WIDE-AREA TRANSPORT OF Chattonella RED TIDE FROM INLAND AREA OF HARIMA-NADA TO BISAN-SETO DUE TO THE RAINFALL AND FLOOD

Abstract

 The outflow process of harmful red tide Chattonella from Katakami Bay in the northwestern part of the Harima-nada Sea to the Bisan-Seto Sea area was investigated using actual measurements and a numerical flow model. The distribution of red tide was investigated in the bay and surrounding coastal area at least twice a month from June to August from 2018 to 2023. Although the timing of occurrence was different, red tide occurred in the bay in each year. In 2018, 2018, and 2023, runoff from the bay to the coastal area was observed, and before the runoff, river flow had increased due to rainfall. In particular, in 2018, due to the influx of freshwater caused by “2018 western Japan floods”, red tide reached the aquaculture fishing grounds of Bisan Seto, more than 50 km away. When Lagrangian particles were generated and tracked in the bay under the same conditions at the same time, they showed a distribution similar to the actual measurement results, suggesting the transport of harmful red tide across the Bay due to rainfall and flooding.