Constructed wetlands can play an active role in improving the water quality of urban rivers. In this study, a sequential series system of the floating-bed constructed wetland (FBCW), horizontal subsurface flow constructed wetland (HSFCW), and surface flow constructed wetland (SFCW) were constructed for the urban river treatment in the cold regions of North China, which gave full play to the combined advantages. In the Yitong River, the designed capacity and the hydraulic loading of the system was 100 m³/d and 0.10 m³/m²d, respectively. The hydraulic retention time was approximately 72 h. The monitoring results, from April to October in 2016, showed the multiple wetland ecosystem could effectively remove chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N), total nitrogen (TN), total phosphate (TP), and suspended solids (SS) at average removal rates of 74.79%, 80.90%, 71.12%, 78.44%, and 91.90%, respectively. The removal rate of SS in floating-bed wetland was the largest among all the indicators (80.24%), which could prevent the block of sub-surface flow wetland effectively. The sub-surface flow wetland could remove the NH₄-N, TN, and TP effectively, and the contribution rates were 79.20%, 64.64%, and 81.71%, respectively. The surface flow wetland could further purify the TN and the removal rate of TN could reach 23%. The total investment of this ecological engineering was $12,000. The construction cost and the operation cost were $120 and $0.02 per ton of polluted water, which was about 1/3 to 1/5 and 1/6 to 1/3 of the conventional sewage treatment, respectively. The results of this study provide a technical demonstration of the restoration of polluted water in urban rivers in northern China.

Hydropower is a clean and renewable energy source, and cascade hydropower stations have been developed to enhance water energy utilization efficiency. While small hydropower stations have a smaller scale and environmental impact compared to large ones, the cumulative effects of cascade development on river ecosystems should not be overlooked. In this study, flow cytometry was used to classify picoplankton from water samples collected at four small cascade hydropower stations on a Pearl River tributary into six microbial groups: Virus, LNA (Low Nucleic Acid), HNA (High Nucleic Acid), Cyanobacteria, Algae, and Fungi. Four ecological assessment indices were calculated: Photosynthetic Autotrophic Capacity (PAC), Bacterial Activity Index (BAI), Virus Regulatory Capacity (VRC), and Fungal Metabolic Capacity (FMC). By analyzing trends in microbial abundance and ecological indices and their correlations with environmental factors, the results showed that along the small cascade hydropower stations, dissolved oxygen (DO) and electrical conductivity (EC) increased from 5.71 mg/L and 49.87 μS/cm upstream to 6.80 mg/L and 56.18 μS/cm downstream, respectively. In contrast, oxidation-reduction potential (ORP) and total organic carbon (TOC) concentrations decreased from 3.81 mV and 1.59 mg/L to −8.05 mV and 1.08 mg/L, respectively. Among the microbial groups, the abundance of Virus, LNA, and Fungi decreased by 30.9%, 30.5%, and 34.9%, respectively, along the cascade system. EC, TOC, and NO₃⁻-N were identified as key drivers of changes in the abundance of the Virus, LNA, and Fungi groups. The concentrations of carbon and nitrogen nutrients significantly influenced the ecological assessment indices. Cascade hydropower stations had a significant impact on PAC, BAI, and VRC, while their influence on FMC was relatively small. The VRC showed a decreasing trend, suggesting a weakening effect of the stations on VRC. This study offers new perspectives and methods that facilitate the rapid and quantitative assessment of the ecological impacts of cascade hydropower stations.

This study aimed to evaluate which environmental factors and genetic groups were important in explaining measured activity of Ammonia-oxidizing archaea (AOA) and bacteria (AOB), which play important roles in global nitrogen cycle, providing a new insight into the mechanism of archaeal and bacterial ammonia oxidation. We sampled 62 soil/sediment samples from coastal wetlands of the Bohai area of China and assessed the abundance of functional genes involved in the nitrogen cycle, soil/sediment characteristics and the potential activity of AOA (PAOA) and AOB (PAOB) using specific inhibitors. At last, we introduced the structure equation model (SEM) to infer direct and indirect effects of variables on potential activities. The results indicated that the change in AOA-amoA gene abundance may be more independent, while AOB-amoA was closely associated with the change in abundance of amx and denitrifier. PAOA was mainly defined by AOA-amoA abundance and partially influenced by the norA gene, suggesting coupling of archaeal ammonia oxidation with nitrite oxidation. PAOB was significantly defined by the abundance of amx and denitrifier, indirectly mediated by AOB-amoA. The activity of AOA seemed to be more independent of other microbial activities, while the activity of AOB varied closely with fluctuations of other microbial species. PAOA was mediated directly by the C/N ratio and indirectly by nitrite concentration and TOC value, while PAOB was mediated directly by ammonium concentration and TOC value and indirectly by C/N ratio. The activity of AOB may be determined by several other functional gene groups and had little correlation with AOB abundance while the activity of AOA was mostly controlled by itself.