Nutrient criteria for surface waters under the European Water Framework Directive: Current state-of-the-art, challenges and future outlook

Reduced greenhouse gas emissions from particulate organic matter degradation in iron-enriched sediments

Iron (Fe) plays an important role in the biogeochemical cycling of carbon and nutrients in aquatic systems. Reactive Fe phases can interact with organic carbon and facilitate the removal of carbon from the biogeochemical cycle; however, this important ecosystem function is often strongly controlled by Fe availability. Due to pollution from lignite mining in the Lusatian province in Northeast Germany, large amounts of iron and sulfate are released into the fluvial-lacustrine system of the Spree River. It was hypothesized that the input of freshly precipitated iron oxyhydroxides from mining areas (e.g., ferrihydrite) alter the biodegradation of particulate organic matter (POM) in downstream lacustrine sediments. To investigate the Fe-dependent degradation of POM, slurries mimicking iron-polluted sediments (85 mg Fe per g, 116 mg Fe per g, and 149 mg Fe per g dry weight) were incubated with plankton or leaf POM under anoxic and oxic headspace conditions, and CO2 and CH4 emissions, water chemistry, and stable isotopes of dissolved inorganic carbon were measured. The experiments revealed that (i) with an increasing Fe content, the CO2 and CH4 emissions were gradually reduced, (ii) CO2 and CH4 production was higher during plankton degradation than during leaf decomposition, and (iii) under oxic conditions, CO2 production was higher and CH4 production was lower when compared to the treatments under anoxic conditions. These findings demonstrate that while benthic mineralization of fresh POM typically releases greenhouse gases into the water column, the availability of iron oxyhydroxides can contribute to reduced greenhouse gas emissions from sediments. This is of considerable relevance for future carbon budgets of similar mining-affected, iron-polluted fluvial-lacustrine river systems.

Toxicity threshold and ecological risk of nitrate in rivers based on endocrine-disrupting effects: A case study in the Luan River basin, China

Nitrate, as a crucial nutrient, is consistently targeted for controlling water eutrophication globally. However, there is considerable evidence suggesting that nitrate has endocrine-disrupting potential on aquatic organisms. In this study, the sensitivity of various adverse effects to nitrate nitrogen (nitrate-N) was compared, and a toxicity threshold based on endocrine-disrupting effects was derived. The spatiotemporal variations of nitrate-N concentrations in the Luan River basin were investigated, and the associated aquatic ecological risks were evaluated using a comprehensive approach. The results showed that reproduction and development were the most sensitive endpoints to nitrate, and their distribution exhibited significant differences compared to behavior. The derived threshold based on endocrine-disrupting effects was 0.65 mgL-1, providing adequate protection for the aquatic ecosystem. In the Luan River basin, the mean nitrate-N concentrations during winter (4.4 mgL-1) were significantly higher than those observed in spring (0.7 mgL-1) and summer (1.2 mgL-1). Tributary inputs had an important influence on the spatial characteristics of nitrate-N in the mainstream, primarily due to agricultural and population-related contamination. The risk quotients (RQ) during winter, summer, and spring were evaluated as 6.7, 1.8, and 1.1, respectively, and the frequency of exposure concentrations exceeding the threshold was 100 %, 64.3 %, and 42.5 %, respectively. At the ecosystem level, nitrate posed intermediate risks to aquatic organisms during winter and summer in the Luan River basin and at the national scale in China. We suggest that nitrate pollution control should not solely focus on water eutrophication but also consider the endocrine disruptive effect on aquatic animals.

An optimized crop-livestock system can achieve a safe and just planetary boundary for phosphorus at the sub-basin level in China

The contribution of crop and livestock production to the exceedance of the planetary boundary for phosphorus (P) in China is still unclear, despite the country's well-known issues with P fertilizer overuse and P-related water pollution. Using coupled models at sub-basin scales we estimate that livestock production increased the consumption of P fertilizer fivefold and exacerbated P losses twofold from 1980 to 2017. At present, China's crop-livestock system is responsible for exceeding what is considered a 'just' threshold for fertilizer P use by 30% (ranging from 17% to 68%) and a 'safe' water quality threshold by 45% (ranging from 31% to 74%) in 25 sub-basins in China. Improving the crop-livestock system will keep all sub-basins within safe water quality and just multigenerational limits for P in 2050.

Four decades of region- and species-specific trends in lowland stream Ephemeroptera abundance

Lowland stream ecosystems are under threat from climate change, industrialization, urbanization, and intensive agriculture. Since the 1980s, improvements in water quality have led to an increase in lowland stream biodiversity. Despite restoration efforts, however, further recovery is often hampered by the presence of region-specific (combinations of) stressors, and species-specific stressor responses. Identification of these stressors may not be achieved by the analysis of abundance data over large areas for entire communities or species assemblages. Therefore, our study introduces an alternative in-depth approach, selecting Ephemeroptera as a model organism group and analyzing 41 years of species abundance data across distinct geographical regions. Our findings revealed that 15 Ephemeroptera species had already disappeared before 1985, emphasizing the importance of evaluating an extended historical period when analyzing biodiversity trends. While biodiversity was generally characterized by an initial recovery that stagnated over time, the analysis of the past 41 years of Ephemeroptera abundance data revealed strong differences in species' abundance trends between periods, regions, and species. Certain species were likely to have benefitted from local restoration measures in specific geographical regions, while others may have declined due to the presence of region-specific stressors. Our approach underscores the importance of studying the development of region- and species-specific stream biodiversity trends over time to aid the selection of the appropriate restoration measures to recover lowland stream biodiversity.

Azolla cultivation enables phosphate extraction from inundated former agricultural soils

To combat the global loss of wetlands and their essential functions, the restoration and creation of wetlands is imperative. However, wetland development is challenging when soils have been in prolonged agricultural use, often resulting in a substantial nutrient legacy, especially of phosphorous (P). Inundating these soils typically leads to P mobilization, resulting in poor water quality and low biodiversity recovery. As a potential novel means to overcome this challenge, we tested whether cultivation of the floating fern Azolla filiculoides could simultaneously extract and recycle P, and provide a commercial product. Azolla has high growth rates due to the nitrogen fixing capacity of its microbiome and is capable of luxury consumption of P. Azolla cultivation may also accelerate soil P mobilization and subsequent extraction by causing surface water anoxia and the release of iron-bound P. To test this approach, we cultivated Azolla on 15 P-rich former agricultural soils in an indoor mesocosm experiment. Soils were inundated and either left unvegetated or inoculated with A. filiculoides during two 8-week cultivation periods. Biomass was harvested at different intervals (weekly/monthly/bimonthly) to investigate the effect of harvesting frequency on oxygen (O2) and nutrient dynamics. We found that Azolla attained high growth rates only on soils with high mobilization of labile P, as plant cover did not reduce surface water O2 concentrations in the first phase after inundation. This concurred with low porewater iron to P ratios (<10) and high porewater P concentrations. A. filiculoides cultivation substantially reduced surface water nutrient concentrations and extracted P at rates up to 122 kg ha-1 yr-1. We conclude that rapid P extraction by A. filiculoides cultivation is possible on soils rich in labile P, offering new perspectives for wetland rehabilitation. Additional field trials are recommended to investigate long-term feasibility, seasonal variations, and the influence of potential grazers and pathogens.

TP or Not TP? Successful Comparison of Two Independent Methods Validates Total Phosphorus Inference for Long-Term Eutrophication Studies

Validating paleo total phosphorus (TP) inference methods over long time scales is essential for understanding historic changes in lake P supply and the processes leading up to the present-day global lake eutrophication crisis. Monitored lake water TP time series have enabled us to identify the drivers of eutrophication over recent decades. However, over longer time scales, the lack of reliable TP inference means our understanding of drivers is speculative. Validation of lake water TP reconstruction, therefore, remains the "ultimate aim" of eutrophication studies. Here, we present the first critical comparison of two fully independent paleo TP inference approaches: the well-established diatom method (DI-TP) and a recently developed sediment geochemical method (SI-TP). Using lake sediment records from a small eutrophic U.K. lake (Crose Mere), we find a statistically significant agreement between the two inferred TP records with greater than 60% shared variance. Both records show identical timings, with a 19th century acceleration in TP concentration and subsequent declines following a peak in 1930. This significant agreement establishes the validity of long-term paleo TP inference for the first time. With this, we can now test assumptions and paradigms that underpin understanding of catchment P sources and pathways over longer time scales.

