Hydrological Seasons Research Articles

Combined monthly GRACE-FO gravity fields for a Global Gravity-based Groundwater Product

SUMMARY The Combination Service for Time-variable Gravity fields (COST-G) operationally provides combinations of monthly Earth gravity field models derived from observations of the microwave ranging instrument of the GRACE Follow-on (GRACE-FO) satellite mission, applying the quality control and combination methodology originally developed by the Horizon 2020 project European Gravity Service for Improved Emergency Management for the data of the GRACE satellites. In the frame of the follow-up Horizon 2020 project Global Gravity-based Groundwater Product (G3P), the GRACE-FO combination is used to derive global grids of groundwater storage anomalies. To meet the user requirements and achieve optimal signal-to-noise ratio, the combination has been further developed and extended to incorporate: • new time-series based on the alternative accelerometer transplant product generated in the frame of the project by the Institute of Geodesy at the Graz University of Technology, which specifically improves the estimation of the C30 coefficient and also reduces the noise at medium to short wavelengths, and • the new time-series AIUB–GRACE-FO–RL02 of monthly GRACE-FO gravity fields, which is derived at the Astronomical Institute of the University of Bern by applying empirical noise modelling techniques. The COST-G quality control confirms the consistency of the contributing GRACE-FO time-series concerning the signal amplitude of seasonal hydrology in large river basins and the secular mass change in polar regions, but it also indicates rather diverse noise characteristics. The difference in the noise levels is taken into account in the combination process by relative weights derived by variance component estimation on the solution level. The weights are expected to be inverse proportional to the noise levels of the individual gravity field solutions. However, this expectation is violated when applying the weighting scheme as developed for the GRACE combination. The reason is found in the high-order coefficients of the gravity field, which are poorly determined from the low–low range-rate observations due to the observation geometry and suffer from aliasing due to the malfunctioning accelerometer onboard one of the GRACE-FO satellites. Hence, for the final G3P-combination a revised weighting scheme is applied where the gravity field coefficients beyond order 60 are excluded from the determination of the weights. The quality of the combined gravity fields is assessed by comparison of the noise content and the signal-to-noise ratio with the individual time-series. Independent validation is provided by the COST-G validation centre at the GFZ German Research Centre for Geosciences, where orbit fits of the low-flying Gravity and steady-state Ocean Circulation Explorer satellite are performed that confirm the high quality of the combined GRACE-FO gravity fields. By the end of the G3P project, the new combination scheme is implemented by COST-G as the new COST-G–GRACE-FO–RL02 and continued to be used for the operational GRACE-FO combination.

Seasonal hydrology and gas transport in a composite cover on sulfide tailings

This study presents the field performance of a five-layer composite cover to mitigate acid mine drainage in legacy sulfide tailings in northern Ontario, Canada. Installed in 2008, this cover comprised sand, clay, geosynthetic clay liner, sand, and waste rock layers. To evaluate the effectiveness of the cover in reducing water and oxygen ingress, groundwater and vadose zone hydrological characterization, stable water isotope analysis, pore-gas measurements, oxygen flux calculations, and variably saturated flow modeling were conducted. The results indicate that the clay layer stayed nearly saturated in the spring, fall, and winter, but temporary desiccation occurred during the summer. Compared to uncovered tailings, the cover significantly lowered diffusive oxygen flux. In the summer, fall, and winter, the capillary barrier effect of the cover functioned effectively and inhibited percolation. Atmospheric pore-gas oxygen concentrations at one out of the three monitoring locations indicate potential cover imperfections that enabled oxygen transport into the tailings. In the spring and early summer, snowmelt infiltration resulted in percolation that compromised the capillary barrier effect, as well as lateral drainage. The resulting increase in water saturation in the cover limited oxygen transport. Despite potential cover imperfections, this composite cover reduced oxygen and water ingress a decade after installation.

