Environmental DNA metabarcoding revealing the distinct responses of phytoplankton and zooplankton to cascade dams along a river-way

Despite providing significant assistance to human society, cascade dams can also have negative impacts on river ecosystems. As the crucial components of river ecosystem, the responses of phytoplankton and zooplankton to cascade dams have rarely been studied simultaneously, and thus, lacking the understanding of the difference in succession between them. Here, we investigated the phytoplankton and zooplankton communities in a river-way with cascade dams using an environmental DNA metabarcoding technology. Along the reservoir areas separated by dams, we found an obvious downward trend in diversity of plankton communities with significant variations in their compositions. The relative abundances of Bacillariophyta and Chlorophyta continued to decrease while Intramacronucleata increased along the river-way. Results of association analyses recognized temperature and flow rate as potential factors resiling the impacts of cascade dams. Beta diversity decomposition indicated species replacement as the main mechanism for variations in plankton communities with higher contribution for phytoplankton. Additionally, we detected wider environmental adaptation (broader environmental breadth, phylogenetic single, and niche breadth) and stronger dispersal ability in phytoplankton than in zooplankton. Environmental variables showed a stronger effect for variations in phytoplankton than zooplankton. Furthermore, we observed that community assembly of phytoplankton and zooplankton was, based on the null model, by heterogeneous selection and drift, respectively. These results suggested differences in phytoplankton and zooplankton response to cascade dams and highlighted the stronger environmental filtering in phytoplankton.