River Nutrient Concentrations Research Articles

Disentangling the contributions of anthropogenic nutrient input and physical forcing to long-term deoxygenation off the Pearl River Estuary, China

Deoxygenation in estuarine and coastal waters worldwide has been largely attributed to the increasing anthropogenic nutrient input, whereas the contribution by long-term (decadal) changes in physical forcing is less investigated. This study aims to disentangle the impacts of three-decade changes in summer river nutrient concentration and physical forcing on the deoxygenation off a large eutrophic estuary, the Pearl River Estuary (PRE) in China. Using a coupled physical-biogeochemical model, we reproduce the observed summer oxygen conditions under the historical (the 1990s) and present (the 2020s) status of river nutrient concentration, freshwater discharge, and wind forcing. We show that the bottom hypoxic (dissolved oxygen < 2 mg/L) area off the PRE in the 2020s has increased by 73 % relative to the 1990s. The expansion is a result of the increased bottom water oxygen consumption outweighing the enhanced vertical oxygen supply, with the former driven by the sharp increase in inorganic nitrogen and phosphorus concentrations (160 %) and the latter caused by the decadal decline in both freshwater discharge (38 %) and wind speed (12.5 %) in summer. Model experiments suggest that if the observed changes in physical forcing had not occurred, the dramatic increase in anthropogenic nutrient concentrations from the 1990s to 2020s could have led to a much greater expansion of hypoxic area (249 %). On the contrary, the decadal decrease in summer freshwater discharge alone (while keeping the nutrient loading the same as in the 1990s) almost eliminates hypoxia off the PRE by weakening water column stratification and limiting the offshore spread of nutrients and organic matter, whereas the declined wind speed increases the hypoxic area by 247 % mainly through enhancing water column stability. Our results reveal that long-term changes in physical forcing are confounding the effects of anthropogenic nutrient input on deoxygenation, underlining the need to consider regional forcing changes in nutrient management to meet water quality goals.

Nutrient dynamics in small west-flowing tropical mountainous rivers of India

In this study, the distribution of dissolved inorganic nitrate (DIN), phosphorus (DIP), and silicate (DSi) in surface waters of 70 coastal rivers in India's Western Ghats (WG) has been determined. The nutrient concentrations in individual rivers appear to be influenced by catchment geology, land use, and climate. The entire WG region contributes 0.31 Tg of DIN, 0.03 Tg of DIP, and 5.65 Tg of DSi annually to the coastal Arabian Sea. The annual ratios of N:P, Si:N, and Si:P suggest that most of these rivers are limited by P rather than N, and there is an excess supply of Si, primarily from natural sources. P limitation is severe during monsoon periods, while N limitation is observed in the Deccan Traps region. The annual export of 0.31 Tg of DIN from WG rivers could support 2.05 Tg of carbon (new production) on the Arabian Sea coast, indicating that the riverine export of DIN can significantly remove atmospheric CO2 through biological pumps in the Arabian Sea. The study also confirms that the excess input of Si from natural processes relative to the anthropogenic supply of limited P and N further supports the idea that riverine nutrient inputs from small west-flowing tropical mountainous rivers do not contribute to coastal eutrophication along the Eastern Arabian Sea coast.

Assessment of nitrogen budget in detailed spatial pattern using high precision modeling approach with constructed accurate agricultural behavior

Changes in the nitrogen cycle due to fertilizer use can cause severe environmental pollution, particularly groundwater pollution, and threaten biosphere integrity. There are many difficulties and limitations in assessing groundwater pollution and a detailed nitrogen budget in an agricultural catchment. Previous methodologies have failed in an accurate assessment of the nitrogen budget in detailed spatial patterns. Herein, we designed a new modeling approach to assess the nitrogen budget using detailed spatial patterns in an agricultural catchment in the Nara Basin. We revised the Soil and Water Assessment Tool file output format, added the results for river nutrient concentrations and ammonia volatilization to the original output file. In this study, we calibrated and validated crop harvests, paddy evapotranspiration, streamflow, and river water concentrations of nitrate–nitrogen and total nitrogen to improve model accuracy as much as possible. Among them, data for evapotranspiration was obtained from a newly released Landsat dataset. The results showed that the amount of nitrogen leaching in rice paddies was 42 kg/ha, accounting for 65 % of total leaching in the study catchment. Cambisols and Fluvic Gleysols were prone to denitrification, and nitrogen leaching or denitrification occurred relatively more readily in low-slope areas. Furthermore, a detailed analysis of nitrogen cycle processes with high spatial precision indicates that areas with severe surface water pollution may also exhibit significant groundwater pollution. Our findings provide new solutions for assessing the nitrogen budget and groundwater pollution in catchments.

