Stratification Period Research Articles

Unraveling Interactions between Asymmetric Tidal Turbulence, Residual Circulation, and Salinity Dynamics in Short, Periodically Weakly Stratified Estuaries

AbstractAsymmetric tidal turbulence (ATT) strongly influences estuarine health and functioning. However, its impact on the three-dimensional estuarine dynamics and the feedback of water motion and salinity distribution on ATT remain poorly understood, especially for short estuaries (estuarine length ≪ tidal wavelength). This study systematically investigates the abovementioned interactions in a short estuary for the first time, considering periodically weakly stratified conditions. This is done by developing a three-dimensional semi-analytical model (combining perturbation method with finite element method) that allows a dissection of the contributions of different processes to ATT, estuarine circulation, and salt transport. The generation of ATT is dominated by (i) strain-induced periodic stratification and (ii) asymmetric bottom-shear-generated turbulence, and their contributions to ATT are different both in amplitude and phase. The magnitude of the residual circulation related to ATT and the eddy viscosity–shear covariance (ESCO) is about half of that of the gravitational circulation (GC) and shows a “reversed” pattern as compared to GC. ATT generated by strain-induced periodic stratification contributes to an ESCO circulation with a spatial structure similar to GC. This circulation reduces the longitudinal salinity gradients and thus weakens GC. Contrastingly, the ESCO circulation due to asymmetric bottom-shear-generated turbulence shows patterns opposite to GC and acts to enhance GC. Concerning the salinity dynamics at steady state, GC and tidal pumping are equally important to salt import, whereas ESCO circulation yields a significant seaward salt transport. These findings highlight the importance of identifying the sources of ATT to understand its impact on estuarine circulation and salt distribution.

Upwelling-induced trace gas dynamics in the Baltic Sea inferred from 8 years of autonomous measurements on a ship of opportunity

Abstract. Autonomous measurements aboard ships of opportunity (SOOP) provide in situ data sets with high spatial and temporal coverage. In this study, we use 8 years of carbon dioxide (CO2) and methane (CH4) observations from SOOP Finnmaid to study the influence of upwelling on trace gas dynamics in the Baltic Sea. Between spring and autumn, coastal upwelling transports water masses enriched with CO2 and CH4 to the surface of the Baltic Sea. We study the seasonality, regional distribution, relaxation, and interannual variability in this process. We use reanalysed wind and modelled sea surface temperature (SST) data in a newly established statistical upwelling detection method to identify major upwelling areas and time periods. Large upwelling-induced SST decrease and trace gas concentration increase are most frequently detected around August after a long period of thermal stratification, i.e. limited exchange between surface and underlying waters. We found that these upwelling events with large SST excursions shape local trace gas dynamics and often lead to near-linear relationships between increasing trace gas levels and decreasing temperature. Upwelling relaxation is mainly driven by mixing, modulated by air–sea gas exchange, and possibly primary production. Subsequent warming through air–sea heat exchange has the potential to enhance trace gas saturation. In 2015, quasi-continuous upwelling over several months led to weak summer stratification, which directly impacted the observed trace gas and SST dynamics in several upwelling-prone areas. Trend analysis is still prevented by the observed high variability, uncertainties from data coverage, and long water residence times of 10–30 years. We introduce an extrapolation method based on trace gas–SST relationships that allows us to estimate upwelling-induced trace gas fluxes in upwelling-affected regions. In general, the surface water reverses from CO2 sink to source, and CH4 outgassing is intensified as a consequence of upwelling. We conclude that SOOP data, especially when combined with other data sets, enable flux quantification and process studies addressing the process of upwelling on large spatial and temporal scales.

Paleoceanographic significance of Globigerinoides ruber (white) morphotypes from the Andaman Sea

