Journal of Hydroecology

Cascade reservoir construction significantly alters river hydrological regimes and nutrient transport processes. However， the quantitative responses of phytoplankton communities to nitrogen (N) and phosphorus (P) under such development remain poorly understood. To investigate the impact mechanisms， this study conducted six field sampling campaigns from 2021 to 2023 in the upper reaches of the Yangtze River mainstem， from Zhimenda to Zhutuo. Using Threshold Indicator Taxa Analysis (TITAN)， the response thresholds of phytoplankton communities to total nitrogen (TN) and total phosphorus (TP) were compared， and indicator taxa were identified across three distinct river sections: an upper natural reach， a middle cascade reservoir reach， and a lower flowing reach. The results showed significant differences in community structure among the sections， with TN and TP identified as key driving factors. The upper natural reach exhibited the lowest TN (0.659–0.737 mg/L) and TP (0.011–0.017 mg/L) ecological thresholds， with indicator taxa dominated by clean-water diatoms (e.g.， Ceratoneis， Diatoma)， indicating high sensitivity to nutrient inputs. The middle cascade reservoir reach displayed the narrowest threshold ranges (TN: 0.702–0.709 mg/L; TP: 0.018–0.019 mg/L)， suggesting a highly concentrated community response. Indicator taxa here were differentiated， including positive responders like Coelastrum and negative responders like Asterionella， indicating a critical transitional state of the community. The lower flowing reach showed a divergence in nitrogen thresholds (negative: 1.165 mg/L; positive: 1.385 mg/L) with phosphorus thresholds similar to the middle reach (TP: 0.019–0.020 mg/L). Eutrophic taxa such as Pseudanabaena and Cryptomonas erosa emerged as positive indicators. Mechanistic analysis suggests that cascade development restructures habitat heterogeneity by altering hydrodynamics (slowed flow velocity， increased water residence time) and nutrient cycling (retention effects， internal nutrient loading). This not only screens for different species compositions but also drives shifts in species adaptive strategies， as evidenced by the reversed response direction of certain taxa like Nitzschia across reaches. This study provides a quantitative basis and mechanistic insights for nutrient management and ecological risk assessment in rivers under cascade development.