Nutrient metrics to compare algal photosynthetic responses to point and non-point sources of nitrogen pollution

Point- and non-point source nutrients are likely to have different ecological impacts in receiving waters, due to differences in the concentration and proportions of nutrient fractions. However, the direct comparison of their ecological impacts in receiving waters has barely been quantified. We undertook algal bioassays with algal communities from river sites and showed that there was a photosynthetic yield (Fv/Fm) response to nutrient enrichment when river nutrient concentrations were relatively low, but not at higher nutrient concentrations. To combat this variability in the photosynthetic state of algae, we developed a standardized algal bioassay (3-day), using a cultured species of algae which was starved of nitrogen, to compare the photosynthetic response to three nitrogen sources: treated wastewater, aquaculture farm discharges, and soil erosion-derived nutrient sources. This study showed that the nutrient parameter that had the highest correlation with algal photosynthetic response was total dissolved nitrogen (TDN), i.e., the sum of dissolved inorganic and organic nitrogen, rather than dissolved inorganic nitrogen alone. This was true across all three nutrient sources (R2 = 0.58–0.78). Additionally, the same concentrations of TDN from soil erosion-derived and aquaculture samples resulted in a significantly higher algal photosynthetic response, compared to the treated wastewater. This indicates that TDN from soils and aquaculture farms was significantly more bioavailable to the cultured algae than treated wastewater. When a range of parameters were correlated with algal responses, organic and inorganic nutrients, and organic carbon were the parameters that had the highest explanatory power for soil erosion-derived and aquaculture samples (R2 = 0.75–0.87). The importance of organic compounds in these equations points to the potential of microbial transformation of organic nutrients into more bioavailable forms during the 3-day bioassay. This highlights the need to understand the relationship between algal and microbial communities in natural systems for nutrient source impact assessment. This study provides an improved understanding and metrics for comparing the algal growth response to different nutrient sources.

Assessment of land use and land cover, water nutrient and metal concentration related to illegal mining activities in an Austral semi–arid river system: A remote sensing and multivariate analysis approach

The mining sector in various countries, particularly in the sub–Saharan African region, faces significant impact from the emergence of small–scale unlicensed artisanal mines. This trend is influenced by the rising demand and prices for minerals, along with prevalent poverty levels. Thus, the detrimental impacts of these artisanal mines on the natural environment (i.e., rivers) have remained poorly understood particularly in the Zimbabwean context. To understand the consequences of this situation, a study was conducted in the Umzingwane Catchment, located in southern Zimbabwe, focusing on the variations in water nutrient and metal concentrations in rivers affected by illegal mining activities along their riparian zones. Using multi–year Sentinel–2 composite data and the random forest machine learning algorithm on the Google Earth Engine cloud–computing platform, we mapped the spatial distribution of illegal mines in the affected regions and seven distinct land use classes, including artisanal mines, bare surfaces, settlements, official mines, croplands, and natural vegetation, with an acceptable overall and class accuracies of ±70 % were identified. Artisanal mines were found to be located along rivers and this was attributed to their large water requirements needed during the mining process. The water quality analysis revealed elevated nutrient concentrations, such as ammonium and nitrate (range 0.10–20.0 mg L−1), which could be attributed to mine drainage from the use of ammonium nitrate explosives during mining activities. Additionally, the prevalence of croplands in the area may have potentially contributed to increased nutrient concentrations. The principal component analysis and hierarchical cluster analysis revealed three clusters, with one of these clusters showing parameters like Ca, Mg, K, Hg and Na, which are usually associated with mineral gypsum found in the drainage of artisanal mines in the selected rivers. Cluster 2 consisted of B, Cu, Fe, Pb, and Mn, which are likely from the natural environment and finally, cluster 3 contained As, Cd, Cr, and Zn, which were likely associated with both legal and illegal mining operations. These findings provide essential insights into the health of the studied river system and the impacts of human activities in the region. They further serve as a foundation for developing and implementing regulatory measures aimed at protecting riverine systems, in line with sustainable development goal 15.1 which focuses on preserving and conserving terrestrial and inland freshwater ecosystems, including rivers. By acting on this information, authorities can work towards safeguarding these vital natural resources and promoting sustainable development in the area.

