid-high latitudes experience a greater relative increase in heat release, primarily through upward thermal radiation, compared to lakes at lower latitudes with comparable surface water temperature increases. Additionally, seasonal variations in latent heat flux intensify the heat release during warmer seasons compared to colder ones. Future projections suggest substantially greater heat release compared to historical trends.

Significant spatiotemporal pattern of nitrous oxide emission and its influencing factors from a shallow eutropic lake in Inner Mongolia, China

Eutrophic shallow lakes are generally considered as a contributor to the emission of nitrous oxide (N2O), while regional and global estimates have remained imprecise. This due to a lack of data and insufficient understanding of the multiple contributing factors. This study characterized the spatiotemporal variability in N2O concentrations and N2O diffusive fluxes and the contributing factors in Lake Wuliangsuhai, a typical shallow eutrophic and seasonally frozen lake in Inner Mongolia with cold and arid climate. Dissolved N2O concentrations of the lake exhibited a range of 4.5 to 101.2 nmol/L, displaying significant spatiotemporal variations. The lowest and highest concentrations were measured in summer and winter, respectively. The spatial distribution of N2O flux was consistent with that of N2O concentrations. Additionally, the hotspots of N2O emissions were detected within close to the main inflow of lake. The wide spatial and temporal variation in N2O emissions indicate the complexity and its relative importance of factors influencing emissions. N2O emissions in different lake zones and seasons were regulated by diverse factors. Factors influencing the spatial and temporal distribution of N2O concentrations and fluxes were identified as WT, WD, DO, Chl-a, SD and COD. Interestingly, the same factor demonstrated opposing effects on N2O emission in various seasons or zones. This research improves our understanding of N2O emissions in shallow eutrophic lakes in cold and arid areas.

Disruption of boreal lake circulation in response to mid-Holocene warmth; evidence from the varved sediments of Lake Nautajärvi, southern Finland

Future climate projections are expected to have a substantial impact on boreal lake circulation regimes. Understanding lake sensitivity to warmer climates is therefore critical for mitigating potential ecological and societal impacts. The Holocene Thermal Maximum (HTM; ca 7-5 ka BP) provides a valuable analogue to investigate lake responses to warmer climates devoid of major anthropogenic influences. Here, we analyse the micro-X-ray core scanning profiles (μ-XRF) of the annually laminated (varved) sediments from Lake Nautajärvi (NAU-23) in southern Finland to elucidate changes in lake circulation and sedimentation patterns. Principal component analysis (PCA) identifies two key components in the geochemical data associated with the nature of the sediments, i.e. detrital vs organic sedimentation (PC1), and hypolimnetic oxidation (PC2). Our findings reveal that during the HTM, the lake became more sensitive to changes in oxygenation and mixing intensity. These changes were triggered by a warmer climate, which increased organic matter and redox sensitive metal solute concentrations in the water column, strengthening lake stratification and weakening dimictic circulation patterns. Superimposed on HTM weakened circulation are distinct phases of increased oxidation and iron-rich varve formation that do not happen when the background conditions are cooler (i.e. the early and late Holocene). This is driven by temporary strengthening of the mixing regime in response to climatic variability and storminess cycles across southern Scandinavia. These findings demonstrate that whilst warmer conditions weaken boreal lake circulation regimes, they can also make them increasingly vulnerable to short term oscillations in prevalent climatic conditions and weather patterns, which could have significant impacts on lake water quality and aquatic ecosystems. These findings underscore the non-stationary nature of lake sensitivity to short-term climatic variability and emphasize the potential for similar shifts to occur under future warming scenarios.

Asymmetric impacts of climate change on thermal habitat suitability for inland lake fishes

Climate change is altering the thermal habitats of freshwater fish species. We analyze modeled daily temperature profiles from 12,688 lakes in the US to track changes in thermal habitat of 60 lake fish species from different thermal guilds during 1980-2021. We quantify changes in each species' preferred days, defined as the number of days per year when a lake contains the species' preferred temperature. We find that cooler-water species are losing preferred days more rapidly than warmer-water species are gaining them. This asymmetric impact cannot be attributed to differences in geographic distribution among species; instead, it is linked to the seasonal dynamics of lake temperatures and increased thermal homogenization of the water column. The potential advantages of an increase in warmer-water species may not fully compensate for the losses in cooler-water species as warming continues, emphasizing the importance of mitigating climate change to support effective freshwater fisheries management.

