Deep Lake Research Articles

Quantifying the functional genes of C, N, P, and S cycling in a deep lake: Depth patterns and drivers

Microbial functional composition is important for biogeochemical cycles, and is usually constrained by taxonomic species pool and natural environmental gradients. However, the distribution of functional genes in deep lakes and the driving factors remains elusive. Here, we quantified the abundance of 71 functional genes relevant to carbon degradation, carbon fixation, methane metabolism, nitrogen cycling, phosphorus cycling, and sulfur cycling in 38 sediments along the water depth ranging from 0 to 90 m in Lugu Lake, China, using Quantitative Microbial Ecology Chip (QMEC) and then explored their water-depth diversity pattern and abiotic and biotic drivers. Functional gene diversity showed a hump-shaped pattern along the depth and peaked at around 50 m which is the low boundary of thermocline layer. There were specific environmental preferences among functional gene subgroups such as nitrogen and sulfur cycling genes preferring to deeper and shallow waters, respectively. The dissimilarity of total functional genes increased with water depth distance indicating a distance-decay relationship. There was a congruence between functional and taxonomic composition by showing the positive correlations between the compositions of functional genes and bacteria or archaea. This phenomenon is consistently observed for the six functional gene subgroups, and the congruence strength was highest for nitrogen cycling while lowest for sulfur cycling. Compared to abiotic factors, biotic factors were more relevant to the functional gene diversity and composition. Biotic factors explained 28.5 % of the variance of functional gene diversity, while water depth and other environmental factors such as water total phosphorus and sediment carbon explained 4.2 % and 3.8 %, respectively. For functional gene composition, biotic factors accounted for 25.2 % of the variance, whereas water depth and other environmental factors contributed to 0.7 % and 4.4 %, respectively. Among all explanatory variables for function genes, bacterial composition had the highest contribution, which was further supported by showing its direct effects of 1.45 and 0.86 on functional diversity and composition, respectively. This study for the first time quantified the microbial functional genes along water depth in deep lakes and provided a more comprehensive understanding of the functional dynamics in microbial communities within aquatic ecosystems.

Seasonal Dynamics of the Silica-Scaled Chrysophytes as Potential Markers of Climate Change in Natural Model: Deep Cold Lake–Shallow Warmer Reservoir

In the context of global climate changes, it is important to assess the sustainability perspective of aquatic ecosystems based on marker organisms. In this work, we analyzed seasonal dynamics of silica-scaled chrysophytes in freshwater communicating environments which have considerable differences in water temperature between two ecosystems: the deep and cold oligotrophic Lake Baikal versus the shallower and warmer downstream Irkutsk Reservoir having mesotrophic features. During three seasons of the open water period of 2023, 38 species of silica-scaled chrysophytes were observed at 17 stations using scanning and transmission microscopy. The distribution of silica-scaled chrysophytes was shown to correlate with the water temperature. The greatest species richness was observed in the spring season in a large bay of the Irkutsk Reservoir (23 species), the smallest in the cold spring waters of Southern Baikal (up to 7 species). Widespread species living in Southern Baikal continued to grow in warmer waters of the reservoir. Using the example of silica-scaled chrysophytes, the stability of the high-latitudinal freshwater ecosystems affected by climate change is discussed. Continuous increment of the water temperature can lead to an increased abundance of widespread species and the displacement of psychrophilic species, affecting the overall biodiversity in such ecosystems.

