Riverine Total Phosphorus Research Articles

Toward Large‐Scale Riverine Phosphorus Estimation Using Remote Sensing and Machine Learning

AbstractPhosphorus pollution is a major water quality issue impacting the environment and human health. Traditional methods limit the frequency and extent of total phosphorus (TP) measurements across many rivers. However, remote sensing can accurately estimate riverine TP; nevertheless, no large‐scale assessment of riverine TP using remote sensing exists. Large‐scale models using remote sensing can provide a fast and consistent method for TP measurement, important for data generalization and accessing extensive spatial‐temporal change in TP. Our study uses remote sensing and machine learning to estimate the TP in rivers in the contiguous United States (CONUS). Initially, we developed a national scale matchup data set for Landsat detectable rivers (river width >30 m) using in situ TP and surface reflectance. We used in situ data from the Water Quality Portal (WQP), alongside water surface reflectance data from Landsat 5, 7, and 8 spanning from 1984 to 2021. Then, we used this data set to develop a machine learning (ML) model using different preprocessing methods and algorithms. We found that using high‐level vegetation in the clustering approach and over‐sampling or under‐sampling our training data in the sampling approach improved our model estimation accuracy. We compared XGBLinear, XGBTree, Regularized Random Forest (RRF), and K‐Nearest neighbors ML algorithms and selected XGBLinear as the best model with an R2 of 0.604, RMSE of 0.103 mg/L, mean average error of 0.83, and NSE of 0.602. Finally, we identified human footprint, elevation, river area, and soil erosion as the main attributes influencing the accuracy of estimated TP from the ML model.

The spatial and seasonal variability of nutrient status in the seaward rivers of China shaped by the human activities

Understanding the riverine nutrient status is of paramount importance to the management of coastal ecosystems. This study investigated the spatial and seasonal variability of nutrient status in the seaward rivers of China at a national scale, using the latest observational data (2020–2023). A total of 126 rivers were considered, covering nearly all rivers in China that enter adjacent seas. The average flux of total nitrogen (TN) to the sea amounts to about 8968 tons d-1, while the total phosphorus (TP) and ammonium (AN) flux are about 329 tons d-1 and 276 tons d-1, respectively. In general, the river nutrient fluxes are higher in summer, with the Southern China (SC) showing higher levels than the Northern China (NC). This pattern is largely regulated by the spatio-temporal characteristics of runoff. By contrast, the spatio-temporal pattern of riverine nutrient concentration differs from it slightly. Riverine TN/TP concentration typically shows higher levels in NC, peaking in winter/summer, while the spatio-temporal variations of AN are minimal. The coal-fired heating prevalent in NC during winter causes severe atmospheric nitrogen pollutions, contributing to the heightened TN concentration in local rivers. At the meantime, the vast cropland in NC creates a substantial demand for the phosphorus fertilizer, responsible for the elevated TP concentration in northern rivers. The seasonal variation of TP concentration is mainly driven by rainfall, which exacerbates the loss of phosphorus from croplands into nearby rivers, leading to a simultaneous increase in riverine TP and turbidity levels during summer. This process is particularly pronounced in NC, due to elevated phosphorus levels in croplands and more concentrated summer rainfall. The present study suggests the crucial role of human activities in shaping the riverine nutrient status in China, which will be of practical significance for the future environmental management.

Long-term variations in external phosphorus inputs and riverine phosphorus export in a typical arid and semiarid irrigation watershed