Dissolved silicon and nitrogen in glacial rivers and water of Blago bay (Russian Arctic, Novaya Zemlya): origin, variability and spreading

Hydrochemical studies of watercourses and the water area of Blagopoluchia bay (Novaya Zemlya, Arctic, Russia) have been carried out. The concentrations of nutrients in rivers and streams are higher than those in the water area of Blagopoluchia bay. It is shown that the concentration of silicon in constantly flowing rivers is 1–13 μM, the concentration of NO3 — 0.5–8, for small and temporary streams these values are higher and are in the range of 18–46 μM Si, 1–11 μM NO3– . The influence of streams and rivers flowing into Blagopoluchia Bay on the water area of the bay is local and extends to 1 km from the mouth, and does not influence the Kara Sea nutrient content.

Temporal patterns and causal drivers of aboveground plant biomass in a coastal wetland: Insights from time-series analyses

Salt marshes play a crucial role in coastal biogeochemical cycles and provide unique ecosystem services. Salt marsh biomass, which can strongly influence such services, varies over time in response to hydrologic conditions and other environmental drivers. We used gap-filled monthly observations of Spartina alterniflora aboveground biomass derived from Landsat 5 and Landsat 8 satellite imagery from 1984-2018 to analyze temporal patterns in biomass in comparison to air temperature, precipitation, river discharge, nutrient input, sea level, and drought index for a southeastern US salt marsh. Wavelet analysis and ensemble empirical mode decomposition identified month to multi-year periodicities in both plant biomass and environmental drivers. Wavelet coherence detected cross-correlations between annual biomass cycles and precipitation, temperature, river discharge, nutrient concentrations (NOx and PO43–) and sea level. At longer periods we detected coherence between biomass and all variables except precipitation. Through empirical dynamic modeling we showed that temperature, river discharge, drought, sea level, and river nutrient concentrations were causally connected to salt marsh biomass and exceeded the confounding effect of seasonality. This study demonstrated the insights into biomass dynamics and causal connections that can be gained through the analysis of long-term data.

Assessing the impact of watershed characteristics and management on nutrient concentrations in tropical rivers using a machine learning method

Excessive loadings of terrestrial nitrogen and phosphorus, as well as their imbalances with silicon, have been recognized as one of the major causes of water quality and ecosystem deterioration in receiving waters. In this study, a periodic water quality monitoring was conducted in the rivers and streams of a tropical island (Ishigaki Island, Japan) to identify the factors controlling the concentrations of dissolved inorganic nitrogen (DIN), total phosphorus (TP) and dissolved silicon (DSi) with a special focus on the catchment characteristics (e.g., land use, surface geology, topography). Random Forest (RF) machine learning algorithm was employed to develop predictive models for nutrient concentrations from the catchment properties. The developed models could predict nutrient concentrations with sufficient accuracy, demonstrating that the studied nutrients are strongly affected by catchment properties. Agricultural land uses (e.g., livestock barn, sugarcane field) were ranked as the most important parameters for DIN and TP, while broadleaf forest was the most influential factor for DSi. Using the RF models, the contributions of DIN originating from sugarcane fields (i.e., fertilizers) and barns (i.e., manure) to riverine DIN were estimated, which were up to 60% in total in the studied river basins. Furthermore, the yield of DIN from sugarcane fields, calculated as the concentration of DIN derived from sugarcane fields divided by the percent area of sugarcane fields, strongly positively correlated with the areal coverage of limestone, suggesting that fertilizer-derived DIN is more prone to leaching out from cropland soil to groundwater and rivers in catchments with a higher dominance of calcareous geology. These results, including the methodology employed, have implications for water quality assessment and management in inland and coastal waters not only at the study site but also other regions.

