Distribution of microcystins in environmental multimedia and their bioaccumulation characteristics in marine benthic organisms in the Geum River Estuary, South Korea

Highlights: • Concentrations of MCs in water were strongly related to cyanobacterial density and biomass. • MC-LR was the most abundant MC variant in environmental multimedia and organisms. • Water-SPM distribution coefficients of MCs decreased from inner to outer estuary areas. • The half-life of dissolved MCs in the estuary was longer than that of particulate MCs. • Species-specific bioaccumulation of MCs occurred in tidal flat organisms. Spatio-temporal distributions and bioaccumulation characteristics of freshwater cyanobacterial toxins, such as microcystins (MCs) in the Geum River Estuary, South Korea, were investigated during summer. Environmental multimedia samples (water, suspended particulate matter (SPM), and sediments) and tidal flat organisms (polychaetes, decapods, amphipods, and bivalves) were collected from regions inside and outside of the estuary dam for MCs analysis. Phytoplankton communities in the Geum River (freshwater) and estuarine area (brackish water) were also analyzed in order to understand the relationship with MCs concentrations. Seasonal variation in the structure of phytoplankton communities was detected in the Geum River, with a relatively high density of Cyanophyta in summer. MC concentrations were strongly correlated to water temperature, chlorophyll a, and cyanobacterial density. MC-LR was the most abundant MC variants in environmental samples. Dissolved MCs remained for longer periods and were more widely distributed in the coastal environments compared to particulate MCs. The distribution coefficients between water and SPM (K_d-SPM) and between water and sediments (K_d-sediment) of MCs showed that the phase shift of MCs in the environmental samples occurred in the estuary. K_d-SPM declined from the inside to outside regions of the estuary dam, and was mainly attributed to differences in the half-lives of MCs in dissolved (4.7 d for MC-LR) and particulate phases (0.44–0.52 d for MC-LR). Species-specific bioaccumulation of MCs occurred in tidal flat organisms, with relatively high bioaccumulation factors of MCs being detected in polychaetes and decapods compared to amphipods and bivalves. Overall, this study advances our understanding on the distribution, transport, fate, and bioaccumulation of MCs in estuarine and coastal environments.