Spatial dynamics of pCO2 and CO2 emissions from eutrophic lakes

Eutrophication of lakes exhibits a more serious trend in a global scale under the climate change. Eutrophic lakes also exhibit a unique role in global carbon, yet, data on carbon dioxide (CO2) emissions from these eutrophic lakes are too scarce to accurately upscale CO2 emissions for large scale or global estimation. This study presented the results of a partial pressure of carbon dioxide (pCO2) survey covering 43 eutrophic lakes in China in the same season across diverse ecosystem types and climate zones. The first estimation of CO2 flux from Chinese eutrophic lakes was reported as − 0.7 to 1.0 Tg C a−1 based on the surveyed pCO2 values, indicating some lakes act as CO2 sinks while others are sources. The results showed that pCO2 in Chinese eutrophic lakes ranged from 0.9 to 1366.4 μatm with the mean of 356.9 ± 310.3 μatm, almost 69.3 % of the sampling sites in eutrophic lakes were undersaturated with CO2 to the atmosphere. The mean pCO2 variations in eutrophic lakes were consistent with the amount of precipitation across different climate zones, with the trend of semiarid > semihumid > humid regions, and all lower than that for atmosphere. Meanwhile, both the mean and median values of pCO2 went down with the decrease of the lake mean depth and the increasing lake area, and the highest value in deep lakes (mean depth > 10 m, 548.3 ± 353.7 μatm) and small lakes (376.6 ± 329.8 μatm). In all the studied eutrophic lakes, pCO2 values showed significant seasonal variations, and in summer and autumn showed an under-saturated seasonal mean value. This study proved that CO2 undersaturation was prevalent in the eutrophic conditions. Due to the CO2 source-sink conversion role of eutrophic lakes, it is recommended to focus on CO2 fluxs from eutrophic lakes in the first place during the research of inland waters carbon emissions.

Causal feedback loops modify lake chlorophyll a–nutrient relationships over two decades of nutrient reductions and climate warming

AbstractUnderstanding how the causal feedback between phytoplankton and environmental drivers controlling the chlorophyll a (Chl a, as a proxy of phytoplankton biomass)–nutrient relationships are modulated under different ecosystem conditions is a major challenge in aquatic ecology. Using an empirical dynamic model (convergent cross mapping) on a 20‐yr dataset on 20 Danish lakes, we quantified hypothesized causal feedback networks for each lake and related them to lake system properties (e.g., mean water depth, nutrient concentrations and extent of reduction, climate warming) vs. the Chl a–nutrient relationship (estimated from generalized least square models). The results showed prevalent causal feedback across the studied lakes, which demonstrated clear patterns for the tested ecosystem variations. Weaker causal feedbacks were found in deeper lakes and lakes with larger warming trends, while stronger causal feedbacks appeared in lakes experiencing greater reductions of TP (total phosphorus) and TN (total nitrogen). Moreover, these causal feedbacks showed a strong and positive coupled pattern. Most of the causal feedbacks worked as enhancement loops, which promote the sensitivity of phytoplankton to TP, not least in shallow lakes with a high TP reduction, and as regulatory loops, which force a shift in the Chl a–TN relationship from a more negative slope in lakes experiencing a high nutrient reduction and weak warming to a positive slope in lakes with low nutrient reduction and stronger warming. Our findings suggest a mechanistic explanation of how internal feedbacks regulate the Chl a–nutrient relationships across a broad gradient of nutrient reductions, climate warming, and lake morphologies.

Phytoplankton in Deep Lakes of the Dinaric Karst: Functional Biodiversity and Main Ecological Features.

Phytoplankton is a polyphyletic group of organisms that responds rapidly to environmental conditions and provides a reliable response to changes, making it a good ecological indicator for water quality monitoring. However, a gradient is almost essential for a reliable relationship between pressure and impact. In a low-gradient environment, ingenuity is required to outsmart the limitations of the commonly used linear relationship. Here, we examine changes in biomass and functional biodiversity by analysing larger data sets (2013-2022) in six ecologically diverse, natural, deep Croatian karst lakes with low nutrient gradients using nonlinear correlation coefficients and multivariate analyses in 209 samples. We found that phytoplankton biomass was most strongly influenced by nutrients, salinity and alkalinity, while light availability and total nitrogen strongly influenced phytoplankton functional biodiversity. An additional analysis of the TN:TP ratio revealed that the oligotrophic Lake Vransko is nitrogen-limited, and lakes Kozjak and Prošće are phosphorus-limited. This further clarified the relationship of phytoplankton to nutrients despite the low gradient. The complex analysis in this study provides a new perspective for predicting changes in the structure and succession of phytoplankton in deep karst lakes for successful management under apparent anthropogenic pressure and climate change.

