The impact of rainfall events, catchment characteristics and estuarine processes on the export of dissolved organic matter from two lowland rivers and their shared estuary

Enhancing machine learning models for total organic carbon prediction by integrating geospatial parameters in river watersheds

This study utilizes machine learning (ML) algorithms to develop a robust total organic carbon (TOC) prediction model for river waters in the Geumho River sub-basins, South Korea, considering both non-rain and rain events. The model incorporates geospatial parameters such as land use, slope, flow rate, and basic water quality metrics including biochemical oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and suspended solids (SS). A key aspect of this research is examining how land use information enhances the model's predictive accuracy. We compared two ML algorithms-extreme gradient boosting (XGBoost) and deep neural networks (DNN)-with a traditional multiple linear regression (MLR) approach. XGBoost outperformed the others, achieving an R2 value between 0.61 and 0.68 in the test dataset and demonstrating significant improvement during rain events with an R2 of 0.77 when including land use data. In contrast, this enhancement was not observed with the MLR model. Feature importance analysis using Shapley values highlighted COD as the primary predictor for non-rain events, while during rain events, COD, TP, TN, SS and agricultural land collectively influenced TOC levels. This study significantly advances understanding of TOC variability across different land use scenarios in river systems and underscores the importance of integrating geospatial and water quality parameters to enhance TOC prediction, particularly during rain events. This methodology provides a valuable framework for developing river management strategies and monitoring long-term TOC trends, especially in scenarios with gaps in essential monitoring data.

The multiscale nexus among land use-land cover changes and water quality in the Suquía River Basin, a semi-arid region of Argentina

Agricultural intensification and urban sprawl have led to significant alterations in riverscapes, and one of the critical consequences is the deterioration of water quality with significant implications for public health. Therefore, the objectives of this study were the assessment of the water quality of the Suquía River, the assessment of LULC change at different spatial scales, and the analysis of the potential seasonal correlation among LULC change and Water Quality Index (WQI). The Sample Sites (SS) 1 and 2 before Cordoba city had the highest WQI values while from SS3 the WQI decreased, with the lowest WQI close to the wastewater treatment plant (SS7) after Cordoba city. From SS8 in a agricultural context, the WQI increases but does not reach the original values. In light of analysis carried out, the correlation between water quality variables and the different LULC classes at the local and regional scales demonstrated that WQI is negatively affected by agricultural and urban activities, while natural classes impacted positively. The spatialization of the results can help strongly in assessing and managing the diffusion of point and non-point pollution along the riverscape. The knowledge gained from this research can play a crucial role in water resources management, which supports the provision of river ecosystem services essential for the well-being of local populations.

Stormwater discharge: An overlooked source of disinfection byproduct precursors

Discharge from the stormwater system is as an important pathway for contaminant transport, impacting the quantity and characteristics of dissolved organic matter (DOM) in surface water, and thus the formation of disinfection byproducts (DBPs) during downstream drinking water disinfection. In this study, DOM in stormwater pipes was characterized by size-exclusion chromatography, and the formation of 27 DBPs and halogen-specific total organic halogen (TOX) following chlorination was investigated. Overall, DOM in stormwater pipes was characterized by low molecular weight compounds and microbial-derived organics. Total DBP concentrations in chlorinated stormwaters were ∼1-15 times higher than in chlorinated surface waters. DBPs formed in stormwaters were dominated by trihalomethanes and haloacetic acids. Moreover, the DBP-associated toxicity of chlorinated stormwaters was ∼1-38 times higher than in chlorinated surface waters, and mainly due to the presence of large amount of haloacetaldehydes and haloacetonitriles. Sampling during a rainfall event suggested that stormwater discharge significantly increased DBP precursors in the surface water. The high formation and estimated toxicity of DBPs in stormwater discharge indicates this is an overlooked source of DBP precursors, posing a threat to the aquatic environment and potentially drinking water quality.