Seepage-mediated export of bacteria from soil is taxon-specific and driven by seasonal infiltration regimes

The seepage-mediated translocation of soil microorganisms across the soil-regolith boundary plays a central role in connecting surface and subsurface habitats. However, field studies targeting the factors controlling the dynamics of this translocation are still rare. We aimed to disentangle the effects of taxonomic identity, soil type, land use, and hydrological season on the translocation of bacteria over 1.5 years. Seepage-mediated bacterial transport was assessed using tension-controlled lysimeters in various soil types of a low-mountain groundwater recharge area under a temperate climate. Susceptibility to transport with seepage was primarily taxon-specific while influence by land use played only a minor role. The estimated total export of cells from soil ranged from 5.2 × 1011 to 1.3 × 1012 cells per m2 during a six-months-period and was 1.5 times higher during hydrological winter than in summer. In winter, low ionic strength seepage likely favored the export of bacterial groups with cell sizes of less than 1 μm such as Cand. Patescibacteria or Bdellovibrionaceae, or extracellular stages of potential pathogens of eukaryotes (Chlamydiales, Rickettsiales). Cand. Patescibacteria alone accounted for 40% of the cells exported in winter. Mobile cells contributed 1.3–2.1% to the total organic carbon exported from topsoil and subsoil. Our results demonstrate that substantial bacterial biomass is continuously exported from soils to subsurface environments. Export is strongly driven by the susceptibility of specific bacterial taxa to undergo translocation, and by seasonal infiltration regime. Enhanced introduction of bacterivorous or potential pathogenic taxa from soil to groundwater upon recharge may represent an important controlling factor for bacterial interaction networks or trophic interactions in groundwater food webs.

Dissolved Carbon Concentrations and Emission Fluxes in Rivers and Lakes of Central Asia (Sayan–Altai Mountain Region, Tyva)

The carbon (C) cycle in inland waters, including carbon concentrations in and carbon dioxide (CO2) emissions from water surfaces, are at the forefront of biogeochemical studies, especially in regions strongly impacted by ongoing climate change. Towards a better understanding of C storage, transport and emission in Central Asian mountain regions, an area of knowledge that has been extremely poorly studied until now, here, we carried out systematic measurements of dissolved C and CO2 emissions in rivers and lakes located along a macrotransect of various natural landscapes in the Sayan–Altai mountain region, from the high mountains of the Western Sayan in the northwest of Tyva to the arid (dry) steppes and semideserts in the intermountain basins in the southeast of Tyva on the border with Mongolia. New data on major hydrochemical parameters and CO2 fluxes (fCO2) gathered by floating chambers and dissolved organic and inorganic carbon (DOC and DIC, respectively) concentrations collected over the four main hydrological seasons allowed us to assess the current C biogeochemical status of these water bodies in order to judge possible future changes under climate warming. We further tested the impact of permafrost, river watershed size, lake area and climate parameters as well as ‘internal’ biogeochemical drivers (pH, mineralization, organic matter quality and bacterial population) on CO2 concentration and emissions in lakes and rivers of this region and compared them with available data from other subarctic and mountain settings. We found strong environmental control of the CO2 pattern in the studied water bodies, with thermokarst lakes being drastically different from other lakes. In freshwater lakes, pCO2 negatively correlated with O2, whereas the water temperature exerted a positive impact on pCO2 in large rivers. Overall, the large complexity of counteracting external and internal drivers of CO2 exchange between the water surfaces and the atmosphere (CO2-rich underground DIC influx and lateral soil and subsurface water; CO2 production in the water column due to dissolved and particulate OC biodegradation; CO2 uptake by aquatic biota) precluded establishing simple causalities between a single environmental parameter and the fCO2 of rivers and lakes. The season-averaged CO2 emission flux from the rivers of Tyva measured in this study was comparable, with some uncertainty, to the C uptake fluxes from terrestrial ecosystems of the region, which were assessed in other works.

Углеводы в поверхностных водах Карелии и их связь с содержанием биогенных и органических веществ