Eutrophication and Dissolved Organic Matter Exacerbate the Diel Discrepancy of CO2 Emissions in China's Largest Urban Lake

The large variability in the emissions of carbon dioxide (CO2) from urban lakes remains a challenge for partitioning these sources at meaningful spatial and temporal scales. Dissolved organic matter (DOM) governs the spatial and temporal variations in CO2, yet relationships of the CO2 concentration (cCO2) and emission flux (FCO2) with DOM in urban lakes have rarely been reported. In this study, we monitored levels of cCO2, FCO2, and the composition of DOM over a 24 h period at three sites during the dry and wet seasons in China's largest urban lake, Tangxun Lake. Our study found the ratio of day/night FCO2 (millimoles per square meter per day) decreased from the dry season (0.79; 7.68/9.68) to the wet season (0.25; 6.05/24.16), averaging 0.42 (6.77/15.97), implying that accounting for nighttime CO2 emissions can increase regional estimates by 70%. This study revealed that eutrophication affected diurnal CO2 emissions with greater algal growth enhancing daytime CO2 uptake and subsequently increasing nighttime CO2 emissions via DOM degradation (larger protein-like DOM fraction). We anticipate that the relative magnitude of FCO2 between day and night from lakes is likely to increase due to urbanization and climate change, underscoring the importance of treating urban lakes as a distinct group and integrating DOM dynamics into carbon cycling in future research.

The relationship between climate classes and particulate matters over Europe

Present and future Köppen-Geiger climate maps and time series of particulate matter (PM10) at continental scale were used to investigate the relationship between spatial patterns of PM10 and climate zones. Five main patterns (spatial clusters) of PM10 were found over the European continent, which show a good spatial overlap with the main Köppen-Geiger climate zones. The map of future climate, which shows a poleward movement of the warmer climate zones, an expansion of the (semi-)arid zones and a shrinking of the polar and tundra zones, indicates a higher PM10 concentration, especially in the northern part of Europe. The results also show that there is a temporal shift (later/earlier) of PM10 extremes in the period 2013-2022. This insight into the relationship between climate zones and clustered PM10 time series and the use of high-quality future climate maps based on different scenarios can be used to estimate possible changes, such as annual averages or seasonal cycles, in PM10 concentrations.

Lake Surface Water Temperature in high altitude lakes in the Pyrenees: Combining satellite with monitoring data to assess recent trends

Lake Surface Water Temperature (LSWT) influences critical bio-geological processes in lake ecosystems, and there is growing evidence of rising LSWT over recent decades worldwide and future shifts in thermal patterns are expected to be a major consequence of global warming. At a regional scale, assessing recent trends and anticipating impacts requires data from a number of lakes, but long term in situ monitoring programs are scarce, particularly in mountain areas. In this work, we propose the combined use of satellite-derived temperature with in situ data for a five-year period (2017-2022) from 5 small (<0.5km2) high altitude (1880-2680 masl) Pyrenean lakes. The comparison of in situ and satellite-derived data in a common period (2017-2022) during the summer season showed a notably high (r = 0.94, p < 0.01) correlation coefficient, indicative of a robust relationship between the two data sources. The root mean square errors ranged from 1.8 °C to 3.9 °C, while the mean absolute errors ranged from 1.6 °C to 3.6 °C. We applied the obtained in situ-satellite eq. (2017-2022) to Landsat 5, 7 and 8/9 data since 1985 to reconstruct the summer surface temperature of the five studied lakes with in situ data and to four additional lakes with no in situ monitoring data. Reconstructed LSWT for the 1985-2022 showed an upward trend in all lakes. Moreover, paleolimnological reconstructions based on sediment cores studies demonstrate large changes in the last decades in organic carbon accumulation, sediment fluxes and bioproductivity in the Pyrenean lakes. Our research represents the first comprehensive investigation conducted on high mountain lakes in the Pyrenees that compares field monitoring data with satellite-derived temperature records. The results demonstrate the reliability of satellite-derived LSWT for surface temperatures in small lakes, and provide a tool to improve the LSWT in lakes with no monitoring surveys.

An optimized NARX-based model for predicting thermal dynamics and heatwaves in rivers

The thermal dynamics within river ecosystems represent critical areas of study due to their profound impact on overall aquatic health. With the rising prevalence of heatwaves in rivers, a consequence of climate change, it is imperative to deepen our understanding through comprehensive research efforts. Despite this urgency, there remains a noticeable dearth in studies aimed at refining modeling techniques to precisely characterize the duration and intensity of these events. In response to this gap, the present study endeavors to augment the NARX-based model (Nonlinear Autoregressive network with Exogenous Inputs) to enhance predictive capabilities regarding thermal dynamics and river heatwaves. The optimized NARX-based model included the Bayesian Optimization (BO) algorithm, which allows fine-tuning the number of NARX hidden nodes and lagged input/target values, and the Bayesian Regularization (BR) backpropagation algorithm to improve the NARX calibration process. A long-term dataset spanning from 1991 to 2021, encompassing 18 rivers across the expansive Vistula River Basin, one of Europe's largest river systems, was employed for this study. The performance of the BO-NARX-BR model was compared with that of the widely utilized air2stream model for modeling river water temperature (RWT). The results unequivocally demonstrated the superior performance of the NARX-based model across the calibration and validation periods, and four heatwave years. In the context of river heatwaves, the study revealed an escalating frequency and intensity within the Vistula River Basin. Furthermore, the NARX-based model exhibited superior proficiency in characterizing river heatwaves compared to the air2stream model. This study, as the inaugural examination of river heatwaves in Poland and one of the few globally, furnishes crucial reference points for subsequent research endeavors on this phenomenon.

