Downstream Nutrient Research Articles

Landscape positions dictating crop fertilizer responses in wheat-based farming systems of East African Highlands

AbstractImproving fertilizer use efficiency has remained a challenge, particularly for small-scale farming in undulating ‘abnormal’ landscapes of East Africa. Milne's 1930s concept on ‘Catena’ was considered as a breakthrough in understanding soil variability and its implication on productivity in East African highlands. However, there is limited information on how the ‘Catena’ features could be used for fine tuning fertilizer recommendations. We initiated multiple on-farm replicated experiments in three wheat-growing districts (Endamohoni, Lemo and Worreilu) in the Ethiopian highlands in 2014, 2015 and 2016 to assess landscape positions affecting crop-nutrient responses, identify yield limiting nutrients across the ‘Catena’ (N, P, K, S and Zn) and quantify effects of landscape positions on resources use efficiency. We clustered farmlands across the ‘Catena’ (Hillslopes, Midslopes and Footslopes) based on land scape positions in the respective locations. Wheat yield was more strongly and significantly affected by landscape positions (P < 0.001) than by nutrient sources or rates. The crop response to fertilizers was 50 to 300% higher in foot slopes than in hillslopes, depending on locations and inputs levels. With increasing slope, there was a decrease in a crop fertilizer response due to a significant decrease in soil organic carbon, clay content and soil water content, with r2 of 0.95, 0.86 and 0.96, respectively. The difference in the crop response between landscape positions was significantly higher (P < 0.05) with higher rates of nutrient applications (>N92 P46) while differences between landscape positions diminish at lower rates. Yield benefits due to application of K was significant only in the dry years (P < 0.05), while there was hardly any yield benefit from the application of zinc and sulfur. The crop nitrogen recovery fraction and crop water productivity decreased with an increasing slope regardless of nutrient combinations. The results indicated that the landscape position could be considered as a proxy indicator for targeted fertilizer application, particularly in farms with undulating topographic features. Hillslopes are better served by the application of organic fertilizers along with conservation measures as applying higher rates of mineral fertilizer in hillslopes would rather increase the risk of downstream nutrient movement.

Influence of land use and hydrologic variability on seasonal dissolved organic carbon and nitrate export: insights from a multi-year regional analysis for the northeastern USA

Land use/land cover (LULC) change has significant impacts on nutrient loading to aquatic systems and has been linked to deteriorating water quality globally. While many relationships between LULC and nutrient loading have been identified, characterization of the interaction between LULC, climate (specifically variable hydrologic forcing) and solute export across seasonal and interannual time scales is needed to understand the processes that determine nutrient loading and responses to change. Recent advances in high-frequency water quality sensors provide opportunities to assess these interannual relationships with sufficiently high temporal resolution to capture the unpredictable, short-term storm events that likely drive important export mechanisms for dissolved organic carbon (DOC) and nitrate (NO3−–N). We deployed a network of in situ sensors in forested, agricultural, and urban watersheds across the northeastern United States. Using 2 years of high-frequency sensor data, we provide a regional assessment of how LULC and hydrologic variability affected the timing and magnitude of dissolved organic carbon and nitrate export, and the status of watershed fluxes as either supply or transport controlled. Analysis of annual export dynamics revealed systematic differences in the timing and magnitude of DOC and NO3−–N delivery among different LULC classes, with distinct regional similarities in the timing of DOC and NO3−–N fluxes from forested and urban watersheds. Conversely, export dynamics at agricultural sites appeared to be highly site-specific, likely driven by local agricultural practices and regulations. Furthermore, the magnitude of solute fluxes across watersheds responded strongly to interannual variability in rainfall, suggesting a high degree of hydrologic control over nutrient loading across the region. Thus, there is strong potential for climate-driven changes in regional hydrologic cycles to drive variation in the magnitude of downstream nutrient fluxes, particularly in watersheds where solute supply and/or transport has been modified.

