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Climate change is expected to significantly deteriorate water quality in heavily managed agricultural landscapes, however, the exact mechanisms of these impacts are unknown. In this study we adopted a modelling approach to predict the multiple effects of climate change on hydrological and biogeochemical responses for dominant solutes and particulates in two agriculture-dominated temperate headwater catchments. We used climatic projections from three climatic models to simulate future flows, mobilisation and delivery of solutes and particulates. This allowed an examination of potential drivers by identifying changes in flow pathway distribution and key environmental variables. We found that future climate conditions will lead to a general increase in stream discharge as well as higher concentrations and loads of solutes and particulates. However, unlike previous studies, we observed a higher magnitude of change during the warmer part of the year. These changes will reduce the relative importance of winter flows on solute and particulate transport, leading to both higher and more evenly distributed concentrations and loads between seasons. We linked these changes to the higher importance of superficial flow pathways of tile and surface runoff driven by more rapid transition from extremely wet to dry conditions. Overall, the observed increase in solute and particulate mobilisation and delivery will lead to widespread water quality deterioration. Mitigation of this deterioration would require adequate management efforts to address the direct and indirect negative effects on stream biota and water scarcity., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Heavily modified headwater streams and open ditches carry high nitrogen loads from agricultural soils that sustain eutrophication and poor water quality in downstream aquatic ecosystems. To remediate agricultural streams and reduce the export of nitrate (NO 3 - ), phosphorus and suspended sediments, two-stage ditches with constructed floodplains can be implemented as countermeasures. By extending hydrological connectivity between the stream channel and riparian corridor within constructed floodplains, these remediated ditches enhance the removal of NO 3 - via the microbial denitrification process. Ten remediated ditches were paired with upstream trapezoidal ditches in Sweden across different soils and land uses to measure the capacity for denitrification and nitrous oxide (N 2 O) production and yields under denitrifying conditions in stream and floodplain sediments. To examine the controls for denitrification, water quality was monitored monthly and flow discharge continuously along reaches. Floodplain sediments accounted for 33 % of total denitrification capacity of remediated ditches, primarily controlled by inundation and stream NO 3 - concentrations. Despite reductions in flow-weighted NO 3 - concentrations along reaches, NO 3 - removal in remediated ditches via denitrification can be masked by inputs of NO 3 - -rich groundwaters, typical of intensively managed agricultural landscapes. Although N 2 O production rates were 50 % lower in floodplains compared to the stream, remediated ditches emitted more N 2 O than conventional trapezoidal ditches. Higher denitrification rates and reductions of N 2 O proportions were predicted by catchments with loamy soils, higher proportions of agricultural land use and lower floodplain elevations. For realizing enhanced NO 3 - removal from floodplains and avoiding increased N 2 O emissions, soil type, land use and the design of floodplains need to be considered when implementing remediated streams. Further, we stress the need for assessing the impact of stream remediation in the context of broader catchment processes, to determine the overall potential for improving water quality., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personalrelationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

The Arctic environment harbors a complex mosaic of mercury (Hg) and carbon (C) reservoirs, some of which are rapidly destabilizing in response to climate warming. The sources of riverine Hg across the Mackenzie River basin (MRB) are uncertain, which leads to a poor understanding of potential future release. Measurements of dissolved and particulate mercury (DHg, PHg) and carbon (DOC, POC) concentration were performed, along with analyses of Hg stable isotope ratios (incl. ∆ 199 Hg, δ 202 Hg), radiocarbon content (∆ 14 C) and optical properties of DOC of river water. Isotopic ratios of Hg revealed a closer association to terrestrial Hg reservoirs for the particulate fraction, while the dissolved fraction was more closely associated with atmospheric deposition sources of shorter turnover time. There was a positive correlation between the ∆ 14 C-OC and riverine Hg concentration for both particulate and dissolved fractions, indicating that waters transporting older-OC ( 14 C-depleted) also