Variability of winter cooling affects intensity of phytoplankton spring blooms – how resilient is the ciliate assemblage composition to changes in food availability?

After years of partial winter mixing in Lake Zurich (Switzerland), a complete turnover of the water column reoccurred during winter/spring 2021. It was favored by a cold, windy winter and a small difference of water temperatures between the surface zone and a hypolimnion (deep water zone) that had gradually warmed during the previous years. The trend of declining phytoplankton spring blooms due to incomplete winter mixing was interrupted by mass development of algae due to the upwelling of nutrients accumulated in the hypolimnion. The effects of this singular deep mixing on the microbial food web during spring were studied in a high-frequency sampling campaign and compared with data from two years of partial winter mixing (2020 and 2022). A particular focus was put on the quantitative composition of the ciliate assemblage. Our results showed that not all organisms reacted equally to the nutrient (phosphorus) boost in the surface zone. Centric diatoms and cryptophytes profited most directly from the deep mixing, outcompeting the otherwise dominant cyanobacterium Planktothrix rubescens. Heterotrophic bacteria and their top predators, the ‘heterotrophic nanoflagellates’ trophic guild, were less affected by the nutrient supply and showed only short-lived increases of maximal biomass. The assemblage composition of ciliate morphotypes was highly resilient over the three years, presumably due to the range of acceptable food items of the predominant omnivorous species. However, numerous ciliate morphotypes showed brief mass development in 2021, and Balanion planctonicum, small Urotricha species and tintinnids were significantly more frequent than in 2020/2022. Small interception-feeding morphotypes apparently profited from the rich supply of their cryptomonad food, and tintinnid morphotypes additionally benefited from the availability of building material (e.g., centric diatom shells) for their loricae. In summary, we show that effects of lake warming in deep stratifying lakes are not as unidirectional as previously presumed, and we reveal resilience of the pelagic ciliate morphotype assemblage to lake warming related interannual variability in Lake Zurich.

Environmental factors affecting the spatio-temporal distribution of zooplankton functional groups in a deep alkaline lake

Environmental factors affecting the spatio-temporal distribution of zooplankton functional groups in a deep alkaline lake

Lake Champlain Zooplankton Community Dynamics Following an Extreme Flood Event

Lake Champlain, the sixth largest freshwater lake in the U.S., is a deep thermally stratified temperate zone lake system. Recently, flood events have impacted the Northeastern U.S. more frequently than in past decades, resulting in increased turbidity and other impacts in northern temperate lakes. This study represents an unprecedented analysis of the impacts of major spring and summer flooding in 2011 on Lake Champlain zooplankton communities. Few studies exist on flood impacts on lake systems, especially in natural un-impounded lakes. Our results illustrate the impacts of large-scale flooding on planktonic communities in deep stratified temperate lakes and the differential responses among species based on autecological traits. Community responses include flood-adapted increase during the flood event (Ceriodaphnia reticulata and Eubosmina coregoni), a flood-intolerant decline (Asplanchna spp. and Keratella cochlearis) or a delayed flood intolerance (Daphnia retrocurva, Mesocyclops edax, and Polyarthra spp.). Our results suggest that large, temperate lakes such as Lake Champlain will experience community shifts in zooplankton composition during future extreme flood events associated with climate change-related weather patterns in the Northeastern U.S.

Distinct ecological niches and community dynamics: understanding free-living and particle-attached bacterial communities in an oligotrophic deep lake.