Effect of dam construction on nutrient deposition from a small agricultural karst catchment

Abstract Assessing the effects of dam construction on nutrient deposition in small catchments is important to our understanding of the influence of dam interception on downstream nutrient transport as well as of the link between nutrient loss from sloping land and the eutrophic status of large rivers. This study investigates the effect of dam construction on nutrient deposition from a small agricultural karst catchment over the past 50 years by examining the nutrients preserved in reservoir sediments. A new method is proposed to determine the historical nutrient deposition rate ( NDR ) in a small man-made reservoir. We analyzed the concentrations of total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) in three sediment cores retrieved from the reservoir, and estimated the mean annual NDRs across the whole reservoir for five depth ranges represented deposition over the periods 1959–1962, 1963–1972, 1973–1985, 1986–1997, and 1998–2012. The relatively low TN, TP, and TOC concentrations below about 30 cm in the three cores reflect the small contribution from anthropogenic nutrient inputs prior to the 1990s. In contrast, the rapid up-core increases in TN, TP, and TOC above 30 cm reflect the increasing supply of anthropogenic nutrients since the 1990s. Natural nutrients were the main contributor to nutrient deposition in the reservoir over the past 50 years. The lack of coupling between sedimentation rates ( SRs ) and nutrient concentrations suggests that the sediment yield has had little influence on the changing pattern of nutrient concentrations preserved within the reservoir deposits, and this provides a new insight into the relationship between sediment yield and nutrient concentration in reservoir sediments. The deposition rates of TN, TP, and TOC over the past 50 years show similar patterns; i.e., they decreased between 1959 and 1972, and then generally increased thereafter. The temporal patterns of these deposition rates were controlled mainly by the sediment yield over the first four periods, whereas anthropogenic nutrient sources (e.g., fertilizer application) were the main cause of the increased NDRs during the final period. These results provide an improved understanding of the environmental factors controlling changes in NDRs . Overall, the profiles of TN, TP, and TOC concentrations, and the NDRs estimated from the three cores, reflect the processes of nutrient loss from the catchment and enrichment in the reservoir in light of environmental change influenced by human activity, and indicate the effect of dam construction on nutrient deposition in the catchment over the past 50 years. This is the first study to document historical NDRs in a small dam reservoir, and the newly proposed method could be applied to other reservoirs in karst and non-karst areas.

Capacity for bioreactors and riparian rehabilitation to enhance nitrate attenuation in agricultural streams

Globally, small agricultural waterways fed by springs, tile drains, and seeps can disproportionately contribute to downstream nutrient loading, which is associated with declines in water quality and ecosystem functions. Treating nitrate using a multiple tool, multiple-scale approach in small waterways could offer improved management of these sources. We used a before-after-control-impact design to test the suitability of three small (<30 m3) edge-of-field denitrifying woodchip bioreactors and stream bank re-shaping and riparian planting. Over three-and-a-half-years, riparian rehabilitation enhanced nitrate flux attenuation compared to pre-rehabilitation, but only under relatively low flow conditions. In comparison, there were no significant changes in nitrate flux in a control waterway under any flow condition. N fluxes always increased in both the control and treatment waterways when reaches gained water downstream. Nitrate removal efficiencies for all three bioreactors ranged from <10 to >99%, with performance variations due to short residence times and fluctuations in source water chemistry. A single tile drain bioreactor removed 0.41 kg NO3-N d−1, equivalent to ∼10% of the mean daily tile drain nitrate load. Greenhouse gas fluxes from the tile drain bioreactor were similar to the surrounding pasture (CO2-C mean: 185–286 mg C m2 h−1; N2O-N mean: 49–90 μg N m2 h−1), suggesting no negative impacts from the bioreactor. Overall, our results suggest a multiple-tool, multiple-scale application of rehabilitation tools can reduce downstream N fluxes, but only under certain flow conditions. Thus, local rehabilitation tools, like those trialed here, will need to be scaled appropriately if they are to significantly attenuate nutrient losses from small agricultural waterways. Moreover, these will not replace catchment-scale nutrient plans to address losses from land and legacy groundwater N pollution.