contained higher levels of Hg. In the dissolved fraction, older DOC was also associated with higher molecular weight, aromaticity and humic content, which are likely associated with higher Hg-binding potential. Riverine PHg concentration increased with turbidity and SO 4 concentration. There were large contrasts in Hg concentration and OC age and quality among the mountain and lowland sectors of the MRB, which likely reflect the spatial distribution of various terrestrial Hg and OC reservoirs, including weathering of sulfate minerals, erosion and extraction of coal deposits, thawing permafrost, forest fires, peatlands, and forests. Results revealed major differences in the sources of particulate and dissolved riverine Hg, but nonetheless a common positive association with older riverine OC. These findings reveal that a complex mixture of Hg sources, supplied across the MRB, will contribute to future trends in Hg export to the Arctic Ocean under rapid environmental changes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Agricultural land use covers almost half of the EU territory and reducing nutrient and pesticide losses to freshwaters is central to existing EU policy. However, the progress of improving freshwater quality and reducing eutrophication is slow and lags behind targets. The Green Deal is a key element of the EU plans to implement the United Nation's Sustainable Development Goals. Here, we discuss the opportunities that the Green Deal and associated strategies may provide for the achievement of the water quality goals of the Water Framework Directive in agricultural landscapes. We welcome Green Deal's aspirational stated goals. However, the reliance of mitigation of diffuse agricultural pollution on the reform of the Common Agricultural Policy represents grave risks for practical implementation and the achievement of the Green Deal objectives. We also argue that the new strategies should be targeted at tackling and understanding the sources of water quality problems along the full pollution continuum. To maximise the opportunities for tackling diffuse pollution from agricultural land use and achieving the delayed water quality targets, we stress that a range of targeted new instruments will be needed to close the gaps in the pollution continuum 'from source to impact'. These gaps include: (I) smart and standardised monitoring of the impacts of proposed eco-schemes and agri-environment-climate measures, (ii) active restoration of agricultural streams and ditches and their floodplains to reduce secondary pollution sources, (iii) options to draw down nutrient levels to or below the agronomic optimum that reduce legacy sources, (iv) integrating farm-scale and catchment-scale analysis of trade-offs in reducing different pollutants and their combined effects, and finally (v) accounting for emerging pressures to freshwater quality due to climate change. Incorporation of the pollution continuum framework into tackling diffuse agricultural pollution will ensure that the European water-related policy goals are achieved., Competing Interests: Declaration of competing interest The authors express no conflict of interest including any financial and personal relationships with other people or organizations that could inappropriately influence (bias) their work., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Nutrient losses from headwater catchments (<50 km 2 ) cause eutrophication problems downstream. Catchment properties are strongly reflected in the levels of nutrient concentrations in headwater streams. Based on measurements of total and dissolved nitrogen (TN, DN) and phosphorus (TP, DP) in 235 small headwater streams, we showed that proportion of arable land in a catchment had the strongest positive effect on nutrient concentrations, with coefficient of determination (R 2 ) of 0.54, 0.64, 0.45, and 0.51 for TN, DN, TP, and DP, respectively. In contrast, increased proportion of forest and wetland led to lower nutrient concentrations in streams. The geological composition of catchments had a major influence on the soil properties. In turn, certain soil properties, such as clay content and content of aluminum (Al), an important binding agent of P, influenced losses of particulate P (PP) and DP, respectively. Consequently, by using soil properties as a link between geology and water quality, areas potentially sensitive to nutrient losses were identified by classifying bedrock categories into three geological groups. Approximately 25% of Swedish arable land was identified as potentially sensitive. Sensitive catchments were found in regions with sedimentary bedrock and showed higher concentrations of dissolved nutrient fractions even when the proportion of agricultural land was small, indicating higher background concentrations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Recent technological breakthroughs of optical sensors and analysers have enabled matching the water quality measurement interval to the time