Influence of stormflow and baseflow phosphorus pressures on stream ecology in agricultural catchments

Perspectives on total phosphorus response in rivers: Examining the influence of rainfall extremes and post-dry rainfall

Eutrophication is a significant environmental problem caused by nutrient loads from both point and non-point sources. Weather variables, particularly precipitation, affect the concentration of nutrients in water bodies, particularly those from non-point sources, in two contrasting ways. Heavy precipitation causes surface runoff which transports pollutants to rivers and increases nutrient concentration. Conversely, increased river flow can dilute the concentration, lowering it. This study investigates the impact of extreme precipitation, prolonged precipitation, and precipitation after a dry period on the total phosphorus concentration in the Moehne and Erft rivers in Germany, given the projected increase in frequency of extreme precipitation events and long drought periods due to climate change. The study comprises two parts: selecting extreme weather days from 2001 to 2021 and comparing observed Total Phosphorus concentrations with estimated concentrations derived from Generalized Additive Models and linear regression based on the discharge-concentration relationship. Changes in river TP concentration in response to continuous precipitation and precipitation after a dry period were also studied. Our results showed that during wet extreme and post-dry period rainfall events, TP concentration consistently surpassed expected values, underscoring the profound influence of intense rainfall on nutrient mobilization. However, we observed the impact of continuous rainfall to be non-unidirectional. Our work is distinguished by three key innovations: 1) addressing limitations in studying the effects of extreme weather on water quality due to limited temporal resolution, 2) incorporating both linear and non-linear modeling approaches for discharge-concentration relationships, and 3) performing a comprehensive analysis of temporal and spatial patterns of Total Phosphorus concentrations in response to varying rainfall patterns.

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.

The spatial and temporal dynamics of sediment phosphorus attenuation and release in impacted stream catchments

Sediments can attenuate phosphorus (P) from overlying water and reduce trophic status in zero and first order ditches and streams. These features can be considered as intermediate mitigation features between P mobilised from land, and onward delivery to river systems, if the risk of chemical P release from sediments is minimal. However, risk assessments are rarely based on temporal scale dynamics and especially at fine scale in both sediment and water column environments. In this study, in eutrophic stream catchments, bed sediments were tested fortnightly and spatially over one year for EPC0 (to derive phosphate exchange potential-PEP) and for P across a spectrum from labile to recalcitrant fractions. At the same time stream discharge and P concentrations were measured synchronously at high frequency and resolved to 1-hour intervals and indicated high water quality pressures at all flow rates. PEP indicated spatial and temporal changes most likely caused by periods of source disconnection/reconnection and sediment mobilisation during storm events, moving from periods of high attenuation potential to near saturation. Despite these spatial and temporal changes, PEP did not indicate much potential for chemical P release from the sediments (distributing mostly below or close to zero). However, this may be a misleading risk assessment by itself as physical P release, especially of the labile bicarbonate-dithionite (B-D) P fraction of sediments, was a more dominant process mobilised during storm events reducing by up to 84 % during a succession of summer storm events. The total P and total reactive P loads monitored leaving the catchments were coincident with these changes. The specific downstream trophic effects of this episodic P release will need to be assessed in terms of its bioavailability, in combination with other more noted diffuse and point P source processes.

Sources of contamination in sediments of retention tanks and the influence of precipitation type on the size of pollution load

Densification of cities and urban population contributes to increased runoff and suspended solids and alteration of the urban water cycle. Nowadays, Blue-Green Infrastructure is promoted to increase a city's resilience to floods; however, stormwater drainage systems, supported with retention tanks are still important in protecting urban areas against floods. Sediment accumulation in stormwater infrastructure relates to an issue of pollutants such as heavy metals, nutrients etc. Research on the origin of the pollutants associated with the suspension and ultimately sediment accumulated in sewage can bring new insights about processes in urban catchment areas. This is the first study, which is focused on the analysis of stable carbon and nitrogen isotopes in bottom sediments collected from municipal retention tanks to verify the origin of the deposited pollutants immediately after pluvial floods. The research was additionally extended with water quality analyzes immediately after three types of weather: a dry period, typical precipitation (< 30 mm) and torrential rainfalls (2 events with daily precipitation over 30 mm which caused pluvial flooding of the city area). Analyses of sediments indicated that the main source of carbon and nitrogen in the bottom of the retention tanks had been brought with stormwater runoff from the city area. Organic nitrogen fertilizers appeared to be the main source of nitrogen, while the sources of organic carbon were mixed: C3 land plants, wood, and oil. Additionally, it was found that torrential rainfall caused a 23-fold increase of N-NO₃ concentration, a sevenfold increase of P-PO₄ concentration, and an over fivefold increase of concentration of organic matter, in comparison to typical precipitation.