Effects of Drying and Re-Wetting on Litter Decomposition and Nutrient Recycling: A Manipulative Experiment

Climate change and water abstraction may change stream flow from perennial into intermittent lotic systems, modifying their abiotic and biotic benthic environment and impacting ecosystem processes such as nutrient turnover. We conducted a microcosm experiment to investigate the interactive effect of water intermittency, macrofauna and leaf size (Populus nigra leaves) on nutrient mineralization and recycling. Leaf disks (1 or 5 cm diameter) were incubated for 40 days with or without the leaf-consumer, Potamophylax cingulatus larvae (Trichoptera, Limnephilidae) and with or without an intervening, 10-days simulation of stream drying and subsequent rewetting. Nutrient fluxes, residual leaf biomass and leaf elemental composition were measured to evaluate how intermittency, macrofauna and leaf size affect organic matter mineralization rates and stoichiometry. Results suggest that drying slows decomposition rates, impacting both the microbial and setting to zero macrofauna activities. The presence of macrofauna increases mineralization and nutrient (C, N and P) regeneration rates. Our findings also suggest that leaf disks with higher diameter display higher microbial activity and NH4+ regeneration. During the experiment, the C:N:P ratios of residual litter changed, as the leaf material became enriched with N and P. Our study suggests that increasingly frequent dry events might slow mineralization rates and downstream nutrient transport.

Quantifying the contribution of tile drainage to basin-scale water yield using analytical and numerical models

The Des Moines Lobe (DML) of north-central Iowa has been artificially drained by subsurface drains and surface ditches to provide some of the most productive agricultural land in the world. Herein we report on the use of end-member mixing analysis (EMMA) models and the numerical model Soil and Water Assessment Tool (SWAT) to quantify the contribution of tile drainage to basin-scale water yields at various scales within the 2370 km2 Boone River watershed (BRW), a subbasin within the Des Moines River watershed. EMMA and SWAT methods suggested that tile drainage provided approximately 46 to 54% of annual discharge in the Boone River and during the March to June period, accounted for a majority of flow in the river. In the BRW subbasin of Lyons Creek, approximately 66% of the annual flow was sourced from tile drainage. Within the DML region, tile drainage contributes to basin-scale water yields at scales ranging from 40 to 16,000 km2, with downstream effects diminishing with increasing watershed size. Developing a better understanding of water sources contributing to river discharge is needed if mitigation and control strategies are going to be successfully targeted to reduce downstream nutrient export.

Techno-economic feasibility of recovering phosphorus, nitrogen and water from dilute human urine via forward osmosis

Due to high phosphorus (P) and nitrogen (N) content, human urine has often proven to suitable raw material for fertiliser production. However, most of the urine diverting toilets or male urinals dilute the urine 2 to 10 times. This decreases the efficiency in the precipitation of P and stripping of N. In this work, a commercial fertiliser blend was used as forward osmosis (FO) draw solution (DS) to concentrate real diluted urine. During the concentration, the urea in the urine is recovered as it diffuses to the fertiliser. Additionally, the combination of concentrate PO43-, reverse Mg2+ flux from the DS and the Mg2+ presents in the flushing water, was able to recover the PO43- as struvite. With 50% concentrated urine, 93% P recovery was achieved without the addition of an external Mg2+. Concurrently, 50% of the N was recovered in the diluted fertiliser DS. An economic analysis was performed to understand the feasibility of this process. It was found that the revenue from the produced fertilisers could potentially offset the operational and capital costs of the system. Additionally, if the reduction in the downstream nutrients load is accounted for, the total revenue of the process would be over 5.3 times of the associated costs.

