Phytoplankton similarity indices as ecological metrics in a fragmented river system

Abstract This study investigated the impacts of dam-induced river fragmentation on phytoplankton community dynamics in the East River (Dongjiang River), a tributary of the Pearl River Basin in China. Phytoplankton communities were categorized into four taxonomic groups: ‘total community composition’, ‘ Bacillariophyta ’, ‘ Chlorophyta ’ and ‘other algal groups’. Beta diversity patterns across seasons and spatial gradients were quantified using Jaccard and Bray–Curtis indices. Results revealed that dam construction significantly altered phytoplankton community structures, with the most pronounced changes observed in the ‘other algal groups’—a category often overlooked in ecological assessments. While spatial distance exhibited weak correlations with community similarity, environmental factors such as phosphorus (total phosphorus, phosphate) and nitrogen levels, alongside light availability, emerged as critical drivers shaping phytoplankton distribution. Notably, non-dominant algal groups (‘other algal groups’) demonstrated superior predictive power for environmental parameters compared to dominant taxa or the total community. These findings highlight the ecological significance of rare or less-abundant species in fragmented river systems, underscoring their potential as sensitive indicators for monitoring anthropogenic impacts. This study provides novel insights into the complex interactions between hydrological alterations and microbial community responses, emphasizing the need to incorporate non-dominant taxa into water quality assessment frameworks.