Excessive phosphorus (P) along with drained water from farmland in the arid and semiarid watersheds when entering into water bodies brings about serious environmental problems in the aquatic ecosystem. It is critical to explore variations in watershed P balance and the relationship between anthropogenic P input and riverine total phosphorus (TP) export in a typical irrigation watersheds. In this study, long-term anthropogenic P variations in Ulansuhai Nur watershed (UNW), a typical irrigation watershed in Yellow River basin, was investigated using a quantitative Net Anthropogenic Phosphorus Input (NAPI) budget model. The results showed that annual NAPI exhibited a significant upward trend with a multi-year average of 2541.6 kg P km−2 yr−1 in the UNW. Hotspots for watershed NAPI were discovered in Linhe and Hangjin Houqi counties. Chemical P fertilizers and livestock breeding were two dominated sources of NAPI. Annual riverine TP export showed a significantly declined trend with a net decrease of 80.6%. The export ratio of watershed NAPI was 0.6%, lower than those reported for other watersheds worldwide. There was a significant positive linear correlation between NAPI and riverine TP export from 2005 to 2009. However, after 2009, riverine TP export exhibited a decreased trend with increasing watershed NAPI, which was attributed to environmental treatment measures. By reconstructing riverine TP export without the impact of pollution treatment measures, annual average reduction amount of riverine TP export from 2009 to 2019 was estimated to be 237.2 ton, 47.2% and 52.8% of which were attributed to the point and nonpoint sources measures. This study not only widens the application scope of NAPI budget method, but also provides useful information of nutrient management and control in the arid and semiarid irrigation watershed.

Machine learning-based estimation of riverine nutrient concentrations and associated uncertainties caused by sampling frequencies

Accurate and sufficient water quality data is essential for watershed management and sustainability. Machine learning models have shown great potentials for estimating water quality with the development of online sensors. However, accurate estimation is challenging because of uncertainties related to models used and data input. In this study, random forest (RF), support vector machine (SVM), and back-propagation neural network (BPNN) models are developed with three sampling frequency datasets (i.e., 4-hourly, daily, and weekly) and five conventional indicators (i.e., water temperature (WT), hydrogen ion concentration (pH), electrical conductivity (EC), dissolved oxygen (DO), and turbidity (TUR)) as surrogates to individually estimate riverine total phosphorus (TP), total nitrogen (TN), and ammonia nitrogen (NH4+-N) in a small-scale coastal watershed. The results show that the RF model outperforms the SVM and BPNN machine learning models in terms of estimative performance, which explains much of the variation in TP (79 ± 1.3%), TN (84 ± 0.9%), and NH4+-N (75 ± 1.3%), when using the 4-hourly sampling frequency dataset. The higher sampling frequency would help the RF obtain a significantly better performance for the three nutrient estimation measures (4-hourly > daily > weekly) for R2 and NSE values. WT, EC, and TUR were the three key input indicators for nutrient estimations in RF. Our study highlights the importance of high-frequency data as input to machine learning model development. The RF model is shown to be viable for riverine nutrient estimation in small-scale watersheds of important local water security.

Modeling the sensitivity of cyanobacteria blooms to plausible changes in precipitation and air temperature variability

Many recent studies have attributed the observed variability of cyanobacteria blooms to meteorological drivers and have projected blooms with worsening societal and ecological impacts under future climate scenarios. Nonetheless, few studies have jointly examined their sensitivity to projected changes in both precipitation and temperature variability. Using an Integrated Assessment Model (IAM) of Lake Champlain's eutrophic Missisquoi Bay, we demonstrate a factorial design approach for evaluating the sensitivity of concentrations of chlorophyll a (chl-a), a cyanobacteria surrogate, to global climate model-informed changes in the central tendency and variability of daily precipitation and air temperature.An Analysis of Variance (ANOVA) and multivariate contour plots highlight synergistic effects of these climatic changes on exceedances of the World Health Organization's moderate 50 μg/L concentration threshold for recreational contact. Although increased precipitation produces greater riverine total phosphorus loads, warmer and drier scenarios produce the most severe blooms due to the greater mobilization and cyanobacteria uptake of legacy phosphorus under these conditions. Increases in daily precipitation variability aggravate blooms most under warmer and wetter scenarios. Greater temperature variability raises exceedances under current air temperatures but reduces them under more severe warming when water temperatures exceed optimal values for cyanobacteria growth more often. Our experiments, controlled for wind-induced changes to lake water quality, signal the importance of larger summer runoff events for curtailing bloom growth through reductions of water temperature, sunlight penetration and stratification. Finally, the importance of sequences of wet and dry periods in generating cyanobacteria blooms motivates future research on bloom responses to changes in interannual climate persistence.