The results of a long-term research of the content of carbohydrates in natural waters of Republic of Karelia (Russia) from 2011 to 2020 are summarized. In total, 34 water bodies (29 lakes and 5 rivers) were studied in different hydrological seasons. The presented sample of water bodies captures almost all types of surface waters in the humid zone in terms of trophicity and humus content. It was found that the carbohydrates concentration varies widely from 0.7 to 53.1 mg/L (average 3.3 mg/L) and its share of total organic matter averages 16%. It was revealed that the content and distribution of carbohydrates in surface waters depends both on natural (the level of trophicity and humus content in the reservoir) and anthropogenic factors (eutrophication, trout farms, industrial wastewater discharge, seepage waters of solid waste landfills, communal wastewater). In addition, all the studied water bodies were categorized into two groups according to the water pollution degree: contaminated and uncontaminated. It was revealed that the concentration of carbohydrates in contaminated water bodies of Karelia is on average 1.4 times higher than in uncontaminated ones. In addition, the content of carbohydrates increases together with such chemical parameters as 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD Cr), permanganate oxygen demand (COD Mn), chlorophyll a, mineral and total phosphorus (Pmin and Р total relatively) and ammonium (N-NH 4+). In particular, these relations are pronounced in contaminated water bodies, where the highest values of the correlation coefficient between the above parameters are noted. The content of carbohydrates in surface waters is a complex indicator of both organic and biogenic water pollution.

Environmental and spatial factors play different roles in phytoplankton community assembly in different hydrological seasons in Lake Wuchang, China

Lake phytoplankton communities are affected by environmental and spatial factors. We studied the relative importance of environmental and spatial factors on the phytoplankton community assembly in Lake Wuchang across three hydrological seasons, which were divided into dry (December to March), normal (April to June, October to November) and wet seasons (July to September) based on the water level and depth. Spatial and temporal patterns of environmental factors and phytoplankton community composition and diversity were studied using Kruskal–Wallis test, Wilcoxon test and NMDS. CCA, Mantel and partial Mantel tests, and PLS-PM were used to investigate the effects of environmental and spatial factors on phytoplankton community characteristics. Results showed that phytoplankton assemblages at the eight study sites were composed of totally 244 species belonging to 9 phyla, which changed from Bacillariophyta and Chlorophyta to Cyanophyta across the whole hydrological period. There was significantly higher abundance and biomass in the normal and wet seasons than that in the dry season. Phytoplankton alpha diversity exhibited uniform temporal distribution patterns with higher values in the dry season than in the normal and wet seasons. The Mantel and partial Mantel tests revealed that environmental (physicochemical conditions of lake water) and spatial factors (geographic distances among sites) jointly affected the phytoplankton community structure and beta diversity across the hydrological seasons, while spatial factors were more important in the wet season. Partial least squares path models showed that spatial factors exhibited a significant positive correlation with the phytoplankton diversity with the path coefficients of 0.53 and 0.71 in the normal and wet seasons, respectively. Phytoplankton composition had significant correlation with on phytoplankton diversity with the path coefficient of −0.75 and 0.61 in the normal and wet seasons, respectively. Our findings revealed that both environmental and spatial factors affected the phytoplankton community assembly in Lake Wuchang. Environmental factors played a more important role in the dry season, while spatial factors were more important in the wet season. With the exception of the abiotic factors (environmental and spatial), the impacts of biotic factors on phytoplankton community cannot be ignored. Therefore, it is also necessary to strengthen further research on the top-down control over phytoplankton communities in Lake Wuchang.

Artificial water-level fluctuation modulates trophic niches of benthic fish assemblages in one of the world's largest reservoirs

The impoundment of the Yangtze River and construction of the Three Gorges Dam and Reservoir created an unnatural flow regime with a 30 m water-level fluctuation and four artificial seasons that differ from the river's natural flow-regime timing. Using stable isotopes, we selected 15 common benthic fish species representing five trophic guilds to analyze seasonal variation in production sources, trophic niche width, and niche overlap. We hypothesized that, similarly to a natural flow regime, artificial hydrological seasons effectively influence fish trophic niches. We found consistency in seasonal shifts in the contribution of aquatic and terrestrial producers to fish biomass. δ13C and δ15N depicted seasonal changes in intra-guild trophic niche width and intra- and inter-guild niche overlap. Considering that the Three Gorges Reservoir's flow regime is artificially regulated, our findings suggest that water-level fluctuation is one of the key mechanisms driving such trophic shifts and allowing multiple species to coexist. Given the increased cascaded large reservoir construction and the concomitant creation of unnatural flow regimes in large rivers worldwide, these findings are timely. Our findings contribute to improving water-level management by optimizing rhythmicity while balancing flood control and fish protection.

Why the 2022 Po River drought is the worst in the past two centuries.