Baseflow estimation based on a self-adaptive non-linear reservoir algorithm in a rainy watershed of eastern China

Accurate baseflow estimation is critical for water resources evaluation and management, and non-point source pollution quantification. Nonlinear reservoir algorithm (NRA) has been increasingly applied to baseflow separation because of its good approximation to the real groundwater discharge (commonly dominated by the unconfined aquifer) in most watersheds. However, in the rainy regions, large uncertainties may remain in the traditional NRA-separated baseflow sequences due to its empirical transition function for the rising limb of discharge process, and the evident variations of baseflow recession in the initial period of the falling limb caused by the disturbance from surface flow or rainfall events. To improve the reliability of baseflow separation, a self-adaptive non-linear reservoir algorithm (SA-NRA) was developed in this study based on the NRA, a self-adaptive groundwater discharge modified parameter, and the Particle Swarm Optimization algorithm (PSO). The validation of SA-NRA in a rainy watershed of eastern China showed that SA-NRA could be the approach to provide a goodness-of-fit for baseflow recession behaviors in the rainy regions. The traditional NRA and Eckhardt's two-parameter recursive digital filter (ERDF), calibrated (or validated) only with the pure baseflow recession data, can hardly provide reliable baseflow predictions for the non-pure baseflow recession periods (including the rising limb and the falling limb with surface flow or rainfall disturbance) due to the apparent variations of baseflow recession behavior. Therefore, more attentions should be paid to the uncertainties of baseflow separation for the non-pure baseflow recession periods in the rainy regions.

Advances in Catchment Science, Hydrochemistry, and Aquatic Ecology Enabled by High-Frequency Water Quality Measurements

High-frequency water quality measurements in streams and rivers have expanded in scope and sophistication during the last two decades. Existing technology allows in situ automated measurements of water quality constituents, including both solutes and particulates, at unprecedented frequencies from seconds to subdaily sampling intervals. This detailed chemical information can be combined with measurements of hydrological and biogeochemical processes, bringing new insights into the sources, transport pathways, and transformation processes of solutes and particulates in complex catchments and along the aquatic continuum. Here, we summarize established and emerging high-frequency water quality technologies, outline key high-frequency hydrochemical data sets, and review scientific advances in key focus areas enabled by the rapid development of high-frequency water quality measurements in streams and rivers. Finally, we discuss future directions and challenges for using high-frequency water quality measurements to bridge scientific and management gaps by promoting a holistic understanding of freshwater systems and catchment status, health, and function.

Combined Effect of Climate and Anthropopressure on River Water Quality

This study was a continuation of our investigation of the spatio-temporal variability of the Bzura River's water chemistry. Our research is of particular importance in the context of the recent ecological disaster on the Oder River and concerns the international problem of surface water contamination. The study area was a 120 km section of the Bzura River. We tested more measurement points and with a higher sampling frequency than those used in the national monitoring of river water quality. During two hydrological years, 360 water samples were collected. The selected parameters: electrical conductivity, temperature, dissolved oxygen, dissolved organic carbon, nitrates, phosphates, bicarbonates, chlorides, sodium, potassium, calcium, and magnesium were determined. Numerous results exceeded the Polish threshold limits. Spatio-temporal variability and water quality were assessed using principal component analysis (PCA), cluster analysis (CA), and water quality index (WQI) approaches. Many point sources of pollution related to urbanization, agriculture, and industry were detected. Moreover, due to the changing climatic conditions, a significant difference between temporal variability in both years was observed. Our results indicated that it is necessary to increase the number of measurement stations for surface water monitoring; it will allow for a faster detection of the threat.

Assessment of Phosphorus Input from Urban Areas in the Passaúna River and Reservoir