Landscape patterns of catchment and land-use regulate legacy phosphorus releases in subtropical mixed agricultural and woodland catchments

Landscape composition and configuration determine the exchange of matter and energy among different landscape patches and may affect riverine phosphorus (P) exports derived from watershed legacy sources. However, a lack of understanding of landscape pattern effects on legacy P releases has yielded large uncertainties in mitigating watershed water quality using management practices or landscape planning. This study revealed the significance of legacy effect in the headwater catchments through the time-lag response of the long-term trend of river P exports to the change of net anthropogenic P input (NAPI). By constructing empirical statistical models that incorporated NAPI, hydroclimatic, terrain factors, soil chemical properties, and land use variables, the sources of annual riverine total phosphorus (TP) and dissolved inorganic phosphorus (DIP) exports were divided into current annual NAPI input and legacy sources inputs. The model estimations indicated that the contribution of legacy sources to riverine TP exports was 0.33–1.12 kg ha−1 yr−1 (50.7–82.8%), which was significantly higher than the contribution to DIP exports (0.18–0.49 kg ha−1 yr−1, 42.4–81.4%) in 2012–2017. Redundancy analysis (RDA) and variance partitioning analysis (VPA) methods were used to quantify the relative contribution of landscape patterns, soil P content, and terrain factors to legacy P releases. Results revealed that the relative contribution of the landscape composition and configuration to the total variations of legacy P releases was greater than that of the soil P and terrain factors. For different land use patches, a large area of woodland with a high aggregation degree and a large area of ponds with multiple net structures may significantly alleviate legacy P releases. In contrast, the legacy P releases were significantly positively associated with highly aggregated agricultural, tea plantation, and residential patches. This study provides theoretical support for strategies aiming to control legacy P from the perspective of landscape planning.

Modeling flow-related phosphorus inputs to tropical semiarid reservoirs

Hydrological data and total phosphorus (TP) concentration at reservoirs’ outlet were combined in a transient complete-mix model to obtain mean input loads and inlet concentration-flow relationships. This approach was designed to investigate the issue of phosphorus pollution in semiarid regions with intermittent rivers. The methodology was applied for twenty reservoirs in the State of Ceará, Brazilian semiarid. The modeled TP loads correlated well (R2 = 0.74) with reference loads estimated from environmental inventories, with only 10% of underestimated results. The average input loads per unit area of the catchments ranged from about 4 to 40 kg km−2 yr−1, which were considerably lower than the national average of about 500 kg km−2 yr−1. This was attributed to lower precipitation indexes, intermittent river regime and a high-density reservoir network, peculiar of the Brazilian semiarid. Meanwhile, the input load per unit area of a small and highly populated urban catchment, with higher precipitation indexes and deficient sanitation was substantially higher (2626 kg km−2 yr−1). Moreover, the fitted TP concentration-flow relationships directly reflected different TP input sources: strong u-shaped behavior marked the curves of highly non-point source dominated catchments, whereas a dilution pattern prevailed in those with significant point source inputs. The model validation with measured riverine TP concentration reached a NSE of 0.63. However, peak values in TP concentration during low flow rates sensitively affected the fitting of the models. In spite of non-point source dominance in the catchments, some relationships presented a slight signal of this use type. The variation range of the fitting parameters in comparison with other studies, as well the expected behavior of the curves in light of land use characteristics, strongly support the methodology applied in this study. The proposed approach will potentially help address the TP issue in tropical semiarid regions. Furthermore, the paper presents a simple way to deal with the challenging lack of monitored data in such environments.

Variation of net anthropogenic phosphorus inputs (NAPI) and riverine phosphorus fluxes in seven major river basins in China

