Widespread dispersal and bio-accumulation of toxic microcystins in benthic marine ecosystems

Freshwater cyanobacteria produce toxic microcystins (MCs), which travel from freshwater areas into the sea. The MCs produced by cyanobacteria in a freshwater reservoir were discharged frequently into the adjacent Isahaya Bay, remained in the surface sediments, and then accumulated in various macrobenthic animals on the seafloor. The MCs were transported further outside of Isahaya Bay (Ariake Bay), and the median values of the MC contents in the sediments were in the same levels in both bays, while their temporal variations were also similar during the study period. Therefore, the fluctuations of the MC contents in the surface sediments were physically controlled by the timing of the discharge from the reservoir. The MC contents in polychaetes and oysters collected in Isahaya Bay increased markedly during winter. The median values of the carbon-based MC contents in the sediments, primary consumers, and secondary consumers in the bay were 87, 160, and 250 ngMC gC−1, respectively. These results demonstrated bio-accumulation at lower trophic levels in benthic marine ecosystems. An understanding of the processes occurring between sediments and macrobenthic animals is important for clarifying MC dynamics in ecosystems.