Elevated phosphorus loads play an important role in the deterioration of water quality and can subsequently pose a threat to the aquatic organisms in a river or a standing water body. The accurate assessment of total phosphorus (TP) fluxes from a catchment is of high importance to the well-being of the entire river ecosystem. In this study, we assessed the yearly input of TP from the urban areas of the Passaúna catchment in southern Brazil. The catchment drains into the eponymous reservoir, which provides drinking water for more than 800,000 inhabitants of the Curitiba Metropolitan region. The protection of the water quality in the river as well as in the reservoir is of paramount importance, yet high phosphorous inputs have been detected. For adequate protection, the catchment emissions need to be accurately assessed. Initially, the TP concentration in the river sediment was determined in order to assess the relationship between the TP export of the urban areas and the TP stock of the river. It was found that in areas with a higher share of urban land cover and especially in areas with a lack of sewage treatment, the TP concentration in the sediment reached up to 6700 mg/kg. The assessment of the overall TP input from urban areas was based on a regionalized emission-modeling approach, combined with data from long-term water quality monitoring of the river. The monitoring station established upstream of the Passaúna Reservoir inflow provided an initial assessment and the necessary output for the validation and calibration of the model. From the drainage basin of the monitoring station, an overall TP input of 2501 kg/a (0.31 kg/(ha a)) was measured between 1 May 2018 and 1 May 2019 (3508 kg TP/a or 0.23 kg/(ha a) when extrapolating the overall catchment of the Passaúna Reservoir). The monitoring data indicated that the TP input increases during the wet months of the year. The sediment stock of the river also plays an important role in the interannual budget of TP. During the timespan of one year, many deposition–resuspension events happen. The resuspended material is included in the baseflow and hinders the differentiation between urban and nonurban input. After calibration, the model was able to predict the yearly input of TP from the urban areas of the Passaúna catchment. In addition, the share of inhabitants who are not connected to the sewer system was assessed. Overall, the combination of monitoring and modeling in this study offers a valuable overview of the TP dynamics of the system, while the model ensures reproducibility with high accuracy at the same time.

Integrated watershed process model for evaluating mercury sources, transport, and future remediation scenarios in an industrially contaminated site

We used the Soil Water Assessment Tool (SWAT) as a framework to develop an empirical Hg flux model for Upper East Fork Poplar Creek (UEFPC), a Hg-contaminated watershed in Oak Ridge, Tennessee. By integrating long-term Hg monitoring data with simulated flow and suspended solid loads in a site-specific empirical Hg transport model, we (1) quantified the spatial, temporal, and flow regime controls on daily Hg flux (adjusted R² = 0.82) and (2) made predictions about Hg flux under future climate, land use, and management scenarios. We found that 62.79% of the average daily Hg flux in the watershed is currently driven by base flow, whereas variability in Hg flux is driven by storm and extreme flow. We estimate an average annual Hg flux of 28.82 g day^-1 leaving the watershed under baseline precipitation, with an estimated 43.73% reduction in daily Hg flux under drought conditions and a 296% increase in daily Hg flux in extreme precipitation scenarios. We estimated that a new mercury treatment facility would result in a 24.7% reduction in Hg flux under baseline conditions and a 33.4% reduction under extreme precipitation scenarios. The study demonstrated the merit of this approach, which can be replicated for sites where information on flow, suspended solids, and Hg concentrations is available.

Biomass and nutrient dynamics of major green tides in Ireland: Implications for biomonitoring

The control of macroalgal bloom development is central for protecting estuarine ecosystems. The identification of the nutrients limiting the development of macroalgal blooms, and their most likely sources is crucial for management strategies. Three Irish estuaries (Argideen, Clonakilty and Tolka) affected by green tides were monitored from June 2016 to August 2017. During each sampling occasion, biomass abundances, tissue N and P contents, and δ¹⁵N were determined for tubular and laminar morphologies of Ulva. All estuaries showed maximum biomass during summer and minimum during winter. Tissue nutrient contents revealed P rather than N limitation. The δ¹⁵N during the peak bloom indicated agriculture as the most likely source of nitrogen in the Argideen and Clonakilty, and urban wastewaters in the Tolka. No differences in the δ¹⁵N, and the tissue nutrients content were observed between morphologies. The period between May and July is most suitable for bioassessment of green tides.

Charting a perfect storm of water quality pressures

The agri-food economy can be a significant driver of water quality pressures but the role of hydro-meteorological patterns in a changing climate also requires consideration. For this purpose, an assessment was made of a ten-year synchronous high temporal resolution water quality and hydro-meteorological dataset in Irish agricultural catchments. Changes occurring to rainfall intensity and soil temperature patterns were found to be important drivers of nutrient mobility in soils. There were links between the intensity of the North Atlantic Oscillation over the decade and large shifts in baseline nutrient concentrations in catchments. The data also revealed extreme weather impacts to pollution patterns including short periods of rain induced nutrient flux, that exceeded average annual mass loads in these catchments, and drought influences on point source pollution. These influences need consideration, and may require different mitigation strategies, as links between water quality land use pressure and water quality state in regulatory reviews. In a decade of both increased land use source and hydro-meteorological transport pressures, water quality natural capital in Ireland has faced a perfect storm. Such conditions are difficult to model and only revealed in high temporal resolution datasets.

