Downstream Lakes Research Articles

Unravelling the role of sulphate in reed development in urban freshwater lakes

Many European lakes have suffered from reed die-back since the 1950s. Previous studies have concluded that this is due to a combination of several interacting factors, but possibly also a single threat with high impact might be responsible for the phenomenon. In this study, we investigated 14 lakes in the Berlin area differing in reed development and sulphate concentration from 2000 to 2020. To unravel the decline of reed beds in some of the lakes with coal mining activities in the upper watershed, we compiled a comprehensive data set. Thus, the littoral zone of the lakes was divided into 1302 segments considering the reed ratio relative to segment area, water quality parameters, littoral characteristics and bank usage of the lakes which all have been monitored for 20 years. We ran two-way panel regressions with a within estimator to consider the spatial variation between and within the segments over time. The regression results revealed a strong negative relationship between reed ratio and sulphate concentrations (p<0.001) as well as tree shading (p<0.001) and a strong positive relationship with brushwood fascines (p<0.001). Taking only sulphate into account, reeds would have covered an additional area of 5.5 ha or 22.6% in 2020 (total reed area: 24.3 ha) in the absence of increased sulphate concentrations. In conclusion, changes in water quality upstream the catchment cannot be ignored in the development of management plans for downstream lakes.

Key factors driving dissolved organic matter composition and bioavailability in lakes situated along the Eastern Route of the South-to-North Water Diversion Project, China

The Eastern Route of the South-to-North Water Diversion Project (SNWDP-ER) is a large scale multi-decade infrastructure project aiming to divert substantial amounts of water (≈45 billion m3 yr−1) to alleviate water shortage in comparatively arid regions of northern China. The project has ramifications for hydrological connectivity and biogeochemical cycling of dissolved organic matter (DOM) in regional lakes affected by the project. We carried out an extensive field sampling campaign along the SNWDP-ER in different hydrological seasons of 2018 and monthly observations in Lake Hongze and Lake Luoma from April 2018 to June 2021. We found the lakes connecting to the SNWDP-ER had higher mean DOC, specific UV absorbance, higher ratio of humic-like to protein-like fluorophores (Humic : Protein), and shallower spectral slope (S275–295) in the wet season compared to the wet-to-dry transition, and dry seasons. The southern lakes and Yangtze River had lower DOC concentration, bioavailable DOC (BDOC), and higher DOM aromaticity compared to the northern two downstream lakes. Ultrahigh-resolution mass spectrometry (FT-ICR MS) revealed higher relative abundance of CHO-containing and aromatic compounds in the Yangtze River and the southern three upstream lakes compared to the northern two lakes. The data from Lake Hongze and Lake Luoma, studied in different hydrological seasons, suggest that water delivery had high consistency in DOM composition and BDOC over the season. We conclude that positioning along the watercourse and seasonally variable hydrological conditions play an important role in influencing the DOM composition and bioavailability of key lakes connecting to the SNWDP-ER. Our results indicated that the water diversion project delivers water with low DOC concentration and higher aromaticity and thus is of higher quality since it has higher DOM removal potential during drinking water treatment.

Evaluating the effectiveness of sediment retention by comparing the spatiotemporal burial of sediment carbon, nitrogen and phosphorus in a plateau lake and its affiliated reservoirs