Optimisation of a forward osmosis and membrane distillation hybrid system for the treatment of source-separated urine

The high concentration of nitrogen, phosphorous and potassium in human urine makes it a suitable raw material for fertiliser production. However, urine is often diluted with a significant amount of flushing water which increases the costs for the downstream nutrients recovery process. Re-using the water and the nutrients in the urine is paramount for enhancing the sustainability of our waste management system. In this work, a combination of forward osmosis (FO) and membrane distillation (MD) was used to extract distilled water from human urine. FO was chosen as MD pre-treatment to increase the overall nitrogen rejection and to prevent wetting of the MD membrane. The goal of this investigation was to tune the FO and MD operating parameters to reduce the nitrogen transport to the MD permeate. Urine pH, draw solution (DS) salt concentration and operating pressure were varied as a means to enhance the FO performances. On the other hand, feed temperature, nitrogen concentration and membrane characteristics were investigated to optimise the MD process. With 2.5 M NaCl as DS commercial FO membranes achieved a water flux between 31.5 and 28.7 L m−2 h−1 and a minimum nitrogen flux of 1.4 g L−1. An additional 33% reduction in the nitrogen transport was observed by applying minimal hydraulic pressure on the DS. However, this was also found to significantly reduce the net transmembrane water flux. Acidification of the feed was also beneficial for both FO and MD nitrogen rejection. Finally, we demonstrated that, by tuning the MD membrane porosity and thickness, higher MD permeate quality could be achieved. To conclude, the hybrid FO-MD process is expected to be an effective solution for the production of clean water and concentrated fertiliser from human urine. This double barrier separation process could be suitable for both water reclamation in space application and resource recovery in urban application.

Influence of Particle Size and Pressure Drop in Cake Filtration Process on Removal of Suspended Solids in Anaerobically Digested Palm Oil Mill Effluent

Palm oil mill effluent (POME) poses a great threat to the environment. However, it contains valuable resources such as energy, water and nutrients that could be recovered for sustainable development. Currently, anaerobic digester has been employed to recover the energy potential in POME. However, the presence of suspended solids in the digestate hinders the downstream nutrients recovery process. In that light, cake filtration process appears to be an attractive option for the removal of suspended solids in the digestate. Hence, this paper studied the performance of cake filtration in removing suspended solids at different pressure condition and particle size of perlite. The effectiveness of cake filtration process was evaluated based on the quality of filtrate (turbidity and total suspended solids (TSS)) and filtration flux. In this study, perlites of different particle size distribution (FP3, FW6, FW20, and FW50) were used as both precoat and body feed. The amount of precoat and body feed were chosen as 1 g each. The filtration process was carried out at different pressure condition (2 – 5 bar). It was found that perlite with the finest particle size (FP3) achieved up to 90% of turbidity and TSS removal due to the formation of more compact cake filtration layer. On the other hand, larger perlite FW50 recorded lowest removal efficiency due to its porous cake layer, though this resulted in higher filtration flux. Generally, the increase in pressure drop resulted in higher flux but at the same time led to drastic initial flux decline due to the quick cover up of filtration voids. The outcomes from this study show that it is wise to consider the effect of particle size distribution and pressure drop in order to achieve high clarity of filtrate as well as high filtration flux.

Evaluating the nutrient reduction and water supply benefits of an on-farm water storage (OFWS) system in East Mississippi

