Impacts of hydraulic engineering construction on the aquatic environment and ecosystem of Nansi Lake (Shandong Province, China)

Nansi Lake, located in the southwestern region of Shandong Province, China, consists of four interconnected lakes—Nanyang, Dushan, Zhaoyang, and Weishan—and functions as an essential water conveyance and storage reservoir for the East Route of the South-to-North Water Diversion Project. The construction of the Erji Dam hydraulic complex between 1958 and 1973 resulted in the division of the lake into two distinct sub-lakes: Shangji Lake (upper lake) and Xiaji Lake (lower lake). This development significantly altered the hydrological connectivity and ecological conditions of the area. This research examines the effects of hydraulic engineering on the aquatic environment and ecosystem of Nansi Lake by analyzing water quality parameters and sedimentary diatom communities, with an emphasis on spatial variations between the northern and southern regions. The results demonstrate that Nansi Lake exhibits mild pollution, characterized by mesotrophic to eutrophic conditions. Sediments primarily consist of clay and silt, with minimal sand content, while organic matter originates from both autochthonous and allochthonous sources. A total of 181 diatom species from 37 genera were identified, revealing distinct dominant taxa between Shangji Lake and Xiaji Lake. Redundancy analysis (RDA) indicated that total nitrogen (TN) and Ca2+ were the primary factors influencing diatom communities in Shangji Lake, whereas total organic carbon (TOC) and dissolved oxygen (DO) predominantly governed those in Xiaji Lake. The research indicates that the water quality, nutrient levels, sedimentation patterns, and diatom distributions of Nansi Lake demonstrate considerable spatial heterogeneity, primarily influenced by anthropogenic activities such as hydraulic engineering and aquaculture, leading to marked variations between the northern and southern regions.