The riverine phosphorus (P) import resulting from human activities is always a worldwide concern for environmental management due to the effect of eutrophication. In this study, we made modification of the NAPI method to make the results closer to the actual situation. We collected the data of seven major outflow rivers in China to have a comprehensive understanding of P export and P inputs and build the quantitative relationship between them. We estimated the net anthropogenic phosphorus inputs (NAPI), including fertilizer P (Pfert), net food and feed import P and non-food P using by human (Pim+nf), in seven major river basins in China and the corresponding riverine total phosphorus (TP) fluxes. The relationship between NAPI and riverine TP flux was also explored. NAPI in seven river basins presented an obvious uneven distribution. Huaihe River basin showed the highest NAPI of 4005.09 kg P km−2 yr−1 due to its highest intensities of human-activities, and the lowest NAPI was observed in Songhua River basin as 334.36 kg P km−2 yr−1. Pim+nf occupied a larger proportion of NAPI in the Pearl River and Liaohe River basins (> 65%), while Pfert contributed more to NAPI in the other basins (nearly 60%) with an exception of the Yangtze River basin (where Pim+nf and Pfert approximately contributed the same). Different contributions of NAPI components were mainly attributed to the different land uses. The total TP flux of all the seven rivers was 117.10 × 103 t P yr−1, with the highest flux in the Yangtze River (77.42 × 103 t P yr−1), contributed 72.88% to the total TP fluxes in China. Change in riverine TP flux could be well described by NAPI, river discharge, and percentage of lake area in the basin and this provided an effective way to predict TP fluxes in rivers.

Long-term (1980–2015) changes in net anthropogenic phosphorus inputs and riverine phosphorus export in the Yangtze River basin

Quantitative information on long-term net anthropogenic phosphorus inputs (NAPI) and its relationship with riverine phosphorus (P) export are critical for developing sustainable and efficient watershed P management strategies. This is the first study to address long-term (1980–2015) NAPI and riverine P flux dynamics for the Yangtze River basin (YRB), the largest watershed in China. Over the 36-year study period, estimated NAPI to the YRB progressively increased by ∼1.4 times, with NAPIA (chemical fertilizer input + atmospheric deposition + seed input) and NAPIB (net food/feed imports + non-food input) contributing 65% and 35%, respectively. Higher population, livestock density and agricultural land area were the main drivers of increasing NAPI. Riverine total phosphorus (TP), particulate phosphorus (PP) and suspended sediment (SS) export at Datong hydrological station (downstream station) decreased by 52%, 75% and 75% during 1980–2015, respectively. In contrast, dissolved phosphorus (DP) showed an increase in both concentration (∼7-fold) and its contribution to TP flux (∼16-fold). Different trends in riverine P forms were mainly due to increasing dam/reservoir construction and changes in vegetation/land use and NAPI components. Multiple regression models incorporating NAPIA, NAPIB, dam/reservoir storage capacity and water discharge explained 84% and 92% of the temporal variability in riverine DP and PP fluxes, respectively. Riverine TP flux estimated as the sum of DP and PP fluxes showed high agreement with measured values (R2 = 0.87, NSE = 0.84), indicating strong efficacy for the developed models. The model forecasted an increase of 50% and 7% and a decrease of 15% and 22% in riverine DP flux from 2015 to 2045 under developing, dam building, NAPIA and NAPIB reduction scenarios, respectively. This study highlights the importance of including enhanced P transformation from particulate to bioavailable forms due to river regulation and changes in land-use, input sources and legacy P pools in development of P pollution control strategies.

An ensemble modeling framework to study the effects of climate change on the trophic state of shallow reservoirs

Our understanding of the potential impact of climatic change on catchment hydrology and aquatic system dynamics has been advanced over the past decade, but there are still considerable knowledge gaps with respect to its effects on water quality vis-à-vis the increasing demands for drinking water. In this study, we developed an integrated hydrological-water quality (SWAT-YRWQM) model to elucidate the effects of a changing climate on the trophic state of the shallow Yuqiao Reservoir. Using a two-step downscaling process, we reproduced the prevailing meteorological conditions, as well as the streamflows in three major tributaries of the study area. A sensitivity analysis exercise showed that the nature of the calibration dataset used, namely the range of flows (i.e., dry versus wet years) included, can profoundly influence the predictive power of our modeling framework. Our climatic scenarios projected a minor change of the streamflow rates, but a variant degree of increase of the riverine total phosphorus (TP) concentrations and associated loading rates into the reservoir. Consequently, a significant rise of in-lake TP concentrations is projected for the near (2016–2030) and distant (2031–2050) future compared to the reference (2006–2015) conditions. Interestingly, the ambient TP levels appear to be lower in the distant relative to the near future, owing to changes in the magnitude and relative contribution of both external and internal nutrient loading sources. Our analysis also highlights the importance of reservoir operation practices to regulate water levels as a means for mitigating the climate change impact on the trophic status of the Yuqiao Reservoir, given that the diversion of low-nutrient water from the upstream basin can significantly reduce (30–40%) the TP concentrations. Our findings are highly relevant to the on-going debate about the potential implications of climate change for water availability, highlighting the importance of adaptation strategies to optimize the water resources management.

