Chronic increasing nitrogen and endogenous phosphorus release from sediment threaten to the water quality in a semi-humid region reservoir.

The water quality in the drinking water reservoir directly affects people's quality of life and health. When external pollution input is effectively controlled, endogenous release is considered the main cause of water quality deterioration. As the major nitrogen (N) and phosphorus (P) sources in reservoirs, sediment plays a vital role in affecting the water quality. To understand the spatial and temporal variation of N and P in the sediment, this study analyzed the current characteristics and cumulative effects of a semi-humid reservoir, Yuqiao Reservoir, in North China. The N and P concentrations in the reservoir sediment were decreased along the flow direction, while the minimum values were recorded at the central sediment profile. External input and algal deposition were the main factors leading to higher sediment concentrations in the east (Re-E) and west (Re-W) areas of reservoir sediment profiles. According to the long-term datasets, the peaks of both sediment total nitrogen content and deposition rate were observed in the 2010s, which has increased about three times and six times than in the1990s, respectively. Therefore, the increase in phosphorus concentration may be the main reason for eutrophication in water in recent years. The mineralization of organic matter has a significant promoting effect on releasing N and P from sediments, which will intensify eutrophication in water dominated by P and bring huge challenges to water environment management. This study highlights that the current imbalance in N and P inputs into reservoirs and the endogenous P release from sediment will have a significant impact on water quality.

The Effect of Rainfall on Aquatic Nitrogen and Phosphorus in a Semi-Humid Area Catchment, Northern China.

The impact of rainfall on water quality may be more important in semi-arid regions, where rainfall is concentrated over a couple of months. To explore the impact of rainfall changes on water quality, e.g., nitrogen (TN) and phosphorous (TP), the diversion from Luan River to Tianjin Watershed in the northern semi-humid area was selected as the study area. TN and TP concentrations in rivers and the Yuqiao Reservoir during the three-year high-flow season (2019-2021) were analyzed. The response relationship and influencing factors among the watershed's biogeochemical process, rainfall, and water quality were clarified. The results showed that rainfall in the high flow season mainly controlled the river flow. The concentration of TN and TP in the inflow rivers is regulated by rainfall/flow, while the concentration of TN and TP in the water diversion river has different variation characteristics in the water diversion period and other periods. The lowest annual concentrations of TN and TP were observed in the normal year, while the highest annual concentration was observed in the wet year, indicating that the hydrological process drove the nutrient transport in the watershed. For the tributaries, the Li River catchment contributed a large amount of N and P to the aquatic environment. For the reservoir, the extreme TN concentrations were the same as the tributaries, while the extremes of TP concentrations decreased from the dry year to wet year, which was in contrast to the tributaries. The spatial variation of TN and TP concentrations in the reservoir showed that the concentration decreased following the flow direction from the river estuary to the reservoir outlet. Considering climate change, with the increase of rainfall in North China in the future, the TN and TP transport fluxes in the watershed may continue to increase, leading to the nitrogen and phosphorus load of the downstream reservoir. To ensure the impact of the increase of potential N and P output fluxes in the watershed on the water quality of the reservoir area, it is necessary to strengthen the effective prevention and control of non-point source pollution in the watershed.

Simplified validation of the ELISA kit determination of Microcystins in surface water.

The enzyme-linked immunosorbent assay (ELISA), as a universal method for the determination of Microcystins, is of great significance for the rapid detection of Microcystins pollution. This study aimed to propose a simplified validation method for Microcystins ELISA kit by summarizing related documents and guidelines. After summarizing and clarifying from 20 validation parameters, 11 parameters were selected to simplify the validation of Microcystins ELISA kit. In addition, the acceptable range and validation details of each parameter were analyzed. The results indicated that the coefficient of determination of the Microcystin-LR standard curve was higher than 0.99. The concentration of quality control samples was within control limits. The accuracy of spiked and proficient samples was within 70%-130%. The variability of intra-assay, inter-assay, and reproducibility was less than 11, 15 and 21%, respectively. The LOD and LLOQ were 0.002 µg/L and 0.05 µg/L, respectively. When the concentration of Microcystins exceeded 5 µg/L, it was recommended to dilute the samples to the working range before detection. The specificity was estimated with seven Microcystin analogues and three amino acids, indicating that the cross-reactivity was less than 30%. These results revealed that the ELISA kit was satisfactory for detecting Microcystins in water.

[Bioavailability of dissolved organic nitrogen components in the lake sediment to alage].

Samples in the sediments of Wuliangsuhai and Erhai Lake were selected, and the technologies of XAD-8 resins separation and three dimensional fluorescence excitation-emission matrix (3DEEM) spectra were applied, in order to study the bioavailability of dissolved organic nitrogen components to alage under the room cultivation. The obtained results showed that: (1) Average loss of DON and DOC from sediments was below 5% after dividing DON into different groups, which means the technology of XAD- 8 resins separation could be used in the study of DON components in the lake sediment. (2) Through 3DEEM analysis, hydrophilic and hydrophobic DON was dominated by protein-like and humic-like materials in the lake sediment, respectively. (3) Under the hydrophilic component condition, growth curves of algae tended to show an "S" shape with a straight upward trend in the sediments of Wuliangsuhai and Erhai lake,with the maximum algal density reaching 535.5 x 10(4) and 709.5 x 10(4) per milliliter, respectively. Meanwhile, the DON concentrations were significantly reduced after cultivation to 2.46 and 2.98 mg x L(-1), respectively, which suggests that hydrophilic DON in the lake sediment was the bioavailable organic nitrogen for alage. (4) Under the hydrophbic component conditions, growth curves of algae tended to show a "unimodal" shape in the sediments of Wuliangsuhai and Erhai lake,with the maximum algal density reaching 113.5 x 10(4) and 275.5 x 10(4) per milliliter,respectively. The DON concentrations were significantly reduced during the early cultivation period, and then kept stable in the late period, which suggests that the hydrophobic DON component bioavailable to alage was low in short-term, and the hydrophobic DON component had hardly any positive effect on the growth of algae.