Hydrological controls on cascade reservoirs regulating phosphorus retention and downriver fluxes

The cumulative effects of cascade reservoirs control nitrogen and phosphorus flux: Base on biogeochemical processes

The reservoir serves as a water source, a flood control structure, a navigational aid, and also impacts the downstream ecosystem as well as the reservoir zone. However, debate exists about effectiveness of cascade reservoirs in controlling the transportation of nutrients, particularly in the Yangtze River basin, which has been significantly affected by reservoir development. This research develops a new model X-NPSEM (X with Nitrogen and Phosphorus Steady-state Reservoir Model) based on biogeochemical processes of nitrogen and phosphorus reaction for investigating the dynamic storage capacity of cascade reservoirs at both reservoir- and watershed scales. Then the cumulative effects of cascade reservoirs and the related mechanism were investigated in Fujiang watershed, China. Based on the results, cascade reservoirs retained 16.3 % of nitrogen fluxes and 37.6 % of phosphorus fluxes annually. Downstream reservoirs have higher retention rates of phosphorus (0.48/d) compared to upstream reservoirs (0.10/d), mainly due to inflow sediment. Nitrogen retention rates show seasonal variations: wet season (0.21/d) and dry season (0.17/d). These fluctuations in nitrogen retention are primarily influenced by changes in temperature rather than other factors such as operation period, nitrogen and phosphorus concentration, or the nitrogen/phosphorus ratio. In upstream, the concentration of sediment entering the reservoir plays a decisive role in the transformation of P retention from sink to source. The X-NPSRM coupler model could be used for global reservoir operation and watershed management.

Discharge dynamics of agricultural diffuse pollution under different rainfall patterns in the middle Yangtze river

Rainfall plays a crucial role in influencing the loss of agricultural diffuse pollution. The middle Yangtze River region is well-know for its humid climate and numerous agricultural activities. Thus, this study quantitatively analyzed the concentration and distribution of nitrogen (N) and phosphorus (P) load and loss in a major tributary of the middle Yangtze River under different rainfall patterns by using sampling analysis and SWAT model simulation. The total nitrogen (TN) and nitrate-nitrogen (NO3-) concentrations were 1.604-3.574 and 0.830-2.556 mg/L, respectively. The total phosphorous (TP) and Soluble Reactive Phosphorus (SRP) were 2-148 and 2-104 μg/L, respectively. The modeling results demonstrated that higher rainfall intensity led to greater load and loss flux of diffuse pollutant at the outlet. Organic nitrogen (ORGN) is the main nitrogen form transported from the subbasin to the reach, while organic phosphorus (ORGP) and inorganic phosphorus (INORGP) were transported at similar amounts. Under the condition of conventional rainfall, the outlet reaches mainly transported NO3-, and ORGN gradually increased when rainstorm events occurred. The ratio of INORGP to ORGP was relatively stable. During extreme rainfall event, rainfall is the dominant element of agricultural diffuse pollution. A strong positive correlation exists between rainfall intensity and pollution loss during rainstorms. Storm rain events were the main source of TN and TP losses. Few storm rain days generated pollutants that accounted for a large proportion of the total loss, and their impact on TP loss was significantly greater than that of TN. The influence of storm rain on TN is mainly the increase in runoff, while TP is sensitive to the runoff and sediment transport promoted by rainfall. By setting different precipitation scenarios, it was confirmed that under the same rainfall amount, short-term storm rain has the most significant impact on the TN load, whereas TP load may be influenced more by the combined effects of rainfall duration and intensity. Therefore, to reduce the impact of agricultural diffuse pollution, it is important to take targeted measures for the rainstorm days.

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

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

Human disturbance on phosphorus sources, processes and riverine export in a subtropical watershed

Phosphorus (P) is a key nutrient in freshwater systems, often acting as the limiting nutrient. The dominant sources of P in the Jiulong River watershed (S.E. China) are anthropogenic. Dissolved and particulate P species were measured in the West (WJR) and North (NJR) rivers during the wet and dry seasons of 2018 and at their river outlets during a storm (June 2019). Sources of P pollution were characterized from mainly single source subcatchments (dry season). The Agriculture source (WJR) had a total P of 114.7 ± 13.1 μg P L^-1, which was mainly dissolved inorganic P (DIP) from excess fertilizer washed from the fields. By contrast, the West Urban source (sewage effluent) was mainly particulate (POP) and dissolved organic P (DOP). The effect of reservoirs in the main NJR was to decrease total particulate P (TPP) and DIP and increase POP, due to increased sedimentation of particles and biological uptake. An increase in all P species was observed at the beginning of the storm, followed by a decrease on the rising hydrograph due to dilution. The final concentration of all P species was higher than baseflow, confirming that storms increase the P flux out of the watershed. P was initially washed off the fields during the storm, and during the falling hydrograph P increased due to interflow and other longer-term sources. The high DIN:DIP ratio confirmed the key importance of P inputs from human activities in substantially altering P sources and cycling, and hence the importance of science-based management to alleviate the eutrophication problem.