River metabolic fingerprints and regimes reveal ecosystem responses to enhanced wastewater treatment.

Although many studies have examined how improvements in wastewater treatment impact river nutrient concentrations and loads, there has been much less focus on measuring river metabolism to evaluate the wider aquatic ecosystem benefits of reducing nutrient inputs to rivers. The objectives of this study were to evaluate the effects of enhanced wastewater treatment (nitrification) on river metabolism in the Grand River, Canada's largest river draining into Lake Erie. Metabolic fingerprints and regimes (calculated from high-frequency dissolved oxygen [DO] measurements) were used to visualize whole-river ecosystem functional responses to these wastewater treatment upgrades. There was a 60% reduction in ecosystem respiration during summer, in response to reductions in effluent total ammonia inputs, causing a shift from net heterotrophy to net autotrophy, and contraction of river metabolic fingerprints. This resulted in major improvements in summer DO concentrations, with reductions in the percentage of days during summer that DO minima fell below water-quality guidelines for protection of aquatic early life stages, from 88% to ≤16%. The results also point to potential cascading impacts on coupled phosphorus and nitrogen cycles, which may generate further improvements in river water quality. During the summer, high rates of river metabolism and nutrient retention may result in measured water-column nutrient concentrations potentially underestimating nutrient pressures. This study also demonstrates the value of combining river metabolism with nutrient monitoring for a more holistic understanding of the role of nutrients in river ecosystem health and function.

Italian reference rivers under the Water Framework Directive umbrella: do natural factors actually depict the observed nutrient conditions?

BackgroundDespite the efforts made in the last century to counteract the nutrient enrichment from diffuse and point-sources, the excess of nitrogen and phosphorous is among the main causes of degradation of European rivers. In this context, determining natural background concentrations of nutrients in rivers is crucial for a correct definition of their ecological status. In the most anthropized regions, this is a difficult task. This study provides a nation-wide assessment of the nutrient concentration variability between Italian river reference sites.ResultsWe applied the Affinity Propagation technique to identify groups of river sites classified as reference based on measured nutrients and oxygen water saturation. The role of natural and anthropogenic factors determining differences in nutrients concentration between groups of sites was explored. Nitrate concentrations varied from 0.01 mg N l−1 to more than 5 mg N l−1. Ammonia and total phosphorous varied between 0.001 and 0.12 mg l−1. Observed nutrient levels, although in line with those identified for reference sites in other countries, largely exceed the ranges reported for natural basins. Atmospheric deposition of inorganic N and artificial and/or high-impact agricultural land use are the major factors determining differences in nutrient concentration. Factors like, e.g. catchment size, precipitation amount and altitude do not play a relevant role in explaining nutrient differences between groups of reference sites.ConclusionsWe especially focused on (i) major causes of failure in the selection of appropriate reference sites in Italy; (ii) the potential of setting higher NO3-N thresholds for the classification of ecological status in specific areas, and (iii) the prospective of a regionalization approach, in which human effects are accepted to a low degree for reference site selection or when setting thresholds for peculiar geographical areas.

