Influence of suspended particulate matter input on phytoplankton community structure in estuarine environments

Riverine suspended particulate matter (SPM) inputs play a crucial role in regulating phytoplankton sedimentation and stability in estuarine ecosystems, thereby mediating organic matter cycling. However, our understanding of how the physicochemical properties of SPM influence the short-term response mechanisms of phytoplankton remains insufficient. This study focuses on the sedimentation phase following SPM input and employs a controlled microcosm experimental system. Through short-term sedimentation experiments, it thoroughly investigates the effects of SPM particle size and surface charge on the sedimentation behavior and community composition of marine phytoplankton. The results show that SPM sedimentation substantially decreased phytoplankton biomass in seawater, with sedimentation rates increasing significantly as SPM particle size decreased. Surface charge also strongly enhanced phytoplankton sedimentation. In terms of community composition, SPM markedly altered phytoplankton structure by significantly reducing the relative abundance of Bacillariophyta and Dinophyta while increasing that of Heterokontophyta. Among the examined factors, SPM particle size emerged as a primary driver of these changes. Additionally, SPM inputs increased the relative abundance of microalgae-associated symbiotic bacteria, and combined with the changes in community composition and the results of co-occurrence network analysis, it is hypothesized that the enriched microalgae-associated bacteria may form potential ecological associations with phytoplankton under SPM disturbance conditions. Overall, this study provides new insights into the short-term responses of marine phytoplankton to riverine SPM input during the initial sedimentation phase, and offers preliminary mechanistic references for understanding particulate matter-driven plankton dynamics in estuarine systems.