Impact of a large sub-tropical reservoir on the cycling of nutrients in a river

The quantity and composition of nutrients carried by rivers play an important role in maintaining the ecosystem of downstream rivers and marginal seas. To reveal the impact of damming on the composition and flux changes of nutrients in rivers, this study conducted a detailed survey of a large sub-tropical reservoir (Xinanjiang Reservoir, XAJR) in eastern China from August 2013 to June 2014 obtaining samples at bi-monthly intervals. The thermal stratification, water quality in situ parameters, and the contents of nutrients in the water column of the river inflow, transition area, central reservoir area, and discharge water of the XAJR were analyzed in detail along the fluvial direction. The results show that the thermal stratification of the XAJR had seasonal and spatial heterogeneity. Accordingly, the pH and dissolved oxygen saturation degree in water also showed a similar stratification phenomenon. The analysis of nutrient limitation for primary productivity indicated that in different seasons, varying limiting degrees of the silicon or phosphorus were developed in different locations along the XAJR- river system. Among them, the river area and transition area were more susceptible to silicon restriction in winter, whereas phosphorus restriction mainly occurred during the warm seasons from April to October. XAJR had a retention effect on nitrogen, phosphorus, and silicon, among which phosphorus was more easily retained by the reservoir. In addition, nitrogen underwent transformation processes between different forms inside the reservoir. In the reservoir, the different degrees of retention effect of nutrients also led to a significant increase in the ratio of silicon to phosphorus and nitrogen to phosphorus in the discharged water. This study confirmed that thermal stratification has important control over the contents and forms of nutrients in water. This finding provided an idea for the restoration of the fluxes and stoichiometric ratios of the nutrients in the downstream river using the reservoir's capacity for artificial regulation.

Long-term and longitudinal nutrient stoichiometry changes in oligotrophic cascade reservoirs with trout cage aquaculture

The potential nutrient stoichiometry changes caused by trout cage aquaculture is concerned especially in oligotrophic waters. Long-term total nitrogen (N), total phosphorus (P) and N:P ratio changes in 6 cascade reservoirs with rainbow trout cage aquaculture in the oligotrophic upstream Yellow River (UYR) were studied from 2013 to 2017 in this paper. The 5-year monitoring results showed that N, P and N:P ratio levels showed no obvious long-term changes in high-altitude oligotrophic waters with rainbow trout cage aquaculture. No obvious longitudinal N, P and N:P ratio level changes were observed in the 6 cascade reservoirs from upstream Longyangxia Reservoir (LYR) to downstream Jishixia Reservoir (JSR). The increased N and P resulting from the cage aquaculture accounted only for 1.74% and 5.2% of the natural N and P levels, respectively, with a fish production of 10,000 tonnes. The upstream Yellow River remained oligotrophic and phosphorus-limited. Results in this study proved that trout cage aquaculture do not necessarily cause nitrogen, phosphorus and N:P ratio changes even in oligotrophic waters. Phosphorus should be considered first when identifying priority nitrogen and phosphorus sources and the corresponding control measures in waters with high N:P ratio.

Variation of biogeochemical cycle of riverine dissolved inorganic carbon and silicon with the cascade damming

