Short-term dynamics of cyanobacterial toxins (microcystins) following a discharge from a coastal reservoir in Isahaya Bay, Japan

•Toxic cyanobacteria have been bloomed in the reservoir of Isahaya Bay, Japan.•Microcystins produced by toxic cyanobacteria has been discharged into the bay.•We conducted the field monitoring in the bay just after the discharge.•The microcystins were detected in the seawater, and were deposited on the sea floor.•They were moved away from the reservoir, and spread throughout the coastal area. Freshwater cyanobacteria produce highly toxic substances such as microcystins (MCs), and water containing MCs is often discharged to downstream and coastal areas. We conducted field monitoring in Isahaya Bay to clarify the short-term dynamics of MCs discharged from a reservoir following a cyanobacteria bloom in the warm season. MCs were detected in the seawater of the bay (max. 0.10μgL−1), and were deposited on the sea floor, with the MC content of the surface sediment increasing by approximately five times (0.11±0.077–0.53±0.15μgkgww−1, mean±SD) at the four stations near the reservoir drainage gate before and after the discharge. The MCs was then transported from the mouth of the bay by tidal currents during the period of the study. Therefore, the MCs were moved away from the closed water area where the cyanobacteria blooms, and spread throughout the coastal area.