Spatial heterogeneity of tributary ecosystems in a channel-type reservoir: Linking physicochemistry to phytoplankton functional groups

River damming has altered riverine biogeochemical cycles, yet we still lack a mechanistic understanding of how modified hydrodynamic conditions reshape mainstream-tributary ecosystems, particularly the coupling between physicochemistry processes and phytoplankton functional groups in channel-type reservoirs. Here, through high-frequency sampling and multidimensional analysis of the Xiangjiaba Reservoir in the upper Yangtze River, China, we explored spatial heterogeneity of tributary ecosystems and its underlying mechanisms. Distinct spatial patterns emerged - while maintaining strong connectivity between mainstream and tributaries (connectivity index: 0.85), the inter-tributary connectivity remained notably weak (0.26-0.34). Intriguingly, adjacent tributaries (Xining and Zhongdu Rivers) developed markedly different ecological characteristics, whereas geographically distant tributaries (Zhongdu and Dawenxi Rivers) displayed unexpected ecological convergence, challenging conventional spatial distance-ecological similarity paradigms. This spatial heterogeneity was reflected in both biogeochemical processes and phytoplankton functional groups: restricted water exchange in tributaries may promote nutrient accumulation (TN: 1.35-1.45 mg/L), leading to distinct shifts in functional group composition (ρ = 0.574, p < 0.001). We identified a critical threshold in relative water column stability (RWCS = 5.111/m) beyond which bloom-forming functional groups became dominant. Temporal analysis revealed synchronized patterns where tributary algal biomass peaked when system connectivity reached its minimum (0.30) in May, highlighting the cascading effects from hydrodynamics to ecosystem functions. These findings provide fresh perspectives on tributary ecosystem heterogeneity in regulated rivers, with important implications for reservoir management under global change.