The causes of recent hydrological droughts and their future evolution under a changing climate are still poorly understood. Banking on a 216-year river flow time series at the Po River outlet, we show that the 2022 hydrological drought is the worst event (30% lower than the second worst, with a six-century return period), part of an increasing trend in severe drought occurrence. The decline in summer river flows (-4.14 cubic meters per second per year), which is more relevant than the precipitation decline, is attributed to a combination of changes in the precipitation regime, resulting in a decline of snow fraction (-0.6% per year) and snowmelt (-0.18 millimeters per day per year), and to increasing evaporation rate (+0.013 cubic kilometers per year) and irrigated areas (100% increment from 1900). Our study presents a compelling case where the hydrological impact of climate change is exacerbated by local changes in hydrologic seasonality and water use.

Sedimentology of the Jura Molasse: Miocene tidal clastics and freshwater carbonates from the Tramelan-2 Borehole, NW Switzerland

The Tramelan-2 borehole (Canton Bern, Switzerland) continuously cored a 275.60 m sequence of Palaeogene to Neogene sediments, providing a rare opportunity for sedimentological analysis of the Jura Molasse. Lithostratigraphy, sedimentary facies and heavy minerals allow correlation with the classical Swiss Molasse. Evidence for clastic input from Alpine and non-Alpine sources is consistent with deposition at the northern feather edge of the Molasse Basin. Grey sandstones at the base of the succession are tentatively interpreted as fluvial facies of the Lower Freshwater Molasse (USM). These are overlain by erosively-based conglomerates, interpreted as winnowed storm lags of locally-derived clasts and distantly-sourced Alpine material at the transgressive base of the Upper Marine Molasse (OMM). Above this, a range of plane-laminated and cross-bedded bioclastic sandstones are interpreted as the deposits of a meso- or macrotidal flat or estuarine complex. Facies evolution in the upper OMM records a series of stacked regressive cycles within an overall upward trend of reducing current energy, reflecting a transition from offshore to nearshore shallow marine environments. The unconformably overlying Upper Freshwater Molasse (OSM) passes upwards from micaceous siltstones and marls into a series of freshwater carbonates, dominated by lacustrine limestones which are arranged in repeated regressive cycles, each capped by organic-rich horizons. The middle part of the carbonate interval comprises palustrine limestones with pedogenetic fabrics recording periodic subaerial exposure. Stable isotope signatures from the palustrine facies reflect subaerial pedogenetic overprint, while the lacustrine carbonates record cyclical variations in δ13C which may reflect increases in organic material during progressive lake fill and/or an increased influence of aquifer waters during flooding events. The Jura Molasse onlapped a Mesozoic carbonate pediment hosting a regional karst system which remained active at least until the Burdigalian transgression on footwall highs bounding the Delémont Basin, and locally into the Langhian near Tramelan. This configuration controlled subsequent deposition of OSM carbonates which onlapped erosional and potentially early tectonic relief, and were deposited in groundwater-fed seasonal lacustrine/palustrine environments, where the karstic aquifer controls on seasonal hydrology appear to have been comparable to Caribbean freshwater carbonate wetland systems today.

Geomorphological habitat type drives variation in temporal species turnover but not temporal nestedness in Amazonian fish assemblages

Explaining the mechanisms underlying spatial and temporal variation in community composition is a major challenge. Nevertheless, the processes controlling temporal variation at a site (i.e. temporal β‐diversity, including its turnover and nestedness components) are less understood than those affecting variation among sites (i.e. spatial β‐diversity). Short‐term temporal turnover (e.g. throughout an annual cycle) is expected to correlate positively with seasonal environmental variability and landscape connectivity, but also species pool size (γ‐diversity). We use the megadiverse Amazonian freshwater ichthyofauna as a model to ask whether seasonality and landscape connectivity drive variation in temporal species turnover among geomorphological habitat types, while accounting for between‐habitat variation in γ‐diversity. We collected 11 397 fish representing 260 species during a year‐long sampling program from an area containing the lowland Amazon's four major geomorphological habitat types: rivers, floodplains, terra firme streams, and shield streams. River‐floodplain systems exhibit strong but predictable seasonality (via a high‐amplitude annual flood pulse), high connectivity, and high species richness with many rare species. Terra firme and shield streams exhibit low seasonality, low connectivity, and low species richness with proportionally fewer rare species. Based on these parameters we predicted that river‐floodplain systems should have higher temporal turnover than stream systems. Using a null model approach combined with β‐deviation calculations, we confirmed that rivers and floodplains do exhibit higher turnover (but not nestedness) than terra firme and shield streams, even when controlling for the potentially confounding effect of higher species richness in river‐floodplain systems. All habitats exhibit low temporal nestedness, indicating that short‐term changes in community composition result primarily from temporal species turnover. Our results provide a timely reminder that efforts to conserve the Amazon's threatened aquatic biodiversity should account for the distinct temporal dynamics of habitat types and variation in hydrological seasonality.

