Remote estimates of CDOM using Sentinel-2 remote sensing data in reservoirs with different trophic states across China.

Chromophoric dissolved organic matter (DOM) is called as CDOM which could affect the optical properties of surface waters, and is a useful parameter for monitoring complex inland aquatic systems. Large-scale monitoring of CDOM using remote-sensing has been a challenge due to the poor transferability of CDOM retrieval models across regions. To overcome these difficulties, a study is conducted using Sentinel-2 images, in situ reflectance spectral data, and water chemical parameters at 93 water reservoirs across China classified by trophic state. Empirical algorithms are established between CDOM absorption coefficient aCDOM(355) and reflectance band ratio (B5/B2,vegetation Red Edge/Blue) acquired in situ and via Sentinel-2 MSI sensors. Relationships are stronger (r2 > 0.7, p < 0.05) when analysis is conducted separately by trophic states. Validation models show that, by accounting for trophic state of reservoirs and using B5/B2 band ratios, it is possible to expand the geographical range of remote sensing-based models to determine CDOM. However, the accuracy of model validation decreased from oligotrophic (r2: 0.86) to eutrophic reservoirs (r2: 0.82), likely due to increased complexity of CDOM sources in nutrient-rich systems. This study provides a strategy for using local and remote-sensing data to monitor the spatial variations of CDOM in reservoirs based on different trophic states, and will contribute to water resources management.

Influence of vegetation type and temperature on the performance of constructed wetlands for nutrient removal.

In this study, the influence of vegetation type and environmental temperature on performance of constructed wetlands (CWs) was investigated. Results of vegetation types indicated that the removal of most nutrients in polyculture was greater than those in monoculture and unplanted control. The greatest removal percentages of NH4+-N, total nitrogen (TN) and total phosphorus (TP) in polyculture were 98.7%, 98.5%, and 92.6%, respectively. In experiments of different temperatures, the removal percentages of NH4+-N, NO3--N, TN and TP in all CWs tended to decrease with the decline of temperature. Especially, a sharp decline in the removal percentages of NO3--N (decreased by above 13.8%) and TN (decreased by above 7.9%) of all CWs was observed at low temperature (average temperature of 8.9 °C). Overall, the performance of CWs was obviously influenced by temperature, and the polyculture still showed best performance in the removal of nitrogen when the average temperature dropped to 19.8 °C. Additionally, the variations of urease activities in rhizosphere soil tended to decrease with the decreasing temperature. Overall, a substantial enhancement for nitrogen and TP removal in polyculture (Canna indica + Lythrum salicaria) was observed. In conclusion, CW cultivated with polyculture was a good strategy for enhancing nutrient removal when temperature was above 19.8 °C.

Remote sensing of fluorescent humification levels and its potential environmental linkages in lakes across China.

The pollution or eutrophication affected by dissolved organic matter (DOM) composition and sources of inland waters had attracted concerns from the public and government in China. Combined with remote sensing techniques, the fluorescent DOM (FDOM) parameters accounted for the important part of optical constituent as chromophoric dissolved organic matter (CDOM) was a useful tool to trace relative DOM sources and assess the trophic states for large-scale regions comprehensively and timely. Here, the objective of this research is to calibrate and validate a general model based on Landsat 8 OLI product embedded in Google Earth Engine (GEE) for deriving humification index (HIX) based on EEMs in lakes across China. The Landsat surface reflectance was matched with 1150 pairs fieldtrip samples and the nine sensitive spectral variables with good correlation with HIX were selected as the inputs in machine learning methods. The calibration of XGBoost model (R2 = 0.86, RMSE = 0.29) outperformed other models. Our results indicated that the entire dataset of HIX has a strong association with Landsat reflectance, yielding low root mean square error between measured and predicted HIX (R2 = 0.81, RMSE = 0.42) for lakes in China. Finally, the optimal XGBoost model was used to calculate the spatial distribution of HIX of 2015 and 2020 in typical lakes selected from the Report on the State of the Ecology and Environment in China. The significant decreasing of HIX from 2015 to 2020 with trophic states showed positive control of humification level of lakes based on the published document of Action plan for prevention and control of water pollution in 2015 of China. The calibrated model would greatly facilitate FDOM monitoring in lakes, and provide indicators for relative DOM sources to evaluate the impact of water protection measures or human disturbance effect from Covid-19 lockdown, and offer the government supervision to improve the water quality management for lake ecosystems.

