Damming Exacerbates the Discontinuities of Phytoplankton in a Subtropical River in China

Phytoplankton are sensitive to changes in river ecosystems. However, the spatial impacts of dams on phytoplankton communities have not been well documented, and the relationships between environmental drivers and phytoplankton communities in different types of river reaches need to be further explored to manage dammed rivers. Here, we used multiple statistical analysis methods based on collected samples to reveal the effects of damming on the longitudinal continuity of phytoplankton by comparing those communities and their drivers in the natural background reaches, reservoirs and corresponding post-dam reaches in the Jiulong River of China. The result illustrated that overall, phytoplankton richness and abundance in the reservoirs were higher than those in the natural background reaches and the corresponding downstream reaches, and the longer the reservoir water was retained, the more noticeable those differences were. Throughout the year, the relative abundance in the natural background reaches was dominated by Cyanophyta and diatoms, in the reservoirs by Cyanophyta and Chlorophyta, and in the post-dam reaches by diatoms. β-diversity between the natural background reaches and the reservoirs increased due to species replacement, while the β-diversity between the reservoirs and the downstream reaches mainly consisted of species richness. In natural background reaches, water environmental factors and hydraulic parameters were the main drivers of phytoplankton community variability, whereas in reservoirs the importance of hydraulic parameters and nutrients was greatly increased. Our findings reveal that damming exacerbates longitudinal discontinuities in phytoplankton communities and in their drivers. Management measures should focus on the main factors driving the phytoplankton communities in different river reaches such as reducing water retention time and limiting nutrient inputs around reservoirs to minimize the risk of algal blooms