Assessing the Potential Impact of River Chemistry on Arctic Coastal Production

The Arctic coastal margin receives a disproportionately large fraction of the global river discharge. The bio-geochemistry of the river water as it empties into the marine environment reflects inputs and processes that occur as the water travels from its headwaters. Climate-induced changes to Arctic vegetation and permafrost melt may impact river chemistry. Understanding the impact of river nutrients on coastal marine production, and how this may change in the future, are important for resource managers and community members who monitor and rely on coastal food resources. Using the Energy Exascale Earth System Model we explore the impact of timing and river nutrient concentrations on primary production in each coastal Arctic region and then assess how this influences secondary production and particle fluxes supporting the benthic food web. Our results indicate that while the concentration of Arctic river nitrogen can have a significant impact on annual average nitrogen and primary production in the coastal Arctic, with production increases of up to 20% in the river influenced interior Seas, the timing of the river nutrient inputs into the marine environment appears less important. Bloom timing and partitioning between small and large phytoplankton were minimally impacted by both river nutrient concentration and timing, suggesting that in general, coastal Arctic ecosystem dynamics will continue to be primarily driven by light availability, rather than nutrients. Under a doubling river nutrient scenario, the percentage increase in the POC flux to the benthos on river influenced Arctic coastal shelves was 2-4 times the percentage increase in primary production, suggesting changes to the river nutrient concentration has the potential to modify the Arctic food web structure and dynamics. Generally, the nutrient-induced changes to primary production were smaller than changes previously simulated in response to ice reduction and temperature increase. However, in the Laptev Sea, the production increase resulting from a doubling of river nutrients exceeded the production increase simulated with an atmospheric warming scenario. Dissolved organic carbon is presently poorly represented in the model so its impact on production is hard to simulate. Applying established relationships between modeled DOC, total DOC, and light absorption we illustrate that DOC could play a very important role in modulating production. Our findings highlight the importance of developing more realistic river nutrient and discharge forcing for Earth System Models such that their impact on the critical Arctic coastal domain can be more adequately resolved.

Ultraviolet‐visual spectroscopy estimation of nitrate concentrations in surface waters via machine learning

AbstractHigh‐frequency acquisition of nutrient concentrations in rivers is needed to generate nutrient loading estimates commensurate with flow and discharge data. Although the combination of field sampling and laboratory analysis is the standard approach to riverine water quality analysis, this strategy is expensive and can miss important storm‐related events. Ultraviolet‐visual (UV–Vis) spectroscopy is widely used in drinking water and wastewater systems for high‐frequency concentration estimates. However, surface waters present a unique challenge as co‐occurring constituents in environmental samples cause spectral interference at the wavelengths used to measure concentrations of dissolved nutrients. Partial least squares regression (PLSR), Lasso regression (Lasso), and stepwise multivariate linear regression (Stepwise) models can be effective predictors of nitrate concentrations using UV–Vis absorbance and are used in many available in‐situ nitrate sensors; however, the proliferation of user‐friendly open‐source machine learning (ML) algorithms offers an opportunity to use sophisticated big‐data techniques to predict nutrient concentrations in surface waters. We collected samples from four rivers across southern Ontario with a variety of nitrate concentrations, flow regimes, and interfering co‐contaminants. We demonstrated that ML applications of random forest and gradient boosting models significantly outperformed PLSR, Lasso, and Stepwise methodologies to estimate nitrate concentrations in complex environmental samples via UV–Vis absorbance. Importantly, ML applications outcompete current models at low concentrations. This new predictive methodology provides regulators and stakeholders an opportunity to establish low cost, continuous monitoring environmental programs using UV–Vis approaches.

Effects of Land Use on Nutrient Concentrations in the Inflow River of Lake Taihu, China

Land use is an important factor affecting non-point nutrient loading. Here, the Wuxi River basin was selected to analyze the influence of sub-basin land use on nutrient concentrations using remotely sensed land use data and monthly river water quality variables from October 2019 to September 2020. The results showed that the water quality of the river was closely related to land-use type. Specifically, dryland farmland, villages, and building land have a strong promoting influence on nitrogen, phosphorus, organic carbon, and phytoplankton chlorophyll a. The proportion of orchard land was also positively correlated with river nutrient concentrations. A negative correlation was observed between the proportion of forest land and nutrient concentrations. Moreover, the proportion of the water area in rivers and reservoirs was negatively correlated with the total dissolved nitrogen and nitrate concentrations in the river, and the proportion of the water area in natural pits and fishponds was negatively correlated with river nitrate and ammonia concentrations. Furthermore, the proportion of river and fishpond areas was positively correlated with the concentration of dissolved total phosphorus, dissolved organic carbon, and the permanganate index, while the proportion of the natural pond area was positively correlated with the concentration of particulate phosphorus and phytoplankton chlorophyll a. The influence of land-use types on water quality was also affected by distance from the river. This research indicates that the appropriate utilization of land and wetlands is key to controlling non-point nutrient loading in the river network, including Lake Taihu. Specifically, the self-purification capacity of wetland waters should be incorporated into nutrient control schemes, and special attention should be paid to the reduction of non-point source pollution in the drylands along the downstream riverbanks and urbanized areas.