An On-Farm Water Storage (OFWS) system is a structural Best Management Practice (BMP) that prevents downstream nutrient loading by capturing irrigation tailwater and storm runoff from agricultural fields. OFWS systems, as a result, also act as a source of water for irrigation with the potential to recycle nutrients captured in runoff events. A monitoring study was conducted for an OFWS system located on a corn and soybean farm in East Mississippi from June 2014 to August 2016 to analyze the effectiveness of the system for reducing downstream nutrient runoff, supplying water for irrigation, and recycling nutrients in captured water that is reapplied to the field. Nitrate and dissolved phosphorus (DP) concentrations in the storm runoff events captured by the OFWS system storage pond and prevented from going downstream measured as high as 179 mg L−1 and 0.69 mg L−1, respectively. Water can be lost downstream from the storage pond overflow pipe when the pond is at its maximum capacity in March-April of each year, but nitrate concentrations were less than 10 mg L−1 in the storage pond in March-April for both years of the study, and DP concentration was less than 0.053 mg L−1 in the water that could be lost downstream, which showed that OFWS systems can be effective in reducing downstream nutrient loading by capturing storm runoff events. Over three growing seasons, roughly 357,000 m3 of water was used for irrigation from the OFWS storage pond in a region which has traditionally been under dryland production. This shows that OFWS systems can serve a dual purpose of reducing nutrient runoff and providing water for irrigation in East Mississippi, where groundwater is not a cost-efficient source of water for irrigation. Irrigated corn yields were higher than non-irrigated corn yields by an average of 1532; 2285; and 3950 kg ha−1 in 2014, 2015, and 2016, respectively; and irrigated soybean yields were higher than non-irrigated soybean yields over the same years by an average of 302; 1411; and 800 kg ha−1, respectively, demonstrating the importance of irrigation in East Mississippi. Analysis of nutrient concentrations in water samples collected simultaneously from both the irrigation system (sprinkler), which is fed from the bottom of the pond, and the storage pond grab samples showed that nitrate concentrations in the irrigation samples were lower than in the storage pond, but ammonia concentrations were higher in the irrigation water samples. Low nitrate concentrations and variability in nitrate concentration in the irrigation water as compared to the storage pond water showed that some of the nitrogen load is being recycled but not enough for the producer to reduce commercial fertilizer application.

Response of epilithic diatom communities to environmental gradients along an Ecuadorian Andean River

Epilithic diatom communities are particularly suitable for the evaluation of freshwater quality. In Ecuador, however, no water quality index includes this biotic parameter. This work is the first attempt in the country to determine the composition of epilithic diatom communities associated with different degrees of eutrophication. This was accomplished by measuring physical, chemical and microbiological variables at five sampling sites along the Pita River, Ecuador, from August to December 2016. The results indicate a clear gradient of eutrophication from sampling sites P1 (good water) in the upper reaches to P5 (bad water) located in the lower reaches. Concerning diatom analyses, the results indicated a high diversity for tropical areas in terms of species richness, varying from S=34 in headwaters to S=42 downstream. Moreover, the results obtained suggest a lack of concordance with the trophic values given to some of the epilithic diatoms in the literature. There were also species that seem to be sensitive to downstream nutrient increases that were not considered as bioindicators in previous studies. We concluded that the trophic values of diatom species available in the scientific literature are not directly applicable to their sites in the Pita River. Hence, it is necessary to establish a trophic diatom index for the Andean region of Ecuador.

Right practice, right place: A conservation planning toolbox for meeting water quality goals in the Corn Belt

O n August 2, 2014, the residents of Toledo, Ohio, were warned that their drinking water was potentially contaminated with microcystin, an algal-related toxin that can cause various illnesses and liver damage, and unsafe to drink (Bullerjahn et al. 2016). They were the latest victims of a series of water quality crises in the United States, many (including the Toledo event) attributable in large part to nutrients lost from agricultural landscapes. Meanwhile, hypoxia in the Gulf of Mexico continues unabated (USEPA 2015), with the 2017 dead zone being the largest recorded (NOAA 2017). In response to these crises, states in the Mississippi River and Great Lakes drainage basins are undertaking ambitious efforts to achieve 40% to 45% reductions in nutrient losses from agriculture. There is a growing consensus that in-field conservation practices, such as improved nutrient management, conservation tillage, and cover crops, will not, singly or together, achieve the 45% nutrient reduction goals (Schilling and Wolter 2009; Iowa Nutrient Reduction Strategy 2013). Meeting these water quality goals will require combining in-field nutrient management practices with downstream nutrient removal practices, such as bioreactors and filter strips at the edges of fields and wetland creation and floodplain restoration below fields (McLellan et al.