Wadden Sea Eutrophication: Long-Term Trends and Regional Differences

The Wadden Sea is a shallow intertidal coastal sea, largely protected by barrier islands and fringing the North Sea coasts of The Netherlands, Germany and Denmark. It is subject to influences from both the North Sea and major European rivers. Nutrient enrichment from these rivers since the 1950s has impacted the Wadden Sea ecology including loss of seagrass, increased phytoplankton blooms and increased green macroalgae blooms. Rivers are the major source of nutrients causing Wadden Sea eutrophication. The nutrient input of the major rivers impacting the Wadden Sea reached a maximum during the 1980s and decreased at an average pace of about 2.5 % per year for total Nitrogen (TN) and about 5 % per year for total Phosphorus (TP), leading to decreasing nutrient levels but also increasing N/P ratios. During the past decade, the lowest nutrient inputs since 1977 were observed but these declining trends are levelling out for TP. Phytoplankton biomass (measured as chlorophyll a) in the Wadden Sea has decreased since the 1980s and presently reached a comparatively low level. In tidal inlet stations with a long-term monitoring, summer phytoplankton levels correlate with riverine TN and TP loads but stations located closer to the coast behave in a more complex manner. Regional differences are observed, with highest chlorophyll a levels in the southern Wadden Sea and lowest levels in the northern Wadden Sea. Model data support the hypothesis that the higher eutrophication levels in the southern Wadden Sea are linked to a more intense coastward accumulation of organic matter produced in the North Sea.

Nexus of water, energy and ecosystems in the upper Mekong River: A system analysis of phosphorus transport through cascade reservoirs

The nexus of water, energy and ecosystems in the context of large-scale hydropower development is of worldwide concern; however, few quantitative studies have been conducted based on dynamic modeling. This study develops a system modeling approach to address the water-energy-ecosystem (WEE) nexus associated with the operation of cascade reservoirs. The approach couples multiobjective reservoir operation optimization with dynamic mass balance calculation of total phosphorus (TP). The approach was applied in the upper Mekong River Basin (i.e., the Lancang River Basin). The impacts of the cascade reservoirs on riverine TP transport were systematically investigated through a series of numerical experiments. The major study findings include the following. First, TP export from the Lancang River is closely related to characteristics of the cascade reservoirs, including the storage capacity and residence time of the reservoirs, as well as their locations. The two largest reservoirs demonstrate a much stronger retention effect than the other reservoirs. Second, although reservoir operations have significant impacts on the WEE nexus, the potential to enhance TP export is limited under traditional reservoir operations using total hydropower production and firm power as the objectives. Third, when TP export is incorporated as an environmental objective in the operation of the cascade reservoirs, strong tradeoffs among TP export and the two traditional objectives are revealed, and no synergetic effects are identified. Thus, a balance among the conflicting objectives must be carefully determined with compromise within the socioeconomic and political backgrounds of the complex nexus. Overall, this study establishes a general framework for quantifying the WEE nexus in the context of large-scale hydropower development and the results provide insights into the transboundary management of the Mekong River Basin.