Modelling the impact of river flow, macronutrients and solar radiation on the eutrophication status of small shallow estuaries

Small, semi-enclosed basins have often been the location of human settlements; however, this has subjected them to extensive anthropogenic use, negatively impacting the water quality and increasing their susceptibility to eutrophication. A coupled depth-averaged hydrodynamic-biogeochemical model has been configured of the shallow, microtidal Christchurch Harbour estuary, Dorset UK, to investigate processes driving declines in ecosystem health with particular emphasis on understanding the impact of changing river flows, river nutrient inputs and solar radiation. Instances of summer oxygen undersaturation and increased levels of chlorophyll were found to coincide with regions within the estuary yielding long residence times, even under low nutrient conditions. Inverse relationships between time undersaturated and both river flow and river nutrient concentration were observed but with no significant correlation between time undersaturated and solar irradiance which we attribute to the estuary's shallow nature. Our results showed that although river flow controls estuarine renewal, river nutrient concentration plays the greatest role in driving eutrophication development in small, shallow semi-enclosed basins.

Freshwater salinization syndrome: from emerging global problem to managing risks

Freshwater salinization is an emerging global problem impacting safe drinking water, ecosystem health and biodiversity, infrastructure corrosion, and food production. Freshwater salinization originates from diverse anthropogenic and geologic sources including road salts, human-accelerated weathering, sewage, urban construction, fertilizer, mine drainage, resource extraction, water softeners, saltwater intrusion, and evaporative concentration of ions due to hydrologic alterations and climate change. The complex interrelationships between salt ions and chemical, biological, and geologic parameters and consequences on the natural, social, and built environment are called Freshwater Salinization Syndrome (FSS). Here, we provide a comprehensive overview of salinization issues (past, present, and future), and we investigate drivers and solutions. We analyze the expanding global magnitude and scope of FSS including its discovery in humid regions, connections to human-accelerated weathering and mobilization of ‘chemical cocktails.’ We also present data illustrating: (1) increasing trends in salt ion concentrations in some of the world’s major freshwaters, including critical drinking water supplies; (2) decreasing trends in nutrient concentrations in rivers due to regulations but increasing trends in salinization, which have been due to lack of adequate management and regulations; (3) regional trends in atmospheric deposition of salt ions and storage of salt ions in soils and groundwater, and (4) applications of specific conductance as a proxy for tracking sources and concentrations of groups of elements in freshwaters. We prioritize FSS research needs related to better understanding: (1) effects of saltwater intrusion on ecosystem processes, (2) potential health risks from groundwater contamination of home wells, (3) potential risks to clean and safe drinking water sources, (4) economic and safety impacts of infrastructure corrosion, (5) alteration of biodiversity and ecosystem functions, and (6) application of high-frequency sensors in state-of-the art monitoring and management. We evaluate management solutions using a watershed approach spanning air, land, and water to explore variations in sources, fate and transport of different salt ions (e.g. monitoring of atmospheric deposition of ions, stormwater management, groundwater remediation, and managing road runoff). We also identify tradeoffs in management approaches such as unanticipated retention and release of chemical cocktails from urban stormwater management best management practices (BMPs) and unintended consequences of alternative deicers on water quality. Overall, we show that FSS has direct and indirect effects on mobilization of diverse chemical cocktails of ions, metals, nutrients, organics, and radionuclides in freshwaters with mounting impacts. Our comprehensive review suggests what could happen if FSS were not managed into the future and evaluates strategies for reducing increasing risks to clean and safe drinking water, human health, costly infrastructure, biodiversity, and critical ecosystem services.