A short-cut methodology for the spatial assessment of the biochemical river quality

The deterioration of superficial water quality is a significant concern in water management. Currently, most European rivers do not achieve qualitative standards defined by Directive 2000/60/EC (Water Framework Directive, WFD), while the health status of many surface water bodies remains unknown. Within this context, we propose a new methodology to perform a semi-quantitative analysis of the pressure state of a river, starting from easily accessible data related to anthropic activities. The proposed approach aims to address the endemic scarcity of monitoring records. This study proposes a procedure to (i) evaluate the relative pressure of different human activities, (ii) identify allocation points of different pollutant sources along the river using a raster-based approach, and (iii) determine a spatial biochemical water quality index. The developed index expresses the overall biochemical state of surface water induced by pollutant sources that may simultaneously impact a single river segment. This includes establishments under the so-called Seveso Directive, activities subjected to the IPPC-IED discipline, wastewater treatment plants, and contaminated sites. The methodology has been tested over three rivers in Northern Italy, each exposed to different industrial and anthropogenic pressures: Reno, Enza, and Parma. A comparison with monitored data yielded convincing results, proving the consistency of the proposed index in reproducing the spatial variability of the river water quality. While additional investigations are necessary, the developed methodology can serve as a valuable tool to support decision-making processes and predictive studies in areas lacking or having limited water quality monitoring data.

Recognizing Agricultural Headwaters as Critical Ecosystems

Agricultural headwaters are positioned at the interface between terrestrial and aquatic ecosystems and, therefore, at the margins of scientific disciplines. They are deemed devoid of biodiversity and too polluted by ecologists, overlooked by hydrologists, and are perceived as a nuisance by landowners and water authorities. While agricultural streams are widespread and represent a major habitat in terms of stream length, they remain understudied and thereby undervalued. Agricultural headwater streams are significantly modified and polluted but at the same time are the critical linkages among land, air, and water ecosystems. They exhibit the largest variation in streamflow, water quality, and greenhouse gas emission with cascading effects on the entire stream networks, yet they are underrepresented in monitoring, remediation, and restoration. Therefore, we call for more intense efforts to characterize and understand the inherent variability and sensitivity of these ecosystems to global change drivers through scientific and regulatory monitoring and to improve their ecosystem conditions and functions through purposeful and evidence-based remediation.

Water quality trends of streams in Puerto Rico: Evaluating 50 years of the Clean Water Act

Water quality regulations entail a substantial commitment of resources from governments and private entities. It is important to continually evaluate the effectiveness of these regulations to ensure they are having the intended impact. In this paper, we evaluated nutrient data as indicators of primary productivity and dissolved oxygen (DO) concentrations and pH as response variables to assess historical water quality trends from 55 stations of Puerto Rico. The stations were divided into impaired versus non-impaired categories based on their historical total phosphorus (TP) mean concentration. Mean TP and total nitrogen (TN) concentrations were significantly higher in the impaired stations relative to the non-impaired stations. In contrast, DO mean concentrations and mean pH values were significantly lower in the impaired stations. A generalized additive mixed model was used to demonstrate temporal trends. A significant decrease in TP and TN concentrations was observed with time at the impaired stations. This was accompanied by significant increases in DO concentrations and pH. The non-impaired stations showed a marginal (statistically nonsignificant) decreasing trend with time. The large reductions in nutrient concentrations observed at the impaired stations seem to be related to the closure of several primary wastewater treatment plants (WWTPs) across the island. The conversion of abandoned crop agricultural lands into secondary forest in recent decades has resulted in small but significant decreases in TN (not TP) in receiving streams. We conclude that the Clean Water Act has promoted improvements in water quality in Puerto Rico by advancing upgrades in sanitary infrastructure and the regulation of point sources of pollution.

Uptake and cellular responses of Microcystis aeruginosa to PFOS in various environmental conditions

Although PFOS has been banned as a persistent organic pollutant, it still exists in large quantities within the environment, thus impacting the health of aquatic ecosystems. Previous studies focused solely on high PFOS concentrations, disregarding the connection with environmental factors. To gain a more comprehensive understanding of the PFOS effects on aquatic ecosystems amidst changing environmental conditions, this study investigated the cellular responses of Microcystis aeruginosa to varying PFOS concentrations under heatwave and nutrient stress conditions. The results showed that PFOS concentrations exceeding 5.0 µg/L had obvious effects on multiple physiological responses of M. aeruginosa, resulting in the suppression of algal cell growth and the induction of oxidative damage. However, PFOS concentration at levels below 20.0 µg/L has been found to enhance the growth of algal cells and trigger significant oxidative damage under heatwave conditions. Heatwave conditions could enhance the uptake of PFOS in algal cells, potentially leading to heightened algal growth when PFOS concentration was equal to or less than 5.0 µg/L. Conversely, deficiency or limitation of nitrogen and phosphorus significantly decreased algal abundance and chlorophyll content, inducing severe oxidative stress that could be mitigated by exposure to PFOS. This study holds significance in managing the impact of PFOS on algal growth across diverse environmental conditions.

Setting nutrient boundaries to protect aquatic communities: The importance of comparing observed and predicted classifications using measures derived from a confusion matrix

Defining nutrient thresholds that protect and support the ecological integrity of aquatic ecosystems is a fundamental step in maintaining their natural biodiversity and preserving their resilience. With increasing catchment pressures and climate change, it is more important than ever to develop clear methods to establish thresholds for status classification and management of waters. This must often be achieved using complex data and should be robust to interference from additional pressures as well as ameliorating or confounding conditions. We use both artificial and real data to examine challenges in setting nutrient thresholds in unbalanced and skewed data. We found significant advantages to using binary logistic regression over other techniques. However, one of the key challenges is objectively selecting a probability from which to derive the nutrient threshold. For this purpose, the examination of the proportions of matching and mismatching status classifications of nutrients and a biological quality element using a confusion matrix is a key step that should be more widely adopted in threshold selection. We examined a large array of statistical measures of classification accuracy and their performance over combinations of skewness and imbalance in the data. The most appropriate threshold probability is a compromise between maximising overall classification accuracy and reducing mismatches expressed as commission (false positives) without excessive omission (false negatives). An application to a lake type indicated total phosphorus thresholds that would be around 50 μg l-1 lower than the threshold achieved by an 'unguided' approach, indicating that this approach is a very significant development meriting attention from national authorities responsible for water management.

A diatom-based approach to refine nutrient concentrations compatible with the "good" status of Northern Italy rivers

The identification of ecologically sound thresholds represents an important step toward improving the ecological status of rivers through appropriate measures to contain nutrient loads. The aim of the present study was to estimate phosphorus and nitrogen concentrations compatible with the achievement of the "good" ecological status of rivers from data collected in the Po River District, the largest hydrographic system in Italy. For this purpose, relationships between the diatom index used in Italy for the national assessment of the stream ecological status, the ICMi (Intercalibration Common Metric index), and total phosphorus and nitrate concentrations were analyzed using monitoring data collected between 2009 and 2019. The Po River Basin encompasses five distinct river types, from Alpine to Mediterranean to Lowlands, characterized by different anthropogenic pressures and water quality. Through regression analysis between the ICMi and nutrient concentrations, we estimated ranges of the latter values corresponding to a "good" ecological status for each river type. The resulting thresholds are far more stringent than the limits set by the Italian legislation for water quality classification. This is particularly true for total phosphorus, whose threshold value should be roughly halved for all river types. For nitrates, the results are more differentiated according to river type: the estimated thresholds are much more stringent than those currently in use for siliceous Alpine and Mediterranean rivers. Moreover, the availability of such a large database allowed also to assess the influence of one nutrient over the other on the diatom community and to highlight some critical issues in the formulation of ICMi for Mediterranean rivers.

Applying water quality standards to pollution from diffuse sources

Water quality standards are instrumental in evaluating the status of water bodies, and in providing protective and restorative endpoints. To date, much of the infrastructure used to implement water quality standards has been directed towards remediating and managing pollution from point source discharges. However, pollution from diffuse sources is the leading cause of water quality impairment, especially by nutrients. Although the effects of nutrient enrichment on streams is well studied, and ecological thresholds identified, those thresholds have not been widely adopted as standards primarily because they are not attainable by point sources. Clearly, a framework for adopting and applying standards to manage pollution from diffuse sources needs to be decoupled from those intended for point sources. This paper argues for a relatively unstructured distributional approach to predict how ecological responses might shift in response to management of diffuse sources. The approach calls for first developing a deterministic model of stressor and response variables, followed by a reformulation as a Bayesian model. In the case here, a structural equation model was developed that linked nutrient enrichment, habitat quality, and chloride and manganese concentrations to an index of macroinvertebrate quality. Results from the Bayesian representation suggest that in landscapes where the drainage network has been highly modified for agricultural production, reduction in total phosphorus alone is expected to have a modest (but non-trivial) effect on macroinvertebrate condition, shifting the distribution of scores up by 1 point. The addition of habitat restoration is likely to shift the distribution upwards by 4 points, an effect size observed in Ohio, USA from other large-scale restoration efforts.