Dissolved phosphorus export through baseflow in an intensively cultivated agricultural watershed of eastern China

Inputs of phosphorus (P) during baseflow period usually come from groundwater, bed sediments, and some other sources. Baseflow P can have critical effects on nutrient enrichment of surface waters in some intensively cultivated agricultural watersheds. This study was conducted to estimate the baseflow dissolved phosphorus (DP) export in a typical rainy agricultural watershed of eastern China using a recursive tracing source algorithm (RTSA) and reveal the rules and trends of baseflow DP loads and concentrations. Results indicated that RTSA provided a satisfactory prediction for baseflow DP load (Nash-Sutcliffe efficiency (NSE) = 0.72, R2 = 0.74). From 2003 to 2012, the annual baseflow DP loads ranged from 0.159 (2004) to 0.771 (2012) kg/ha which contributed about 64.3% of the mean total annual DP export in stream (0.597 kg/ha). The annual flow-weighted DP concentrations in streamflow (0.076-0.125 mg/L) and baseflow (0.076-0.137 mg/L) far exceeded the eutrophication threshold of DP (0.01 mg/L). Significantly increasing trends were obtained in the streamflow and baseflow DP loads and the flow-weighted concentrations (Mann-Kendall test, Zs > 2.56, p < 0.01) because of the changes of hydro-meteorological conditions. This indicates that, in the context of global climate change, baseflow DP export would be one of key issues for nonpoint source pollution control in the intensive agricultural watershed.

A Tale of Two Rivers: Can the Restoration Lessons of River Thames (Southern UK) Be Transferred to River Hindon (Northern India)?

This study identifies the basin scale factors and potential remedies to restore the severely polluted Hindon River in India, by comparing with another basin with high population density: the River Thames in the UK. Biochemical oxygen demand (BOD) and dissolved oxygen (DO) in the Thames River are usually around 8 mg/l and 7.5 mg/l respectively, while phosphorus and ammonium range between 0.1-0.6 mg/l and 0.1-0.4 mg/l respectively. The Thames has seen great improvements in water quality over the past decades, due to high levels of sewage treatment and regulation of industrial effluents which have improved water quality conditions. Conversely, the Hindon River suffers from extremely poor water quality and this is mainly attributed to the direct discharge of partially treated or untreated municipal and industrial wastewater into the river. BOD is in the range of 15-60 mg/l and DO is below 5 mg/l. Phosphorus ranges around 2-6 mg/l at most of the monitoring stations and ammonia-nitrogen in the range of 10-40 mg /l in Galeta at Hindon. The analysis of variance also depicts the spatial and temporal variation in water quality in the Hindon River. Besides, non-point sources, pollution from point sources with minimal base flow in the river during dry season, result in low dilution capacity causing high pollutant concentrations which impacts the river ecosystem and fisheries. To restore the Hindon River, resources must be focussed on mainly treating sewage and industrial effluents and by developing appropriate river basin management and regulatory plans.

Seaweed's Bioactive Candidate Compounds to Food Industry and Global Food Security

The world population is continuously growing, so it is important to keep producing food in a sustainable way, especially in a way that is nutritious and in a sufficient quantity to overcome global needs. Seaweed grows, and can be cultivated, in seawater and generally does not compete for arable land and freshwater. Thus, the coastal areas of the planet are the most suitable for seaweed production, which can be an alternative to traditional agriculture and can thus contribute to a reduced carbon footprint. There are evolving studies that characterize seaweed's nutritional value and policies that recognize them as food, and identify the potential benefits and negative factors that may be produced or accumulated by seaweed, which are, or can be, dangerous for human health. Seaweeds have a high nutritional value along with a low caloric input and with the presence of fibers, proteins, omega 3 and 6 unsaturated fatty acids, vitamins, and minerals. Moreover, several seaweed sub-products have interesting features to the food industry. Therefore, the focus of this review is in the performance of seaweed as a potential alternative and as a safe food source. Here described is the nutritional value and concerns relating to seaweed consumption, and also how seaweed-derived compounds are already commercially explored and available in the food industry and the usage restrictions to safeguard them as safe food additives for human consumption.

Drivers of excess phosphorus and stream sediments in a nested agricultural catchment during base and stormflow conditions

A variety of landscape and hydrological characteristics influence nutrient concentrations and suspended sediments in freshwater systems, yet the combined influence of these characteristics within nested agricultural catchments is still poorly understood, particularly across varying flow states. To tease apart potential drivers at within-catchment scales, it is necessary to sample at a spatiotemporal resolution that captures how landscape drivers change with time. The overall objective of this study was to evaluate the relative influence of landscape and hydrological characteristics at sub-catchment scales in relation to total P (TP), soluble reactive P (SRP), the ratio of SRP and TP (SRP/TP), and total suspended solids (TSS) across varying flow conditions. Synoptic surveys were conducted at 13 longitudinal sampling sites under a variety of flow conditions (n = 14) between 2016 and 2017 in the Innisfil Creek watershed, southern Ontario. The surveys were grouped into baseflow and stormflow conditions, and partial least squares regression (PLSR) was used to characterize the relationships between catchment characteristics, median concentrations of P, and TSS. Soil texture (i.e., clay dominated), winter wheat (Triticum aestivum L.), and constructed drain density had the largest influences on stormflow SRP and SRP/TP ratios, but measures of soil erosion, like the Bank Erosion Hazard Index and sinuosity, had the largest influence on stormflow TSS. During baseflow periods, these landscape characteristics were not informative, and they were difficult to tie to in-stream conditions. Overall, our PLSR models indicated that buried tile drainage was a major source of SRP in Innisfil Creek, whereas bank erosion was a dominant source of TSS.