Evaluating the effectiveness of sediment retention by comparing the spatiotemporal burial of sediment C, N, and P in a plateau lake and its affiliated reservoirs is critical in dealing with global eutrophication in freshwater ecosystems. The objective of the study was to estimate the spatial–temporal of C, N and P accumulation rates (CAR, NAR and PAR) in sediment cores of a plateau Lake Dianchi (DC) and its four affiliated reservoirs (Dashiba-DSB, Xibaisha-XBS, Baoxianghe-BXH, and Shuanlong-SHL). The results indicated that the average sediment accumulation rates (SARs) ranged from 0.040 to 1.187 g cm−2 a−1 in DC and 0.201 to 3.965 g cm−2 a−1 in its affiliated reservoirs. Spatially, all of the values of CAR, NAR and PAR exhibited similar distribution characteristics, along with two high-value regions concentrated in the southern and northern parts of the lake DC. Temporally, these three values of Lake DC and its affiliated reservoirs showed upward trends from bottom to top over the past ∼ 150 years, especially occurring in the 1950s. The change in the C/N ratio indicated that intensive anthropogenic factors promoted the CAR, NAR and PAR in sediments. Based on a developed proportional model, the ratios of sediment retention by each reservoir on TOC, TN and TP were calculated to be 2.15 %, 4.38 % and 9.01 % for DSB, 0.14 %, 0.18 % and 0.14 % for XBS, 1.61 %, 5.77 % and 1.23 % for BXH, and 0.97 %, 2.07 % and 0.46 % for SHL, respectively. Reservoirs located in the upper and middle reaches of the lake catchment play a critical role in sediment retention and prevent a potential risk from water deterioration of the downstream Lake DC. Linkages between reservoir-sources and lake-sinks not only provide valuable information for managing the environmental changes within the catchment, but also contribute to new insight into the regulation of water eutrophication in global lake ecosystem.

Heavy metal deposition dynamics under improved vegetation in the middle reach of the Yangtze River

Estuarine heavy metal deposition processes in tributaries can reflect the environmental changes in the basin and the contribution of tributaries to the pollution of downstream lakes. The dynamic processes of heavy metal deposition in two major tributaries of the middle reaches of the Yangtze River were explored using sediment cores. The relationships between heavy metals (HMs) and various physicochemical properties were analyzed using Pearson correlation analysis. The sediment chronological sequences were determined using Pb isotope dating, and the sediment fluxes of heavy metals were calculated. The differences in the driving factors of the two watersheds were analyzed using redundancy analysis. The results showed that heavy metal in both sediment cores were significantly higher than the corresponding background values and showed a relatively stable trend from deep to shallow, with Cd being the most exceeded heavy metal in both tributaries. The average sediment deposition rate was 1.31 cm/year. The Pearson correlation analysis (PCA) results between the HMs indicated similar sources, and the correlation analysis between HM and environmental variables showed that the HMs in both cores, especially Cu and Cr, were significantly correlated with phosphorus, suggesting a synergistic loss of HMs and P. Heavy metal deposition and the normalized difference vegetation index (NDVI) in watersheds showed a negative correlation. This negative correlation is more pronounced in watersheds with higher vegetation cover, where heavy metal deposition is more driven by natural factors. The redundancy analysis (RDA) results indicated that the transport of Cd and Cu was influenced by precipitation and runoff. Heavy metal deposition processes in lake estuaries under improved terrestrial vegetation show the historical contribution of tributaries to lakes, which is important for studying pollution and ecological restoration in watersheds.

Estimates, spatial variability, and environmental drivers of mercury biomagnification rates through lake food webs in the Canadian subarctic

Biomagnification of mercury (Hg) through lake food webs is understudied in rapidly changing northern regions, where wild-caught subsistence fish are critical to food security. We investigated estimates and among-lake variability of Hg biomagnification rates (BMR), relationships between Hg BMR and Hg levels in subsistence fish, and environmental drivers of Hg BMR in ten remote subarctic lakes in Northwest Territories, Canada. Lake-specific linear regressions between Hg concentrations (total Hg ([THg]) in fish and methyl Hg ([MeHg]) in primary consumers) and baseline-adjusted δ15N ratios were significant (p < 0.001, r2 = 0.58–0.88), indicating biomagnification of Hg through food webs of all studied lakes. Quantified using the slope of Hg-δ15N regressions, Hg BMR ranged from 0.16 to 0.25, with mean ± standard deviation of 0.20 ± 0.03). Using fish [MeHg] rather than [THg] lowered estimates of Hg BMR by ∼10%, suggesting that the use of [THg] as a proxy for [MeHg] in fish can influence estimates of Hg BMR. Among-lake variability of size-standardized [THg] in resident fish species from different trophic guilds, namely Lake Whitefish (Coregonus clupeaformis) and Northern Pike (Esox lucius), was not significantly explained by among-lake variability in Hg BMR. Stepwise multiple regressions indicated that among-lake variability of Hg BMR was best explained by a positive relationship with catchment forest cover (p = 0.009, r2 = 0.59), likely reflecting effects of forest cover on water chemistry of downstream lakes and ultimately, concentrations of biomagnifying MeHg (and percent MeHg of total Hg) in resident biota. These findings improve our understanding of Hg biomagnification in remote subarctic lakes.