Oligotrophic deep-water lakes are unique and sensitive ecosystems with limited nutrient availability. Understanding bacterial communities within these lakes is crucial for assessing ecosystem health, biogeochemical cycling, and responses to environmental changes. In this study, we investigated the seasonal and vertical dynamics of both free-living (FL) and particle-attached (PA) bacteria in Lake Fuxian, a typical oligotrophic deep freshwater lake in southeast China. Our findings revealed distinct seasonal and vertical dynamics of FL and PA bacterial communities, driven by similar physiochemical environmental factors. PA bacteria exhibited higher α- and β-diversity and were enriched with Proteobacteria, Cyanobacteria, Firmicutes, Patescibacteria, Planctomycetota, and Verrucomicrobiota, while FL bacteria were enriched with Actinobacteria and Bacteroidota. FL bacteria showed enrichment in putative functions related to chemoheterotrophy and aerobic anoxygenic photosynthesis, whereas the PA fraction was enriched with intracellular parasites (mainly contributed by Rickettsiales, Chlamydiales, and Legionellales) and nitrogen metabolism functions. Deterministic processes predominantly shaped the assembly of both FL and PA bacterial communities, with stochastic processes playing a greater role in the FL fraction. Network analysis revealed extensive species interactions, with a higher proportion of positively correlated edges in the PA network, indicating mutualistic or cooperative interactions. Cyanobium, Comamonadaceae, and Roseomonas were identified as keystone taxa in the PA network, underscoring potential cooperation between autotrophic and heterotrophic bacteria in organic particle microhabitats. Overall, the disparities in bacterial diversity, community composition, putative function, and network characteristics between FL and PA fractions highlight their adaptation to distinct ecological niches within these unique lake ecosystems.IMPORTANCEUnderstanding the diversity of microbial communities, their assembly mechanisms, and their responses to environmental changes is fundamental to the study of aquatic microbial ecology. Oligotrophic deep-water lakes are fragile ecosystems with limited nutrient resources, rendering them highly susceptible to environmental fluctuations. Examining different bacterial types within these lakes offers valuable insights into the intricate mechanisms governing community dynamics and adaptation strategies across various scales. In our investigation of oligotrophic deep freshwater Lake Fuxian in China, we explored the seasonal and vertical dynamics of two bacterial types: free-living (FL) and particle-attached (PA). Our findings unveiled distinct patterns in the diversity, composition, and putative functions of these bacteria, all shaped by environmental factors. Understanding these subtleties provides insight into bacterial interactions, thereby influencing the overall ecosystem functioning. Ultimately, our research illuminates the adaptation and roles of FL and PA bacteria within these unique lake environments, contributing significantly to our broader comprehension of ecosystem stability and health.

Mineralogical and geochemical characteristics of mudstones from the Lower Cretaceous Prosopis Formation in the Bongor basin, Chad: Implications for provenance, paleoenvironment and organic matter enrichment

The Cretaceous greenhouse represents the largest global warming event since the Phanerozoic, significantly affecting the provenance, weathering, regional climate, and paleoenvironment of lake basins globally. During the period of global warming, the control of regional paleoenvironment over organic matter enrichment is still unclear. The Lower Cretaceous Prosopis Formation in the Bongor Basin features thick lacustrine organic-rich mudstones as major effective hydrocarbon source rocks with excellent hydrocarbon potential in the basin. However, the provenance, paleoclimate, paleoenvironment and mechanism of organic matter accumulation in this location have not yet been systematically studied. In this study, 22 dark gray mudstone samples were collected from three cored wells. Clay mineralogy, along with organic and inorganic elemental geochemical analyses were jointly carried out to determine the characteristics of the provenance, paleoclimate and paleoenvironment as well as the major controlling factors for organic matter enrichment. The results imply that the Prosopis Formation mudstones are mainly composed of clay minerals such as illite, kaolinite, chlorite, and illite/smectite mixed layers (I/S). Organic geochemistry indicated that all the studied mudstones reached the standard of good to excellent source rocks (the total organic carbon content ranged from 1.27% to 7.41%). Inorganic elemental geochemistry is characterized by high enrichment of Fe, Mg, P, Mn, Ti, Na, Ca, V, Co, Cu, Sr, Ba and B relative to UCC and PAAS, whereas rare earth elements (REEs) show enrichment of light rare earth elements (LREEs) and a deficiency of heavy rare earth elements (HREEs) relative to chondrite. The source composition of the mudstones is predominantly intermediate-felsic igneous rocks that have not undergone sediment recycling. The parent rocks experienced weak to moderate weathering, corresponding to semiarid paleoclimatic conditions of transitional nature. The organic-rich mudstones were deposited in fresh to brackish waters with paleoredox conditions ranging from weakly oxic to anoxic semi-deep to deep lake environments having relatively low initial productivity. Comprehensive study indicates that paleoredox and paleoclimate are important parameters affecting the enrichment of organic matter in lake basins during global warming. The aridification of climate is one of the most important reasons for the high input and efficient preservation of organic matter. The relatively semiarid climate both increased the supply of terrigenous nutrients to the lake basins and provided a driving force for the increase in salinity, thereby providing better sources and preservation conditions for organic matter.