Establishing nationally representative benchmarks of farm-gate nitrogen and phosphorus balances and use efficiencies on Irish farms to encourage improvements

Agriculture faces considerable challenges of achieving more sustainable production that minimises nitrogen (N) and phosphorus (P) losses and meets international obligations for water quality and greenhouse gas emissions. This must involve reducing nutrient balance (NB) surpluses and increasing nutrient use efficiencies (NUEs), which could also improve farm profitability (a win-win). To set targets and motivate improvements in Ireland, nationally representative benchmarks were established for different farm categories (sector, soil group and production intensity). Annual farm-gate NBs (kg ha^-1) and NUEs (%) for N and P were calculated for 1446 nationally representative farms from 2008 to 2015 using import and export data collected by the Teagasc National Farm Survey (part of the EU Farm Accountancy Data Network). Benchmarks for each category were established using quantile regression analysis and percentile rankings to identify farms with the lowest NB surplus per production intensity and highest gross margins (€ ha^-1). Within all categories, large ranges in NBs and NUEs between benchmark farms and poorer performers show considerable room for nutrient management improvements. Results show that as agriculture intensifies, nutrient surpluses, use efficiencies and gross margins increase, but benchmark farms minimise surpluses to relatively low levels (i.e. are more sustainable). This is due to, per ha, lower fertiliser and feed imports, greater exports of agricultural products, and for dairy, sheep and suckler cattle, relatively high stocking rates. For the ambitious scenario of all non-benchmark farms reaching the optimal benchmark zone, moderate reductions in farm nutrient surpluses were found with great improvements in profitability, leading to a 31% and 9% decrease in N and P surplus nationally, predominantly from dairy and non-suckler cattle. The study also identifies excessive surpluses for each level of production intensity, which could be used by policy in setting upper limits to improve sustainability.

Intra-population strain variation in phosphorus storage strategies of the freshwater cyanobacterium Raphidiopsis raciborskii

Several cyanobacteria, including diazotrophic Raphidiopsis raciborskii, can form harmful blooms when dissolved inorganic phosphorus concentrations are very low. We hypothesized that R. raciborskii strains would vary in phosphorus (P) allocations to cell growth and storage, providing resilience of populations to continuously low or variable P supplies. We tested this hypothesis using six toxic strains (producing cylindrospermopsins) isolated from a field population using batch monocultures with and without P and dissolved inorganic nitrogen (DIN). Treatments replete with DIN, irrespective of P addition, had similar exponential growth rates for individual strains. P storage capacity varied 4-fold among strains and was significantly higher in DIN-free treatments than in replete treatments. P was stored by all R. raciborskii strains, in preference to allocation to increase growth rates. P stores decreased with increased growth rate across strains, but weeere not related to the time to P starvation in P-free treatments. The storage capacity of R. raciborskii, combined with strategies to efficiently uptake P, means that P controls may not control R. raciborskii populations in the short term. Intra-population strain variation in P storage capacity will need to be reflected in process-based models to predict blooms of R. raciborskii and other cyanobacteria adapted to low-P conditions.

Drivers of water pollution and evaluating its ecological stress with special reference to macrovertebrates (fish community structure): a case of Churni River, India

The focus of this study is to measure ecological stress of Churni River based on the estimates of dissolved oxygen (DO), pH, biological oxygen demand (BOD), chemical oxygen demand (COD), nutrients imbalances of dissolved inorganic nitrogen or DIN (NO₃^- N and NO₂^- N) and dissolved inorganic phosphate or DIP (PO₄^3-). The present water quality measured in terms of overall index of pollution (OIP), eutrophication index (EI), organic pollution (OPI) and water pollution indexes for ecological status (WPI) portrays that the river is polluted having a high concentration of BOD, COD, nutrients (DIP and DIN) and a very low concentration of DO. Fish community structure taken as most sensitive indicator of ecological stress of water pollution depicts that out of 44 species, 28 fish species (63.63%) comprising 20.0% planktivore (PL), 9.09% benthic feeder (BE), 18.18% omnivorous (OM) and 15.90% carnivorous (CA) at Majhdia and 21 fish species (47.72%) comprising 18.18% PL, 4.59% BE, 13.63% OM and 11.36% CA at Ranaghat have been disappeared. The present investigation has found that anthropogenic interventions like disposal of industrial effluents and agricultural run-off from on-bed and off bed land use are the main drivers of the pollution. Furthermore, natural forcing in the form of neotectonic movements and monsoon regimes has intensified the problem.