Downstream effects of the Three Gorges Dam on larval dispersal, spatial distribution, and growth of the four major Chinese carps call for reprioritizing conservation measures

Larval drift and dispersal are critical processes that affect recruitment success for many riverine fishes. Hypolimnetic discharge from the Three Gorges Dam (TGD) lowers river temperature and reduces downstream nutrients, inducing distinct shifts in habitat conditions downstream of the dam. The inflow of major tributaries buffers these influences and creates physiochemical gradients according to the distance from the dam. We investigated the abundance, feeding, and growth of larvae of four major Chinese carps in three sections of the middle Yangtze River. Water temperature and transparency showed clear spatial gradients. Larvae in the river section closest to the dam tended to be lower in abundance and temporally delayed peak abundance and showed lower feeding intensity, poorer condition, and slower growth than those further from the dam. Our results demonstrate that physiochemical gradients influenced by the TGD have strong effects on abundance, feeding, and growth of the drifting larvae. We recommend that river sections farther from the TGD, particularly around the mouth of Poyang Lake, should become high-priority conservation areas to enhance protection of critical aquatic species.

Spatial variability of ocean fertilizing nutrients in the dust‐emitting ephemeral river catchments of Namibia

AbstractResearch into global hot spots of dust emission has focused on exposed fine‐grained sediments in palaeo‐ or ephemeral dryland lake basins including Etosha (Namibia) and Makgadikgadi (Botswana) in southern Africa. Namibia's western ephemeral river valleys are also known to produce dust but have remained largely overlooked as a regionally significant source. Nutrient enrichment of valley sediments and proximity to the South Atlantic suggests aeolian dust could play an important role in ocean fertilization. The fertility of valley dust is dependent on fluvial sediments originating in the upper catchments on the Southern African Central Plateau. In this study we investigate climate, geology, vegetation and land use variability and how these may influence the nitrogen, phosphorus and iron availability in the catchments. We intensely sampled the Huab, Kuiseb and Tsauchab river systems to map the spatial distribution of nutrients from upper catchments to river termini. Samples were analysed for the bioavailable fractions of iron, nitrogen and phosphorus as well as total nitrogen and phosphorus. Results show that the lower valley reaches are sources of aeolian dust enriched in nutrients. Nitrogen levels correlate with precipitation and vegetation levels and phosphorus levels with geology. However, differences in upper catchment sediment nutrient levels were not representative of downstream nutrient differences between valleys. Rather, it is the hydrological and geomorphological processes of the ephemeral river systems that are key for producing the enriched sediments in the lower reaches. We demonstrate that the ephemeral river valleys of western Namibia are an extensive and enriched source of mineral dust that could play a critical role in marine productivity of the southern Atlantic. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Surface water retention systems for cattail production as a biofuel

Surface water retention systems act to reduce nutrient pollution by collecting excess nutrients within a watershed via runoff. Harvesting aquatic biomass, such as the invasive cattail, from retention systems removes nutrients absorbed by the plant from the ecosystem permanently. Harvested biomass can be used as a renewable energy source in place of fossil fuels, offsetting carbon emissions. The purpose of this research was to simulate cattail harvest from surface water retention systems to determine their ability to provide suitable growing conditions with annual fluctuations in water availability. The economic and environmental benefits associated with nutrient removal and carbon offsets were also calculated and monetized. A proposed upstream and existing downstream water retention system in southern Manitoba were modelled using a system dynamics model with streamflow inputs provided by a physical hydrologic model, Modélisation Environmentale Communautaire - Surface and Hydrology (MESH). Harvesting cattail and other unconventional feedstocks, such as reeds, sedges, and grasses, from retention systems provided a viable revenue stream for landowners over a ten-year period. This practice generates income for landowners via biomass and carbon credit production on otherwise underutilized marginal cropland invaded with cattail. The economic benefits promote wetland habitat restoration while managing cattail growth to maintain biodiversity. Excess nitrogen and phosphorus are also removed from the ecosystem, reducing downstream nutrient loading. Utilizing surface water retention systems for cattail harvest is a best management strategy for nutrient retention on the landscape and improving agricultural resilience.