Sources and composition of organic matter as a tool for understanding the complex variation in paleoenvironments and the connectivity of an epicontinental basin: The Miocene in the northern Pannonian Basin

Epicontinental basins are extremely prone to major paleogeographic changes, and this will directly affect any organic matter (OM) preserved in the depositional record. In this study the Middle–Late Miocene successions in the northern Pannonian Basin System were investigated via sedimentological, petrographic, and geochemical analyses of cores from the Danube Basin to reveal the interplay of factors driving the character of the OM. In the late Middle Miocene (~12.3 Ma), the Central Paratethys Sea maintained normal marine salinity, with dysoxic bottom waters in a distal basin floor environment rich in aquatic OM. The last rifting phase followed during the Late Miocene and led to formation of the deep Lake Pannon. Like seawater, the brackish lake water still also contained sulfate. These open lacustrine deposits (~11.6–10.0 Ma) reveal OM sourced from submerged/floating macrophytes and algae, and humid conditions are indicated by the preponderance of deciduous trees and shrubs on shores. The study identifies hybrid event beds (HEBs) on Lake Pannon's floor (~10.0–9.3 Ma), with currents redepositing mud and OM, resulting in similarities between the Middle Miocene and Late Miocene successions. Turbidite deposition (~9.3–9.0 Ma) from the paleo-Danube induced a shift in OM, replacing algae with terrestrial input. Complete isolation from the main water masses of Lake Pannon (~9.0–8.9 Ma) altered its sources of OM, transitioning from algae to macrophytes, and caused a drop in salinity, likely associated with a humidity peak. The subsequent deltaic dominance (~8.9–8.6 Ma) features well-developed topset lakes, swamps, and floodplain forests, reflecting warm temperate to subtropical climates. The Middle–Upper Miocene deposits studied here are source rocks with fair to very good richness and poor to fair generative potential, and contain kerogen types III and IV, while type II is rare. The rapid paleoenvironmental changes observed over the order of ~100 kyr caused the complete switching of OM type and delivery, giving an indication of the complexity of epicontinental basins.

Year-round sampling of the fish community in a boreal lake: differences between summer and winter influence estimates of species composition, catch, and fish size.

Boreal lakes experience pronounced seasonal variation in abiotic factors, especially light, temperature, and oxygen. A deep boreal humic lake was sampled year-round to test putative changes in total fish catch, species composition, catch-per-unit-effort (CPUE), habitat use, fish size, and condition. Monthly sampling was conducted in Lake Pääjärvi, southern Finland, during one full year in 2020-2021 as well as in March and August 2021 and 2022. The fish community was dominated by cyprinid species in all months, but the percentage of percid fish caught increased during the warm summer period. Most fish were caught in littoral habitats and the highest catches occurred in summer, but some species (e.g., ruffe, Gymnocephalus cernua, and pikeperch, Sander lucioperca) remained abundant in the winter catch. The body size of fish was larger in the winter catch, while condition factor was higher in summer for most species. Fish species proportions in total catch, CPUE, and average size of fish were closest to the annual mean values in September, which may be used as the optimal period to monitor fish communities of similar deep boreal lakes. Our findings highlight the need for year-round research to reveal the impacts of rising temperatures and diminishing ice-covered periods in fish communities and lake food webs.