Lake browning counteracts cyanobacteria responses to nutrients: Evidence from phytoplankton dynamics in large enclosure experiments and comprehensive observational data

Lakes worldwide are affected by multiple stressors, including climate change. This includes massive loading of both nutrients and humic substances to lakes during extreme weather events, which also may disrupt thermal stratification. Since multi-stressor effects vary widely in space and time, their combined ecological impacts remain difficult to predict. Therefore, we combined two consecutive large enclosure experiments with a comprehensive time-series and a broad-scale field survey to unravel the combined effects of storm-induced lake browning, nutrient enrichment and deep mixing on phytoplankton communities, focusing particularly on potentially toxic cyanobacterial blooms. The experimental results revealed that browning counteracted the stimulating effect of nutrients on phytoplankton and caused a shift from phototrophic cyanobacteria and chlorophytes to mixotrophic cryptophytes. Light limitation by browning was identified as the likely mechanism underlying this response. Deep-mixing increased microcystin concentrations in clear nutrient-enriched enclosures, caused by upwelling of a metalimnetic Planktothrix rubescens population. Monitoring data from a 25-year time-series of a eutrophic lake and from 588 northern European lakes corroborate the experimental results: Browning suppresses cyanobacteria in terms of both biovolume and proportion of the total phytoplankton biovolume. Both the experimental and observational results indicated a lower total phosphorus threshold for cyanobacterial bloom development in clearwater lakes (10-20 μg P L-1 ) than in humic lakes (20-30 μg P L-1 ). This finding provides management guidance for lakes receiving more nutrients and humic substances due to more frequent extreme weather events.

Looking back to move forward: Restoring vegetated canals to meet missing Water Framework Directive goals in agricultural basins

Nitrate pollution and eutrophication remain pressing issues in Europe regarding the quality of aquatic ecosystems and the safety of drinking water. Achieving water quality goals under the Water Framework Directive (WFD) has proven to be particularly challenging in agricultural catchments, where high nitrate concentrations are the main reason for the failure of many water bodies to meet a good ecological status. Canals and ditches are common man-made features of irrigated and drained landscapes and, when vegetated, have recently been identified as denitrification hotspots. By combining experimental data and GIS-based upscaling estimation, the potential capacity of the canal network to reduce nitrate loads was quantified in several scenarios differing in the level of nitrate pollution and in the extent of the canal network length where conservative management practices are implemented. The analysis was carried out in the irrigated lowlands of the Po River basin, which is the largest hydrographic system in Italy and a global hotspot for nitrogen inputs and eutrophication. Scenario simulations showed that maintaining aquatic vegetation in at least 25 % of the canal network length, selecting sites with high nitrate availability (>2.4 mg N L-1), would promote a greater potential for permanent N removal. The increased denitrification capacity would meet the load reduction target required to achieve a WFD good ecological status in waters draining into the Adriatic Sea during the spring-summer months, when the eutrophication risk is higher. Promoting denitrification in the canal network by postponing the mowing of in-stream vegetation to the end of the growing season could be an effective mitigation strategy to improve water quality in agricultural basins and contribute to achieving the WFD goals.

Effect of climate change on the water quality of Mediterranean rivers and alternatives to improve its status

Surface water (SW) quality is particularly vulnerable to increased concentrations of nutrients, and this issue may be exacerbated by climate change. Knowledge of the effects of temperature and rainfall on SW quality is required to take the necessary measures to achieve good SW status in the future. To address this, the aims of this study were threefold: (1) to assess how a changing climate may alter the nitrate, ammonium, phosphorus and biological oxygen demand status (BOD5) of SW; (2) assess the relationship between water quality and flow; and (3) simulate diffuse and point source pollution reduction scenarios in the Júcar River Basin District in the Mediterranean region. A regionalised long-term climate scenario was used following one Representative Concentration Pathway (RCP8.5) with the data incorporated into the coupling of hydrological and water quality models. According to these climate change scenarios, SW with poor nitrate, ammonium, phosphorus and BOD5 status are expected to increase in the future by factors of 1.3, 1.9, 4 and 4, respectively. Furthermore, median ammonium and phosphorus concentration may be doubled in months with low flows. Additional measures are required to maintain current status in the water bodies, and it is necessary to reduce at least 25% of diffuse nitrate pollution, and 50% of point loads of ammonium, phosphorus, and BOD5.

Phosphorus conditions change the cellular responses of Microcystis aeruginosa to perfluorooctanoic acid

Perfluorooctanoic acid (PFOA), a widespread and emerging organic contaminant of aquatic environments, has high bioaccumulation potential and high toxicity. Consequently, major concerns have been raised worldwide regarding the management of this pollutant in aquatic ecosystems. To thoroughly understand PFOA's toxic effects on aquatic organisms, systematic investigations were conducted on the cellular responses of Microcystis aeruginosa to the environmental concentrations of PFOA under various concentrations as well as phosphorus (P) conditions (concentrations and forms). The results showed that P conditions remarkably affected cyanobacterial growth as well as photosynthetic pigment content, triggered oxidative stress to disrupt the function and structure of the cell membrane, and caused changes in the extracellular and intracellular contents of microcystin-LR (MC-LR). Furthermore, PFOA (100 μg/L) was absorbed by cyanobacterial cells through the stimulation of the secretion of extracellular polymeric substances (EPS) by M. aeruginosa. After entering the cyanobacterial cells, PFOA inhibited photosynthesis, reduced P absorption, induced oxidative damage, lead to a loss of cell integrity evident in scanning electron microscope images, and increased mcyA gene expression to promote MC-LR production. Moreover, the limited P concentration and forms conditions led to increased PFOA absorption by cyanobacterial cells, which further upregulated mcyA gene expression and increased the risk of MC-LR diffusion into the aquatic environment. Our present study provided a theoretical basis and new ideas for understanding and addressing safety issues related to the presence of PFOA in aquatic environments with varying nutritional statuses.

Development of standards for assessing water quality in marine coastal waters of Bahrain

Marine coastal waters of Bahrain are under pressure due to human activities and climate change. We used marine monitoring data (2005-2020) from 27 sites to establish baseline conditions and develop standards for assessments of water quality. Five hydrodynamic regions were identified: Oyster Beds, North, West, East, East (Coastal). Data from Oyster Beds sites, likely to be less impacted by human activities, were used to determine baseline conditions. For most parameters, candidate thresholds were based on 50 % and 100 % variation from baseline and 95th percentiles. Comparisons of data against different thresholds showed different outcomes. Overall, results indicate good water quality, with potential concerns in East (Coastal). Trend analyses showed some significant trends in all regions: downward (favourable) for some parameters (e.g. turbidity: North) and upward for others (e.g. nitrate: Oyster Beds, East and East (Coastal)). Future work requires greater understanding around optimum guidelines that protect and mitigate any adverse ecological impacts.

Establishing riverine nutrient criteria using individual taxa thresholds

Nutrient enrichment is one of the most pervasive impacts on aquatic ecosystems globally. Approaches to establish nutrient criteria that safeguard aquatic ecosystem health are highly variable and, in many instances, criteria are derived from correlations between in-situ nutrient concentrations and biological indices. Summarising entire assemblages with a single index can result in a substantial loss of information and potentially weaker relationships. In this study, we compared the derivation of nutrient criteria using biological indices and those from individual taxa for rivers and streams in New Zealand. Random forest models, including nutrient concentrations, were built to predict two biological indices and individual taxa across New Zealand's river monitoring network. For all acceptable models, the response of the biological indices and individual taxa to increasing Dissolved Inorganic Nitrogen (DIN) and Dissolved Reactive Phosphorus (DRP) were then predicted for every river reach across the nation, and nutrient concentrations that protected 80% of taxa were then identified. Models for the biological indices were poor but were good for most of the taxa, with nutrient concentrations almost always being the most influential factor. To ensure persistence of at least 80% of the taxa within a river reach, we estimated that DIN (Dissolved Inorganic Nitrogen) concentrations would need to be below 0.57-1.32 mg/L, and DRP (Dissolved Reactive Phosphorus) concentrations below 0.019-0.033 mg/L, depending on the river type. In general, high order, low slope rivers and streams required more stringent nutrient criteria than steep, low order streams. The link between nutrient concentrations and biological indices were weak and likely suffer from the loss of information from summarising an entire assemblage into a single numeric. We consider that the derivation of nutrient criteria for waterways should also examine the individual relationships with the taxa in a river system to establish protection for a desired proportion of taxa.