Comprehensive insights into the occurrence, distribution, risk assessment of typical pesticides and their environmental driving effects in a large drinking water reservoir

The problem of pesticide pollution in large drinking water reservoirs is becoming increasingly prominent due to agricultural production. To systematically evaluate the distribution characteristics, seasonal fluctuations, environmental behaviors, and ecological risks of pesticides in the Fengshuba Reservoir (China), we conducted a comprehensive investigation of the seasonal changes in the concentrations of 31 pesticides, including 8 organochlorine pesticides (OCPs), 16 organophosphates pesticides (OPPs), and 7 synthetic pyrethroids (SPs). The results showed the concentrations of OCPs, OPPs, and SPs in the water phase were 121.98–321.64 ng/L, 279.28–371.75 ng/L, and 118.96–186.65 ng/L, respectively. The concentrations in the porewater phase were 3363.34–7105.00 ng/L, 8648.98–14,265.95 ng/L, and 2740.83–7738.28 ng/L, respectively. The concentrations in the sediment phase were 47.92–88.28 μg/kg, 159.87–446.64 μg/kg, and 29.43–99.62 μg/kg, respectively. The concentrations in the soil phase were 23.62–50.76 μg/kg, 96.85–184.19 μg/kg, and 21.02–37.02 μg/kg, respectively. Seasonal hydrology, hydraulic retention time, precipitation, temperature, and biological metabolism all impacted the concentration and composition of pesticides, but no apparent environmental gradient effect was found within the reservoir. Redundancy analysis indicated that NO3-, Dissolved oxygen (DO), Dissolved organic carbon (DOC), Organic nitrogen (ON), and Inorganic carbon (IC) were the main environmental parameters affecting pesticide concentrations. The pseudo-partitioning values in water and porewater ranged from 1.2 to 3.3 × 104 and 22.0–61.7 L/kg, respectively. The ecological risk was low for OCPs, moderate for OPPs, and high for SPs in the water phase. SPs should be considered a priority indicator for risk assessment. Future conservation efforts for reservoir system should focus on pollution prevention in the soil phase.

Effects of Hydrological Season on the Relationship between Land Use and Surface Water Quality

In recent decades, land use patterns have changed significantly in highly urbanized areas, which is usually linked with the spatial variation of surface water quality at the catchment scale, but little attention has been paid to how hydrological seasons affect this relationship. Taking Pudong New Area of Shanghai, China, as an example, this paper evaluated the influence of hydrological seasons on the relationship between land use and water quality under different hydrological buffers. It was shown that the contribution of land use to the spatial variation of water quality is approximately 30%. In addition, the explanatory ability was greatest in the average season while it was smaller in the dry and wet seasons. Land uses showed scale effects; at a smaller scale, urban areas, agricultural land and water areas were the most important land uses affected by water quality. As the buffers changed from 500 to 1500 m, the impact of urban areas decreased significantly, while that of agricultural land and water areas increased rapidly; however, when the buffer was greater than 1000 m, the explanatory ability of water areas did not increase further but remained stable. Green space is only significant at the 200 m and 500 m scales, which showed the effect of improving river quality. This study is expected to provide references for future decision making of urban construction, environmental planning and management.