Recent and projected changes in climate patterns in the Middle East and North Africa (MENA) region

Observational and reanalysis datasets reveal a northward shift of the convective regions over northern Africa in summer and an eastward shift in winter in the last four decades, with the changes in the location and intensity of the thermal lows and subtropical highs also modulating the dust loading and cloud cover over the Middle East and North Africa region. A multi-model ensemble from ten models of the Coupled Model Intercomparison Project-sixth phase gives skillful simulations when compared to in-situ measurements and generally captures the trends in the ERA-5 data over the historical period. For the most extreme climate change scenario and towards the end of the twenty-first century, the subtropical highs are projected to migrate poleward by 1.5°, consistent with the projected expansion of the Hadley Cells, with a weakening of the tropical easterly jet in the summer by up to a third and a strengthening of the subtropical jet in winter typically by 10% except over the eastern Mediterranean where the storm track is projected to shift polewards. The length of the seasons is projected to remain about the same, suggesting the warming is likely to be felt uniformly throughout the year.

Influence of hydrological features on CO2 and CH4 concentrations in the surface water of lakes, Southwest China: A seasonal and mixing regime analysis

Due to the large spatiotemporal variability in the processes controlling carbon emissions from lakes, estimates of global lake carbon emission remain uncertain. Identifying the most reliable predictors of CO2 and CH4 concentrations across different hydrological features can enhance the accuracy of carbon emission estimates locally and globally. Here, we used data from 71 lakes in Southwest China varying in surface area (0.01‒702.4 km2), mean depth (< 1‒89.6 m), and climate to analyze differences in CO2 and CH4 concentrations and their driving mechanisms between the dry and rainy seasons and between different mixing regimes. The results showed that the average concentrations of CO2 and CH4 in the rainy season were 23.9 ± 18.8 μmol L-1 and 2.5 ± 4.9 μmol L-1, respectively, which were significantly higher than in the dry season (10.5 ± 10.3 μmol L-1 and 1.8 ± 4.2 μmol L-1, respectively). The average concentrations of CO2 and CH4 at the vertically mixed sites were 24.1 ± 21.8 μmol L-1 and 2.6 ± 5.4 μmol L-1, being higher than those at the stratified sites (14.8 ± 13.4 μmol L-1 and 1.7 ± 3.5 μmol L-1, respectively). Moreover, the environmental factors were divided into four categories, i.e., system productivity (represented by the contents of total nitrogen, total phosphorus, chlorophyll a and dissolved organic matter), physicochemical factors (water temperature, Secchi disk depth, dissolved oxygen and pH value), lake morphology (lake area, water depth and drainage ratio), and geoclimatic factors (altitude, wind speed, precipitation and land-use intensity). In addition to the regression and variance partitioning analyses between the concentrations of CO2 and CH4 and environmental factors, the cascading effects of environmental factors on CO2 and CH4 concentrations were further elucidated under four distinct hydrological scenarios, indicating the different driving mechanisms between the scenarios. Lake morphology and geoclimatic factors were the main direct drivers of the carbon concentrations during the rainy season, while they indirectly affected the carbon concentrations via influencing physicochemical factors and further system productivity during the dry season; although lake morphology and geoclimatic factors directly contributed to the carbon concentrations at the vertically mixed and stratified sites, the direct effect of system productivity was only observed at the stratified sites. Our results emphasize that, when estimating carbon emissions from lakes at broad spatial scales, it is essential to consider the influence of precipitation-related seasons and lake mixing regimes.

Arctic warming drives striking twenty-first century ecosystem shifts in Great Slave Lake (Subarctic Canada), North America's deepest lake

Great Slave Lake (GSL), one of the world's largest and deepest lakes, has undergone an aquatic ecosystem transformation in response to twenty-first-century accelerated Arctic warming that is unparalleled in at least the past two centuries. Algal remains from four high-resolution palaeolimnological records retrieved from the West Basin provide baseline limnological data that we compared with historical phycological surveys undertaken on GSL between the 1940s and 1990s. We document the rapid restructuring of algal community composition ca 2000 CE that is consistent with recent increases in regional air temperature and declines in ice cover and wind speed, that collectively altered habitats for aquatic biota. This new limnological regime initiated the first observation of scaled chrysophytes and favoured the rapid proliferation of small planktonic cyclotelloid diatoms which replaced the long-established dominance of large filamentous Aulacoseira islandica in West Basin sedimentary records. Such abrupt transformations in the primary producers of this socioecologically valuable 'northern Great Lake' may have widespread implications for the entire food web with unknown consequences for aquatic ecosystem functioning and fisheries, which First Nations, Métis and other northern communities depend upon, pointing to the need for new studies.