Does anoxia promote the mobilization of P-bearing colloids from dam reservoir sediment?

In the context of a reservoir, the anoxia that develops in the bottom sediment induces the release of phosphorus (P) into the overlying water, thus supporting eutrophication. Most studies focusing on P dynamic in an aquatic environment fail to consider the "truly" dissolved and colloidal fractions, hence the colloidal P has gone largely unexplored. The aim of this study was to investigate the release of sedimentary P under oscillating aerobic, anoxic and aerobic conditions, in taking into account the colloidal (10 kDa-1 µm) and truly dissolved (< 10 kDa) fractions. Laboratory incubations of wet sediment originating from a dam reservoir were performed over 63 days, consisting of 25 days of aerobic conditioning (lasting 2 periods) and 38 days of anoxia. Results showed that oxic conditions induced a very limited release of phosphorus, both in truly dissolved and colloidal forms. In turn, the development of anoxic conditions caused a large release of P, mainly in the colloidal fraction, representing about 90 % of the total water-mobilizable P (PWM < 1 µm). The initial release of truly dissolved P during the anoxic stage gradually diminished over time, possibly due to the formation of secondary minerals or re-adsorption processes. Approximately half of the PWM released during anoxia persisted under subsequent oxic conditions and consisted solely of colloidal P. The dynamics of PWM were primarily influenced by two main factors: (i) the reductive dissolution of iron, which released both dissolved and colloidal P, and (ii) the release of indigenous organic matter, which impacted the stability of the released colloids through bridging mechanisms.

Linking denitrification and pesticide transformation potentials with community ecology and groundwater discharge in hyporheic sediments in a lowland stream

Contamination of rivers by nitrate and pesticides poses a risk for aquatic ecosystems in lowland catchments that are often intensively used for agriculture. Here, the hyporheic zone, the streambed underneath the stream, plays a vital role due to its efficient self-purification capacity. The present study aims to evaluate the denitrification and transformation potential of 14 pesticides and three transformation products in the hyporheic sediment from a lowland stream with a high N load and by comparing an agricultural straightened section to a natural meandering part of the stream influenced by different groundwater discharges. Batch experiments were set up to evaluate the denitrification and pesticide transformation potentials in hyporheic sediment from two depths (5-15 cm (a) and 15-25 cm (b)). Our results revealed that (i) differences between the agricultural and natural sections of the river did not influence pollutant attenuation, (ii) both the nitrate and pesticide attenuation processes were more rapid in the upper "a" layer compared to the "b" layer due to higher microbial abundance, (iii) high groundwater discharge reduced the denitrification potential while pesticide transformation was unaffected, (iv) denitrification correlated with denitrifier abundance (nirK) in the "b" layer, while this correlation was not seen in the "a" layer, and (v) a microbial community with low diversity can explain limited transformation for the majority of tested pesticides. Overall, our results suggest that high groundwater discharge zones with reduced residence time in the hyporheic zone can be an important source of pesticides and nitrate to surface water.

Environmental degradation of the Mexican Caribbean reef lagoons

Insufficient attention to the large volumes of wastewater produced by expansive tourism and urban development in the north of the Mexican Caribbean has increased concerns on the ecological and economic sustainability of this important tourist destination, which is currently threatened by massive arrivals of pelagic Sargassum. Comparing environmental descriptions for sites exposed to contrasting anthropogenic pressure and before and during massive Sargassum tides, uncovered significant shifts in the environmental conditions in the last 20 years, from oligotrophic to mesotrophic-eutrophic conditions. The most significant changes were observed in the north, for habitats exposed to high anthropogenic pressure. Accordingly, the severe threat that massive Sargassum beaching currently represents for the survival of Caribbean coral reefs cannot be considered the only driver of reef eutrophication in the Mexican Caribbean, as the habitat degradation documented here has an important contribution from anthropogenic fertilization.

Evaluation of the costs of agricultural diffuse water pollution abatement in the context of Lithuania's water protection goals and climate change

This study aimed at evaluating the scale and costs of an environmentally and economically optimal set of Best Management Practices (BMPs) for agricultural pollution abatement in Lithuania in order to reach water protection goals in both inland and marine waters by distributing BMPs optimally in space, while taking climate change impacts into consideration. The assessment of BMPs impact involved the use of the SWAT model by applying two climate change representative concentration pathways (RCP4.5 and RCP8.5) and two time horizons (mid-century and end-century), as well as five BMPs (arable land conversion to grasslands, reduced fertilization, no-till farming, catch-crops, and stubble fields throughout winter). The optimization of the set of BMPs employed a genetic algorithm. The results suggest that the need for BMPs application will increase from 52% of agricultural areas in the historical period up to 65% by the end of century in the RCP8.5 scenario. This means less arable land could actually be used for crop production in the future if water protection targets are met. The high costs for reaching water targets would rise even more, i.e. by 173% for RCP4.5, and by 220% for the RCP8.5 scenario, reaching approximately 200 million euros/year. In such a context, the BMP optimization approach is essential for significant reduction of the costs. Winter cover crops and reduced fertilization show the best effectiveness and cost balance, and will therefore be essential in pursuing water protection targets.

Long-term water quality monitoring in agricultural catchments in Sweden: Impact of climatic drivers on diffuse nutrient loads

Water quality related to non-point source pollution continues to pose challenges in agricultural landscapes, despite two completed cycles of Water Framework Directive actions by farmers and landowners. Future climate projections will cause new challenges in landscape hydrology and subsequently, the potential responses in water quality. Investigating the nutrient trends in surface waters and studying the efficiency of mitigation measures revealed that loads and measures are highly variable both spatially and temporally in catchments with different agro-climatic and environmental conditions. In Sweden, nitrogen and phosphorus loads in eight agricultural catchments (470-3300 ha) have been intensively monitored for >20 years. This study investigated the relationship between precipitation, air temperature, and discharge patterns in relation to nitrogen (N) and phosphorus (P) loads at catchment outlets. The time series data analysis was carried out by integrating Mann-Kendall test, Pettitt break-points, and Generalized Additive Model. The results showed that the nutrient loads highly depend on water discharge, which had large variation in annual average (158-441 mm yr^-1). The annual average loads were also considerably different among the catchments with total N (TN) loads ranging from 6.76 to 35.73 kg ha^-1, and total P (TP) loads ranging from 0.11 to 1.04 kg ha^-1. The climatic drivers were highly significant indicators of nutrient loads but with varying degree of significance. Precipitation (28-962 mm yr^-1) was a significant indicator of TN loads in five catchments (loamy sand/sandy loam) while annual average temperature (6.5-8.7 °C yr^-1) was a significant driver of TN loads in six out of eight catchments. TP loads were associated with precipitation in two catchments and significantly correlated to water discharge in six catchments. Considering the more frequent occurrence of extreme weather events, it is necessary to tailor N and P mitigation measures to future climate-change features of precipitation, temperature, and discharge.

Phosphorus reference conditions in lowland streams: Survey in old forests and anaerobic sediment release

The reference (pristine) concentrations of total dissolved phosphorus (TDP) in surface waters are difficult to define in lowland regions because of their high population density. Here, we estimated reference conditions of TDP from headwaters and their riverbed sediments (n = 140) in old forests. Surface waters and sediments were collected in the lowland region of Belgium as transects starting in 20 different old forests (before 1775; pristine) to up to 1.5 km outside the forest edge (disturbed). Sediment P concentrations in disturbed samples readily increased with increasing distance from the borders of old forests. The P mobilized from the sediments to the overlying water was also measured in laboratory incubations to mimic P release during seasonal redox cycles. The sediment respiration was larger in disturbed than in the reference sediments. Disturbed sediments released more P during anaerobic than aerobic incubations, while reference sediments did not show such a difference. The mean TDP in the reference surface water samples is 57 μg TDP L^-1 [95 % confidence interval 41; 72], very close to the pre-industrial background previously estimated for the Scheldt river. The 90th percentile of the reference water is 137 [110; 180] μg TDP L^-1. That P90 is just under the prevailing TP limit of 140 μg P L^-1, illustrating that this limit is very close to the upper baseline of reference conditions. The P75, often proposed as the threshold for high ecological status, was 84 [55; 122] μg P L^-1. This study showed that soil or sediments in old forests might serve to identify reference conditions. The well-established summer peaks of TDP in lowland rivers are unlikely to occur in pristine areas.