Exploring the use of multi-source high-resolution satellite data for snow water equivalent reconstruction over mountainous catchments

Abstract. The hydrological cycle is strongly influenced by the accumulation and melting of seasonal snow. For this reason, mountains are often claimed to be the “water towers” of the world. In this context, a key variable is the snow water equivalent (SWE). However, the complex processes of snow accumulation, redistribution, and ablation make its quantification and prediction very challenging. In this work, we explore the use of multi-source data to reconstruct SWE at a high spatial resolution (HR) of 25 m. To this purpose, we propose a novel approach based on (i) in situ snow depth or SWE observations, temperature data and synthetic aperture radar (SAR) images to determine the pixel state, i.e., whether it is undergoing an SWE increase (accumulation) or decrease (ablation), (ii) a daily HR time series of snow cover area (SCA) maps derived by high- and low-resolution multispectral optical satellite images to define the days of snow presence, and (iii) a degree-day model driven by in situ temperature to determine the potential melting. Given the typical high spatial heterogeneity of snow in mountainous areas, the use of HR images represents an important novelty that allows us to sample its distribution more adequately, thus resulting in highly detailed spatialized information. The proposed SWE reconstruction approach also foresees a novel SCA time series regularization technique that models impossible transitions based on the pixel state, i.e., the erroneous change in the pixel class from snow to snow-free when it is expected to be in accumulation or equilibrium and, vice versa, from snow-free to snow when it is expected to be in ablation or equilibrium. Furthermore, it reconstructs the SWE for the entire hydrological season, including late snowfall. The approach does not require spatialized precipitation information as input, which is usually affected by uncertainty. The method provided good results in two different test catchments: the South Fork of the San Joaquin River, California, and the Schnals catchment, Italy. It obtained good agreement when evaluated against HR spatialized reference maps (showing an average bias of −22 mm, a root mean square error – RMSE – of 212 mm, and a correlation of 0.74), against a daily dataset at coarser resolution (showing an average bias of −44 mm, an RMSE of 127 mm, and a correlation of 0.66), and against manual measurements (showing an average bias of −5 mm, an RMSE of 191 mm, and a correlation of 0.35). The main sources of error are discussed to provide insights into the main advantages and disadvantages of the method that may be of interest for several hydrological and ecological applications.

Implications of submonthly oxygen and carbon isotope variations in late Pleistocene Melanopsis shells for regional and local hydroclimate in the upper Jordan River valley

AbstractMany water-stressed regions of the globe have a highly seasonal precipitation regime. However, seasonality in the past and under changing climates is little studied. Submonthly records of sclerochronological δ18O and δ13C values of Melanopsis shells from the Jordan River Dureijat archaeological site (JRD) in the upper Jordan River valley presented here document the hydrology of paleo-Lake Hula. These records were assessed for changes in seasonal hydrology in the lake and compared with modern shells collected from present-day waterbodies in northern Israel and with models of δ18Oshell. Results from shells in sediments dating from the last glacial maximum (LGM) to the Bølling-Allerød imply changes in waterbody size that qualitatively parallel changes in the late Pleistocene Lake Lisan levels; Hula Lake was well buffered when Lake Lisan stood at a high stand and poorly buffered when water levels were lower. Furthermore, data from shells dated to the LGM suggest inflowing water with lower δ18O values than local rainfall, providing evidence for a greater proportion of snow in the catchment than today. Reconstruction of water δ18O and mixing-model calculations suggest that snowmelt contribution to spring water during the LGM may have been more than twice the amount in the modern-day catchment.

Extreme Low Flow during Long-Lasting Phases of River Runoff in the Central Part of the East European Plain

In the rivers of the central part of the East European Plain (the Volga at Staritsa, the Oka at Kaluga, and the Don at Stanitsa Kazanskaya), long phases (10–15 years or more) of increased/decreased annual and seasonal runoff have occurred, as well as differences in the frequencies of extremely low flow conditions from the late 19th century to 2020. Phase boundaries were identified by cumulative deviation curves and statistical homogeneity. The frequencies of specific water flow values were estimated using the empirical curves of the exceedance probability of annual and seasonal water flows based on their long-term time series. In the century-long changes of rivers considered, two long contrasting phases were revealed. These phases are characterized by increased and decreased runoff of hydrological seasons. Near simultaneously, a phase of increased runoff was first observed for the freshet season. On the contrary, phases of decreased runoff were first observed for low-water seasons. The runoff phases differ significantly in duration and differences in flow. Significant differences were revealed in the frequency of low-water years for a low runoff with an exceedance probability above or equal to 75% and above or equal to 95%.