Chronic nutrient inputs affect stream macroinvertebrate communities more than acute inputs: An experiment manipulating phosphorus, nitrogen and sediment

Freshwaters worldwide are affected by multiple stressors. Timing of inputs and pathways of delivery can influence the impact stressors have on freshwater communities. In particular, effects of point versus diffuse nutrient inputs on stream macroinvertebrates are poorly understood. Point-source inputs tend to pose a chronic problem, whereas diffuse inputs tend to be acute with short concentration spikes. We manipulated three key agricultural stressors, phosphorus (ambient, chronic, acute), nitrogen (ambient, chronic, acute) and fine sediment (ambient, high), in 112 stream mesocosms (26 days colonisation, 18 days of manipulations) and determined the individual and combined effects of these stressors on stream macroinvertebrate communities (benthos and drift). Chronic nutrient treatments continuously received high concentrations of P and/or N. Acute channels received the same continuous enrichment, but concentrations were doubled during two 3-hour periods (day 6, day 13) to simulate acute nutrient inputs during rainstorms. Sediment was the most pervasive stressor in the benthos, reducing total macroinvertebrate abundance and richness, EPT (mayflies, stoneflies, caddisflies) abundance and richness. By contrast, N or P enrichment did not affect any of the six studied community-level metrics. In the drift assemblage, enrichment effects became more prevalent the longer the experiment went on. Sediment was the dominant driver of drift responses at the beginning of the experiment. After the first acute nutrient pulse, sediment remained the most influential stressor but its effects started to fade. After the second pulse, N became the dominant stressor. In general, impacts of either N or P on the drift were due to chronic exposure, with acute nutrient pulses having no additional effects. Overall, our findings imply that cost-effective management should focus on mitigating sediment inputs first and tackle chronic nutrient inputs second. Freshwater managers should also take into account the length of exposure to high nutrient concentrations, rather than merely the concentrations themselves.

Modeling the Ecological Impact of Phosphorus in Catchments with Multiple Environmental Stressors

The broken phosphorus (P) cycle has led to widespread eutrophication of freshwaters. Despite reductions in anthropogenic nutrient inputs that have led to improvement in the chemical status of running waters, corresponding improvements in their ecological status are often not observed. We tested a novel combination of complementary statistical modeling approaches, including random-effect regression trees and compositional and ordinary linear mixed models, to examine the potential reasons for this disparity, using low-frequency regulatory data available to catchment managers. A benthic Trophic Diatom Index (TDI) was linked to potential stressors, including nutrient concentrations, soluble reactive P (SRP) loads from different sources, land cover, and catchment hydrological characteristics. Modeling suggested that SRP, traditionally considered the bioavailable component, may not be the best indicator of ecological impacts of P, as shown by a stronger and spatially more variable negative relationship between total P (TP) concentrations and TDI. Nitrate-N ( < 0.001) and TP ( = 0.002) also showed negative relationship with TDI in models where land cover was not included. Land cover had the strongest influence on the ecological response. The positive effect of seminatural land cover ( < 0.001) and negative effect of urban land cover ( = 0.030) may be related to differentiated bioavailability of P fractions in catchments with different characteristics (e.g., P loads from point vs. diffuse sources) as well as resilience factors such as hydro-morphology and habitat condition, supporting the need for further research into factors affecting this stressor-response relationship in different catchment types. Advanced statistical modeling indicated that to achieve desired ecological status, future catchment-specific mitigation should target P impacts alongside multiple stressors.