Iron-enhanced sand filters: Multi-year urban runoff (stormwater) quality performance

Untreated urban runoff (stormwater) is a major pathway for contaminants, e.g., nutrients and metals, to receiving waters. Where eutrophication occurs, dissolved phosphorus (DP) treatment is often necessary to protect receiving waters, yet few practical methods exist. Iron-enhanced sand filters (IESFs) have successfully treated DP in laboratory and limited field studies. Yet, multi-year-IESF studies to understand reportedly variable performance are unavailable. Herein, nine IESFs were sampled from 2015 to 2018 (528 samples; 70 rainfall-runoff events). Analysis focused on influent/effluent concentrations and removal efficiencies alongside design and catchment parameters. Overall, IESFs significantly removed most total and dissolved metal analytes. Generally, phosphorus removal efficiencies correlated positively with influent concentrations and IESF:catchment area ratios, demonstrating the importance of proper sizing and siting. For all paired influent-effluent samples, respective median total phosphorus, orthophosphate, and DP removal efficiencies were 33 %, 41 %, and 13 %, and respective median effluent concentrations were 120, 25, and 75 (μg/L); with two malfunctioning sites omitted, these respective concentrations were 92, 11, and 47, which better matched relevant goals and (indirectly applicable) standards. Nonetheless, phosphorus removal efficiency and effluent concentrations varied significantly across IESFs and events. Seasonality appeared influential, yet variable influent concentrations confounded spatiotemporal removal efficiency comparisons. Thus, compared to removal efficiencies, effluent concentrations may be better indicators of receiving water risk/benefit and of equal importance for water quality crediting. Although 122 influent-effluent pairs were analyzed, a greater sample size would allow multivariate hypothesis tests with additional predictors. Overall, in this multi-site-year study, most IESFs performed at (n = 5) or near (n = 2) phosphorus effluent concentration and less-so, removal efficiency benchmarks. This research provides new quantitative knowledge on long-term IESF performance for real-world conditions and goals. Research recommendations include multivariate dimension reduction studies and comprehensive, effective information transfer to improve IESF understanding and performance and address practitioner needs, e.g., for refined design, operation, and assessment guidance.

Time-course biofilm formation and presence of antibiotic resistance genes on everyday plastic items deployed in river waters

The plastisphere has been widely studied in the oceans; however, there is little information on how living organisms interact with the plastisphere in freshwater ecosystems, and particularly on how this interaction changes over time. We have characterized, over one year, the evolution of the eukaryotic and bacterial communities colonizing four everyday plastic items deployed in two sites of the same river with different anthropogenic impact. α-diversity analyses showed that site had a significant role in bacterial and eukaryotic diversity, with the most impacted site having higher values of the Shannon diversity index. β-diversity analyses showed that site explained most of the sample variation followed by substrate type (i.e., plastic item) and time since first colonization. In this regard, core microbiomes/biomes in each plastic at 1, 3, 6 and 12 months could be identified at genus level, giving a global overview of the evolution of the plastisphere over time. The measured concentration of antibiotics in the river water positively correlated with the abundance of antibiotic resistance genes (ARGs) on the plastics. These results provide relevant information on the temporal dynamics of the plastisphere in freshwater ecosystems and emphasize the potential contribution of plastic items to the global spread of antibiotic resistance.

Influence of social and environmental drivers on nutrient concentrations and ratios in lakes: A comparison between China and Europe

Global anthropogenic flows of nitrogen (N) and phosphorus (P) have exceeded planetary boundaries with significant implications for pollution of the freshwater resources in the world. Understanding the global patterns and drivers of N and P concentrations and their ratios in the lakes could help design more effective management and remediation strategies to mitigate the impacts of eutrophication. While a suite of drivers are associated with the sources of nutrients, their transport and internal processes that control concentrations of N and P in the lakes, much less is known about the relative importance of different drivers in explaining spatial variations of lake nutrients and ratios. In this study, we compared N and P concentrations and their ratios in the lakes across China and Europe and examined the differences of dominant environmental and social drivers on lake nutrients. Our comparison showed that total nitrogen (TN) and total phosphorus (TP) concentrations were much higher in the lakes in China compared to those in Europe (i.e., TN: 1.13 mg/L in China vs. 0.64 mg/L in Europe; TP: 35.83 μg/L in China vs. 19.38 μg/L in Europe, the median value). However, lake N/P ratios for both regions were not statistically different. Concentrations of TN and TP showed decoupling in both regions, with the majority of lakes having high N/P mass ratios when evaluated by the commonly accepted threshold of 23 (i.e., 61% in China and 68% in Europe), indicating that phytoplankton are more P limited relative to N. Agricultural activity in the lake catchment is an important predictor for both nutrient concentrations and their ratio in Europe. This reflects successful investments in infrastructure and policy prescriptions in addressing point sources of pollution. In comparison, lake depth and water residence time are important in the decoupling of N and P concentrations in China. The regional difference between the dominant drivers can provide important insights into development of effective water pollution control measures. It is necessary for policy makers and water resource managers to be aware of large-scale imbalance of nutrients in lake due to the potential environmental consequences. A set of spatially flexible policies for water quality controls would be beneficial for sustaining the ecological integrity and future health of lakes.

Evaluating the biological validity of European river typology systems with least disturbed benthic macroinvertebrate communities

Humans have severely altered freshwater ecosystems globally, causing a loss of biodiversity. Regulatory frameworks, like the Water Framework Directive, have been developed to support actions that halt and reverse this loss. These frameworks use typology systems that summarize freshwater ecosystems into environmentally delineated types. Within types, ecosystems that are minimally impacted by human activities, i.e., in reference conditions, are expected to be similar concerning physical, chemical, and biological characteristics. This assumption is critical when water quality assessments rely on comparisons to type-specific reference conditions. Lyche Solheim et al. (2019) developed a pan-European river typology system, the Broad River Types, that unifies the national Water Framework Directive typology systems and is gaining traction within the research community. However, it is unknown how similar biological communities are within these individual Broad River Types. We used analysis of similarities and classification strength analysis to examine if the Broad River Types delineate distinct macroinvertebrate communities across Europe and whether they outperform two ecoregional approaches: the European Biogeographical Regions and Illies' Freshwater Ecoregions. We determined indicator and typical taxa for the types of all three typology systems and evaluated their distinctiveness. All three typology systems captured more variation in macroinvertebrate communities than random combinations of sites. The results were similar among typology systems, but the Broad River Types always performed worse than either the Biogeographic Regions or Illies' Freshwater Ecoregions. Despite reaching statistical significance, the statistics of analysis of similarity and classification strength were low in all tests indicating substantial overlap among the macroinvertebrate communities of different types. We conclude that the Broad River Types do not represent an improvement upon existing freshwater typologies when used to delineate macroinvertebrate communities and we propose future avenues for advancement: regionally constrained types, better recognition of intermittent rivers, and consideration of biotic communities.

From planetary to regional boundaries for agricultural nitrogen pollution

Excessive agricultural nitrogen use causes environmental problems globally¹, to an extent that it has been suggested that a safe planetary boundary has been exceeded². Earlier estimates for the planetary nitrogen boundary^3,4, however, did not account for the spatial variability in both ecosystems' sensitivity to nitrogen pollution and agricultural nitrogen losses. Here we use a spatially explicit model to establish regional boundaries for agricultural nitrogen surplus from thresholds for eutrophication of terrestrial and aquatic ecosystems and nitrate in groundwater. We estimate regional boundaries for agricultural nitrogen pollution and find both overuse and room for intensification of agricultural nitrogen. The aggregated global surplus boundary with respect to all thresholds is 43 megatonnes of nitrogen per year, which is 64 per cent lower than the current (2010) nitrogen surplus (119 megatonnes of nitrogen per year). Allowing the nitrogen surplus to increase to close yield gaps in regions where environmental thresholds are not exceeded lifts the planetary nitrogen boundary to 57 megatonnes of nitrogen per year. Feeding the world without trespassing regional and planetary nitrogen boundaries requires large increases in nitrogen use efficiencies accompanied by mitigation of non-agricultural nitrogen sources such as sewage water. This asks for coordinated action that recognizes the heterogeneity of agricultural systems, non-agricultural nitrogen losses and environmental vulnerabilities.

