Deciphering the effects of cascade reservoirs on phytoplankton communities in a karst river: Importance of super-large reservoirs

Abstract

Rapid dam construction is fragmenting river networks and altering phytoplankton communities, yet how dense cascades of different-sized reservoirs in karst rivers affect ecosystems remains poorly quantified. We investigated how reservoir scale and associated hydraulic conditions regulate phytoplankton succession in the upper-middle Wujiang River, a densely dammed karst region in China. A total of 100 sampling stations were surveyed across wet and dry seasons, with phytoplankton diversity and community-environment relationships were quantified using multivariate analyses. Phytoplankton density was higher in the wet than in the dry season (5.2 × 10⁹ vs. 4.1 × 10⁸ cells/L), with Cyanophyta dominating lentic reservoir sections and Bacillariophyta dominating lotic river sections in the dry season. In riverine stations, community composition was primarily shaped by physicochemical variables and hydraulic conditions, whereas in reservoirs hydraulic attributes (storage capacity, hydraulic retention time and age) explained the largest fraction of community variation (11 %), exceeding the contributions of nutrients (6 %) and other environmental factors. Super-large reservoirs (storage capacity >1 km³) and reservoir chains with combined capacity >1 km³ promoted pronounced convergence between in-reservoir and surrounding riverine communities, reduced β-diversity, and enhanced basin-scale biotic homogenization relative to medium-large reservoirs. These findings show that reservoir scale, through its hydraulic attributes, is a major control on phytoplankton communities and the associated ecological effects of cascade damming, and should inform ecological risk assessment and sustainable management of densely dammed karst systems.