Globigerinoides ruber (white), a tropical to sub-tropical planktonic foraminiferal species which dwells in the mixed layer, has garnered significant attention of paleoceanographic investigations. Recent studies on G. ruber (w) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) have inferred differences in seasonal calcification depths and offsets in isotopic and elemental data. To decipher if the morphotype abundances vary in response to changes in Indian Summer Monsoon (ISM), two well-dated sediment cores from the Andaman Sea, a core monsoon area in the northeastern Indian Ocean, are investigated. The abundance variations of G. ruber s.s. are very similar to total G. ruber (w), whereas G. ruber s.l. is very sensitive to the hydrologic changes linked to the ISM variability. Increased abundances of G. ruber s.l. were observed during periods of weak stratification and the resultant increase in productivity. The last glacial maximum (LGM), deglacial, Younger Dryas (YD) and late Holocene periods with weak ISM are characterized by a high abundance of G. ruber s.l. Conversely, this morphotype abundance decreased from 30 to 23 ka, Bølling/Allerød (B/A), early and mid-Holocene when the ISM was enhanced. The large variation of G. ruber morphotypes in the Andaman Sea is attributed to the selective preference of nutrient-rich water which varied with water column changes (salinity-controlled stratification) associated with ISM evolution. The elevated abundances of G. ruber s.l. during the glacial period closely match with the available records from other regions, possibly making them potential climatic markers.

Seed Dormancy and Soil Seed Bank of the Two Alpine Primula Species in the Hengduan Mountains of Southwest China.

The timing of germination has long been recognized as a key seedling survival strategy for plants in highly variable alpine environments. Seed dormancy and germination mechanisms are important factors that determining the timing of germination. To gain an understanding of how these mechanisms help to synchronize the germination event to the beginning of the growing season in two of the most popular Primula species (P. secundiflora and P. sikkimensis) in the Hengduan Mountains, Southwest China, we explored their seed dormancy and germination characteristics in the laboratory and their soil seed bank type in the field. Germination was first tested using fresh seeds at two alternating temperatures (15/5 and 25/15°C) and five constant temperatures (5, 10, 15, 20, and 25°C) in light and dark, and again after dry after-ripening at room temperature for 6 months. Germination tests were also conducted at a range of temperatures (5–30, 25/15, and 15/5°C) in light and dark for seeds dry cold stored at 4°C for 4 years, after which they were incubated under the above-mentioned incubation conditions after different periods (4 and 8 weeks) of cold stratification. Base temperatures (T b) and thermal times for 50% germination (θ 50) were calculated. Seeds were buried at the collection site to test persistence in the soil for 5 years. Dry storage improved germination significantly, as compared with fresh seeds, suggesting after-ripening released physiological dormancy (PD); however, it was not sufficient to break dormancy. Cold stratification released PD completely after dry storage, increasing final germination, and widening the temperature range from medium to both high and low; moreover, the T b and θ 50 for germination decreased. Fresh seeds had a light requirement for germination, facilitating formation of a persistent soil seed bank. Although the requirement reduced during treatments for dormancy release or at lower alternating temperatures (15/5°C), a high proportion of viable seeds did not germinate even after 5 years of burial, showing that the seeds of these two species could cycle back to dormancy if the conditions were unfavorable during spring. In this study, fresh seeds of the two Primula species exhibited type 3 non-deep physiological dormancy and required light for germination. After dormancy release, they had a low thermal requirement for germination control, as well as rapid seed germination in spring and at/near the soil surface from the soil seed bank. Such dormancy and germination mechanisms reflect a germination strategy of these two Primula species, adapted to the same alpine environments.

Mechanism of nitrous oxide (N2O) production during thermal stratification of a karst, deep-water reservoir in southwestern China

Reservoirs are hotspots of greenhouse gas (GHG) emission, and the global warming potential of nitrous oxide (N2O) is higher than that of CO2 and CH4. To promote our understanding of the relationship of biotic and abiotic environments with the mechanism of N2O production in a quintessential karst reservoir, water samples were collected in a weak thermal stratification period (WTSP) and a strong thermal stratification period (STSP) in 2019 in the Longtan (LT) Reservoir. In the LT Reservoir, higher N2O concentrations and functional gene abundances (nitrification and denitrification genes) were found in the STSP than in the WTSP. In detail, the N2O concentrations were 10.45 ± 6.99 nmol L−1 and 23.77 ± 7.06 nmol L−1 in the WTSP and STSP, respectively; the functional gene abundances were higher in the STSP than in the WTSP in each water layer. During the STSP, the nitrification gene abundances were higher at a 30 m depth than at a 100 m depth, while denitrification gene abundances were higher at 100 m than at 30 m. Along the river, N2O flux was significantly higher in the LT Reservoir’s released water than in upstream water in the STSP, while it exhibited no significant difference in whole river-reservoir system in the WTSP. In conclusion, stratification and nitrogen (N) loading facilitated higher N2O production in STSP than in WTSP; different N transformation processes and sources of N2O were present between the water layers above and below the thermocline (in deep water); finally, the river-reservoir system here acted as a source of N2O release to the atmosphere in the STSP, while it exhibited different emission patterns in the WTSP. This work highlights the mechanism of N2O production and functional gene abundances in water layers during different thermal stratification periods, which will help to elucidate the biotic and abiotic environment in a karst deep-water reservoir and will help to inform N2O production mechanisms in other reservoirs worldwide. More sampling campaigns and the denitrification potential of sediment were worthy of conducting to comprehend the mechanisms of N2O production from stratification to turnover period in the reservoir.

