Assessing the Ecosystem Health of Large Drinking-Water Reservoirs Based on the Phytoplankton Index of Biotic Integrity (P-IBI): A Case Study of Danjiangkou Reservoir

As an important component of reservoir ecosystems, phytoplankton is often used as an indicator to assess the health of water ecosystems such as lakes and reservoirs. The exploration the phytoplankton index of biotic integrity (P-IBI) has been proposed to assess the ecological health of the large drinking-water reservoirs. This study investigated phytoplankton communities and environmental variables at 19 sampling sites in the Danjiangkou Reservoir from October 2019 to July 2021. Results showed that 170 species of phytoplankton from 9 phyla were detected in Danjiangkou Reservoir, and the total density varied from 0.61 x 10(5) to 36.64 x 10(5) cells/L, with the mean value of 8.83 x 10(5) cells/L. The P-IBI values were higher in winter and lower in spring in terms of time, and the spatial trend of P-IBI values from high to low was outlet of the reservoir > entrance of Dan Reservoir > entrance of Han Reservoir > Han Reservoir > Dan Reservoir. Linear regression analysis showed that the evaluation results of P-IBI and the comprehensive trophic level index (TLI) evaluation were generally consistent. Redundancy analysis (RDA) showed significant correlations between P-IBI and candidate indicators and major environmental factors with significant differences between seasons. The P-IBI is an effective tool to evaluate the ecological health of large drinking-water reservoirs and could provide some scientific reference for the ecological health assessment of large drinking-water reservoirs.