An evaluation of high frequency turbidity as a proxy for riverine total phosphorus concentrations

Surface water eutrophication resulting from excessive phosphorus (P) inputs is one of today's most challenging environmental issues. Riverine total phosphorus (TP) concentrations have high temporal variability, which complicates flux estimation. We evaluated the usefulness of high frequency in-situ turbidity measurements as a proxy for TP in Sävjaån, a river draining a mixed land use catchment (722 km2) in central Sweden. Turbidity was monitored every 10th–15th minute during 6 consecutive years (2012–2017). Linear regression showed a good relationship between high frequency turbidity and TP (r2 = 0.64) and could hence be used for comparison of flux estimation methods. Predictive power of the turbidity-TP relationship was not improved by adding seasons, hydrograph rising/falling limb or high/low stream discharge to the model which argues for a single transfer function relating turbidity and TP. Both TP and turbidity were log-normally distributed. However, flux estimates were sensitive to data transformation; predicted TP concentrations and fluxes based on log-transformed data were biased towards lower concentrations and fluxes compared to non-transformed data. In five of six years grab sample and high frequency estimated TP fluxes were similar (grab sample estimates −10% to +13% P transport compared to high frequency flux estimates). The exception was in 2013, when a 50-year spring flood occurred, and the grab sample estimated flux was 56% larger than that estimated from high frequency data. Thus, the flux comparisons were mostly affected by stream discharge, which underlines the importance of capturing high discharge episodes with, e.g. in situ sensors. While uncertainties regarding the use of turbidity as a proxy for TP remain, it is clear that credible water chemistry data can be obtained with current high frequency sensors. We conclude that high frequency data can be used to better understand catchment response to external pressures and gain insights into water quality that will be missed with grab sampling.

Evaluating the relationships between watershed physiography, land use patterns, and phosphorus loading in the bay of Quinte basin, Ontario, Canada

Abstract The Bay of Quinte, a Z-shaped embayment at the northeastern end of Lake Ontario, represents a characteristic case of an area of concern (AOC) where scientific uncertainty underlies management efforts to address eutrophication problems. The present study attempts to examine the relationships between total phosphorus (TP) (i.e., concentrations from point and non-point sources, net export, and yield) and watershed attributes, including land use cover and physiographic characteristics. Dynamic linear modeling is applied to assess temporal trends in the flow rates in five major tributaries of the area. Urbanized catchments near the Bay of Quinte show more dynamic flow patterns with significant within- and among-year variability and stronger response to local precipitation patterns. The “flashy” behavior of urban land is characterized by relatively high net TP export and TP yield. Self-organizing map analysis, classified the watershed into six distinct spatial clusters, representing two agricultural, one forested, one urban, one pasture-dominated, and one transitional area. Our study shows that agricultural and urban sites profoundly shape riverine TP dynamics. We also provide evidence that tributaries draining agricultural areas exhibit considerable variability, depending on management practices and soil properties. Excessive fertilizer or manure application in croplands, management practices (e.g., crop types and tillage methods) and soil hydrological characteristics (e.g., hydraulic conductivity and permeability) generally determine soil phosphorus level and consequent losses through erosion and runoff from agricultural sites. In a similar manner, pasture-based grazing systems appear to play an important role in modulating the fate and transport of phosphorus in the Bay of Quinte watershed.

Antecedent conditions, hydrological connectivity and anthropogenic inputs: Factors affecting nitrate and phosphorus transfers to agricultural headwater streams

This paper examines relationships between rainfall–runoff, catchment connectivity, antecedent moisture conditions and fertiliser application with nitrate-N and total phosphorus (TP) fluxes in an arable headwater catchment over three hydrological years (2012–2014). Annual precipitation totals did not vary substantially between years, yet the timing of rainfall strongly influenced runoff generation and subsequent nitrate-N and TP fluxes. The greatest nitrate-N (>250kgNday−1) and TP (>10kgTPday−1) fluxes only occurred when shallow groundwater was within 0.6m of the ground surface and runoff coefficients were greater than 0.1. These thresholds were reached less frequently in 2012 due to drought recovery resulting in lower annual nitrate-N (7.4kgNha−1) and TP (0.12kgPha−1) fluxes in comparison with 2013 (15.1kgNha−1; 0.21kgPha−1). The wet winter of 2013 with elevated shallow groundwater levels led to more frequent activation of sub-surface pathways and tile drain flow. Throughout the period, dry antecedent conditions had a temporary effect in elevating TP loads. Evidence of TP source exhaustion after consecutive storm events can be attributed to the repeated depletion of temporarily connected critical source areas to the river network via impermeable road surfaces. Fertiliser application varied considerably across three years due to differences in crop rotation between farms, with annual N and P fertiliser inputs varying by up to 21% and 41%, respectively. Proportional reductions in annual riverine nitrate-N and TP loadings were not observed at the sub-catchment outlet as loadings were largely influenced by annual runoff. Nitrate loadings were slightly higher during fertiliser application, but there was little relationship between P fertiliser application and riverine TP load. These data indicate that this intensive arable catchment may be in a state of biogeochemical stationarity, whereby legacy stores of nutrients buffer against changes in contemporary nutrient inputs.

