River regulation effects on zooplankton community assembly: Implications for arid river restoration and conservation

River flow reduction and biodegradation are increasingly threatening ecological processes and the health of rivers. Water replenishment is frequently applied to drought-stricken rivers to conserve biodiversity and restore ecosystems. To evaluate and improve the effects of different water supply regimes on aquatic environments and organisms, we investigated the species composition and community structure of zooplankton in 2020, following water replenishment in the dried section of the Yongding River in Beijing. Slow base flow of water and flow break were trialed in the mountainous and plain river sections, respectively, during the flooding season (June-August), while short-time pulse high flow was applied outside the flooding season. The results indicated a low-density but stable zooplankton community structure, as well as low nutrient loads with water purification outputs in the flooding season at the mountainous sites. Meanwhile, the absence of flow at plain sites drove isolated lakes or pools into rapid eutrophication and concurrent outbreaks of pollution-indicator rotifers. Compared with the disturbances of high flow, continuous base flow was effective in fostering a river system with lower nutrient loads and higher biostability. Our findings support the importance of water-saving base flows for maintaining aquatic habitats and associated biota, whereas high flows can satisfy the nutrient diffusion and special life activities of aquatic organisms in arid river management.