Effect of damming on hydrogeochemical characteristics and potential environmental risks in a large reservoir: Insights from different vertical layer sampling

The water environment of large reservoirs is fragility due to effects from hydrological regulation of damming and anthropogenic inputs. As a critical path to quantify the natural chemical weathering and assess environmental risks, solute chemistry of river has been widely focused on. However, the complexed hydrological conditions of large reservoir affect the chemical compositions, and the significance of solute vertical geochemistry as an indicator of chemical weathering and water quality health remains explore. Therefore, the Three Gorges Reservoir (TGR) was selected as a typical study area, which is the world's largest hydropower project and subject to frequent water quality problems. Then, the chemical compositions in stratified water were determined. Ca2+ (52.8 ± 4.3 mg/L) and HCO3- (180.9 ± 8.9 mg/L) were the most abundant ions among cations and anions, respectively. Incremental mean concentration of total major ions followed with the increase of riverine depth and flow direction. An improved inversion model was used to quantify the source contribution, which weathering of dolomite (34%) and calcite (38%) contributed the most to total cations, and the influences of agriculture and sewage discharge were limited. Additional contributions of evaporite and pyrite oxidation were found in analysis of deeper water samples, which also results in 2%-67% difference in estimated CO2 release flux using data from different depth, indicating additional information about sulfuric acid driven weathering was contained. Finally, the water quality of the reservoir was assessed for irrigation and non-carcinogenic risks. Results showed the stratified water of TGR can be used as a good water source of irrigation. However, NO3- (5.1 ± 1.1 mg/L) may have a potential non-carcinogenic risk to children, especially in surface water. To sum up, this study provided an indispensable supplement to the water chemistry archives in the TGR basin, serving as theoretical references for environmental management of large reservoirs.

Phytoplankton in headwater streams: spatiotemporal patterns and underlying mechanisms

Phytoplankton are key members of river ecosystems wherein they influence and regulate the health of the local environment. Headwater streams are subject to minimal human activity and serve as the sources of rivers, generally exhibiting minimal pollution and strong hydrodynamic forces. To date, the characteristics of phytoplankton communities in headwater streams have remained poorly understood. This study aims to address this knowledge gap by comparing phytoplankton communities in headwater streams with those in plain rivers. The results demonstrated that within similar watershed sizes, lower levels of spatiotemporal variability were observed with respect to phytoplankton community as compared to plain rivers. Lower nutrient levels and strong hydrodynamics contribute to phytoplankton growth limitation in these streams, thereby reducing the levels of spatiotemporal variation. However, these conditions additionally contribute to greater phytoplankton diversity and consequent succession towards Cyanophyta. Overall, these results provide new insights into the dynamics of headwater stream ecosystems and support efforts for their ecological conservation.

Deciphering the water quality impacts of COVID-19 human mobility shifts in estuaries surrounding New York City

The COVID-19 pandemic altered human mobility, particularly in large metropolitan areas. In New York City (NYC), stay-at-home orders and social distancing led to significant decreases in commuting, tourism, and a surge of outward migration. Such changes could result in decreased anthropogenic pressure on local environments. Several studies have linked COVID-19 shutdowns with improvements in water quality. However, the bulk of these studies primarily focused on short-term impacts during shutdown periods, without assessing longer-term impacts as restrictions eased. Here, we examine both concurrent lockdown and societal reopening impacts on water quality, using pre-pandemic baseline conditions, in two highly urbanized estuaries surrounding NYC, the New-York Harbor estuary and Long Island Sound (LIS). We compiled datasets from 2017 to 2021 of mass-transit ridership, work-from-home trends, and municipal wastewater effluent to assess changes in human mobility and anthropogenic pressure during multiple waves of the pandemic in 2020 and 2021. These were linked to changes in water quality assessed using high spatiotemporal ocean color remote sensing, which provides near-daily observations across the estuary study regions. To distinguish anthropogenic impacts from natural environmental variability, we examined meteorological/hydrological conditions, primarily precipitation and wind. Our results show that nitrogen loading into the New York Harbor declined significantly in the spring of 2020 and remained below pre-pandemic values through 2021. In contrast, nitrogen loading into LIS remained closer to the pre-pandemic average. In response, water clarity in New-York Harbor significantly improved, with less of a change in LIS. We further show that changes in nitrogen loading had higher impact on water quality than meteorological conditions. Our study demonstrates the value of remote sensing observations in assessing water quality changes when field-based monitoring is hindered and highlights the complex nature of urban estuaries and their heterogeneous response to changes in extreme events and human behavior.

