Effects of dissolved oxygen changes in the benthic environment on phosphorus flux at the sediment-water interface in a coastal brackish lake.

In semi-enclosed coastal brackish lakes, changes in dissolved oxygen in the bottom layer due to salinity stratification can affect the flux of phosphorus (P) at the sediment-water interface, resulting in short- and long-term water quality fluctuations in the water column. In this study, the physicochemical properties of the water layers and sediments at five sites in Saemangeum Lake were analyzed in spring and autumn for four years, and phosphorus release experiments from sediments were conducted for 20 days under oxic and anoxic conditions during the same period. Sediment total phosphorus (T-P) decreased in autumn compared to spring due to mineralization of organic bound phosphorus, which was the most dominant P fraction. This may be related to the increase in the ratio of PO4-P to T-P in bottom waters in autumn, when hypoxia was frequently observed. The difference in P fluxes between oxic and anoxic conditions indicated that during autumn, as compared to spring, the release of phosphorus could have a more immediate impact on the water column during the formation of hypoxia/anoxia. The main factors influencing changes in P fluxes from sediments were identified through redundancy analysis. Additionally, based on the results of multiple regression analysis, sediment TOC, sediment non-apatite phosphorus, porewater pH, and porewater PO4-P were determined to be the most significant factors affecting P fluxes from sediments, depending on the season or redox conditions. Recently, the increased influx of seawater into Saemangeum Lake has been shown to contribute to water quality improvements in the water column due to a strong dilution effect. However, the sediment environment has shifted towards a more reduced state, leading to increased P release under anoxic conditions. Therefore, for future water quality management within the lake, it is necessary to consistently address the recurring hypoxia and continuously monitor phosphorus dynamics.

Effect of temperature variation on phosphorus flux at the sediment-water interface of the steppe wetlands.

Environmental factors are generally considered to be important factors affecting the release process of phosphorus (P) in sediments. However, little is known about the effect of temperature increased at first and then decreased with the season change on the P flux rate and flux amount at the sediment-water interface in the steppe wetlands. The effects of the temperature variation on P flux at the sediment-water interface in the steppe wetlands during the vegetation growing season under simulated wetland habitat were studied. The results showed that the release of P from sediments to overlying water was greatly affected by temperature changes. When the temperature rose, P was released from the sediment into the overlying water, while P was precipitated from the water into the sediment with the temperature dropped. During simulation period, the total P in water flux rates between sediment and overlying water (FP) was ranged from - 4.51 to 4.99 mg·m-2·day-1, while the dissolved reactive P in water flux rates between sediment and overlying water (FDP) was changed from - 5.37 to 5.14 mg·m-2·day-1. The FP and FDP were negatively correlated with the content of total P in water (WTP), dissolved reactive P in water (WDRP), pH of sediment (pH), and microbial biomass P (MBP), but increased with temperature (T), aluminum phosphate (Al.P), and occluded phosphate (Oc.P). The P flux rates were affected by temperature variation both directly and indirectly; the mechanism of how temperature influenced the fate of P in the wetland is still not clear. Therefore, the physicochemical properties and kinetic, thermodynamic, and microbiology characteristics should be combined together to clarify the mechanism in future research.

Higher Fine Particle Fraction in Sediment Increased Phosphorus Flux to Estuary in Restored Yellow River Basin.

River delta-front estuaries (DEs) are vital interfaces for fluxes between terrestrial and marine environments. However, deep uncertainty exists in estimating the sedimentary pollutant flux from terrestrial environments in DEs due, in part, to a lack of direct measurements in these dynamic and complicated regions and uncertainty in the calculation method. Due to its high sediment content, the Yellow River (YR) has a strong ability to adsorb phosphorus; therefore, it reliably reflects estuarine sedimentary processes. Here, through the comprehensive analysis of field samples, monitoring data and remote sensing images, we conclude that riverine fine particles control the deltaic estuary pollution status and that particle size is the key factor. Based on the stable relationships between phosphorus and heavy metals, with r2 values of 0.990, 0.992, and 0.639 for As, Cd, and Cr, respectively, we estimated that the P flux reached 22.68 g/m2 yr in 2017. Analysis of the YR high-silt sediment load, which has a strong phosphorus adsorption ability and constitutes a substantial fraction of global fluvial sediment transport, revealed a negative correlation between the riverine sediment load and the estuarine phosphorus flux.