Hydrodynamic and ecological 3D modeling in tropical lakes

Conservation and improvement of water quality in water bodies is an important matter to maintain all of its uses as well as other human necessities like microclimate regulation and leisure. Lakes and reservoirs have a complex circulation behavior with vertical temperature profiles changes along the time, resulting in differences in water density and a vertical stratification condition. This characteristic can directly affect the water quality conditions perturbing its main indicators. This study aims to evaluate the quasi-3D models' capacity to represent the hydrodynamic behavior of a tropical lake and its effects on the main variables that characterize its water quality. To achieve this objective, high-frequency monitoring data were collected, the lake was represented in a quasi-3D model, and the accuracy of the result was evaluated by applying statistical indices. The evaluation showed good agreement between field measures and simulated results when compared with other applications. The connections between hydrodynamic behavior and water quality were seen with the simulations results analysis, which showed that mixing events and long stratification periods perturb the water quality, the first with re-suspended bed material and the second blocking the surface and bottom exchanges. The application of a 3D model gives the capacity to reproduce the reservoir spatial variability and its vertical profiles, which is necessary to study the constituents' distributions across the water column. Therefore, the hydrodynamic and water quality behavior of lakes was accurately represented by the model, as well as the importance of improving high-frequency monitoring techniques.

Climate Change and Teleconnections Amplify Lake Stratification With Differential Local Controls of Surface Water Warming and Deep Water Cooling

AbstractTemperate lakes are experiencing increases in warming and stratification duration while global‐scale teleconnections potentially exacerbate effects of climate change. We examined interannual surface and deep water temperature change and stratification phenology in a long‐term, weekly data set (1985–2017) from a dimictic lake in New York State, USA. We developed a metric, called mixing action, to capture multiple facets about the stratified period. We found warming in surface waters and cooling in deep water. Surface warming was positively correlated with air temperature. Deep water cooling was positively correlated with spring mixing period length and deep water spring temperature, indicating a lag effect. Mixing action was correlated with global temperature anomaly and spring North Atlantic Oscillation index. With increasing summer stratification strength and length, dimictic lakes like Mohonk may continue shifting toward monomictic mixing regimes with increasing summer hypolimnetic anoxia and changing lake biogeochemistry, productivity, and habitat for aquatic organisms.

ИСПОЛЬЗОВАНИЕ САЛИЦИЛОВОЙ КИСЛОТЫ В ПРОИЗВОДСТВЕ ПРИВИТЫХ ВИНОГРАДНЫХ САЖЕНЦЕВ

The aim of the study was to evaluate the ef-fect of salicylic acid on the processes of re-generation and root formation in vine grafts during the stratification period. The efficiency of treatment of copulation sections on root-stock cuttings of vines with Salicylic acid (SC) solution was studied) (10-2, 10-4, 10-6, 10-8) during the production of grafted grapevine seedlings. For setting up the experiment, we used the grafted grapevine variety Cabernet Sauvignon, the rootstock - Kober 5BB. In some variants of the experiment, a reduction in the period of vaccination stratification was noted, due to the activation of the process of wound tissue formation. At the same time, the use of salicylic acid stimulated the budding of the scion, the growth of shoots and the devel-opment of the root system. With an increase in the concentration of the solution to 10-2, a slowdown in the growth of shoots was noted. The present results indicate that salicylic acid, being an endogenous stimulator of growth processes, can be used in nursery production.