Water quality and the health of remnant leaf oyster (Isognomon ephippium) populations in four Australian estuaries

Leaf oysters (Isognomon ephippium) are large intertidal bivalves that form shellfish reefs. They have a patchy and restricted distribution in estuaries in northern New South Wales, Australia, where the water quality is impacted by a range of anthropogenic stressors from coastal agriculture, urbanisation, industry and recreational activities, along with natural stochastic events such as flooding. Little, however, is currently known about the tolerance of leaf oysters to poor water quality. This study investigated the condition of leaf oyster populations in four estuaries, by assessing the density, size-frequency and condition index. These biological parameters were modelled against habitat water quality parameters including nutrient loads, organic and inorganic carbon, algal biomass, sedimentation and total alkalinity. The water quality assessments were replicated during dry conditions and after rain in two estuaries. Leaf oyster density, size, and body condition significantly differed among the four estuaries. Density ranged from as few as 0.04 oysters per m² in the heavily impacted Richmond River, up to 267 oysters per m² in the Tweed River. Overall water quality was also significantly different among the four estuaries, and significant declines in water quality were observed after rain. Distance based linear models revealed significant correlations between biological indicators of leaf oyster reef health and a range of water quality parameters. Leaf oysters with higher condition occur in locations with high dissolved oxygen and salinity levels, whereas low pH and high nutrient run-off were associated with low density and poorer oyster condition. Nevertheless, dense populations were found to persist in areas with pH below 7.9, in high turbidity (over 27 NTU) and areas dominated by fine silt. Their ability to form dense reefs on estuarine soft muddy bottom habitats with low water flow, suggests that leaf oysters could be used to complement rock oysters in future intertidal shellfish reef restoration programs.

Spatiotemporal variations of surface water quality in a medium-sized river catchment (Northwestern Germany) with agricultural and urban land use over a five-year period with extremely dry summers

Medium-sized rivers, which are used for intensive agriculture and urban infrastructure, are subject to manifold hydrochemical stressors. Identifying and monitoring these stressors is important for river basin management and a functioning ecosystem. To understand the spatiotemporal variation of surface water quality in a highly modified lowland river, the Münstersche Aa River (Northwestern Germany) with 62% of land used for agriculture and 26% urban/residential area, was exemplarily studied. A total of 519 samples were collected using two automated high-frequency samplers and five catchment-wide sampling campaigns. They covered the five-year period 2015-2020 and included two extremely dry summers. The Münstersche Aa catchment is dominated by low permeable strata resulting in surface water runoff (Baseflow Index: 0.41) which leads to a high amplitude of discharge variation (mean discharge: 0.7 m³/s) with high flow conditions in winter/spring, and low discharge during summer/fall. In wintertime, maximum nitrate concentrations (up to 73 mg NO₃/L) and loads (up to 1300 t NO₃/a; up to 98% in winter) correlate with high-flow conditions. δ¹⁸O and δ¹⁵N isotopic analysis indicated manure from farmland as the major source of nitrate whereas the impact of municipal wastewater treatment plants was neglectable. Increased nitrate concentrations are linked to the higher proportion of farmland in the upper catchment (77%) compared with the lower catchment (47%). In summertime, at extremely low flow conditions, surface water consisted of up to 100% of treated wastewater, resulting in the highest measured chloride, sodium and potassium concentrations. The river is impacted by strongly seasonal and different stressors, which can be expected to intensify with ongoing climate change. Results from this study may help to adapt monitoring schemes for the Münstersche Aa but also for other lowland streams with comparable land-use targeting the goals of the Water Framework Directive.