The problem of agricultural 'diffuse' pollution: Getting to the point

Despite introduction of legislation such as the EU Nitrates and Water Framework Directives (Directives 91/676/EEC and 2000/60/EC respectively), agricultural practices are often still regarded as a major factor in poor water quality across many EU member states. Elevated inputs of nutrients, organic matter and agro-chemicals to receiving waters from agricultural lands in particular are now widely recognised as potentially major causes of deteriorating water quality. Such inputs may emanate from diffuse sources such as agricultural fields, and small point- or intermediate-sources, including farmyards and farm trackways. However, while inputs from these latter intermediate sources may be substantial, their overall contribution to catchment-wide water quality at high temporal or spatial resolution is still largely unknown. In this study, we surveyed water chemistry throughout the multiple natural and artificial watercourses within a single drainage network at high spatial resolution in a predominantly dairy farming area in Southern Ireland. We found that most headwaters at the time of study were impacted by organic inputs via drainage ditches emanating from the vicinity of farmyards. These farmyard drains were found to have elevated concentrations of ammonium, phosphorus, potassium, suspended sediment and biochemical oxygen demand above background levels in the study catchment. Concomitant assessment of macro-invertebrate communities at study sites indicated that the ecological quality of headwaters was also impaired by these inputs. The individual and aggregate contributions of farmyard drains to water quality within a single catchment, when mapped at high spatial resolution, indicates that they constitute a major contribution to catchment scale 'diffuse' agricultural inputs. However, our data also suggest that engineering farmyard drains to maximise their retention and attenuation function may prove to be a cost-effective means of mitigating the effects of point source farmyard inputs.

Sources and Mechanisms of Low-Flow River Phosphorus Elevations: A Repeated Synoptic Survey Approach

High-resolution water quality monitoring indicates recurring elevation of stream phosphorus concentrations during low-flow periods. These increased concentrations may exceed Water Framework Directive (WFD) environmental quality standards during ecologically sensitive periods. The objective of this research was to identify source, mobilization, and pathway factors controlling in-stream total reactive phosphorus (TRP) concentrations during low-flow periods. Synoptic surveys were conducted in three agricultural catchments during spring, summer, and autumn. Up to 50 water samples were obtained across each watercourse per sampling round. Samples were analysed for TRP and total phosphorus (TP), along with supplementary parameters (temperature, conductivity, dissolved oxygen, and oxidation reduction potential). Bed sediment was analysed at a subset of locations for Mehlich P, Al, Ca, and Fe. The greatest percentages of water sampling points exceeding WFD threshold of 0.035 mg L−1 TRP occurred during summer (57%, 11%, and 71% for well-drained, well-drained arable, and poorly drained grassland catchments, respectively). These percentages declined during autumn but did not return to spring concentrations, as winter flushing had not yet occurred. Different controls were elucidated for each catchment: diffuse transport through groundwater and lack of dilution in the well-drained grassland, in-stream mobilization in the well-drained arable, and a combination of point sources and cumulative loading in the poorly drained grassland. Diversity in controlling factors necessitates investigative protocols beyond low-spatial and temporal resolution water sampling and must incorporate both repeated survey and complementary understanding of sediment chemistry and anthropogenic phosphorus sources. Despite similarities in elevation of P at low-flow, catchments will require custom solutions depending on their typology, and both legislative deadlines and target baselines standards must acknowledge these inherent differences.

Designing the National Network for Automatic Monitoring of Water Quality Parameters in Greece

Water quality indices that describe the status of water are commonly used in freshwater vulnerability assessment. The design of river water quality monitoring programs has always been a complex process and despite the numerous methodologies employed by experts, there is still no generally accepted, holistic and practical approach to support all the phases and elements related. Here, a Geographical Information System (GIS)-based multicriteria decision analysis approach was adopted so as to contribute to the design of the national network for monitoring of water quality parameters in Greece that will additionally fulfill the urgent needs for an operational, real-time monitoring of the water resources. During this cost-effective and easily applied procedure the high priority areas were defined by taking into consideration the most important conditioning factors that impose pressures on rivers and the special conditions that increase the need for monitoring locally. The areas of increased need for automatic monitoring of water quality parameters are highlighted and the output map is validated. The sites in high priority areas are proposed for the installation of automatic monitoring stations and the installation and maintenance budget is presented. Finally, the proposed network is contrasted with the current automatic monitoring network in Greece.

Point sources and agricultural practices control spatial-temporal patterns of orthophosphate in tributaries to Chesapeake Bay

Orthophosphate (PO₄) is the most bioavailable form of phosphorus (P). Excess PO₄ may cause harmful algal blooms in aquatic ecosystems. A major restoration effort is underway for Chesapeake Bay (CB) to reduce P, nitrogen, and sediment loading to CB. Although PO₄ cycling and delivery to streams has been characterized in small-scale studies, regional drivers of PO₄ patterns remain poorly understood because most water quality trend assessment focus on total P. Moreover, these trend assessments are usually at an annual timestep. To address this research gap, we analyzed PO₄ patterns over a 9-year period at 53 monitoring stations across the CB watershed to: 1) characterize the role of PO₄ in total P fluxes and trends; 2) describe spatial and temporal patterns of PO₄ concentrations across seasons and streamflow; and 3) explore factors explaining these patterns. Agricultural watersheds exported the most total P compared with watersheds under different land uses (e.g., urban or forest), with PO₄ comprising up to 50% of those exports. Although PO₄ exports are declining at many sites, some agricultural regions are experiencing increasing trends at a rate sufficient to drive total P trends. Regression modeling results suggest that point source load reductions are likely responsible for decreasing PO₄ concentrations observed at many sites. Watersheds with more Conservation Reserve Program enrollment had lower summer PO₄ concentrations, highlighting the effectiveness of this practice. Manure inputs strongly predicted PO₄ concentrations at high flows across all seasons. Both manure applications and conservation tillage were correlated with changes in PO₄ concentrations at high flow, suggesting these activities could contribute to increasing PO₄ concentrations. This study highlights the effectiveness of point source control for reducing PO₄ exports and underscores the need for management strategies to target sources, practices, and landscape factors determining PO₄ loss from soils where manure inputs remain high.