Estimating river nutrient concentrations consistent with good ecological condition: More stringent nutrient thresholds needed

Nutrient pollution remains one of the leading causes of river degradation, making it important to set thresholds that support good ecological condition, which is the main objective of managing Europe's aquatic environment. A wide range of methods has been used by European member states to set river nutrient thresholds in the past, and these vary greatly among countries, even for similar river types. In some countries, thresholds have been set using expert judgement or the statistical distribution of nutrient concentrations. Application of such thresholds creates problems for planning strategies to achieve good ecological status and for managing transboundary river basins. An alternative approach is to examine the statistical relationship between nutrient concentration and one, or more, biological variables. Such relationships can then be used to inform decisions by water managers. We use such 'ecology-based' approaches (univariate regression and mismatch analyses) to derive nutrient thresholds for several river types in Central Europe. Our analysis focused on soluble reactive phosphorus (SRP) and total nitrogen (TN), two variables which were responsible for significant variation (40–55%) in river benthic floras. In this study, for the first time, river nutrient thresholds are estimated using both macrophytes and phytobenthos (EQRs) separately and in combination, calculated as the minimum and the average of the EQRs of the two sub-elements. The resulting thresholds supporting good ecological status range from 21 to 42 µg/L SRP and 0.9–3.5 mg/L TN for the low alkalinity lowland river type, and 32–90 µg/L SRP and 1.0–2.5 mg/L TN for the low alkalinity mid-altitude river type. These targets are compared to the values set by member states. We demonstrate that some national nutrient thresholds fall within the range of predicted values if uncertainty is taken into consideration; however, several threshold values considerably exceed this range. Adopting ecology-based nutrient targets should improve sustainable river management where nutrients are the major pressure preventing the achievement of good ecological status.

Nutrient loss from three small-size watersheds in the southern Baltic Sea in relation to agricultural practices and policy

Agriculture is the major contributor of waterborne nutrient fluxes into the Baltic Sea, one of the world's most eutrophication-sensitive areas. Poland, as a large, densely populated state ohf the Baltic Region, with dominating agricultural land use, largely contributes to riverborne loads of N and P. The aim of our study was to examine the input of nutrients from three small first-order agricultural watersheds (Bladzikowski Stream, Gizdepka river and Mrzezino canal) in the Pomerania region, into the Bay of Puck, inner part of the Gulf of Gdansk. This study attempts to give a partial answer as to the question if inputs of nutrients from the 3 analysed watersheds comply with the targets of the Baltic Sea Action Plan (BSAP) and Country Allocated Reduction Targets (CART). The impact of agricultural practices was assessed on the basis of farm questionnaires and calculations of nutrient balances for the examined farms. The nutrient concentrations in the soil and drainage ditches were examined, followed by an assessment of nutrient concentrations in the watercourses at the sampling points located close to the estuaries. The average mineral N fertiliser consumption (109 kg N/ha) in the analysed watersheds was higher than Poland's average. The average N and P surpluses for surveyed farms (96.4 kg/ha and 4.4 kg/ha, respectively) were higher than the EU mean in case of N and markedly lower in case of P. We used Principal Component Analysis which confirmed that there were correlations between nutrient surpluses and nutrient concentrations in streams and/or drainage ditches. The N–NO3 and Pmin concentrations were also correlated to precipitation. The average N concentrations in the analysed watercourses were equal to 1.53 mg/L for Gizdepka, 1.88 mg/L for Mrzezino canal and 3.52 mg/L for Bładzikowski Stream. The mean P concentrations observed in the investigated watercourses were markedly higher than 0.1 mg/L. With regard to BSAP objectives, as well as CART set for Poland, the average nutrient concentrations in rivers should be approximately at the level of 2.5 mg N/L and 0.07 mg P/L.

Predicting sediment and nutrient concentrations from high-frequency water-quality data.