Dependence of the Photosynthetic Quantum Yield on Phytoplankton Light Absorption: Equations for Assessing Primary Production in the Black Sea

Purpose. Investigations were performed during a scientific cruise to characterize hydrophysical properties, chlorophyll a concentration, photosynthesis-irradiance curves, spectral light absorption coefficients by phytoplankton, and spectral quantum downwelling irradiance. From these results, the dependence of the photosynthetic quantum yield upon environmental factors was studied with the purpose of adapting an algorithm developed for the Baltic Sea so that it can be applied for the Black Sea. Methods and Results. Complex hydrophysical and biological studies were carried out at several depths within the euphotic zone. Spectral bio-optical parameters were measured in accordance with the latest NASA protocols (2018). Experiments to determine the photosynthesis-light relationship were performed under temperature and light conditions similar to those in situ. The quantum yield of photosynthesis was calculated from parameters of photosynthesis-light curves (photosynthesis efficiency, light saturation parameter) and the spectral light absorption coefficients by phytoplankton pigments. It was found out that the main photosynthetic characteristics of phytoplankton, including the maximum photosynthetic quantum yield and the fraction of phytoplankton absorption associated with photoprotective accessory pigments, varied with depth within the euphotic zone, due to phytoplankton acclimation to environment factors during the period of seasonal stratification. The relationship between the photosynthetic quantum yield and the quanta absorbed by photosynthetically active phytoplankton pigments was revealed. The results of this research made it possible to build on the approach developed for other regions and modify the equation for calculating the quantum yield to apply specifically for environmental conditions in the Black Sea. Conclusions. For the first time, comprehensive studies carried out in the Black Sea, including measurements of the photosynthesis-light dependence, spectral light absorption coefficients by phytoplankton and spectral downwelling irradiance as a function of optical depths within the euphotic zone, made it possible to reveal the equation for calculating photosynthetic quantum yield. This new equation can be applied for calculating primary production of the Black Sea using a spectral approach, based both on the results of in situ measurements and remote sensing data.

Assessing the Nutrient Dynamics in a Himalayan Warm Monomictic Lake

Thermal stratification is considered to be the most important limnological feature of deep lake ecosystems affecting a water column’s chemical characteristics thereby directly connected with lake management. The lake is also attracting the attention of management authorities, being a growing tourist destination with sustainable environmental management plan. Lake Manasbal is only true warm monomictic urban lake in Kashmir and remains thermally stratified for 8–9 months, with separate layers of epilimnion, metalimnion and hypolimnion.We noticed that in the lower part of the hypolimnion, the process of stratification is characterized by an oxygen deficit (O2 < 1 mg/L). The rate of oxygen depletion reported in the lake’s bottom layers is indicative of the water body’s more eutrophic nature. A clinograde type of pH curve with a wide range of fluctuation from top to bottom layers of water was observed. Thermal stratification of the lake allows ammonium and phosphates to pool in the hypolimnion for much of the stratification period, resulting in concentrations higher than in the epilimnion. Consequently, the lake was characterized by a nutrient-enriched hypolimnion and an epilimnion deprived of nutrients. With the advent of the lake stratification, the thermocline was formed on 15th March between 2 and 3 m and on 7th April between 4 and 6 m, which gradually moved down to 10 m on 4th July and remained at that depth until circulation. The concentration of phosphorus and nitrogen in Lake Manasbal showed an increase towards the bottom, thus revealing an inverse clinograde depth distribution. Due to stratification phenomena in this lake, the internal release and subsequent distribution of nutrients throughout water column especially phosphorus from sediment water interface after mixing is a cause of concern for management authorities as it has the potential to diminish the ecological and aesthetics of the lake.Therefore, we argue that this study will provide some valuable insights on how stratification is affecting the nutrient dynamics and will offer inputs for better management accordingly both from ecological and economic perspectives.

Terrestrial Inputs Shape Coastal Bacterial and Archaeal Communities in a High Arctic Fjord (Isfjorden, Svalbard).

