Water travel time shapes the biogeography of riverine phytoplankton under the influence of dam and tributary

Abstract Exploring the response of phytoplankton composition and diversity to water travel time (WTT) in rivers is crucial for understanding the biogeographic patterns of these organisms and for protecting watershed ecosystems. This study investigated the effects of WTT on phytoplankton biomass, community composition, and diversity by analyzing 182 samples collected over two seasons from the mainstems and tributaries of the Hanjiang and Danjiang River basins, China. Our study found that, in free‐flowing river reaches, phytoplankton biomass fluctuated and increased progressively from the headwaters to downstream areas along the river flow. Water travel time was the key factor driving phytoplankton biomass and diversity rather than water quality. As WTT increased, phytoplankton biomass, the proportion of Cyanophyta, and the theoretical maximum number of genera ( N ) increased, while the critical percentage of taxon extinction ( a ) and the evenness of the taxon distribution ( k ) decreased. In comparison, the Shannon index showed no distinct pattern of changes. In cascade dam‐controlled river reaches, dams and tributaries disrupted the longitudinal continuity of rivers. Both dam barriers and tributary inflow reduced the contribution rate of upstream phytoplankton communities to downstream reaches. Additionally, dams reduced downstream phytoplankton biomass by altering flow conditions, shifting the dominant control to water quality for phytoplankton community composition and diversity indices. In autumn, increased tributary flow into the mainstem reduced the contribution of upstream phytoplankton communities to downstream areas through a dilution effect. Overall, WTT plays a pivotal role in shaping the biogeography of phytoplankton, while cascade dams and tributaries disrupt the natural riverine longitudinal patterns.