scales of stream flow changes and led to an improved understanding of spatially and temporally heterogeneous sources and delivery pathways for many solutes and particulates. This new ability to match the chemograph with the hydrograph has promoted renewed interest in the concentration-discharge (c-q) relationship and its value in characterizing catchment storage, time lags and legacy effects for both weathering products and anthropogenic pollutants. In this paper we evaluated the stream c-q relationships for a number of water quality determinands (phosphorus, suspended sediments, nitrogen) in intensively managed agricultural catchments based on both high-frequency (sub-hourly) and long-term low-frequency (fortnightly-monthly) routine monitoring data. We used resampled high-frequency data to test the uncertainty in water quality parameters (e.g. mean, 95th percentile and load) derived from low-frequency sub-datasets. We showed that the uncertainty in water quality parameters increases with reduced sampling frequency as a function of the c-q slope. We also showed that different sources and delivery pathways control c-q relationship for different solutes and particulates. Secondly, we evaluated the variation in c-q slopes derived from the long-term low-frequency data for different determinands and catchments and showed strong chemostatic behaviour for phosphorus and nitrogen due to saturation and agricultural legacy effects. The c-q slope analysis can provide an effective tool to evaluate the current monitoring networks and the effectiveness of water management interventions. This research highlights how improved understanding of solute and particulate dynamics obtained with optical sensors and analysers can be used to understand patterns in long-term water quality time series, reduce the uncertainty in the monitoring data and to manage eutrophication in agricultural catchments., (Copyright © 2018 Elsevier B.V. All rights reserved.)

In agricultural catchments, diffuse nutrient fluxes (mainly nitrogen N and phosphorus P), are observed to pollute receiving waters and cause eutrophication. Organic matter (OM) is important in mediating biogeochemical processes in freshwaters. Time series of the variation in nutrient and OM loads give insights into flux processes and their impact on biogeochemistry but are costly to maintain and challenging to analyse for elements that are highly reactive in the environment. We evaluated the capacity of the automated monitoring to capture typically low baseflow concentrations of the reactive forms of nutrients and OM: total reactive phosphorus (TRP), nitrate nitrogen (NO 3 -N) and tryptophan-like fluorescence (TLF). We compared the performance of in situ monitoring (wet chemistry analyser, UV-vis and fluorescence sensors) and automated grab sampling without instantaneous analysis using autosamplers. We found that automatic grab sampling shows storage transformations for TRP and TLF and do not reproduce the diurnal concentration pattern captured by the in situ analysers. The in situ TRP and fluorescence analysers respond to temperature variation and the relationship is concentration-dependent. Accurate detection of low P concentrations is particularly challenging due to large errors associated with both the in situ and autosampler measurements. Aquatic systems can be very sensitive to even low concentrations of P typical of baseflow conditions. Understanding transformations and measurement variability in reactive forms of nutrients and OM associated with in situ analysis is of great importance for understanding in-stream biogeochemical functioning and establishing robust monitoring protocols., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Monthly raw and clarified water samples were obtained for 16 UK surface water treatment works. The fluorescence excitation-emission matrix (EEM) technique was used for the assessment of total organic carbon (TOC) removal and organic matter (OM) characterisation. The impact of algae presence in water on TOC removal, and its relationship with fluorescence, was analysed. Fluorescence peak C intensity was found to be a sensitive and reliable measure of OM content. Fluorescence peak C emission wavelength and peak T intensity (reflecting the degree of hydrophobicity and the microbial fraction, respectively) were found to characterize the OM; the impact of both on TOC removal efficiency was apparent. OM fluorescence properties were shown to predict TOC removal, and identify spatial and temporal variations. Previous work indicates that the trihalomethane (THM) concentration of treated water can be predicted from the raw water TOC concentration. The simplicity, sensitivity, speed of analysis and low cost, combined with potential for incorporation into on-line monitoring systems, mean that fluorescence spectroscopy offers a robust analytical technique to be used in conjunction with, or in place of, other approaches to OM characterisation and THM formation prediction.