Changes in the Dissolved Organic Matter Characteristics Released from Sediment According to Precipitation in the Namhan River with Weirs: A Laboratory Experiment

In this study, changes in the properties of dissolved organic matter (DOM) released from sediments into water layers were investigated. To analyze the spatial and temporal variation in dissolved organic carbon (DOC), sediment and bottom water samples were collected upstream of the Gangcheon, Yeoju, and Ipo weirs of the Namhan River during the rainy and non-rainy seasons. The initial DOC was correlated with precipitation (R² = 0.295, p = 0.034) and residence time (R² = 0.275, p = 0.040). The change in the bottom water DOC concentration resulted from the DOC released from the sediments, which may cause water quality issues in the bottom water. The fluorescence analysis revealed that the DOM contained higher levels of hydrophilic and low-molecular-weight (LMW) organic matter in the non-rainy season and higher levels of hydrophobic and high-molecular-weight (HMW) organic matter in the rainy season. Since the Namhan River is the main resource of drinking water for the Seoul metropolitan area, our results can help to optimize the drinking water treatment process by reflecting the DOM characteristics that vary with the seasons. Furthermore, the statistical analysis confirmed that the nutrient content of pore-water and sediment can be used to estimate the DOM release rate from the sediment to the water layer. The results of this study provide a better understanding of DOM movement in aquatic ecosystems and the influences of rainfall on the water quality of the surface waterbody.

Microbial dynamics and activity of denitrifying anaerobic methane oxidizers in China's estuarine and coastal wetlands

Estuarine and coastal wetlands, which act as large sources of methane (CH₄) and undergo substantial loading of anthropogenic nitrogen (N), provide ideal conditions for denitrifying anaerobic methane oxidation (DAMO) to occur. Yet the microbial mechanisms governing DAMO and the main driving factors in estuarine and coastal ecosystems remain unclear. This study investigated the spatiotemporal distribution and associated activity of DAMO microorganisms along a wide swath of China's coastline (latitudinal range: 22-41°N) using molecular assays and isotope tracing techniques. We uncovered significant spatial and seasonal variation in DAMO bacterial community structure, whereas DAMO archaeal community structure exhibited no seasonal differences. The abundance of DAMO bacterial pmoA gene (2.2 × 10⁵-1.0 × 10⁷ copies g^-1) was almost one order of magnitude higher than that of DAMO archaeal mcrA gene (8.7 × 10⁴ -1.8 × 10⁶ copies g^-1). A significant positive correlation between pmoA and mcrA gene abundances (p < 0.01) was observed, indicating that DAMO bacteria and archaea may cooperate closely and thus complete nitrate elimination. Potential DAMO rates, in the range of 0.09-23.4 nmol ¹³CO₂ g^-1 day^-1 for nitrite-DAMO and 0.03-43.7 nmol ¹³CO₂ g^-1 day^-1 for nitrate-DAMO, tended to be greater in the relatively warmer low-latitudes. Potential DAMO rates were weakly positively correlated with gene abundances, suggesting that DAMO microbial activity could not be predicted directly by gene abundance alone. The heterogeneous variability of DAMO was shaped by interactions among key environmental characteristics (sediment texture, N availability, TOC, Fe³⁺, salinity of water, and temperature). On a broader continental scale, potential N removal rates of 0.1-11.2 g N m^-2 yr^-1 were estimated via nitrite-DAMO activity in China's coastal wetlands. Overall, our results highlight the widespread distribution of DAMO microbes and their potential role in eliminating excess N inputs and reducing CH₄ emissions in estuarine and coastal ecosystems, which could help mitigate global warming.