The Arctic is experiencing dramatic changes including increases in precipitation, glacial melt, and permafrost thaw, resulting in increasing freshwater runoff to coastal waters. During the melt season, terrestrial runoff delivers carbon- and nutrient-rich freshwater to Arctic coastal waters, with unknown consequences for the microbial communities that play a key role in determining the cycling and fate of terrestrial matter at the land-ocean interface. To determine the impacts of runoff on coastal microbial (bacteria and archaea) communities, we investigated changes in pelagic microbial community structure between the early (June) and late (August) melt season in 2018 in the Isfjorden system (Svalbard). Amplicon sequences of the 16S rRNA gene were generated from water column, river and sediment samples collected in Isfjorden along fjord transects from shallow river estuaries and glacier fronts to the outer fjord. Community shifts were investigated in relation to environmental gradients, and compared to river and marine sediment microbial communities. We identified strong temporal and spatial reorganizations in the structure and composition of microbial communities during the summer months in relation to environmental conditions. Microbial diversity patterns highlighted a reorganization from rich communities in June toward more even and less rich communities in August. In June, waters enriched in dissolved organic carbon (DOC) provided a niche for copiotrophic taxa including Sulfitobacter and Octadecabacter. In August, lower DOC concentrations and Atlantic water inflow coincided with a shift toward more cosmopolitan taxa usually associated with summer stratified periods (e.g., SAR11 Clade Ia), and prevalent oligotrophic marine clades (OM60, SAR92). Higher riverine inputs of dissolved inorganic nutrients and suspended particulate matter also contributed to spatial reorganizations of communities in August. Sentinel taxa of this late summer fjord environment included taxa from the class Verrucomicrobiae (Roseibacillus, Luteolibacter), potentially indicative of a higher fraction of particle-attached bacteria. This study highlights the ecological relevance of terrestrial runoff for Arctic coastal microbial communities and how its impacts on biogeochemical conditions may make these communities susceptible to climate change.

Lake thermal structure drives interannual variability in summer anoxia dynamics in a eutrophic lake over 37 years

Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.

THE CONTROL OF APRICOT SEED DORMANCY AND GERMINATION BY LOW TEMPERATURE TREATMENTS

Objective: Freshly harvested seeds of “Local” apricot variety were found to be dormant and did not germinate at all. A specific low-temperature stratification treatment was required to overcome seed dormancy. The most effective temperature for breaking seed dormancy, germination, and seedling growth was 5°C cold stratification (CS). Increased seed germination percentage was recorded when the period of stratification prolonged. Seedling developed from stratified seeds had better growth than those developed from non-stratified seeds.&#x0D; Methods: For stratification treatments, the seeds with removed endocarp were mixed with moistened sand. Afterward, they were subjected to a period of stratification at 5°C. Seeds were stratified in pots of 30 cm×40 cm. Stratified seeds were regularly irrigated once per week. To prevent the water loss during stratification upper surface of pots was covered by a sack. The following stratification was applied for apricot variety: CS for 0, 3, 6, 9, 12, and15 days in 1998 and 1999 years for “Local” variety.&#x0D; Results: Apricot seeds required a CS of about 15 days for “Local” variety to reach maximum germination and normal seedling growth. Moreover, when stratification period was prolonged, some of the chemical constituents of apricot seeds were increased and other was decreased. Therefore, it can be suggested that breaking of dormancy is coincided with several changes in different chemical constituents of seeds. Some of these materials increased (total, reducing and non-reducing sugars, total free amino acids, total indoles, and total and conjugated phenols) and other materials such as free phenols which decreased at seed germinations.&#x0D; Conclusion: The most effective temperature for breaking seed dormancy, germination, and seedling growth was 5°C CS. Increased seed germination percentage was recorded when the period of stratification prolonged. Seedling developed from stratified seeds had better growth than those developed from non-stratified seeds.

Temporal Variability of Planktonic Ciliates in a Coastal Oligotrophic Environment: Mixotrophy, Size Classes and Vertical Distribution