Water-quality monitoring in rivers often focuses on the concentrations of sediments and nutrients, constituents that can smother biota and cause eutrophication. However, the physical and economic constraints of manual sampling prohibit data collection at the frequency required to adequately capture the variation in concentrations through time. Here, we developed models to predict total suspended solids (TSS) and oxidized nitrogen (NOx) concentrations based on high-frequency time series of turbidity, conductivity and river level data from in situ sensors in rivers flowing into the Great Barrier Reef lagoon. We fit generalized-linear mixed-effects models with continuous first-order autoregressive correlation structures to water-quality data collected by manual sampling at two freshwater sites and one estuarine site and used the fitted models to predict TSS and NOx from the in situ sensor data. These models described the temporal autocorrelation in the data and handled observations collected at irregular frequencies, characteristics typical of water-quality monitoring data. Turbidity proved a useful and generalizable surrogate of TSS, with high predictive ability in the estuarine and fresh water sites. Turbidity, conductivity and river level served as combined surrogates of NOx. However, the relationship between NOx and the covariates was more complex than that between TSS and turbidity, and consequently the ability to predict NOx was lower and less generalizable across sites than for TSS. Furthermore, prediction intervals tended to increase during events, for both TSS and NOx models, highlighting the need to include measures of uncertainty routinely in water-quality reporting. Our study also highlights that surrogate-based models used to predict sediments and nutrients need to better incorporate temporal components if variance estimates are to be unbiased and model inference meaningful. The transferability of models across sites, and potentially regions, will become increasingly important as organizations move to automated sensing for water-quality monitoring throughout catchments.

Impacts of Hydrological Changes on Nutrient Transport From Diffuse Sources in a Rural River Basin, Western Japan

AbstractIncreased frequency of heavy rainfall caused by climate change may affect nutrient transport from forests and agricultural lands into downstream rivers. However, little information is available on how the effects of land cover on nutrient concentrations in the downstream rivers change depending on hydrological conditions. To elucidate this, we investigated the effects of changes in precipitation and river discharge on the relationship between river nutrient concentrations (total nitrogen, total phosphorus, nitrate, and phosphate) and land cover in eight subbasins of the Hii River basin, western Japan. We hypothesized that coniferous forests and agricultural lands including poorly managed ones, which require thinning and fertilizer management, respectively, contribute to increases in nutrient concentrations in downstream rivers during storms. Linear mixed‐effects model analyses based on 18‐year observation data were used for testing the hypothesis. Results revealed that the slopes of the multiple regression analysis between nutrient concentrations and coniferous and broad‐leaved forest area ratios were negative and decreased with increasing daily precipitation. This suggests that forests contributed to the dilution of river nutrient concentrations during storms regardless of whether they were coniferous or broad leaved. In contrast, the slope of regression between nutrient concentrations and agricultural area ratio revealed the opposite trend, indicating that even a small percentage of agricultural area makes a large contribution to increases in nutrient concentrations as precipitation increases. Our results imply that storm runoff in response to rainfall results in an elevated supply of nitrogen and phosphorus attached to arable soils to the river.

Modeling the response of river nutrient conditions to land use changes in lowland artificial watersheds (polders)

Excess nutrient export from lowland artificial watersheds (polders) led to severe eutrophication and harmful algal blooms of their surrounding rivers and lakes. Taking River Zhong in Lake Taihu basin as the study area, this study attempted to quantify the impacts of polders’ land use changes on river nutrient conditions. A Nitrogen (N) Dynamic Polder model and a Phosphorus (P) Dynamic Polder model was used to estimate N and P export from lowland polders in the context of land use change. The investigation results showed that N and P contribution to River Zhong increased by 7–30% in case that 30% of agricultural lands (paddy lands or dry lands) were changed into residential areas. In case that 30% agricultural lands were changed into water areas, N and P contribution can be decreased by 6% and 35%, respectively. Fertilization period especially during rainy season (from June to August) were the hot moments of nutrient loss, accounted for above 50%. Infiltration was the main pathway of N and P export from polders, accounting for more than 60%. Therefore, from management perspective, we argued controlling urban areas, maintaining water areas and controlling fertilization for polders could be potential strategies for reducing river nutrient concentration in River Zhong.