Microbial community structure and denitrification responses to cascade low-head dams and their contribution to eutrophication in urban rivers.

Low-head dams are one of the most common hydraulic facilities, yet they often fragment rivers, leading to profound changes in aquatic biodiversity and river eutrophication levels. Systematic assessments of river ecosystem structure and functions, and their contribution to eutrophication, are however lacking, especially for urban rivers where low-head dams prevail. In this study, we address this gap with a field survey on microbial community structure and ecosystem function, in combination with hydrological, environmental and ecological factors. Our findings revealed that microbial communities showed significant differences among the cascade impoundments, which may be due to the environment heterogeneity resulting from the cascade low-head dams. The alternating lentic-lotic flow environment created by the low-head dams caused nutrient accumulation in the cascade impoundments, enhancing environmental sorting and interspecific competition relationships, and thus possibly contributing to the reduction in sediment denitrification function. Decreased denitrification led to excessive accumulation of nutrients, which may have aggravated river eutrophication. In addition, structural equation model analysis showed that flow velocity may be the key controlling factor for river eutrophication. Therefore, in the construction of river flood control and water storage systems, the location, type and water storage capacity of low-head dams should be fully considered to optimize the hydrodynamic conditions of rivers. To summarize, our findings revealed the cumulative effects of cascade low-head dams in an urban river, and provided new insights into the trade-off between construction and decommissioning of low-head dams in urban river systems.