Development of water quality management strategies based on multi-scale field investigation of nitrogen distribution: a case study of Beiyun River, China

Accurately quantifying the distribution of nitrogen (N) contaminants in a river ecosystem is an essential prerequisite for developing scientific water quality management strategy. In this study, we have conducted a series of field investigations along the Beiyun River to collect samples from multiple scales, including surface water, riverbed sediments, vadose zone, and aquifer, for evaluating the spatial distribution of N; besides, column simulation experiments were carried out to characterize the transport behavior of N in riverbed sediments. The surface water of the Beiyun River was detected to be eutrophic because of its elevated total N concentration, which is 33 times of the threshold value causing the potential eutrophication. The hydrodynamic dispersion coefficient (D) of riverbed sediments was estimated by CXTFIT 2.1, demonstrating that the D of upstream section was lower than that of midstream and downstream sections (D_upstream < D_midstream < D_downstream), with the estimated annual N leaching volume of 130,524, 241,776, and 269,808 L/(m²·a), respectively. The average total N concentration in vadose zone and aquifer of upstream Sect. (297.88 mg/kg) was obviously lower than that of midstream Sect. (402.62 mg/kg) and downstream Sect. (447.02 mg/kg). Based on multi-scale investigation data, subsequently, water quality management strategies have been achieved, that is, limiting the discharge of N from the midstream and downstream banks to the river and setting up the impermeable layer in the downstream reaches to reduce infiltration. The findings of this study are of great significance for the improvement of river environmental quality and river management.

An ecotoxicological approach can complement the assessment of natural waters from Portuguese reservoirs?

Within the scope of the European Water Framework Directive (WFD), the scientific community recognized clear opportunities to take advantage of the use of ecotoxicological tools in water quality assessments. In this perspective, bioassays and biomarkers were suggested to contribute to the integration of the chemical and biological conditions, and thus to provide an overall insight into the quality of a water body. This study aimed to assess whether current bioassays as feeding rate assays with Daphnia longispina and growth inhibition assays with Lemna minor are suitable to detect potential ecotoxicity, using waters from Portuguese reservoirs. Several sampling sites were defined in reservoirs (Miranda, Pocinho, and Alqueva). The samplings were conducted in autumn of 2018 and spring of 2019. Total chlorophyll, lipid peroxidation, and proline content were also evaluated in L. minor. Results demonstrated that D. longispina showed some sensitivity to water treatments; however, the results were difficult to interpret since no reason or trend can be accurate. All parameters of L. minor did not show sensitivity to detect potential ecotoxicological risks associated with natural water understudy, since no discrimination among the water treatments was recorded. However, biomarkers/bioassays proved to be concordant to each other. Under the conditions evaluated here (reservoirs and sampling periods), the biological responses observed were not consistent, clear, and coherent with the physical-chemical parameters and chemical analyses performed, suggesting that the ecotoxicological tools selected were not sensitive to assess water quality in this type of ecosystems. In this sense, species of different trophic levels are recommended for ecotoxicological analyses due to differences in species sensitivities.

Assessment of Nutrient Loads into the Ryck River and Options for Their Reduction

A massive shift in agricultural practices over the past decades, to support exceptionally high yields and productivities involving intensive agriculture, have led to unsustainable agriculture practices across the globe. Sustenance of such high yields and productivities demand high use of organic and industrial fertilizers. This acts as a negative pressure on the environment. Excessive use of fertilizers leads to nutrient surplus in the fields, which, as a part of catchment runoff, flows into the water bodies as diffuse pollution. These nutrients through rivers are eventually passed into seas. High nutrients ending up into water bodies cause eutrophication. The situation is worsened when such unsustainable agricultural activities are carried out on drained peatlands. As a result, the nutrients that were not part of the nutrient cycle in the landscape for years begin to leach out due to mineralization of peatlands, thereby putting an additional load of nutrients on the environment, that was already under the negative impact of nutrient surplus. In view of the above, a small lowland catchment of the Ryck river in northeast Germany was assessed for its nitrogen losses from agricultural lands through empirical modelling. Initial empirical modelling resulted in an average annual total nitrogen loss of 14.7 kg ha−1 year−1. After a comparative analysis of these results with procured data, the empirical equation was modified to suit the catchment, yielding more accurate results. The study showed that 75.6% of peatlands in the catchment are under agricultural use. Subsequently, a proposal was made for potential wetland buffer zones in the Ryck catchment. Altogether, 13 peatland sites across 8 sub-catchments were recommended for mitigation of high nutrient runoff. In the end, nutrient efficiency of proposed WBZs in one of the sub-catchments of Ryck has been discussed. The results show that (i) the modified empirical equation can act as a key tool in application-based future strategies for nitrogen reduction in the Ryck catchment, (ii) restoration of peatlands and introduction of WBZs can help in mitigating the nutrient runoff for improved water quality of Ryck, and subsequently (ii) contribute to efficient reduction of riverine loads of nutrients into the Baltic Sea.

Estimating nutrient thresholds for eutrophication management: Novel insights from understudied lake types

Nutrient targets based on pressure-response models are essential for defining ambitions and managing eutrophication. However, the scale of biogeographical variation in these pressure-response relationships is poorly understood, which may hinder eutrophication management in regions where lake ecology is less intensively studied. In this study, we derive ecology-based nutrient targets for five major ecoregions of Europe: Northern, Central-Baltic, Alpine, Mediterranean and Eastern Continental. As a first step, we developed regressions between nutrient concentrations and ecological quality ratios (EQR) based on phytoplankton and macrophyte communities. Significant relationships were established for 13 major lake types; in most cases, these relationships were stronger for phosphorus than for nitrogen, and stronger for phytoplankton than for macrophytes. Using these regressions, we estimated the total phosphorus (TP) and total nitrogen (TN) concentrations at which lakes of different types are likely to achieve good ecological status. However, in the very shallow lakes of the Eastern Continental region, relations between nutrient and biological communities were weak or non-significant. This can be attributed to high nutrient concentrations (in the asymptotic zone of phosphorus-phytoplankton models) suggesting other factors (light, grazing) limit primary production. However, we also show that fish stocking is a major pressure on Eastern Continental lakes, negatively affecting ecological status: lakes with low fish stocking show low chlorophyll-a concentrations and good ecological status despite high nutrient levels, while the lakes with high fish stocking show high chlorophyll-a and low ecological status. This study highlights the need to better understand lakes in biogeographic regions that have been, for historical reasons, less studied. This, in turn, helps reveal factors that challenge the dominant paradigms of lake assessment and management.

Simultaneous biological removal of nitrogen and phosphorus from secondary effluent of wastewater treatment plants by advanced treatment: A review

With the advancement of water ecological protection and water control standard, it is the general trend to upgrade the wastewater treatment plants (WWTPs). The simultaneous removal of nitrogen and phosphorus is the key to improve the water quality of secondary effluent of WWTPs to prevent the eutrophication. Therefore, it is urgent to develop the applicable technologies for simultaneous biological removal of nitrogen and phosphorus from secondary effluent. In this review, the composition of secondary effluent from municipal WWTPs were briefly introduced firstly, then the three main treatment processes for simultaneous nitrogen and phosphorus removal, i.e., the enhanced denitrifying phosphorus removal filter, the pyrite-based autotrophic denitrification and the microalgae biological treatment system were summarized, their performances and mechanisms were analyzed. The influencing factors and microbial community structure were discussed. The advanced removal of nitrogen and phosphorus by different technologies were also compared and summarized in terms of performance, operational characteristics, disadvantage and cost. Finally, the challenges and future prospects of simultaneous removal of nitrogen and phosphorus technologies for secondary effluent were proposed. This review will deepen to understand the principles and applications of the advanced removal of nitrogen and phosphorus and provide some valuable information for upgrading the treatment process of WWTPs.

