Environmental filtering and species interaction shaped the phytoplankton community assembly and diversity in different regulation periods of river-type reservoir.

Abstract

Reservoir regulation significantly alters riverine habitats, profoundly influencing phytoplankton communities, key indicators of aquatic ecosystem health. However, the assembly mechanisms shaping them in large, heterogeneous river-type reservoirs remain understudied. This study investigated the phytoplankton community in Three Gorges Reservoir (TGR), a China's representative river-type reservoir, across flood (FLP) and impoundment (IMP) periods, using samples collecting, alpha diversity, neutral community modeling, Mantel's tests, and co-occurrence network analysis to disentangle the underlying ecological processes. Results revealed a pronounced successional shift, from Bacillariophyta dominating the FLP (66.77 % relative abundance) to Cyanophyta dominating the IMP (68.22 %), and tributary areas showed heightened susceptibility to Cyanophyta blooms, particularly during IMP. Community assembly was overwhelmingly governed by deterministic processes, with neutral models explaining less than 25 % of the variation, and their influence intensified during the more stable IMP. Ammonia nitrogen (NH₄⁺-N), and water temperature (WT) were identified as primary environmental factors driving these changes. Furthermore, network analysis revealed a contrasting pattern between complexity and stability, the FLP's network, despite being more complex (45 nodes, 205 links), was less robust due to its centralization around a few dominant hub species, rendering the fluctuating backwater area (FA) particularly sensitive to disturbance. In contrast, the IMP's network was simpler (14 nodes, 27 links) but more stable. This study quantifies the shifting phytoplankton community assembly mechanisms in complex river-type, and provides a mechanistic explanation for these dynamics. These findings offer a robust foundation for future aquatic ecological health assessments and protection strategies.