To investigate the variation of the biogeochemical cycle of riverine dissolved inorganic carbon (DIC) and silicon (DSi) with the cascade damming, the bicarbonate ([Formula: see text]), dissolved silicon (DSi), and other environmental factors within the cascade reservoirs of the lower reaches of Yalongjiang River, passing through the southeastern Qinghai-Tibet Plateau, were systematically analyzed by collecting water samples during the wet season and dry season from 2018 to 2019, respectively. The results showed that the lower ratio of DSi to[Formula: see text] (0.044 ± 0.001) was mainly controlled by the domination of carbonate mineral in the sedimentary rock of the Yalongjiang River drainage basin. The DSi:[Formula: see text] ratio was positively correlated with discharge (P < 0.05), and negatively correlated with the water retention time (P < 0.01) and chlorophyll a, implying that the variations of DSi:[Formula: see text] ratio were mainly determined by the rock chemical weathering processes and the hydrologic process outside the reservoirs and the biological processes within the cascade reservoirs. The phytoplankton photosynthetic process stoichiometrically assimilated DSi and [Formula: see text], resulted in 3.46 × 10⁴ t·Si a^-1 and 1.89 × 10⁴ t·C a^-1 sequestering in the cascade reservoirs, respectively. Compared with the situation of dam-free in the lower reaches of Yalongjiang River, the export flux of [Formula: see text] and DSi at the mouth of Yalongjiang River was reduced by 11.87% and 62.50%, respectively; the ratio of DSi:[Formula: see text] decreased by 36.01% for only building the Ertan dam and 53.15% for the cascade damming, respectively. The water renewal time prolonged from 45 to 126.6 days due to the regulation of the cascade reservoirs in the mainstream. Ultimately, a conceptual model on migration-transformation of DIC and DSi within the cascade reservoirs in the lower reaches of Yalongjiang River was established. These findings demonstrated that riverine cascade damming could extend the biogeochemical coupling cycle of DIC and DSi within the inland aquatic ecosystems and ensure the ecological environment security in the hot-dry valley.

Distribution of phosphorus species and their release risks in the surface sediments from different reaches along Yellow River

In order to further explore the relationship between water body eutrophication of some reaches of Yellow River and phosphorus species in sediments, and evaluate phosphorus release potential from sediments to overlying water, we investigated the distribution of P species and their release risk in the surface sediments from different reaches along Yellow River, as well as the influence of artificial dam on phosphorus cycle in Yellow River. The results show that the content of calcium phosphorus (P_Ca) is higher than the content of bioavailable phosphorus (BP) (BP = P_ex + P_Fe; in the formula, P_ex means exchangeable phosphorus and P_Fe means iron phosphorus) in the surface sediments from Yellow River. Among all the surface sediment samples from 21 stations, only Dayudu section (H₁₅) has a ratio of W (BP)/W (P_Ca) higher than 0.5; the results show that the intensity of phosphorus release from H₁₅ is high, and there is a potential risk of eutrophication in water, while the phosphorus release level of water sediment in other reaches of Yellow River is low. The content of BP and total phosphorus (∑P) in surface sediments along the Yellow River is in descending order: middle reaches of Yellow River > upper reaches of Yellow River > Yellow River downstream, while the content order of total phosphorus (TP) in the overlying water is as follows: Yellow river downstream > middle reaches of Yellow River > upper reaches of Yellow River (except the H₇ station), indicating that extensive artificial corresponding damming in Yellow River basin makes the concentration of TP increasing significantly in overlying from upstream to downstream. The establishment of Haibowan water conservancy project in Yellow River makes the total particulate matter (TPM) concentration in the water to reduce to very low lever in Wuhai H₇ section, and the phosphorus concentration in the overlying water reaches 0.136 mg L^-1.

Evaluating the retention capacity of a new subtropical run-of-river reservoir

In man-made reservoirs, the sedimentation and assimilation of elements usually prevail as a result of a decrease in the flow regime and an increase in the hydraulic retention time. Thus, the retention capacity derives from hydraulic flushing, as well as chemical and biological reactions. The aim of this study was to assess the element retention capacity of a new subtropical reservoir (Piraju Reservoir situated in São Paulo State, Brazil). Limnological monitoring was performed over four consecutive years (August 2003 to August 2007). We determined 19 variables (chemical, physical, and biological) every 3 months at the inlet (Paranapanema River) and outlet water of the Piraju Reservoir. For each variable, a mass balance was performed and the alpha parameter (i.e., retention capacity) was defined resulting in 323 determinations. From these results, only 10% led to the occurrence of element retention. Retention events were episodic; the fecal coliforms (seven times) and the N-NH₄ (six times) were the variables that presented the highest number of retentions. The results show that different variables can be linked to both the retention and release of elements from the reservoirs. The results show the great significance of the physical processes (in this case, hydraulic retention time and mixing regime) in determining the element retention and exportation from the Piraju Reservoir. The water temperature was a secondary variable for the processes related to retention (such as chemical reactions and biological assimilations).