Assessment of seasonal and spatial water quality variation in a cascading lake system in Chennai, India

Water quality and scarcity are among the most severe problems humans have been facing in the last decades. India, as a fast-developing country, is not an exception. The surface water quality has deteriorated due to anthropogenic activities. Another factor which impacts the water quality is the heavy rainfall during monsoon season. To maintain the quality and the sustainability of water resources, there is the need to study how human activities impact water quality. We hypothesized that the water quality can be impacted by the spatial land use types and by the seasonality. In the present study, seasonal and spatial water quality regarding physical, chemical, and biological parameters from a lake cascading system was assessed monthly from July to December 2019. Land use/cover data was produced by Impact Observatory, Microsoft, and Esri based on the 10-m Sentinel-2 images. Redundancy analysis was applied to investigate the relationship between land use/cover data and water quality in the riparian of 500 and 1000 m to the lakes. Our results showed clear temporal and spatial variation of water quality in 2019, with better water quality in rainy season (Oct.-Dec.) and downstream lakes while relatively worse water quality was recorded in dry season (Jul.-Sep.) and upstream lakes. The water quality variation was explained 27.8 % and 42.7 % by the land use types within 500 m and 1000 m buffer widths, respectively. The outlet of the catchment showed exceptional results due to the impact of a dumpsite. Our findings indicate that the water quality is highly impacted by human-induced land use/cover. The land use/cover types, such as crops, woodland and urban area, show negative impacts and relate to the high level of nutrient concentrations. In opposite, grass land shows positive effects and leads to better water quality. Our study confirms that the lake water quality is distinguished in both spatial and seasonal aspects. Monsoon season improves the water quality.

Optical Characteristics of Chromophoric Dissolved Organic Matter (CDOM) in Upstream and Downstream Lakes of Taihu Lake Basin: New Insights for Water Environmental Management

Optical Characteristics of Chromophoric Dissolved Organic Matter (CDOM) in Upstream and Downstream Lakes of Taihu Lake Basin: New Insights for Water Environmental Management

The Outburst of a Lake and Its Impacts on Redistribution of Surface Water Bodies in High-Altitude Permafrost Region

The lakes distributed in permafrost areas on the Tibetan Plateau (TP) have been experiencing significant changes during the past few decades as a result of the climate warming and regional wetting. In September 2011, an outburst occurred on an endorheic lake (Zonag Lake) in the interior of the TP, which caused the spatial expansion of three downstream lakes (Kusai Lake, Haidingnor Lake and Salt Lake) and modified the four independent lake catchments to one basin. In this study, we investigate the changes in surficial areas and water volumes of the outburst lake and related downstream water bodies 10 years after the outburst. Based on the meteorological and satellite data, the reasons for the expansion of downstream lakes were analyzed. Additionally, the importance of the permafrost layer in determining hydrological process on the TP and the influence of from lake expansion on engineering infrastructures were discussed. The results in this study showed the downstream lakes increased both in area and volume after the outburst of the headwater. Meanwhile, we hope to provide a reference about surface water changes and permafrost degradation for the management of lake overflow and flood on the TP in the background of climate warming and wetting.

Origin, transport, and retention of fluvial sedimentary organic matter in South Africa's largest freshwater wetland, Mkhuze Wetland System

Abstract. Sedimentary organic matter (OM) analyses along a 130 km long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St Lucia, Africa's most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. Combining bulk OM analyses, such as Rock-Eval®, and source-specific biomarker analyses of plant-wax n-alkanes and their stable carbon (δ13C) and hydrogen (δD) isotopic composition showed that fluvial sedimentary OM originating from inland areas is mainly deposited in the floodplain and swamp area of the wetland system but not in the downstream lake area. A distinctly less degraded OM signature, i.e., a considerably lower degree of transformation of unstable components (higher I index) and lower contribution of refractory and persistent fractions (lower R index) as well as recognizably higher δD values compared to samples from upstream sub-environments, characterizes surface sediments of Lake St Lucia. The offset in δD indicates that the contributing vegetation, although similar to upstream vegetation inputs in terms of photosynthetic pathway (δ13C) and alkane distribution pattern, experienced different hydrological growth conditions. The results suggest that under current conditions hinterland sedimentary OM is deposited throughout the wetland system up to the Mkhuze Swamps, which ultimately captures the transported OM. Consequently, samples from the downstream located Lake St Lucia show locally derived signals instead of integrated signals encompassing the river catchment. This finding raises important constraints for future environmental studies as the assumption of watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore might not hold true in certain settings.