Prediction of long-term changes of weak diurnal stratification in shallow lakes using artificial neural networks

ABSTRACT Thermal stratification plays a key role in lakes' ecosystems. In contrast to deep lakes, the thermal structure of shallow polymictic lakes is characterized by a weak stratification with an apparent diurnal cycle. Long-term changes in stratification are governed by climate change and anthropogenic effects such as water level regulation. We developed a simple and robust model system consisting of an energy balance model to estimate depth-averaged water temperatures and an artificial neural network (ANN) model to predict stratification with high temporal resolution. One novelty of our approach is that instead of directly estimating water temperatures at different depths, we simulated the potential energy anomaly index, the indicator of stratification's strength. The ANN-based model's performance was assessed against a physical-based one-dimensional model (General Ocean Turbulence Model) by modeling a 40-year-long period from 1981 to 2020. The new model accurately predicts a shallow lake's weak stratification and its diurnal cycle. Besides, the model proved reliable on longer time scales, capturing the effect of climate change, anthropogenic water level regulation, and their synergistic interaction on the change of stratification's intensity and duration.

Long-term climate warming and extreme cold events driving ecological shifts in a deep oligo-mesotrophic lake.

Deep lakes are critical for freshwater storage, yet they are struggling against major ecological issues from climate change and nutrient pollution. A comprehensive understanding of internal feedback mechanisms is crucial for regulating nutrients in these lakes. A five-year study was conducted on the diatom community and environment in Lake Fuxian, China's largest deep freshwater lake, which is becoming eutrophic. The results indicate a shift in the diatom community from a stable state dominated by a single species to a rapid seasonal fluctuation, and there is a significant increase in diatom biomass. Specifically, stable stratification and low nutrient concentrations are limiting the growth of diatom biomass and maintaining the dominance of Cyclotella. Nutrients in the hypolimnion were replenished in the epilimnion during the extreme cold of winter, triggering a shift in the diatom community. This shift may imply that future climate change will exacerbate the positive feedback of hypoxia-nutrient release of algal blooms, potentially triggering a regime shift in the ecosystem of the entire lake. This study underscores the fact that climate change alters the internal feedback mechanisms of deep lakes, reducing ecosystem stability, and provides a scientific basis for further clarification of protection measures for deep lakes.

Understanding stratification and turnover dynamics of a tropical lake using extensive field observations and 3D hydrodynamic simulations

The stratification and turnover dynamics of a tropical lake were evaluated using field observations and 3D hydrodynamic simulations. Located in the Philippines, Sampaloc Lake is a 104-ha and 27-m deep volcanic crater lake with enclosed watershed, which is at risk of the impacts of intensive aquaculture, rapid urbanization and climate change. Temperature, dissolved oxygen (DO) and chlorophyll-a (Chl-a) were measured at seven sampling stations using a multiprobe. Kruskal-Wallis test revealed that the three parameters are not significantly different among stations, indicating that one sampling station can represent the water quality of the whole lake. Schmidt's Stability Index (SSI) and thermocline strength, together with DO and Chl-a gradients decreased from October 2022 (stratified) to January 2023 (turnover). After successfully verifying the 3D numerical model, sensitivity analyses of water temperature to varying weather, together with particle tracking simulations, were implemented to determine the timing of isothermal state, upwelling, partial mixing, and full turnover. Compared to air temperature, variations in wind speed have more pronounced effects on the delay or progression of isothermal conditions in the lake based on SSI, Lake Number and Wedderburn Number. Isothermal conditions do not necessarily coincide with the timing of full turnover, with the latter being delayed by two days than the former, on average. Results revealed that full turnover can occur several weeks earlier with the decrease in AT and increase in WS. This study can advance the understanding of thermal and turnover dynamics of stratified tropical lakes, leading to better management of the water quality of these water bodies.

Response of aquatic ammonia-oxidizing archaea to thermal stratification and nutrient levels since the Last Glacial Maximum in the deep lake Fuxian, southwestern China