Phosphorus retention along a typical urban landscape river with a series of rubber dams

Small dams are widely constructed in urban rivers as landscape engineering practice, which increasingly cause eutrophication problems. Phosphorus retention in dammed rivers is a critical factor driving eutrophication, but it is little known in urban landscape river systems controlled by small dams. In this study, we investigated the retention of different phosphorus species along an urban landscape river with 30 rubber dams. We found that 42.5% (7.69 metric tons/yr) of the total phosphorus (TP) was trapped within dams, of which total particulate phosphorus (TPP) retention load accounted for 81.5%. From first river segment BBF-4# to the segments further downstream, the TP retention rate sharply decreased from 47.6% to -8.3%-9.2%, and phosphorus was mainly retained in the uppermost segment of the dammed river. The retention rate of dissolved reactive phosphorus (86.3%) was higher than that of TPP (40.3%) because of biological uptake. Further, with a retention rate of -11.3%, the dammed river was a net source of dissolved organic phosphorus. Different hydrological regimes, due to seasonal events and dam management, greatly influenced phosphorus retention within the dammed river, resulting in higher retention loads in the rainy season than in the dry season, and very low retention loads in the snowmelt season, with 1.48, 0.55 and 0.06 t/month, respectively. Our findings imply that management practices should focus on reducing the phosphorus export from the upper watershed and improving the hydrodynamic conditions of the dammed urban landscape river with regard to eutrophication.

Lateral and longitudinal variation in phosphorus fractions in surface sediment and adjacent riparian soil in the Three Gorges Reservoir, China

Hydrological regimes have been significantly altered since the Three Gorges Dam (TGD) raised the water level of the reservoir to the maximum design level of 175 m in October 2010. This change might greatly influence the forms of phosphorus (P) in the sediment and the adjacent riparian soil. The purpose of this study was to reveal the lateral (sediment, water-level-fluctuation zone soil, and upland soil) and longitudinal (from the end of backwater area to the TGD) trends in P factions. Samples from 11 sites located along the main stem and ten sites located along eight tributaries were collected in June 2017. The P fractions were determined using the Standards, Measurements, and Testing (SMT) protocol. The results showed that the order of increase for average pH values was sediment (7.58 ± 0.62), WLFZ soil (7.44 ± 0.29), and adjacent upland soil (7.20 ± 0.68). The total organic carbon in the sediment was also highest with an average of 9.15 ± 2.97 mg·g^-1. The average concentrated HCl-extractable P (total P), organic P (OP), inorganic P (IP), HCl-extractable P (HCl-P), and NaOH-extractable P (NaOH-P) were 630.02 ± 212.24, 161.89 ± 90.77, 468.13 ± 194.92, 335.65 ± 159.88, and 51.40 ± 36.20 mg·kg^-1, respectively. The concentration of both total P and NaOH-P in the sediment of the main stem exhibited an increasing trend from the backwater area to the TGD. The average concentration of P species in the sediment was higher than those in the upland soil and the water-level-fluctuation zone (WLFZ) soil. For all the sediment and soil samples, the rank order of P species concentrations was HCl-P > OP > NaOH-P. Both IP and HCl-P were highly positively correlated with total P in the upland soil, the WLFZ soil and the sediment. However, only in the sediment, NaOH-P was positively correlated with total P and OP. All P species in the upland soil demonstrated greater spatial heterogeneity than those in the WLFZ soil and the sediment. Redundancy analysis revealed that the main variables explaining the variance in P species concentrations were Al in the upland soil and pH in the sediment.

Hydrochemical controls on reservoir nutrient and phytoplankton dynamics under storms

Eutrophication and undesired algal blooms in surface water are common and have been linked to increasing nutrient loading. Effects of extreme events such as storms on reservoir nutrient and phytoplankton remain unclear. Here we carried out continuous high-frequency measurements in a long and narrow dam reservoir in southeast China during a storm period in June-July 2015. Our results show a strong nutrient-phytoplankton relationship as well as a very rapid response to storm runoff. We observed an increase in total suspended matter (TSM), ammonium (NH₄-N), and dissolved reactive phosphate (DRP), with a sharp decline in chlorophyll-a (Chl-a) in the high flow periods. Afterward, Chl-a, total phytoplankton abundance and Cyanophyta fraction elevated gradually. Nitrate was diluted at first with increasing discharge before concentration increased, likely following a delayed input of groundwater. Physiochemical parameters and Chl-a were evenly distributed in the water column during the flooding period. However, 10% of NH₄-N and 25% of DRP were removed in surface water (0-1m) when an algal bloom (Chl-a>30μgL^-1) occurred 10days after peak discharge. Conversely, total particulate P (TPP) of surface water was 58% higher than in the deeper water. Dynamic factor analysis (DFA) revealed that TSM, NH₄-N, DRP, total P and discharge significantly explain Chl-a variations following storms (C_eff=0.89). These findings highlight that the reservoir ecosystem was vulnerable to pulse input from storm runoff and the Cyanophyta bloom was likely fueled by phosphate and ammonium rather than nitrate.