Fine root densities of grasses and perennial sugarcane significantly reduce stream channel erosion in southern China

Stream Channel (SC) erosion results in immense soil and nutrient losses and eutrophication of downstream lakes and rivers. Among other factors, the lack of vegetation cover within SCs and on adjacent soils can accelerate soil and nutrient losses. The combined impact of perennial sugarcane plantation and grass cover on increasing SC erosion losses have not been previously studied. Current study determined SC erosion and associated nutrient losses and clarified the effect of grass roots within SCs and the ratooned sugarcane grown on adjacent lands in Nala watershed in southern subtropics of China. Six SCs in two sub-watersheds (SW1 and SW2) surveyed after revegetation during 2019 were compared with initial survey conducted in 2017 and 2018. The SC erosion was significantly (P < 0.05) reduced by 84.2% (5.19 ± 2.16 Mg ha−1) in entire watershed in 2019 compared to 2018 (43.9 ± 15.2 Mg ha−1). It notably decreased by 99.23% and 96.50% in SC-1-3 at SW1 and SC2-3 at SW2, respectively. Total N and P losses decreased by 84.9% and 82.5%, respectively, in entire watershed. The decreases in SC erosion and nutrient losses are attributed to increasing vegetative cover (R2 = 0.7543, P < 0.001), and grass root densities of <1 mm (R2 = 0.7543, P < 0.001), 1–2 mm (R2 = 0.7051, P < 0.001) and >2 mm (R2 = 0.5746, P < 0.001). Principal component regression analysis confirmed that root densities of SC grasses, perennial sugarcane, and organic matter had positive impacts on controlling SC erosion and consequent nutrient losses.

Spatiotemporal changes of eutrophication and heavy metal pollution in the inflow river system of Baiyangdian after the establishment of Xiongan New Area.

Pollution in inflow rivers seriously endangers the water environment in downstream lakes. In this study, an inflow river system of the Baiyangdian–Fuhe river system (FRS) was investigated to display timely pollution patterns of eutrophication and heavy metals after the establishment of Xiongan New Area, aiming to reveal the weak parts in current pollution treatments and guide the further water quality management. The results showed that the pollution of eutrophication was worse than the heavy metals in FRS, with serious eutrophic parameters of ammonia nitrogen (NH4+-N) and chemical oxygen demand (COD). There were greatly spatiotemporal variations of the pollution in FRS. (1) Concentrations of NH4+-N and total phosphorus were all higher in summer and autumn, whereas, COD contents were higher in spring; the water quality index (WQI) of eutrophication linearly increased along FRS in summer and autumn, with pollution hotspots around the estuary area. (2) The pollution levels of plumbum exceeded cadmium (Cd) and chromium (Cr) but without strongly spatiotemporal changes; however, Cd and Cr in the town area and Cd in spring showed higher concentrations; the WQI of heavy metals showed single peak curves along FRS, with significantly higher values around the town area. Additionally, the four potential pollution sources: domestic sewage, traffic pollution, agricultural wastewater and polluted sediments were identified based on the pollution patterns and pollutant associations. These findings demonstrated current treatments failed to eliminate the pollution in some hotspots and periods, and the in-depth understanding of the pollution spatiotemporal patterns in this study, especially the pollution hotspots, serious periods and potential sources, are crucial to furtherly develop spatiotemporally flexible pollution treatment strategies.