Application of AnnAGNPS to model an agricultural watershed in East-Central Mississippi for the evaluation of an on-farm water storage (OFWS) system

Annualized Agricultural Non-Point Source Pollutant Model (AnnAGNPS) is a watershed-scale, continuous simulation, physical model that has been widely used to simulate runoff, nutrients, sediment, and pesticides in different watersheds. This study applied AnnAGNPS to simulate runoff, nutrients (total Nitrogen and total Phosphorus), and sediment from an agricultural watershed of 30.3ha in East-Central Mississippi. AnnAGNPS was then used to evaluate an On-Farm Water Storage (OFWS) system as a Best Management Practice (BMP) for nutrient and sediment loading control from agricultural fields within this watershed and as a source of water for irrigation. An R2 of 0.85 and E of 0.82 in daily runoff estimation showed that the model can adequately simulate runoff from watersheds in East-Central Mississippi. In addition, an R2 of 0.88 and E of 0.67 for event-based sediment estimation and an R2 of 0.74 and E of 0.54 for monthly phosphorus estimation also showed that the model can satisfactorily simulate sediment and phosphorus. However, the model was not able to simulate nitrogen at a monthly scale, with an R2 of only 0.15 and E of −0.107, because of the lack of site specific and accurate input data. After AnnAGNPS successfully simulated runoff, sediment, and phosphorus, an evaluation of the OFWS system showed that the system was able to capture 220,000m3 of runoff from the monitored watershed that can be stored and used for irrigation. AnnAGNPS estimated that the OFWS system also captured 46 tons of sediment and 558kg of phosphorus during the monitoring period, preventing downstream nutrient and sediment pollution.

Assessment of nutrient loadings of a large multipurpose prairie reservoir

The relatively low water flow velocities in reservoirs cause them to have high capacities for retaining sediments and pollutants, which can lead to a reduction in downstream nutrient loading. Hence, nutrients can progressively accumulate in reservoirs, resulting in the deterioration of aquatic ecosystems and water quality. Lake Diefenbaker (LD) is a large multipurpose reservoir, located on the South Saskatchewan River (SSR), that serves as a major source of freshwater in Saskatchewan, Canada. Over the past several years, changes in land use (e.g. expansion of urban areas and industrial developments) in the reservoir’s catchment have heightened concerns about future water quality in the catchment and in the reservoir. Intensification of agricultural activities has led to an increase in augmented the application of manure and fertilizer for crops and pasture. Although previous research has attempted to quantify nutrient retention in LD, there is a knowledge gap related to the identification of major nutrient sources and quantification of nutrient export from the catchment at different spatial scales. Using the SPAtially Referenced Regression On Watershed (SPARROW) model, this gap has been addressed by assessing water quality regionally, and identifying spatial patterns of factors and processes that affect water quality in the LD catchment. Model results indicate that LD retains about 70% of the inflowing total nitrogen (TN) and 90% of the inflowing total phosphorus (TP) loads, of which fertilizer and manure applied to agricultural fields contribute the greatest proportion. The SPARROW model will be useful as a tool to guide the optimal implementation of nutrient management plans to reduce nutrient inputs to LD.