Integration of best management practices in the Bay of Quinte watershed with the phosphorus dynamics in the receiving waterbody: What do the models predict?

We present a modelling analysis of the management practices that could lead to significant reduction of phosphorus export from the Bay of Quinte watershed and an evaluation of the overall uncertainty associated with the assessment of the Beneficial Use Impairment Eutrophication and Undesirable Algae. Our work highlights the internal recycling as one of the key drivers of phosphorus dynamics in the Bay. The flow from the Trent River is the predominant driver of the upper Bay dynamics until the main stem of the middle area however, the sediments in the same segment release a significant amount of phosphorus and the corresponding fluxes are likely amplified by the macrophyte and dreissenid activity. From a management standpoint, the presence of a significant positive feedback loop in the upper Bay suggests that the anticipated benefits of additional reductions of the exogenous point and non-point loading may not be realized within a reasonable time frame (i.e. 5—10 years). Our analysis of nutrient loading scenarios shows that the restoration pace of the Bay could be slow, even if the riverine total phosphorus concentrations reach levels significantly lower than their contemporary values, <25 µg TP l−1. We believe that the on-going management decisions, monitoring, and modelling should also explicitly consider the role and broader ramifications of internal phosphorus loading into the system. The anticipated lessons from such a multi-faceted exercise are a unique aspect of the Bay of Quinte ecosystem because of the long history of research and monitoring data. This study can produce transferable knowledge to other systems worldwide, experiencing similar hysteresis patterns associated with internal nutrient loading.

Factors controlling phosphorus export from agricultural/forest and residential systems to rivers in eastern China, 1980–2011

Summary This study quantified long-term response of riverine total phosphorus (TP) export to changes in land-use, climate, and net anthropogenic phosphorus inputs to agricultural/forest (NAPIAF) and residential (NAPIR) systems for the upper Jiaojiang watershed in eastern China. Annual NAPIAF rose by 73% in 1980–1999 followed by a 41% decline in 2000–2011, while NAPIR continuously increased by 122% over the 1980–2011 period. Land-use showed a 63% increase in developed land area (D%) and a 91% increase in use of efficient drainage systems on agricultural land area (AD%) over the study period. Although no significant trends were observed in annual river discharge or precipitation, the annual number of storm events rose by 90% along with a 34% increase in the coefficient of variation of daily rainfall. In response to changes of NAPIAF, NAPIR, land-use and precipitation patterns, riverine TP flux increased 16.0-fold over the 32-year record. Phosphorus export via erosion and leaching was the dominant pathway for P delivery to rivers. An empirical model incorporating annual NAPIAF, NAPIR, precipitation, D%, and AD% was developed (R2 = 0.96) for apportioning riverine TP sources and predicting annual riverine TP fluxes. The model estimated that NAPIAF, NAPIR and legacy P sources contributed 19–56%, 16–67% and 13–32% of annual riverine TP flux in 1980–2011, respectively. Compared to reduction of NAPIAF, reduction of NAPIR was predicted to have a greater immediate impact on decreasing riverine TP fluxes. Changes in anthropogenic P input sources (NAPIAF vs. NAPIR), land-use, and precipitation patterns as well as the legacy P source can amplify P export from landscapes to rivers and should be considered in developing P management strategies to reduce riverine P fluxes.