Ammonia-oxidizing archaea (AOA) are the critical nitrifier in lake ecosystems and participate in the global nitrogen cycle. However, the response of aquatic AOA to long-term climate change remains unclear. Here, we obtained a continuous sediment record from a deep, oligotrophic lake (Lake Fuxian, China) and reconstructed changes in AOA productivity over the past 26 thousand years (cal ka BP) using a multi-proxy approach. AOA productivity (estimated using diagnostic archaeal lipids) was low during the Last Glacial Maximum (LGM) before peaking during the cold-dry Heinrich event 1 (H1, 17.7–15.9 cal ka BP). As the Indian Summer Monsoon (ISM) intensified and the climate became warmer and wetter, aquatic AOA productivity progressively declined. We show that a combination of factors modulates millennial-scale AOA productivity. In particular, low lake levels under cold-dry climate conditions strengthen lake mixing and nutrient upwelling, promoting AOA productivity; in contrast, high lake levels during warm-humid climate conditions weaken upwelling, resulting in reduced AOA blooms. Changes in terrestrial nutrient availability also exert a secondary control on aquatic primary production and by extension, AOA productivity. We also show that anthropogenic human activities can potentially alter AOA productivity levels due to climate and land-use change.

Statistical analysis of the Hurst index indicating sedimentary processes' evolution trend in the deep lacustrine mudrock sequences: A case study of the Jiyang Depression, Bohai Bay Basin, eastern China

Quantitative identification of sedimentary processes' evolution trend has substantially influenced the exploration of lacustrine mudrocks. To explore the indicative applicability of the Hurst index (HI) for analyzing the sedimentary processes' evolution trend in a deep lacustrine sequence, lacustrine mudrocks from wells L69, FY1, and NY1 in the Jiyang Depression, eastern China are selected. The comparable fourth-order cycle is divided through lithofacies observation, total organic carbon (TOC), and natural gamma ray (GR) logs combined with previous studies. In addition, the variation of accommodation space is selected to represent the sedimentary processes' evolution trend in the lacustrine environment, characterized by the frequency of oscillation (FST) and directionality (D). Further, HI is introduced to identify the long-term persistence of the sedimentary processes' evolution trend in the lacustrine environment. Through the criteria for the series selections, the TOC series and GR logs passed the Hurst phenomenon test, and the computed HI of those two series was selected to evaluate its effectiveness toward the lacustrine sedimentary processes' evolution trend. The HI computed from the TOC series demonstrates strong negative linear relationships with FST and D, with R2 in 0.833 and 0.814, respectively. Besides, the HI from the more accessible GR logs demonstrates robust negative linear relationships with FST and D, with R2 in 0.981 and 0.976, respectively. The statistical analysis indicates that the content of deep lake subfacies, shale, and mudstone could affect the long-term persistence of the lacustrine mudrock sequence. Thus, the HI could be an alternative indicator to identify the imperceptible sedimentary processes' evolution trend in the deep lacustrine environment.

The ice phenology as a predictor of Planktothrix rubescens bloom in vegetation season in temperate lakes.

Global warming affects air and water temperatures, which impacts the phenology of lakes and aquatic ecosystems. These changes are most noticeable during winter, when the potentially toxic Planktothrix rubescens forms its inoculum for annual blooms. Mostly, research has been conducted on alpine lakes, where blooms have persisted for decades, while a few have focused on temperate lakes. Our study aimed to determine the factors influencing the dynamics of the development of P. rubescens in temperate lakes where blooms occasionally occur, with a particular emphasis on the role of ice phenology. We investigated the vertical distribution of P. rubescens in an annual cycle in three temperate lakes. Samples were collected monthly in the winter and biweekly during the vegetative seasons. Overall, 434 samples were collected and analyzed according to biological and chemical parameters. Physical parameters were measured in situ. The vegetation seasons in temperate lakes showed a similar development pattern in the P. rubescens population as that in alpine lakes. Our results also show the influence of physical and chemical factors on the vertical distribution of this cyanobacterium. These results revealed the significant impact of P. rubescens filaments on phytoplankton biodiversity and biomass. Our data show the role of ice phenology in the establishment of the winter inoculum of P. rubescens and its further mass development until its disappearance in autumn. A climate-zone-independent pattern of P. rubescens blooms was observed during the vegetation periods. The population of P. rubescens was more influenced by physical factors than by the availability of dissolved nutrients in the water. Despite the same etiology, global warming has been shown to cause different responses in aquatic ecosystems, which affect the different nature of P. rubescens appearances. We associated blooms in temperate lakes, in contrast to alpine lakes, mainly with the presence of ice cover during severe winters, when the species establishes its inoculum. Hence, blooms in temperate lakes occur at different time intervals. Therefore, the dynamics of periodic blooms of P. rubescens in temperate lakes provide novel knowledge to the case study and a counterpoint to permanent blooms found in deep alpine lakes.