Real-time monitoring of water quality to identify pollution pathways in small and middle scale rivers

The quality standards for surface waters increase steadily bearing new challenges for water policy. Precise knowledge of the sources and transport pathway of various impacts in a catchment area is of particular importance for any management activities. Online measurements with high temporal resolution are particularly suited for this purpose especially in small and middle scale catchments. In this paper we present an approach applying mobile measuring stations in which commercial available sensors and wet chemical analysers are combined in a new set to enable real-time monitoring of various parameters. The resulting data and the interpretation of their relationships allow the identification of diverse pollution situations in a river. In this paper some examples of impacts from diffuse and point sources are given to illustrate the high information density obtained through the use of this system.

Flow regimes filter species traits of benthic diatom communities and modify the functional features of lowland streams - a nationwide scale study

Changes in land use, climate and flow diversion are key drivers of river flow regime change that may eventually affect freshwater biodiversity and ecosystem functions. However, our knowledge is limited on how the functional features of stream organisms vary along the gradient of hydrological disturbance (i.e. flow regime changes) and how flow regimes mediate the functional features in lowland streams. We analyzed the functional traits of benthic diatoms (unicellular siliceous algae) that are most sensitive and tolerant to flow regime changes along a nationwide scale of 246 sites in Denmark. We combined RLQ and fourth-corner analyses to explore the co-variation between hydrological variables (R table) and species traits (Q table), constrained by the relative abundance of each species (L table) as observed in each of the sampling sites. Further, we examine the relationships between functional features (i.e., functional redundancy and diversity) and hydrological variables by multivariate statistical analyses. Results show that species turnover with displacement of sensitive species by tolerant species was the dominating process in benthic diatom communities during high flow disturbances. Functional features, as indicated by functional diversity and redundancy indices, were mediated mainly by high and low flow magnitude. Median daily flow magnitude shows a consistent positive relationship with functional redundancy and richness indices indicating that larger streams are more resilient to flow perturbations. In addition flow regime changes are less important than median daily flow magnitude and show inconsistent correlation to functional features likely due to the interaction of multiple environmental stressors. Our study highlights the robustness of trait-based approaches for identifying flow regime changes in streams, and strongly suggests that biodiversity conservation and water resource management should focus on protecting natural base flow in headwater streams and generally reduce flow regulation for sustaining stream ecosystems under future global changes.

Multiple-stressor effects of sediment, phosphorus and nitrogen on stream macroinvertebrate communities

Multiple stressors affect stream ecosystems worldwide and their interactions are of particular concern, with gaps existing in understanding stressor impacts on stream communities. Addressing these knowledge gaps will aid in targeting and designing of appropriate mitigation measures. In this study, the agricultural stressors fine sediment (ambient, low, medium, high), phosphorus (ambient, enriched) and nitrogen (ambient, enriched) were manipulated simultaneously in 64 streamside mesocosms to determine their individual and combined effects on the macroinvertebrate community (benthos and drift). Stressor levels were chosen to reflect those typically observed in European agricultural streams. A 21-day colonisation period was followed by a 14-day manipulative period. Results indicate that added sediment had the most pervasive effects, significantly reducing total macroinvertebrate abundance, total EPT abundance and abundances of three common EPT taxa. The greatest effect was at high sediment cover (90%), with decreasing negative impacts at medium (50%) and low (30%) covers. Added sediment also led to higher drift propensities for nine of the twelve drift variables. The effects of nitrogen and phosphorus were relatively weak compared to sediment. Several complex and unpredictable 2-way or 3-way interactions among stressors were observed. While sediment addition generally reduced total abundance at high levels, this decrease was amplified by P enrichment at low sediment, whereas the opposite effect occurred at medium sediment and little effect at high sediment. These results have direct implications for water management as they highlight the importance of managing sediment inputs while also considering the complex interactions which can occur between sediment and nutrient stressors.