Seasonal patterns in the structure of the fish larvae assemblages in the lower Amazon River

Abstract This study assessed the phenology and functional structure of parental stocks of fish larvae, and evaluated how the species modulate their reproductive activities as a function of natural hydrological regimes in the lower Amazon River, Brazil. Larvae were collected using conical plankton nets monthly over two hydrological cycles between January and December 2013, and between May 2017 and April 2018. The collections were conducted around an alluvial island located near the municipality of Santarém, Pará, Brazil. After identification, individuals were classified into four functional groups (e.g., life‐history strategy, feeding habits, type of migration, and swimming‐performance/microhabitat‐use in the water column) based on species traits. The relationships between seasonal hydrology, environmental variables and larval assemblage structure were evaluated using multivariate analyses. A total of 31,347 fish larvae were collected, from nine taxonomic orders, 26 families, 40 genera and 53 species. The larval assemblages showed highly variable taxonomic and functional composition between different phases of the hydrological cycle (rising water, high water, receding water and low water) and were strongly influenced by environmental variables. Fish reproductive phenology in the lower Amazon is complex and distributed among the hydrological phases. The different ecological characteristics allow the species to withstand specific environmental conditions during each phase. Changes in the structure of larval assemblages indicate that hydrological variability is a key ecological factor influencing the reproductive dynamics of fish and reflects adaptive strategies used to reduce interspecific competition for space and food, maximising recruitment. The variation of fish reproduction across the hydrological cycle that we observed is a result of different ecological characteristics enabling species and functional groups to adapt to environmental conditions of each phase.

Colloids in Thermokarst Lakes along a Permafrost and Climate Gradient of Permafrost Peatlands in Western Siberia Using In Situ Dialysis Procedure

Thermokarst lakes in the Western Siberian Lowland (WSL) are major environmental factors controlling organic carbon and trace metal storage in inland waters and greenhouse gas emissions to the atmosphere. In contrast to previously published research devoted to lake hydrochemistry, hydrobiology, sedimentary carbon, and processes controlling the lake total dissolved (<0.45 μm) solute composition, the colloidal forms of organic carbon (ОC), and related elements remain poorly known, especially across the permafrost gradient in this environmentally important region. Here we sampled 38 thermokarst lakes in the WSL, from the continuous to the permafrost-free zone, and we assessed both the total (<0.45 μm) and low-molecular-weight (<1 kDa) concentrations of 50 major and trace elements using conventional filtration and in situ dialysis. We aimed at quantifying the relationships between the colloidal content of an element and the lake surface area, permafrost coverage (absent, sporadic, isolated, discontinuous, and continuous), pH, and the concentrations of the main colloidal constituents, such as OC, Fe, and Al. There was a positive correlation between the lake area and the contents of the colloidal fractions of DOC, Ni, rare earth elements (REE), and Hf, which could be due to the enhanced mobilization of OC, trace metals, and lithogenic elements from silicate minerals in the soil porewater within the lake watershed and peat abrasion at the lake border. In all permafrost zones, the colloidal fractions of alkalis and alkaline-earth metals decreased with an increase in lake size, probably due to a decrease in the DOC concentration in large lakes. There was an increase in the colloidal fractions of DOC, Fe, Al, trivalent and tetravalent trace cations, Mn, Co, Ni, As, V, and U from the southern, permafrost-free zone to the northern, permafrost-bearing zones. This observation could be explained by an enhanced feeding of thermokarst lakes by suprapermafrost flow and the thawing of dispersed peat ice in the northern regions. Considering the large permafrost gradient of thermokarst lakes sampled in the present study, and applying a space-for-time substitution approach, we do not anticipate sizable changes in the colloidal status of DOC or major or trace elements upon climate warming and the permafrost boundary shifting northwards. For incorporating the obtained results into global biogeochemical models of OC, metal micronutrients, and toxicant migration in the permafrost regions, one has to consider the connectivity among lakes, soil waters, and rivers. For this, measurements of lake colloids across the main hydrological seasons, notably the winter period, are necessary.