Seasonal Variation Characteristics and the Factors Affecting Plankton Community Structure in the Yitong River, China.

To explore how environmental factors affected the plankton structure in the Yitong River, we surveyed the water environmental factors and plankton population in different seasons. The results showed high total nitrogen concentrations in Yitong River throughout the year, while the total phosphorus, water temperature (WT), and chemical oxygen demand in summer were significantly higher than those in other seasons (p < 0.05), and the dissolved oxygen (DO) concentrations and TN/TP ratio were significantly lower (p < 0.01) than those in other seasons. There was no significant seasonal change in other environmental factors. Cyanophyta, Chlorophyta, and Bacillariophyta were the main phytoplankton phylum, while Protozoa and Rotifera were the main zooplankton phylum. The abundance and biomass of zooplankton and phytoplankton in the summer were higher than those in other seasons. Non-Metric Multidimensional scaling methods demonstrated obvious seasonal variation of phytoplankton in summer compared to spring and winter, while the seasonal variation of the zooplankton community was not obvious. The results of the redundancy analysis showed that WT, DO and nitrate nitrogen were the main environmental factors affecting phytoplankton abundance. In contrast to environmental factors, phytoplankton was the main factor driving the seasonal variation of the zooplankton community structure. Cyanophyta were positively correlated with the changes in the plankton community.

Removal of chlorpyrifos and its hydrolytic metabolite in microcosm-scale constructed wetlands under soda saline-alkaline condition: Mass balance and intensification strategies.

Chlorpyrifos (CP) is a typical organophosphorus insecticide, which poses serious threats to the natural environment and human health. Strategies for the fast elimination of CP and its toxic hydrolytic metabolite 3,5,6-trichloro-2(1H)-pyridianol (TCP) in drainage water are urgently needed. The fate of CP and TCP in microcosm-scale subsurface batch constructed wetlands (SSBCWs) was quantified with different macrophyte species under soda saline-alkaline (SSA) condition and effective intensification strategies were developed. The macrophyte species Canna indica outperformed Phragmites australis and Typha orientalis for CP and TCP removal in SSBCWs. Mass balance calculation indicates the fate of CP in SSBCWs was residue in water (≤8%), alkaline hydrolysis (18.93-57.42%), microbial degradation (37.75-61.91%), substrate adsorption (~4-14%), and macrophyte uptake (≤3%). The addition of ferric-carbon (Fe-C) as a substrate amendment in SSBCWs increased the CP removal percentage by 35% and reduced the effluent TCP concentration by ~70% during Day 1-4 on average compared with the unintensified control. Fe-C addition simplified the microbial community diversity, while increasing the relative abundance of Proteobacteria which tolerates the microelectrolytic environment. A single application of liquid microbial agent improved CP removal percentage by 84% and decreased the effluent TCP concentration by two orders of magnitude during Day 1-4. The hydraulic retention time for thorough removal of TCP reduced from over 8 d to 4 d. Although only two dominant microbial genera (i.e., Sphingomonas and Pseudomonas) adapted to the environment with CP and SSA, they accelerated CP and TCP degradation via their own metabolism and co-metabolism with other indigenous microorganisms.

Constructed wetlands integrated with microbial fuel cells for COD and nitrogen removal affected by plant and circuit operation mode.