Fluorophores in surface freshwaters: importance, likely structures, and possible impacts of climate change

Fluorescence spectroscopy is one of the most useful techniques currently available for the characterisation of organic matter in natural water samples, because it combines easy availability of instrumentation, high sensitivity and limited requirements for sample treatment. The main fluorophores that can be found in natural waters are usually proteins (and/or free amino acids) and humic substances (humic and fulvic acids). The identification of these fluorescent compounds in water samples helps to obtain information about, among others, biological activity in the water body, possible transport of organic matter from soil, and the phenomenon of photobleaching that decreases both the absorbance and (usually) the fluorescence of natural organic matter. Interestingly, all these phenomena can be affected by climate change, which could alter to different extents the ratio between aquagenic and pedogenic fluorophores. Several events induced by warming in natural waters (and especially lake water) could enhance algal growth, thereby also enhancing the production of aquagenic organic matter. Intense precipitation events could increase the export of pedogenic material to surface waters, while photobleaching would be enhanced in the epilimnion of lakes when summer stratification becomes longer and more stable because of higher temperatures. Interestingly, photobleaching affects humic substances to a higher extent compared to protein-like material, thus protein fluorescence signals could be more preserved in stratified waters.

Foreseen Effects of Climate-Impacted Scenarios on the Photochemical Fate of Selected Cyanotoxins in Surface Freshwaters

Cyanobacteria populate most water environments, and their ability to effectively exploit light and nutrients provide them with a competitive advantage over other life forms. In particular conditions, cyanobacteria may experience considerable growth and give rise to the so-called harmful algal blooms (HABs). HABs are often characterized by the production of cyanotoxins, which cause adverse effects to both aquatic organisms and humans and even threaten drinking water supplies. The concentration of cyanotoxins in surface waters results from the budget between production by cyanobacteria and transformation, including photodegradation under sunlight exposure. Climate change will likely provide favorable conditions for HABs, which are expected to increase in frequency over both space and time. Moreover, climate change could modify the ability of some surface waters to induce phototransformation reactions. Photochemical modeling is here carried out for two cyanotoxins of known photoreaction kinetics (microcystin-LR and cylindrospermopsin), which follow different phototransformation pathways and for particular freshwater scenarios (summertime stratification in lakes, water browning, and evaporative water concentration). On this basis, it is possible to quantitatively predict that the expected changes in water-column conditions under a changing climate would enhance photodegradation of those cyanotoxins that are significantly transformed by reaction with the triplet states of chromophoric dissolved organic matter (³CDOM*). This is known to be the case for microcystin-LR, for which faster photodegradation in some environments would at least partially offset enhanced occurrence. Unfortunately, very few data are currently available for the role of ³CDOM* in the degradation of other cyanotoxins, which is a major knowledge gap in understanding the link between cyanotoxin photodegradation and changing climate.

Strong Precipitation and Human Activity Spur Rapid Nitrate Deposition in Estuarine Delta: Multi-Isotope and Auxiliary Data Evidence

The intensive development of the Yellow River Delta has caused huge transportation of non-point pollutants into the Bohai Sea through source river estuaries and thus poses a considerable threat to eco-environmental security in the region. Long-term irrigation in the Yellow River basin, with occasional heavy rainfall and the related effects of ensuring hydrological processes and human activities in terms of nitrate N transport via surface water systems, is unclear. Using stable isotope (δ2H-H2O and δ18O-H2O, δ15N-NO3− and δ18O-NO3−) and auxiliary geographic data, the ISO source model was run to quantitatively analyze the supply relationship of river systems and the rapid change in the spatial pattern of nitrate N due to heavy rainfall in the estuarine delta. This analysis made clear the dominant contribution of agricultural activities and urbanization to NO3−-N emission, on which basis refined management measures were proposed to deal with NO3− in surface water from the “source-process”. The results of the study show that: (1) The relationship of surface water replenishment in the Yellow River Delta was affected not only by rainfall, irrigation, and other water conservancy measures but also the proportion of water from Yellow River flow declined from the source to estuary; (2) To a certain extent, rainfall diluted the concentration of nitrate N in the river and increased instantaneous flux of nitrate N into the sea, where nitrate N flux continuously increased from upstream to downstream; (3) The rapid deposition of nitrate in the estuary delta was driven by heavy rainfall and human activities such as excessive use of nitrogen fertilizers, rapid urbanization, and livestock waste discharge, and; (4) Scientific measures were needed to realize the interactive effect of the output of non-point source pollutants and the carrying and absorption capacity of coastal fragile ecosystems of the exogenous inputs.