Who spawns where? Temperature, elevation, and discharge differentially affect the distribution of breeding by six Pacific salmonids within a large river basin

Within the geographic range of salmonid fishes, many apparently suitable rivers and streams are used for reproduction by some species but not others. This is widely known but seldom addressed, as studies often examine factors determining the distribution of one or only a few species. We examined physical factors associated with the spawning distribution of six native Pacific salmonids (pink, chum, coho, and Chinook salmon, bull trout, and steelhead) in the Skagit River basin, Washington. Annual mean temperature and catchment elevation had the strongest association with spawning assemblage distribution, but stream length, annual discharge, seasonal hydrology, and land use were also influential. Some species (e.g., pink and Chinook salmon) were more closely associated with each other and with common variables than others, and bull trout were the most distinctive. For interpretation, we investigated the roles of adult body size, timing of spawning, and duration of juvenile rearing, but none of these factors explained groupings in the data. Interspecific differences in habitat association remained, suggesting fundamental constraints on species distributions with implications for conservation and restoration.

Distinct seasonal variations of dissolved organic matter across two large freshwater lakes in China: Lability profiles and predictive modeling

Biological lability of dissolved organic matter (DOM) is a crucial indicator of carbon cycle and contaminant attenuation in freshwater lakes. In this study, we employed a multi-stage plug-flow bioreactor and spectrofluorometric indices to characterize the seasonal variations in DOM composition and lability across Poyang Lake (PY) and Lake Taihu (TH), two large freshwater lakes in China with distinct hydrological seasonality. Our findings showed that the export of floodplain-derived organics and river-lake interaction led to a remarkable increase in terrestrial aromatic and humic-like DOM with high molecular weights and long turnover times in PY. Consequently, the labile fraction was extremely low (average LDOC% of 3%) during the rising-to-flood season (spring and summer). Conversely, autochthonous production in TH considerably enriched semi-labile (average SDOC% of 26%) and biodegradable DOM (average BDOC% of 34%) during the phytoplankton bloom to post-bloom season (summer and autumn). This was reflected by the accumulation of low-light-absorbing and protein-like components with high biological and fluorescence indices. In the dry and non-bloom season (winter), the better preservation of humic substances maintained the high molecular weight and humic degree of DOM in PY, while the decay of aquatic plants strengthened autochthonous production, resulting in a similar BDOC% of PY samples (23%–34%) to TH samples (18%–33%). We further applied partial least squares regression using DOM optical indices as predictive proxies, which generated a greater prediction strength for BDOC% (R2 = 0.80) compared to SDOC% (R2 = 0.57) and LDOC% (R2 = 0.28). The regression model identified aromaticity (SUVA254) as the most effective and negative predictor and low molecular weight (A250/A365) as the highly and positively influential factor. Our study provides new evidence that the seasonality of DOM lability profiles is regulated by the trade-off between flow-related variation and phytoplankton production, and presents an approach to describe and predict DOM lability across freshwater lakes.

Аккумуляция ртути в двустворчатых моллюсках в гидроэкосистеме устьевой области реки Красная (Вьетнам)

The article presents analysis of mercury content in different tissues such as gills, mantle, hepatopancreas and body (leg) in 4 species of bivalve molluscs in the estuary of the Red River in Vietnam. The studies were carried out in 2016–2021 during the main hydrological seasons (during the summer flood and winter low water) at 37 stations in the delta and estuarine coast of the Red River. There are shown the dependences of mercury content in mollusc tissues on the age parameters (length and mass of tissues) and on the content of pollutant in dissolved, suspended forms of near-bottom water layers and in bottom sediments. The mercury content in the tissues of molluscs varies whithin 0,023 ± 0,001 to 0,459 ± 0,007 mg/kg wet weight. The highest content of mercury was found in the hepatopancreas 
 of Austriella corrugata, and the lowest - in the leg of Lutraria rhynchaena. Significant close positive correlations were registered between the mercury concentrations in the mollusc tissues and the mercury content in the near-bottom water layer and in bottom sediments (r from 0.68 to 0.92, p < 0.05). All studied species of molluscs are classified as macroconcentrators of mercury in water; Austriella corrugata can actively accumulate mercury not only from the water, but also from the bottom sediments. Bivalve molluscs can be used as an object of biomonitoring when studying the distribution of mercury in the estuary of the Red River.