The temporal variability of planktonic ciliates was studied in an extreme oligotrophic environment with special focus on trophic modes and size classes. Abundance, biomass, size classes, mixotrophy vs. heterotrophy, and species composition of planktonic ciliates were investigated focusing on temporal (samples collected on a monthly basis during 2019) and vertical (7 depth layers in the euphotic zone, from surface to 120 m) distribution at a coastal station in the oligotrophic Eastern Mediterranean. Abundance was in general very low (20 to 1150 cells L–1), except for September, which presented the highest abundance and biomass. Aloricate species dominated the ciliate community in all months and depths (% contribution from 77% in September to 99% in April). In general, oligotrichs presented maximum abundance at 2–10 m (except for June, July, and November: 100–120 m) whereas choreotrichs were more homogeneously distributed [and showed maxima at deep chlorophyll maximum (DCM)]. Small heterotrophs dominated the ciliate community at all depths and months, on average by 76% (they were 3 times more abundant than mixotrophs in terms of abundance and 2.5 times in terms of biomass). They were equally distributed both vertically and seasonally (and also in terms of size classes). In contrast, mixotrophs were found mainly at the surface layer to 20 m throughout the year, except for June and July (max at 100, 50 m). On average, 63% of integrated aloricate abundance was species &amp;lt;30 μm, of which 25% were mixotrophs. During the stratification period of May to November, the very small (&amp;lt;18 μm) and small (18–30 μm) mixotrophic species were distributed throughout the water column whereas the &amp;gt;50 μm mixotrophic species were found only above the DCM. In contrast, during the mixing period of December to May, mixotrophic ciliates were very few and were dominated by small and medium-sized species. It seems that mixotrophic and heterotrophic ciliates, on one hand, and the four size classes, on the other, are very distinct groups characterized by different distributions both vertically and seasonally. This may have important consequences for the structure and function of the microbial food web of which they are part, as well as for the carbon flow to the higher trophic levels of this oligotrophic environment in which ciliates are the main grazers.

Warming of lowland Polish lakes under future climate change scenarios and consequences for ice cover and mixing dynamics

Study regionThe study region comprises 25 lowland Polish lakes in the northern part of the country. The studied lakes provide domestic, industrial and agricultural water supply, and are major attractions for tourism, thus playing a significant role in the Polish economy. Study focusThe expected impact of future climate change on lake surface water temperature (LSWT) was predicted using the air2water model, which relies solely on daily air temperature (AT) as model input. LSWT and AT observations for the period 1987–2016 were used for model calibration and validation. Then, historical (1987–2005) and future (2006–2100) AT time series from nine EURO-CORDEX climate models were used to project future LSWT under emission scenarios RCP4.5 and RCP8.5. New hydrological insights for the regionThe results showed that air2water can well reproduce daily LWST with root mean square errors lower than 1 °C on average. The warming trends of both AT and LSWT are expected to be lower than those observed in the past decades (after the 1980s), and ice cover and weak stratification conditions are expected to partially buffer the LSWT response to the high AT warming expected in future winters. However, the overall enduring warming will substantially alter future thermal dynamics, leading to a shortening of ice cover and inverse stratification periods, possibly leading to serious consequences for lake water quality and ecosystem health.

Importance of salinity-induced stratification on flocculation in tidal estuaries

Flocculation of suspended particles in tidal estuaries exhibits large spatiotemporal variability due to an interplay of various physical and biogeochemical drivers. Salinity (S) is known to promote flocculation of fine-grained suspended particulate matter (SPM). However, the influence of salinity and salinity-induced stratification on flocculation has not been sufficiently investigated yet. This study aims to understand how these two factors, interactively with turbulent shear (G) and SPM concentration (C), control the vertical variation of floc size and flocculation process in different depth layers in a typical tide-dominated estuarine environment. Analysis of field observation data shows that flocculi (diameter < 20 μm) are mainly affected by C and originate primarily from local resuspension. Macroflocs (> 200 μm) are mainly controlled by stratification that greatly improves aggregate collision efficiency; Microflocs (20–200 μm), as a transition group between flocculi and macroflocs, are affected by dynamics of both sides. They are influenced jointly by C, G and stratification. Besides, the fresh water-dominated surface layer is dominated by small particles (flocculi and microflocs), confined in a relatively narrow particle size range between Ο (100) and Ο (101) as a result of the low level of both C (13–20 mg/L) and S (< 2 practical salinity units). Below the surface layer, floc size increases drastically along with an increased salinity-induced density gradient and achieves maximum particle size (Ο (102)) within the stratified layer. Because of its high efficiency in promoting flocculation and formation of macroflocs, the stratified layer around the halocline can be regarded as an optimal flocculation zone. The benthic layer is characterized by high C (> 30 mg/L), gentle G (~5/s), and periodic stratification, which result in a wide size range between Ο (101) and Ο (102) with microflocs as the dominant group. Finally, we found that the accuracy of flocculation modeling can be significantly improved by integrating a simple relationship between particle collision and stratification.

