Phytoplankton assemblage responses to massive freshwater inputs and anthropogenic toxic substances contamination in the Geum River Estuary, South Korea

This study investigated the relationships between phytoplankton assemblages and water contamination by persistent toxic substances (PTSs) and nutrients in an estuary with an artificial dam over one year. The distribution of PTSs, including 15 polycyclic aromatic hydrocarbons, 6 alkylphenols, and 8 metal(loid)s, along with nutrients, exhibited relatively high concentrations with irregular temporal fluctuations in the inner estuary. During winter and spring, phytoplankton communities showed good ecological quality, with an average of 28 species and a density of 1750 cells L−1. In contrast, during summer, there was a significant increase in the density of freshwater species (max 45,000 cells L−1). These assemblages were categorized into three seasonal groups, featuring dominant taxa like blue-green algae and diatoms. Temperature and nutrient levels were the principal factors influencing phytoplankton assemblages, while PTSs had a minor impact. Overall, phytoplankton assemblages displayed strong seasonal variation, mainly influenced by freshwater input and nutrient availability. [Display omitted] •Phytoplankton assemblage responses to water contamination were investigated.•Persistent toxic substances (PTSs) and nutrients were relatively high in the inner estuary.•Plankton assemblages were divided by season, winter–spring and summer–autumn.•A great density of freshwater species was introduced during the summer monsoon.•Temperature, silicate, and phosphate were major factors, with minimal impact by PTSs.