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.

Soil erosion as a source of sediment and phosphorus in rivers and reservoirs - Watershed analyses using WaTEM/SEDEM.

Spatially distributed modelling of sediment and phosphorus fluxes on a scale of thousands of square kilometers always involves a compromise between the quality of the data input and the complexity of the model that can be applied. WaTEM/SEDEM offers an approach that allows us to target on spatially focused outputs that can easily be implemented in the decision-making process for effective watershed control. The results for a study area covering the watersheds of 58 large reservoirs threatened by eutrophication within the Czech Republic are presented here as an example of the available analyses. The total area of the watersheds is 27,472 km2. After building a complex river topology scheme and estimating the trap efficiencies in all reservoirs within the river networks, we are able to estimate the total transport efficiency of each river unit for any outlet point (terminal reservoir). The sources of the greatest amounts of sediment (phosphorus) can be identified on the scale of single parcels. According the model, the total soil loss in the study area is 7487 Gg year-1 (2.73 Mg ha-1 year-1). The total sediment entry into the river systems in the target area is 1705 Gg year-1 (15.2% of the total soil loss). The total deposition in the 9890 water reservoirs of various sizes in the target area is 1139 Gg year-1. This means that the deposition in the landscape is 5.1× higher than the deposition in the reservoirs within the study area. The mean annual sediment transport by all watershed outlets is 566 Gg year-1. The cost of dredging the sediment would be about 12.8 million EUR year-1. There is great spatial variability in the deposition and transport processes, but it is imperative to provide strengthened soil protection directly on-site, especially in watersheds where the sediment delivery ratio is much higher than the average value. Phosphorus transported by water erosion is an important element in the balances of phosphorus sources in basins. Sewage waters usually play the predominant role in triggering the eutrophication effect, but there are also reservoirs where erosion-based phosphorus plays a major role.

Internal loading of phosphate in Lake Erie Central Basin.

After significant reductions in external phosphorus (P) loads, and subsequent water quality improvements in the early 1980s, the water quality of Lake Erie has declined considerably over the past decade. The frequency and magnitude of harmful algal blooms (primarily in the western basin) and the extent of hypoxic bottom waters in the central basin have increased. The decline in ecosystem health, despite meeting goals for external P loads, has sparked a renewed effort to understand P cycling in the lake. We use pore-water P concentration profiles and sediment cores incubation experiments to quantify the P flux from Lake Erie central basin sediments. In addition, the oxygen isotopes of phosphate were investigated to assess the isotopic signature of sedimentary phosphate inputs relative to the isotopic signature of phosphate in lake water. Extrapolating the total P sediment flux based on the pore-water profiles to the whole area of the central basin ranged from 300 to 1250metric tons per year and using the flux based on core incubation experiments an annual flux of roughly 2400metric tons of P is calculated. These estimates amount to 8-20% of the total external input of P to Lake Erie. The isotopic signature of phosphate in the extractable fraction of the sediments (~18‰) can explain the non-equilibrium isotope values of dissolved phosphate in the deep water of the central basin of Lake Erie, and this is consistent with sediments as an important internal source of P in the Lake.

Enhanced phosphorus flux from overlying water to sediment in a bioelectrochemical system.

This report proposed a novel technique for the regulation of phosphorus flux based on a bioelectrochemical system. In the simulated water system, a simple in situ sediment microbial fuel cell (SMFC) was constructed. SMFC voltage was increased with time until it was 0.23V. The redox potential of the sediment was increased from -220mV to -178mV during the process. Phosphorus concentration in the water system was decreased from 0.1mg/L to 0.01mg/L, compared with 0.09mg/L in the control. The installation of a SMFC produced an external current and internal circuit, which promoted the transfer of phosphate in overlying water to the sediment, enhanced the microbial oxidation of Fe(2+), and increased the formation of stable phosphorus in sediment. In conclusion, phosphorus flux from the overlying water to sediment was enhanced by SMFC, which has the potential to be used for eutrophication control of water bodies.