Phylogenetic diversity of the picocyanobacterial community from a novel winter bloom in Lake Biwa

In Lake Biwa, picocyanobacteria blooms usually occur during the summer–autumn thermal stratification period. Intriguingly, a novel bloom was detected in winter 2015–2016, in which picocyanobacterial cell density increased by one order of magnitude despite lower water temperature, suggesting the possibility that “cold-water-preference” species dominate in the picocyanobacterial community. In the present study, we investigated the phylogenetic diversity of picocyanobacteria in Lake Biwa by analyzing the 16S rRNA gene. We found that the picocyanobacterial community were highly diverse in Lake Biwa, with eight Synechococcus-related operational taxonomic units (OTUs) detected in different seasons. These OTUs fell into distinct phylogenetic groups, and the majority were closely related to clusters reported previously. Notably, OTU04, detected during the winter bloom, was highly affiliated with sequences found in a variety of lakes, such as Tibetan lakes and Lake Superior, where the water bodies generally have a low trophic state and temperature, and different concentrations of total dissolved solids. Thus, we inferred that the group containing OTU04 may be a psychrotolerant lineage that is widely distributed in oligotrophic water systems with low–intermediate salinity.

Zavisimost' kvantovogo vykhoda fotosinteza ot pogloshcheniya sveta fitoplanktonom: polucheniye kolichestvennykh svyazey dlya otsenki pervichnoy produktsii v Chernom more

Purpose. Based on the results of the investigations that were performed during the scientific cruise and included the water hydrophysical characteristics, the chlorophyll a concentration, the photosynthesis-light dependences, the spectral light absorption coefficients by phytoplankton, and the spectral quantum downwelling irradiance, the dependence of the photosynthesis quantum yield upon the environmental factors was studied with the purpose of adaptation of the developed for the Baltic Sea approach for assessing the photosynthesis quantum yield, to the Black Sea. Methods and Results. Complex hydrophysical and biological studies were carried out at several depths within the photosynthesis zone. Spectral bio-optical parameters were measured in accordance with the modern NASA protocol (2018). The experiments on studying the photosynthesis-light relationship were performed under the temperature and light conditions close to the in situ ones. The quantum yield of photosynthesis was calculated based on the parameters of photosynthesis-light dependences (photosynthesis efficiency, light saturation parameter) and the spectral light absorption coefficients by phytoplankton pigments. It was found out that the main photosynthetic characteristics of phytoplankton including the photosynthesis maximum quantum yield and the portion of photoprotective accessory pigments in the total light absorption by phytoplankton varied with depth within the euphotic zone due to phytoplankton adaptation to the environment factors during the period of water seasonal stratification. The relationship between the photosynthesis quantum yield and the number of solar energy quanta absorbed by the photosynthetically active phytoplankton pigments was revealed. The results of the performed research allowed for modifying the equation for calculating the quantum yield for the Black Sea environment conditions according to the approach developed for the other water areas. Conclusions. For the first time, comprehensive studies carried out in the Black Sea and including the measurements of the photosynthesis-light dependences, the spectral light absorption coefficients by phytoplankton and spectral downwelling irradiance at particular optical depths within the euphotic zone enabled to reveal the equation for calculating the photosynthesis quantum yield, which could be applied for calculating primary production of the Black Sea using the spectral approach based both on the results of in situ measurements and the remote sensing data.

Future projections of water level and thermal regime changes of a multipurpose subtropical reservoir (Sao Paulo, Brazil)

The increase in global air temperatures as well as variability in rainfall shifts due to climate change has been affecting the dynamics of water level fluctuations and thermal regimes in lakes and reservoirs. It is expected that at the end of this decade, such impacts will be even more noticeable and may harm the inland waters use. However, little is known about the possible consequences of climate change in multipurpose subtropical reservoirs. Using data generated by a regionalized climate model (RCM) as input to a simple hydrological model and a one-dimensional vertical hydrodynamic model, we forecast potential changes in the Itupararanga reservoir, São Paulo, Brazil, in an exemplary time period (2028–2030) in the next decade. Two Representative Concentration Pathway (RCP) scenarios were considered: an optimistic one corresponding to a CO2 increase of about 650 ppm (RCP 4.5) and a pessimistic scenario where CO2 exceeds 1000 ppm in 2100 (RCP 8.5). We found a significant reduction in the reservoir water level for both scenarios of 35% compared to current conditions. The surface water temperature is expected to increase (+0.6 °C); on the other hand, there would be a cooling of the hypolimnion (RCP 4.5 =-0.3 °C; RCP 8.5 = −1.2 °C). Another consequence is an increase of the duration of stratification periods that would start earlier in the dry period (between July and August), as well as the intensification of the stability of the water column (+43% compared to current conditions) and a deepening of the thermocline. The hydrodynamic modeling results suggest that the water level drop may threaten the reservoir multiple uses, in particular drinking water supply and power generation. Furthermore, the heating of surface water layers and increase of the number of stratified days and thermal stability can have negative impacts on water quality.