Potential mobility of inorganic nutrients and its controls at the sediment-water interface in the main path of Kuroshio Current off eastern Taiwan

Surface sediments were collected from the main path of Kuroshio Current (KC) off eastern Taiwan (ETW), and laboratory incubation was conducted for the study of the nutrient exchange at the sediment-water interface. Exchange fluxes of NO3−-N, NO2−-N, NH4+-N, PO43−-P and SiO32−-Si under simulated site condition were −0.0866, 0.00146, 0.0634, −0.00812 and −0.181mmolm−2d−1, respectively. This indicated that sediments in the studied area might act as a source of NO2−-N and NH4+-N and a sink of NO3−-N, PO43−-P and SiO32−-Si. Factors affecting nutrient exchange fluxes were studied by changing incubation conditions. Compared with some coastal areas in China, the main path of KC off ETW showed relatively low nutrient exchange fluxes at the sediment-water interface. It could be inferred that the sediment-water exchange of nutrients in the main path of KC off ETW had little influence on the downstream nutrient transportation by KC year-round based on the results of this research.

Seasonal water quality changes in on-farm water storage systems in a south-central U.S. agricultural watershed

The objective of this study was to investigate the ability of on-farm water storage (OFWS) systems to mitigate off-site nutrient movement in a south-central U.S. agricultural watershed. We examined the seasonal water quality changes in an OFWS system by measuring several physical and chemical constituents at multiple sampling points throughout the system. Water quality sampling occurred every three weeks during the growing season and every six weeks during the dormant season from February 2012 to December 2014. The collected data were grouped into four seasons and then analyzed using boxplots along with the Wilcoxon and Kruskal-Wallis rank-sum tests for detecting changes in nutrient concentrations. Significant water quality changes were observed in the OFWS system by season and nutrient species, indicating a variation in downstream nutrient reduction with season. The in-ditch median removal efficiency, from the center of the tailwater recovery ditch to the outlet, was 54% during winter and 50% during spring for NO3-N; 60% during spring for NH3-N; 26% during autumn and 65% during winter for ortho-P; and 31% during winter and 10% during spring for TP. The in-pond median concentration removal efficiency was ∼77% during summer for NO3-N, while the concentration remained stable during winter, spring and autumn; 53% from winter to spring and 58% from spring to summer for NH3-N; 70% from winter to spring for ortho-P, while remaining stable during the other seasons; and 28% from winter to spring and 55% from spring to summer for TP. Our results support the hypothesis that OFWS systems could mitigate downstream nutrient-enrichment pollution, especially during spring. The results obtained from this study offer a better insight into the behavior of OFWS systems and help enhance the management of agroecosystems from an ecological and hydrological perspective for water quality pollution control and water resource conservation.

Regional meteorological drivers and long term trends of winter-spring nitrate dynamics across watersheds in northeastern North America

This study evaluated the contribution of winter rain-on-snow (ROS) events to annual and seasonal nitrate (N-NO3) export and identified the regional meteorological drivers of inter-annual variability in ROS N-NO3 export (ROS-N) at 9 headwater streams located across Ontario, Canada and the northeastern United States. Although on average only 3.3 % of annual precipitation fell as ROS during winter over the study period, these events contributed a significant proportion of annual and winter N-NO3 export at the majority of sites (average of 12 and 42 %, respectively); with the exception of the most northern catchment, where total winter precipitation was exceptionally low (average 77 mm). In years with a greater magnitude of ROS events, the timing of the peak N-NO3 export period (during spring melt) was redistributed to earlier in the year. Variability in ROS frequency and magnitude amongst sites was high and a generalised linear model demonstrated that this spatial variability could be explained by interactive effects between regional and site-specific drivers. Snowpack coverage was particularly important for explaining the site-specific ROS response. Specifically, ROS events were less common when higher temperatures eliminated snow cover despite increasing the proportion of winter rainfall, whereas ROS event frequency was greater at sites where sufficient snow cover remained. This research suggests that catchment response to changes in N deposition is sensitive to climate change; a vulnerability which appears to vary in intensity throughout the seasonally snow-covered temperate region. Furthermore, the sensitivity of stream N-NO3 export to ROS events and potential shifts (earlier) in the timing of N-NO3 export relative to other nutrients affect downstream nutrient stoichiometry and the community composition of phytoplankton and other algae.