How good is good ecological status? A test across river typologies, diatom indices and biological elements

Good ecological status is the environmental objective of EU water legislation to be achieved by all European water bodies. However, good ecological status varies depending on national criteria, typology approach, and classification systems used. Since nutrient enrichment is the main cause of river degradation, it is important to establish nutrient criteria that consistently support good ecological status across these influential factors. This study analyzes good ecological status, depending on the typology, classification system and biological element used, and it discusses potential implications of the results for river management. We used a database of 425 sites from northern Spain, corresponding to 11 river types of the Spanish typology derived from physiographic data, or to the four river types resulting from NORTIdiat predictive model, derived from regional diatom reference assemblages. PERMANOVA analysis found significant differences among diatom assemblages across the four river types derived from the NORTIdiat system. Among the classification systems currently in use, or of potential use in the area, the upper P-PO₄^3- threshold, established as the P₉₅ of the class distribution for good ecological status, both NORTIdiat (50.7 μg l^-1) and the Multimetric Diatom index (MDIAT; 26.4 μg l^-1) were close to proposed thresholds for good status at the EU level. However, this value was much higher for the Specific Polluosensitivity Index (IPS; 118.1 μg l^-1). Nutrient thresholds for good status also varied among bioindicators, since the predictive invertebrate-based model NORTI classified 67% of samples with high P-PO₄^3- content in good ecological status, whereas the NORTIdiat classified only 33% of them in good status. Results suggest that current nutrient criteria used to establish good ecological status should be revised, accounting for biological specificity and response of biological elements, to provide a more ecologically coherent approach to preserving or restoring good ecological status.

River and lake nutrient targets that support ecological status: European scale gap analysis and strategies for the implementation of the Water Framework Directive

Eutrophication caused by an excessive presence of nutrients is affecting large portions of European waters with more than 60% of the surface water bodies failing to achieve the primary ambition of water management in Europe, that of good ecological status (GES) with diffuse emission from agriculture being the second most important pressure affecting surface waters. We developed EU wide and regional nutrient targets that define the boundary concentrations between good and moderate status for river and lake total P (TP) and total N (TN) and assessed the gap between actual nutrient concentrations and these targets and considered strategies of nutrient reductions necessary to achieve GES and deliver ecosystem services. The nutrient targets established for rivers ranged from 0.5-3.5 mg/L TN and 11-105 μg/L TP and for lakes 0.5-1.8 mg/L TN and 10-60 μg/L TP. Based on the EU wide targets, 59% of the TN and 57% of the TP river monitoring sites and 64% of the TN and 61% of the TP lake monitoring sites exceed these value and are thus at less than GES. The PCA and step-wise regression for EU basins clearly showed that the basin nutrient export is predominantly related to agricultural inputs. In addition, the step-wise regression models for TN and TP provided the ability to extrapolate the results and quantify the input reductions necessary for reaching the nutrient targets at the EU level. The results suggest that a dual water management strategy would be beneficial and should focus a) on those less polluted rivers and lakes that can easily attain the GES goal and b) on the more highly polluted systems that will improve the delivery of ecosystem services.

Influence of temperature and pH on phosphate removal efficiency of different sorbents used in lake restoration

Phosphorus sorbents (PS) are viewed as a powerful tool to manage eutrophication. Here, we tested three commercially available PS - lanthanum-modified bentonite (LMB), aluminium-modified zeolite (AMZ) and aluminium salts (Al) on their capacity to chemically inactivate soluble reactive phosphorus (SRP) at six different temperatures (5 to 35 °C) and five pH values (6 to 10). We also evaluated if the SRP bound at a neutral pH would be released if pH increases to pH 10. Results showed that temperature affected the SRP binding behavior differently for each PS. For instance, the highest SRP binding capacities of LMB, AMZ and Al were 14.0, 29.9 and 251.1 mg P g^-1 at 30 °C, 35 °C and 30 °C, respectively; and the lowest was at 35 °C for LMB, 25 °C for AMZ and 20 °C for Al (6.3, 4.0 and 205.2 mg P g^-1, respectively). The pH also affected the SRP binding differently. When pH increased from pH 6 to pH 10, LMB and Al decreased their binding capacity from 10.0 to 4.9 and from 571.7 mg P g^-1 to 21.3 mg P g^-1, respectively. The SRP adsorption capacity of AMZ was similar at pH 7 and 10 (6.3 and 6.2 mg P g^-1). We observed that in high pH, LMB did not release the SRP precipitated. In contrast, AMZ and Al desorbed around 39%, and 71% of the SRP adsorbed when pH changed from 7 to 10. Abiotic factors such as pH should be considered when selecting the most promising material in lake restoration.

Improving phosphate use efficiency in the aquatic crop watercress (Nasturtium officinale)

Watercress is a nutrient-dense leafy green crop, traditionally grown in aquatic outdoor systems and increasingly seen as well-suited for indoor hydroponic systems. However, there is concern that this crop has a detrimental impact on the environment through direct phosphate additions causing environmental pollution. Phosphate-based fertilisers are supplied to enhanced crop yield, but their use may contribute to eutrophication of waterways downstream of traditional watercress farms. One option is to develop a more phosphate use efficient (PUE) crop. This review identifies the key traits for this aquatic crop (the ideotype), for future selection, marker development and breeding. Traits identified as important for PUE are (i) increased root surface area through prolific root branching and adventitious root formation, (ii) aerenchyma formation and root hair growth. Functional genomic traits for improved PUE are (iii) efficacious phosphate remobilisation and scavenging strategies and (iv) the use of alternative metabolic pathways. Key genomic targets for this aquatic crop are identified as: PHT phosphate transporter genes, global transcriptional regulators such as those of the SPX family and genes involved in galactolipid and sulfolipid biosynthesis such as MGD2/3, PECP1, PSR2, PLDζ1/2 and SQD2. Breeding for enhanced PUE in watercress will be accelerated by improved molecular genetic resources such as a full reference genome sequence that is currently in development.

Establishing ecologically-relevant nutrient thresholds: A tool-kit with guidance on its use

One key component of any eutrophication management strategy is establishment of realistic thresholds above which negative impacts become significant and provision of ecosystem services is threatened. This paper introduces a toolkit of statistical approaches with which such thresholds can be set, explaining their rationale and situations under which each is effective. All methods assume a causal relationship between nutrients and biota, but we also recognise that nutrients rarely act in isolation. Many of the simpler methods have limited applicability when other stressors are present. Where relationships between nutrients and biota are strong, regression is recommended. Regression relationships can be extended to include additional stressors or variables responsible for variation between water bodies. However, when the relationship between nutrients and biota is weaker, categorical approaches are recommended. Of these, binomial regression and an approach based on classification mismatch are most effective although both will underestimate threshold concentrations if a second stressor is present. Whilst approaches such as changepoint analysis are not particularly useful for meeting the specific needs of EU legislation, other multivariate approaches (e.g. decision trees) may have a role to play. When other stressors are present quantile regression allows thresholds to be established which set limits above which nutrients are likely to influence the biota, irrespective of other pressures. The statistical methods in the toolkit may be useful as part of a management strategy, but more sophisticated approaches, often generating thresholds appropriate to individual water bodies rather than to broadly defined "types", are likely to be necessary too. The importance of understanding underlying ecological processes as well as correct selection and application of methods is emphasised, along with the need to consider local regulatory and decision-making systems, and the ease with which outcomes can be communicated to non-technical audiences.

N and P behaviour in alluvial aquifers and in the soil solution of their catchment areas: How land use and the physical environment contribute to diffuse pollution

The role of land use and the physical environment in N and P pollution of alluvial aquifers was analysed at three levels of information: (1) aquifer (N and P in groundwater), (2) soil transect (potentially leachable N and P in the soil solution) and (3) aquifer's catchment area. The study was carried out in the Oja and Tirón alluvial aquifers and their catchment areas (northern Spain). Nitrate was the dominant N form, both in groundwater and the soil solution of aquifers' catchment areas. Orthophosphate and organic-P were the codominant P forms in the aquifers. Orthophosphate was the main form in the soil solution. During the period 2005-2017 no significant decrease in nitrate pollution was observed, suggesting the need to review current Nitrate Vulnerable Zone (NVZ) designations. Since nitrate is highly mobile, it tended to accumulate in stagnation zones at the lower reaches of the aquifers. P did not accumulate in the same zones due to its low solubility. Principal component analyses (PCAs) of the aquifers, soil transects and aquifers' catchment areas revealed that the observation scale influences the environmental factors that can be detected as intervening in groundwater pollution. At the aquifer scale, links were found between nitrates and land use, topographic, hydrogeological and climatic factors. The protective effect of natural areas against nitrate pollution was noteworthy, while agriculture was associated with pollution. At the soil transect scale, an altitudinal gradient governed soil particle size distribution and land use, separating mountain forest soils from agricultural soils. The negative relationship between clay contents vs. nitrate and orthophosphate in the soil solution pointed to a regulatory role of clay. At the catchment scale, the size and physical characteristics of the catchments and land use distribution determined macronutrient availability in the soil solution and, in turn, N and P groundwater distribution.