Integrated scheduling–assessing system for drought mitigation in the river–connected lake

Lakes are important inland surface water resources and have great influence on the ecological environment as well as the surrounding residential life. However, global lake water resources showed a depleting tendency over the past decades because of the climate change and human activities. To mitigate the drought of lakes linked to a regulated main river, this study proposes an integrated scheduling–assessing system (ISAS) based on the machine learning methodology for a large river–lake system controlled by upstream reservoirs. Closely calibrated to observational data, the ISAS was applied to the middle Yangtze River to mitigate the Poyang Lake drought. The results show that the drought situation in the downstream lake could be improved through the reservoir optimal operation. For the Poyang Lake case, the lowest lake level is not obviously improved, while the starting data of the drought could be delayed by 12, 11, and 17 days, comparing to the conventional scheme in typical dry, normal, and wet years, respectively. Moreover, the duration of the drought could be 20, 19, and 21 days less. It is illustrated that accelerating the reservoir filling speed and decelerating the emptying speed is beneficial to alleviate the drought situation of downstream river-connected lakes.

Long-Distance Pipe Jacking in Complex Urban Geological Environment

A steel underground pipeline with a diameter of 2.4 m and a total length of 3,617 m (plate thickness of 26 mm) has been constructed in a city in central Hubei, and the engineering, procurement, and construction (EPC) project has been lifted from the upstream channel to supplement water to the downstream lake inside the city. Through preliminary geological survey data, site topographic and geomorphic survey, urban construction, as well as the requirements of the construction party, the preliminary arrangement of working wells and receiving wells as well as the selection and customization of pipe jacking machines have been proposed. Frequency conversion motor and remote monitoring technology are adopted for geotechnical change and long-distance pipe jacking. Through detailed survey, the rock and soil change section as well as gradual change conditions have been determined, the accuracy of construction mechanics calculation and construction operation control have improved, the basis and analysis basis are provided, and some experiences in construction operation are summarized.

Proposed Malewa dam in Kenya: Adequate adaptations to original design

Malewa dam in Kenya was first studied in the 1990s at preliminary design stage, through a study funded by Japanese Government who entrusted JICA with it. At that time, its main purpose was water supply to Nakuru, Naivasha and Gilgil cities in the Rift Valley. Thirty years later, its design was reviewed within the frame of a feasibility study funded by World Bank to account for revised needs -now excluding Nakuru- also taking into account increased awareness of environmental impact on downstream Lake Naivasha (Ramsar zone), management of the water scarcity, climate change trends, reliability issue, quality of water, sedimentation, as well as growing concern with operation and maintenance costs. Among studied solutions, focused on supply by gravity for economic reasons, and as ground water poses a serious health problem in this area due to a high fluoride content detrimental to human consumption, the construction of Malewa dam was confirmed to be the best solution after a decision making process, subject to some changes to the basic design, such as using the compensation flow to generate hydro-power to pump water to a WTP nearby the dam, mixing water with groundwater, building check dams and implementing water management and compensation measures.

Persistent contaminants of emerging concern in a great lakes urban-dominant watershed

Within the Great Lakes, contaminants of emerging concern (CECs) have been detected in urban surface waters. Understanding the human and ecological health consequences of environmentally-relevant exposures to persistent CECs, particularly as complex mixtures, is essential. Surface water and a limited number of sediment samples were collected over two years (Spring and Fall 2018–2019) at multiple locations in the Lake Huron to Lake Erie corridor to investigate more than 150 CECs. Surface water was analyzed for pharmaceutical and personal care products (PPCPs), pesticides, and per- and polyfluoroalkyl substances (PFAS). Sediment was analyzed for PFAS. Fifty compounds were detected at ng/L or ng/kg levels. Synthetic sweeteners accounted for 55.7% of the cumulative concentration of all compounds detected across sampling events, followed by pesticides (27.5%), pharmaceuticals (11.7%), and stimulants (3.5%), with 14 compounds consistently detected: acesulfame-potassium, sucralose, sulfamethoxazole, acetaminophen, lidocaine, atenolol, gemfibrozil, iohexol, atrazine, diaminochlorotriazine, deethyl-atrazine, deisopropylatrazine, 2,4-Dichlorophenoxyacetic acid, and caffeine. Compound mixtures became increasingly complex downstream and indicated that the downstream Lake Huron to Erie corridor is wastewater effluent-dominated, whereas the aquatic environment associated with Lake St. Clair is influenced by a mixed urban, suburban, and agricultural landscape. These results will inform follow-up studies on persistent contaminant mixtures and multiple stressors, and offer science-based priorities and evaluation objectives for natural resource and public health agencies/initiatives.