Hydrochemical characteristics in karst reservoirs and its implication for inorganic carbon deposition fluxes

为探究筑坝后不同水库物理、化学、生物过程对水化学和碳循环的影响，本研究对贵州三岔河流域的平寨水库、普定水库以及猫跳河流域的红枫湖水库进行研究，于2018年3月2019年1月分别在入库河流和库区采集了分层水样和沉降颗粒物，并探究水中主要离子及颗粒物通量的时空变化特征及其控制因素.结果表明，水体主要离子的主要来源受碳酸盐溶解影响，并且离子浓度受光合作用控制.红枫湖水库水体水化学类型为Ca-Mg-HCO₃-SO₄型，普定水库、平寨水库水化学类型均为Ca-HCO₃-SO₄.夏季藻类光合作用诱导碳酸盐沉淀导致水体表层Ca²⁺、HCO₃^-及溶解态Si浓度降低，其降低幅度分别为20.87%~44.25%、33.12%~51.18%、48.55%~96.34%.此外，藻类光合作用也影响C、N、Si等生源要素间的化学计量关系.Mg²⁺/Ca²⁺比值在水体垂向剖面上主要受碳酸钙沉淀的控制，而在不同水库之间则主要受流域岩性的控制.根据沉积物捕获器通量计算的平寨水库、普定水库、红枫湖水库夏季颗粒无机碳沉积通量分别为0.74、1.36、0.27 t/（km²·d），而根据水体Ca²⁺浓度降低计算的通量分别为0.31~0.64、0.35~0.99、0.09~0.29 t/（km²·d），根据水体HCO₃^-浓度降低计算的通量分别为0.30~0.65、0.29~1.26、0.12~0.33 t/（km²·d）.其红枫湖水库无机碳沉降通量的实测值与计算值接近，而平寨、普定水库实际沉降通量高于计算值，这可能是有外源输入导致.因此，利用水化学分层数据能对喀斯特水库中的无机碳沉降通量进行合理估算，并且能够得到较好的估算结果，从而指示碳循环的过程.;To explore the impact of changes in physical, chemical and biological processes on hydrochemistry and carbon cycle caused by damming, we investigated the flux of suspended particulate and the spatiotemporal variations of major cations and anions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, SO₄^2-, Cl^-) in three karst reservoirs in Guizhou Province, i.e. Puding Reservoir, Pingzhai Reservoir, and Lake Hongfeng, for a whole hydrological year. The results showed that carbonate weathering and algae photosynthesis controlled the spatiotemporal variations of ion concentrations. The hydro-chemical type of Lake Hongfeng is Ca-Mg-HCO₃-SO₄, while it is Ca-HCO₃-SO₄ in Puding Reservoir and Pingzhai Reservoir. During the summer period, as the result of carbonate precipitation induced by algae photosynthesis, the concentrations of Ca²⁺, HCO₃^-, and SiO₂ in the surface water decreased by 20.87%-44.25%, 33.12%-51.18%, and 48.55%-96.34%, respectively. Our results also indicated that the stoichiometric relationships among C, N, Si could be affected by photosynthesis of aquatic photosynthetic organisms. Additionally, it is found that calcite precipitation regulated the Mg²⁺/Ca²⁺ in water column. Finally, the inorganic carbon fluxes in summer period calculated by sediment traps were 0.74 t/(km²·d), 1.36 t/(km²·d), 0.27 t/(km²·d) for Pingzhai Reservoir, Puding Reservoir and respectively, which are comparable with the fluxes estimated by the concentration differences in Ca²⁺ and HCO₃^- between surface and bottom water layers. The measured fluxes of inorganic carbon in Pingzhai Reservoir and Puding Reservoir by sediment traps are higher than the calculated one, suggesting a certain amount of allochthonous inorganic carbon input due to the strong hydrological condition. Therefore, it is an alternative way to estimate the deposition flux of inorganic carbon in karst reservoirs by using vertical variations in ion concentrations in stratification period.