Organic matter and NH4+-N are two major pollutants in domestic sewage. This study evaluated the influence of plant and circuit operation mode on the performance of constructed wetlands integrated with microbial fuel cells (CW-MFCs) and investigated the removal mechanisms of organic matter and nitrogen. Better chemical oxygen demand (COD) removal was achieved in closed-circuit CW-MFCs regardless of planting or not, with average removal efficiencies of 83.19-86.28% (closed-circuit CW-MFCs) and 76.54-83.19% (open-circuit CW-MFCs), respectively. More than 70% organic matter was removed in the anaerobic region of all CW-MFCs. In addition, the planted CW-MFCs outperformed the unplanted CW-MFCs in ammonium, nitrate, and total nitrogen removal irrespective of circuit connection or not, for example, the NH4+-N removal efficiencies of 95.91-96.82% were achieved in planted CW-MFCs compared with 56.54-59.95% achieved by unplanted CW-MFCs. Besides, 33.14-55.69% of NH4+-N was removed in the anaerobic region. Throughout the experiment, the average voltages of planted and unplanted CW-MFCs were 264 mV and 108 mV, with the corresponding maximum voltage output of 544 mV and 321 mV, respectively. Furthermore, planted CW-MFCs, simultaneously producing a peak power density of 92.05 mW m-3 with a coulombic efficiency of 0.50%, exhibited better than unplanted CW-MFCs (3.29 mW m-3 and 0.21%, respectively) in bioelectricity generation characteristics. Graphical abstract.

Identification and denitrification characteristics of a salt-tolerant denitrifying bacterium Pannonibacter phragmitetus F1.

A salt-tolerant denitrifying bacterium F1 was isolated in this study, which has high nitrite (NO2--N) and nitrate (NO3--N) removal abilities. The salt tolerance capacity of strain F1 was further verified and the effects of initial pH, initial NaNO2 concentration and inoculation size on the denitrification capacity of strain F1 under saline conditions were evaluated. Strain F1 was identified as Pannonibacter phragmitetus and named Pannonibacter phragmitetus F1. This strain can tolerate NaCl concentrations up to 70 g/L, and its most efficient denitrification capacity was observed at NaCl concentrations of 0-10 g/L. Under non-saline condition, the removal percentages of NO2--N and NO3--N by strain Pannonibacter phragmitetus F1 at pH of 10 and inoculation size of 5% were 100% and 83%, respectively, after cultivation for 5 days. Gas generation was observed during the cultivation, indicating that an efficient denitrification performance was achieved. When pH was 10 and the inoculation size was 5%, both the highest removal percentages of NO2--N (99%) and NO3--N (95%) by strain Pannonibacter phragmitetus F1 were observed at NaCl concentration of 10 g/L. When the NaCl concentration was 10 g/L, strain Pannonibacter phragmitetus F1 can adapt to a wide range of neutral and alkaline environments (pH of 7-10) and is highly tolerant of NaNO2 concentration (0.4-1.6 g/L). In conclusion, strain Pannonibacter phragmitetus F1 has a great potential to be applied in the treatment of saline wastewater containing high nitrogen concentrations, e.g. coastal aquaculture wastewater.

Removal of chlorpyrifos and its hydrolytic metabolite 3,5,6-trichloro-2-pyridinol in constructed wetland mesocosms under soda saline-alkaline conditions: Effectiveness and influencing factors.

Chlorpyrifos (CP) is frequently detected in agricultural effluent worldwide. Both CP and its hydrolytic metabolite 3,5,6-trichloro-2-pyridinol (TCP) can cause serious environment hazards, and require removal before discharged into rivers and/or lakes. The effectiveness and main influencing factors of CP and TCP removal in mesocosm-scale subsurface flow constructed wetlands (SSFCWs) were evaluated. Results indicated that CP in SSFCWs reduced to less than detection limit in 4 d and TCP to 2 µg L-1 in 8 d. Higher influent CP concentrations lengthened the degradation process for both CP and TCP. The presence of co-existing inorganic nutrients restrained the degradation of CP during the hydraulic retention time of 2 h to 2 d. A higher pH resulting from the deterioration of soda saline-alkaline level accelerated the degradation of CP through the hydrolysis process. The SSFCWs with slag operating for another 88 d (i.e., 11 trails with HRT of 8 d for each trial) revealed a better and more stable treatment performance compared with previous studies. The results of this study demonstrated the positive feasibility of using SSFCWs with slag for the decontamination of CP-associated agricultural drainage or stormwater runoff.