Transport and partitioning of metals in river networks of a plain area with sedimentary resuspension and implications for downstream lakes

This study showed that metal transport and partitioning are primarily controlled by suspended solids with seasonal flow regimes in plain river networks with sedimentary resuspension. Eight metal species containing iron (Fe), manganese (Mn), cadmium (Cd), chrome (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn), in multiple phases of sediments, suspended solids (>0.7 μm), colloids (1 nm-0.7 μm) and dissolved phase (<1 nm) were analysed to characterize their temporal-spatial patterns, partitioning and transport on a watershed scale. Metal concentrations were associated with suspended solids in the water column and decreased from low flow to high flow. However, metal partitioning between particulate phase (suspended solids) and dissolvable phases (colloids and dissolved phase) was reversed and increased from low flow to high flow with decreased concentration of total suspended solids and median particle size. Partition coefficients (kp) showed differences among metal species, with higher values for Pb (354.3–649.0 L/g) and Cr (54.2–223.7 L/g) and lower values for Zn (2.5–25.2 L/g) and Cd (17.3–21.0 L/g). Metal concentrations in sediments increased by factors of 1.2–3.0 from upstream to downstream in watersheds impacted by urbanization. The behaviours of metals in rivers provide deeper insight into the ecological risks they pose for downstream lakes, where increased redox potential and organic matter may increase metal mobility due to algal blooms. Areas with heavy pollution of metals and the transport routines of metals in the river networks were also revealed in our research.

Constraining the contribution of glacier mass balance to the Tibetan lake growth in the early 21st century

The Tibetan Plateau (TP) hosts numerous glaciers and lakes which are critical natural water reserves but highly vulnerable to changing climate. In contrast to general drying trends in global endorheic basins in recent decades, the widespread lake expansions across the endorheic TP stand out as a puzzling “anomaly”. To quantify the contribution of glacier mass changes to lake expansion at fine basin-scale details, we compute spatially resolved estimation of mass change in both glaciers and lakes across the endorheic TP between 2000 and 2010/14 based on multi-mission remote sensing observations. Our glacier mass balance estimates were based on the differences between the newly released global TanDEM-X DEM and the historical SRTM-C DEM, which provide nearly complete coverage (98%) of the glacierized area on the endorheic TP. We provide lake water storage changes of all lakes >1 km2 on the inner TP. These estimates reveal that the massive lake water increase (9.44 ± 1.43 Gt yr−1) was essentially not from the mass loss of glaciers which represents only about 4.7 ± 8.8% of the lake water change (0.44 ± 0.80 Gt yr−1). The relationship in individual basins was, however, highly heterogeneous. About 20% of total lake storage gain had no causality with glacier feeding. In comparison, for 28% of lake water surplus, mainly in the northwestern TP, the positive glacier mass balance infers that glaciers retained some precipitation surplus that could otherwise have been drained to downstream lakes. For the other 52% of lake storage gain, mostly in southern and eastern regions, the glacier mass loss varied among the basins with limited contributing levels (mostly <20%). Our analyses highlight remarkable spatial and temporal variabilities in lake/glacier changes on the endoreic TP and contribute to a better understanding of the role of glaciers in the recent Tibetan lake growth and the impact of climate change on the two types of water reserves.

Spatial and temporal patterns of urea content in a eutrophic stream continuum on the Northern Great Plains