Net anthropogenic phosphorus inputs and riverine phosphorus fluxes in highly populated headwater watersheds in China

Riverine phosphorus (P) levels in headwaters are a worldwide concern for environmental management due to the sensitivity of freshwater ecosystems to phosphorus loads. Here, we evaluate P in the Huai River Basin of China, a watershed with one of the highest intensities of human-activity in the world. Estimates of net anthropogenic phosphorus inputs (NAPI) were obtained by accounting for the main anthropogenic phosphorus inputs in each watershed of the basin, including fertilizer application, net food and feed import, non-food P and seeding P. Multi-year average (2003–2010) anthropogenic inputs of P to the entire basin were 2700 kg P km−2 year−1, with an average amount of 1800 kg P km−2 year−1 entering its 17 headwater watersheds. Fertilizer application was the largest source of new P across the headwater watersheds (about 70 % of NAPI), followed by P content of imported food and feed (24 %) and non-food P (6 %). Riverine total phosphorus (TP) fluxes showed a significant linear relationship with NAPI, with an average 3.2 % of NAPI exported as riverine TP flux. Our result indicates that NAPI could be a good indicator for assessing the risk of regional P loss, as well as an excellent potential predictor of riverine TP flux. A comparison of our results with other similar analyses suggests that around 3 % of NAPI would be exported as riverine TP loads, although fractional export of P may vary significantly regionally. Corresponding P management should be targeted at the main anthropogenic sources and hot-spot areas.

Reconstructing historical changes in phosphorus inputs to rivers from point and nonpoint sources in a rapidly developing watershed in eastern China, 1980–2010

Quantifying point (PS) and nonpoint source (NPS) phosphorus inputs to rivers is critical for developing effective watershed remediation strategies. This study reconstructed PS and NPS total phosphorus (TP) inputs to the Yongan River in eastern China in 1980–2010 using a load apportionment model (LAM) from paired riverine TP concentrations and river discharge records. Based on the fundamental hydrological differences between PS and NPS pollution, the LAM statistically quantified their individual inputs as a power-law function of river discharge. The LAM-estimated monthly/annual riverine TP loads were in good agreement with results derived from a regression model, Load Estimator (LOADEST). The annual TP load increased from 18.4 to 357.0Mgyr−1 between 1980 and 2010. The PS input contributed 7–45% of annual total TP load and increased 23-fold, consistent with a 20-fold increase in flow-adjusted average chloride concentration during the low flow regime (a proxy for wastewater inputs), as well as measured increases in population, poultry, and industrial production. Inferring from observed TP and chloride ratios, as well as total suspended solids (TSS) and river discharge dynamics, temporally retained P load within the river during the low flow regime was estimated to contribute 18–65% of the annual PS input load. NPS inputs consistently dominated the annual riverine TP load (55–93%) and increased 19-fold, consistent with the strong correlation between riverine TP and TSS concentrations, increasing developed land area, improved agricultural drainage systems, and phosphorus accumulation in agricultural soils. Based on our analysis, TP pollution control strategies should be preferentially directed at reductions in NPS loads, especially during summer high-flow periods when the greatest eutrophication risk occurs.

Temporal and spatial changes in nutrients and chlorophyll-α in a shallow lake, Lake Chaohu, China: An 11-year investigation

Temporal and spatial changes of total nitrogen (TN), total phosphorus (TP) and chlorophyll-a (Chl-a) in a shallow lake, Lake Chaohu, China, were investigated using monthly monitoring data from 2001 through 2011. The results showed that the annual mean concentration ranges of TN, TP, and Chl-a were 0.08–14.60 mg/L, 0.02–1.08 mg/L, and 0.10–465.90 μg/L, respectively. Our data showed that Lake Chaohu was highly eutrophic and that water quality showed no substantial improvement during 2001 through 2011. The mean concentrations of TP, TN and Chl-a in the western lake were significantly higher than in the eastern lake, which indicates a spatial distribution of the three water parameters. The annual mean ratio of TN:TP by weight ranged from 10 to 20, indicating that phosphorus was the limiting nutrient in this lake. A similar seasonality variation for TP and Chl-a was observed. Riverine TP and NH4+ loading from eight major tributaries were in the range of 1.56 × 104−5.47 × 104 and 0.19 × 104−0.51 × 104 tons/yr over 2002–2011, respectively, and exceeded the water environmental capability of the two nutrients in the lake by a factor of 3–6. Thus reduction of nutrient loading in the sub-watershed and tributaries would be essential for the restoration of Lake Chaohu.