Integrated geochemical and microbiological assessments of Astroni lakes reveals Campi Flegrei unrest signatures

Astroni volcano in the Campi Flegrei caldera (southern Italy) is a 2 km wide, densely vegetated tuff ring and hosting several volcanic structures, including tuff cones, scoriae cones, lava domes, and three small lakes. Geochemical data of waters and dissolved gases from the lakes, coupled with microbiological analyses on lake water and sediments, were used to shed light on the possible relationship between the lakes and the hydrothermal fluid circulation system as suggested by previous geophysical surveys. Water chemistry was dominated by solutes, mainly Na+ and HCO3−, deriving from fluids and CO2-rich gases typically found in discharges located at the periphery of hydrothermal-volcanic systems. Lago Grande (LG) lake showed an anoxic hypolimnion with abundant non-atmospheric dissolved gases, consisting of biogenic CH4 and CO2, the latter having a twofold origin, biogenic and hydrothermal. The occurrence of anaerobic methanotrophs coupled with the lack of hydrogenotrophic methanogenic archaea along the whole vertical profile of LG suggested that CH4 was mostly produced from degradation of abundant terrestrial organic matter within the deep lake sediments, and then consumed during its diffusion through the lake. Notwithstanding, the output rate of CH4 from LG surface was anomalously high relative to those commonly measured in lakes. Carbon dioxide from the hydrothermal source and produced by CH4 oxidation was partially fixed in the lake via the acetyl-CoA pathway. Accordingly, the CO2 fluxes from the LG surface were relatively low, in the range of those measured in volcanic lakes dominated by biogenic CO2. The dependence of the chemistry of the Astroni lakes on inputs from the Campi Flegrei hydrothermal system, besides on biogeochemical processes, offers a possible explanation for the anomalous increase of the LG water level occurred in the last years, which was not consistent with the recorded local rainfall but likely caused by an increasing hydraulic pressure related to the enhanced hydrothermal activity recorded at Campi Flegrei in the last decades. According to this hypothesis, the future evolution of the current volcanic unrest may govern the fate of the lake water level with important implications for the functioning of the precious Astroni ecosystem.

Paleoclimatic and paleogeographic significance of the early Santonian ice-rafted dropstones in the Songliao Basin, NE China

Two ice-rafted dropstones were discovered in the Yaojia Formation (early Santonian) of wells Ao14 and F247 in the Songliao Basin, northeastern China. The dropstone from well Ao14 was further studied using computed tomography (CT) scanning, sporopollen, carbon and oxygen isotope, and zircon U–Pb isotope analyses. The results show that the dropstone is composed of quartz monzonite and has an age of 178.0 ± 3.3 Ma. Based on the bedrock age analysis around the Songliao Basin, it is suggested that the dropstone in well Ao14 may have originated from the Zhangguangcai Range. The palynoflora assemblage indicates a mixed survival of cold and warm palynomorphs. The carbon and oxygen isotope compositions of the synsedimentary calcareous clasts surrounding the dropstone exhibit obvious negative biases, suggesting that glacier meltwater flowed into the paleo-lake, probably due to the significant topographic height difference between the Songliao Basin and adjacent areas during the early Santonian. Additionally, it can be inferred that coastal mountains formed in the Zhangguangcai Range area, which is located southeast of the Songliao Basin, and the dropstones derived from this region. During the cold season, the dropstones may be transported to the edge of Songliao Lake by avalanche and mixed with carbonate debris. As the seasons changed, the lake ice broke into pieces and carrying the dropstones and carbonate debris into the deep lake area. The present research provides direct evidence of a brief global cold event and the presence of high mountains in the southeastern Songliao Basin during the early Santonian in the Cretaceous greenhouse world.