Spatiotemporal Organic Carbon Distribution in the Capo Peloro Lagoon (Sicily, Italy) in Relation to Environmentally Sustainable Approaches

Transitional water environments represent very ecologically interesting areas, which provide various ecosystem services, both concerning biodiversity protection and sustainable fruition of resources. In this way, the evaluation of total carbon and its components, chlorophyll, and chemical and physical parameters is of fundamental importance to deepen the dynamics of these peculiar natural areas. Commercial interests linked to the biological resources of these areas are often not well exploited in relation to their sustainability, due to lack of knowledge. In this study, we investigated the distribution of total organic carbon, chlorophyll, and other related physical and chemical parameters in the natural Lagoon of Capo Peloro (Eastern Sicily), to deepen the knowledge on the carbon equilibrium of these transitional basins. Collected data showed different trends for all parameters, mainly related to different seasons and water exchanges with sea. The influences of primary production sources and farmed molluscs were not negligible and deserve to be further investigated in the future. The results obtained reveal good margins for the possibility of environmentally sustainable exploitation of natural resources in both basins, but at the same time, there is a need for a more detailed knowledge of anthropogenic impacts on the area.

A zero-valent iron and zeolite filter for nitrate recycling from agricultural drainage water

Nitrate reduction to ammonium followed by ammonium capture and reuse, represent a new pathway to recycle nitrogen, prevent eutrophication, and to save energy used for industrial ammonium production. The present study investigates the principle of nitrogen recycling to agricultural drainage water using a coupled zero-valent iron (ZVI) and zeolite-based filter column system tested in laboratory and field continuous-flow experiments. A 40-day laboratory test showed 82% nitrate removal, of which 70% was converted to ammonium. In the following pilot scale field test, a total of 59.2 m³ (1700 pore volumes) drainage water with a nitrate concentration of 2-8 mg L^-1 NO₃^--N was filtrated. An oxidizing unit inserted after the ZVI unit removed iron(II) and optimized ammonium retention in the zeolite unit. Nitrate removal efficiency was 94% for the entire 56-day period with a slight pH increase (pH 8.9). All ammonium produced was retained by the zeolite unit. Formation of green rust carbonate (layered Fe^II-Fe^III-hydroxide) was observed on ZVI particle surfaces, which may increase the redox capacity of the filter system by up to 50% and contribute to its cost-efficiency. Moreover, all phosphate in the influent waters with concentrations between 0.1 and 0.5 mg L^-1 was retained due to sorption by iron oxides in the system. Corrosion products formed cause partial filter clogging and should be removed by regular cleaning and backflushing. In conclusion, the ZVI - zeolite coupled filter system serves as a promising and cost-effective technology for nutrient removal and ammonium retention from agricultural drainage water.

Multiple in-stream stressors degrade biological assemblages in five U.S. regions

Biological assemblages in streams are affected by a wide variety of physical and chemical stressors associated with land-use development, yet the importance of combinations of different types of stressors is not well known. From 2013 to 2017, the U.S. Geological Survey completed multi-stressor/multi-assemblage stream ecological assessments in five regions of the United States (434 streams total). Diatom, invertebrate, and fish communities were enumerated, and five types of potential stressors were quantified: habitat disturbance, excess nutrients, high flows, basic water quality, and contaminants in water and sediment. Boosted regression tree (BRT) models for each biological assemblage and region generally included variables from all five stressor types and multiple stressors types in each model was the norm. Classification and regression tree (CART) models then were used to determine thresholds for each BRT model variable above which there appeared to be adverse effects (multi-metric index (MMI) models only). In every region and assemblage there was a significant inverse relation between the MMI and the number of stressors exerting potentially adverse effects. The number of elevated instream stressors often varied substantially for a given level of land-use development and the number of elevated stressors was a better predictor of biological condition than was development. Using the adverse effects-levels that were developed based on the BRT model results, 68% of the streams had two or more stressors with potentially adverse effects and 35% had four or more. Our results indicate that relatively small increases in the number of stressors of different types can have a large effect on a stream ecosystem.

Spatially explicit boundaries for agricultural nitrogen inputs in the European Union to meet air and water quality targets

Agricultural production in the EU has increased strongly since the 1940s, partly driven by increased nitrogen (N) fertiliser and manure inputs. Increased N inputs and associated losses, however, adversely affect air and water quality, with widespread impacts on terrestrial and aquatic ecosystems and human health. Managing these impacts requires knowledge on 'safe boundaries' for N inputs, i.e., N flows that do not exceed environmental thresholds. We used a spatially explicit N balance model for the EU to derive boundaries for N losses and associated N inputs for three environmental thresholds: (i) N deposition onto natural areas to protect terrestrial biodiversity (critical N loads), (ii) N concentration in runoff to surface water (2.5 mg N l^-1) to protect aquatic ecosystems and (iii) nitrate (NO₃^-) concentration in leachate to groundwater (50 mg NO l^-1) to meet the EU drinking water standard. Critical N losses and inputs were calculated for ~40,000 unique soil-slope-climate combinations and then aggregated at country- and EU-level. To respect thresholds for N deposition, N inputs in the EU need to be reduced by 31% on average, ranging from 0% in several countries to 59% in Ireland and Denmark. The strongest reductions are required in intensive livestock regions, such as Benelux, Brittany and the Po valley. To respect thresholds for N concentration in runoff to surface water, N inputs need to be reduced by 43% on average, ranging from 2% in Estonia to 74% in the Netherlands. Average critical N inputs in view of the threshold for NO₃^- concentration in leachate to groundwater are close to actual (year 2010) inputs, even though leaching thresholds are exceeded in 18% of agricultural land. Critical N inputs and their exceedances presented in this paper can inform more targeted mitigation policies than flat-rate targets for N loss reductions currently mentioned in EU policies.

Regime shifts in trophic status and regional nutrient criteria for the Bohai Bay, China

Identifying trophic status shift and developing nutrient criteria were considered important for controlling coastal eutrophication. Based on data from 1980 to 2018, we used sequential t-test analysis of regime shifts and assessment of coastal trophic status models to detect regime shifts in trophic status in Bohai Bay (China). We identified four distinct periods: unimpaired (1980-1984), minimally impacted (1985-1994), tipping point (1995-1999) and severely degraded (2000-2018). Using the reference conditions at different trophic status, a frequency distribution analysis was performed to develop candidate nutrient criteria. By considering other factors, such as the quantity and quality of data and long-term nutrient variations, we determined recommended criteria for dissolved inorganic nitrogen (15.3 μmol L^-1) and phosphorus (0.42 μmol L^-1) in Bohai Bay. This study provides a novel and feasible approach to determine reference conditions for the determination of nutrient criteria for coastal waters.

How EU policies could reduce nutrient pollution in European inland and coastal waters

Intensive agriculture and densely populated areas represent major sources of nutrient pollution for European inland and coastal waters, altering the aquatic ecosystems and affecting their capacity to provide ecosystem services and support economic activities. Ambitious water policies are in place in the European Union (EU) for protecting and restoring aquatic ecosystems under the Water Framework Directive and the Marine Strategy Framework Directive. This research quantified the current pressures of point and diffuse nitrogen and phosphorus emissions to European fresh and coastal waters (2005-2012), and analysed the effects of three policy scenarios of nutrient reduction: 1) the application of measures currently planned in the Rural Development Programmes and under the Urban Waste Water Treatment Directive (UWWTD); 2) the full implementation of the UWWTD and the absence of derogations in the Nitrates Directive; 3) high reduction of nutrient, using best technologies in wastewaters treatment and optimal fertilisation in agriculture. The results of the study show that for the period 2005-2012, the nitrogen load to European seas was 3.3-4.1 TgN/y and the phosphorus load was 0.26-0.30 TgP/y. Policy measures supporting technological improvements (third scenario) could decrease the nutrient export to the seas up to 14% for nitrogen and 20% for phosphorus, improving the ecological status of rivers and lakes, but widening the nutrient imbalance in coastal ecosystems (i.e. increasing nitrogen availability with respect to phosphorus), affecting eutrophication. Further nutrient reductions could be possible by a combination of measures especially in the agricultural sector. However, without tackling current agricultural production and consumption system, the reduction might not be sufficient for achieving the goals of EU water policy in some regions. The study analysed the expected changes and the source contribution in different European regional seas, and highlights the advantages of addressing the land-sea dynamics, checking the coherence of measures taken under different policies.