Urea can degrade water quality and stimulate toxic phytoplankton in P-rich lakes, yet little is known of its sources, abundance, or transportation in lotic systems, particularly within the Northern Great Plains. We measured physico-chemical parameters biweekly during May–September 2010–2012 at 16 stations along a 250 km lotic continuum to quantify spatial and temporal variation in urea concentrations and discharge, and to identify potential regulatory processes. Urea concentrations were similar to those in regional prairie lakes (range 5.2–792.1, median 78.6 μg N L−1) with variable seasonal mean (± SD) concentrations (96.6 ± 96.1 μg N L−1) and fluxes (4.22 × 105 ± 257.6 μg N s−1). Landscape analysis with generalized additive models explained 68.3% of deviance in urea concentrations, with high temporal variability predicted mainly by positive relationships with nutrient content and chlorophyte abundance, but not temperature, dissolved organic matter, bacterial abundance, or urban effluent. Seasonal analysis revealed that during spring, urea content was correlated negatively with leguminous forage cover (% area) and positively with stream discharge, oilseed and cereal crops, and shrubs or deciduous plants, while during summer, urea concentrations were correlated negatively with discharge and leguminous crop cover, as well as nutrient levels. Mean porewater urea concentrations (528.5 ± 229.8 μg N L−1) were over five-fold greater than stream concentrations, suggesting that hyporheic production may offset declining influx from terrestrial sources during summer. We conclude that urea may be ubiquitous in eutrophic prairie streams and that management of its export from land may reduce detrimental effects on downstream lakes.

Simulated Dynamics of Mixed Versus Uniform Grain Size Sediment Pulses in a Gravel‐Bedded River

AbstractMountain rivers often receive sediment in the form of episodic, discrete pulses from a variety of natural and anthropogenic processes. Once emplaced in the river, the movement of this sediment depends on flow, grain size distribution, and channel and network geometry. Here, we simulate downstream bed elevation changes that result from discrete inputs of sediment (∼10,000 m3), differing in volume and grain size distribution, under medium and high flow conditions. We specifically focus on comparing bed responses between mixed and uniform grain size sediment pulses. This work builds on a Lagrangian, bed‐material sediment transport model and applies it to a 27 km reach of the mainstem Nisqually River, Washington, USA. We compare observed bed elevation change and accumulation rates in a downstream lake to simulation results. Then we investigate the magnitude, timing, and persistence of downstream changes due to the introduction of synthetic sediment pulses by comparing the results against a baseline condition (without pulse). Our findings suggest that bed response is primarily influenced by the sediment‐pulse grain size and distribution. Intermediate mixed‐size pulses (∼50% of the median bed gravel size) are likely to have the largest downstream impact because finer sizes translate quickly and coarser sizes (median bed gravel size and larger) disperse slowly. Furthermore, a mixed‐size pulse, with a smaller median grain size than the bed, increases bed mobility more than a uniform‐size pulse. This work has important implications for river management, as it allows us to better understand fluvial geomorphic responses to variations in sediment supply.

Characterizing Sources and Composition of Chromophoric Dissolved Organic Matter in a Key Drinking Water Reservoir Lake Tianmu

Lake Tianmu is an important source of drinking water, and its water quality can influence ecosystem service functions. Unraveling the sources and composition of chromophoric dissolved organic matter (CDOM) that can affect water treatment processes is necessary to maintain water supply safety and ecosystem service functioning of Lake Tianmu. Samples were collected monthly in 2017 and analyzed for CDOM absorbance and fluorescent spectra using parallel factor analysis (PARAFAC) to investigate the spatial and temporal variations of CDOM sources and composition in Lake Tianmu. PARAFAC results showed that CDOM in Lake Tianmu was mainly composed of a microbial humic-like component C1 (44.2%±9.8%), followed by a tryptophan-like component C2 (29.2%±4.3%), tyrosine-like component C3 (17.2%±13.1%), and terrestrial humic-like component, C4 was the lowest (9.4%±2.4%). The CDOM abundance a(254) and fluorescence intensities of C1 and C2 were significantly higher in the river mouths than in the downstream lake regions, whereas the spectral slope S275-295 was significantly lower in the river mouths (t-test, P<0.05), indicating that allochthonous inputs cause an elevated degree of humification and relative increase in the molecular weight of CDOM in the inflowing river mouths. Seasonal differences in CDOM composition were mainly ascribed to the a(254) and fluorescence intensities of C1, C2, and C4 being significantly higher in the summer and autumn than in the winter and spring (t-test, P<0.05). Our results showed that the influences of different seasons on CDOM composition comprise differences in rainfall and runoff input, as well as water temperature, thermal stratification, phytoplankton biomass, and mineralization of CDOM by light and microbes.