Effects of seasonal hydrological regulation of cascade dams on the functional diversity of zooplankton: Implications for the management of massive reservoirs and dams

• Cascade dams cause downstream water quality to deteriorate during the dry season. • Environmental factors and dams had significant effects on the functional diversity. • The number of dams affects water quality and functional diversity. • Suggest reducing the storage time and pollution source input during the dry season. The construction of cascade dams has led to a lack of longitudinal connectivity in rivers, fragmentation of habitats, slowing of the spread of biota, and loss of biodiversity. Here, we examined the zooplankton functional diversity in a typical river with cascade dams and compared the differences between the upstream and downstream parts of the river during the dry and wet seasons. The findings were as follows: (1) during the dry season, the nutrient load in the lower reaches of the river increased, and the water quality deteriorated, while the conditions during the wet season did not change significantly; (2) during the dry season, the functional beta diversity of downstream zooplankton caused by cascade dams was significantly ( p < 0.05) lower than that upstream, but this phenomenon was not prominent during the wet season; (3) during the dry season, environmental factors and dam numbers significantly affected the functional diversity of zooplankton, and the number of dams directly impacted the formation of the distribution pattern of zooplankton functional traits. These findings highlight the effects of the hydrological regulation of cascade dams in different seasons on the functional diversity of zooplankton and suggest that: 1) water retention times and pollution source inputs should be reduced during the dry season, 2) dedicated biological channels should be established to reduce the harm caused by clustered pollution to zooplankton, and 3) thermal pollution and strong scouring caused by large-scale flood discharge simultaneously should be avoided during the rainy season. Zooplankton functional diversity is sensitive to dam hydrological regulation, and we advise it be included as a critical indicator during ecosystem assessment in the dam management process, along with additional biological indicators are recommended.