Presentation Information
[O03-P02]An unprecedented harmful algal bloom on the Pacific shelf off southeastern Hokkaido in 2021★Invited Papers
*Kuroda Hiroshi1, Satomi Takagi2, Takuya Ohnishi2, Takuya Nakanowatari2, Tomonori Azumaya2, Akira Kuwata2, Hiromi Kasai2, Yukiko Taniuchi2 (1.Hokkaido University, 2.Fisheries Resources Institute, Japan Fisheries Research and Education Agency)
Keywords:
Harmful algal blooms,Karenia selliformis,Marine heatwaves and cold spells,Integrated ecosystem analyses
In mid-September 2021, an unprecedented large-scale harmful algal bloom occurred along the Pacific coast of southeastern Hokkaido, Japan, causing severe damage to coastal fisheries. The bloom was dominated by Karenia selliformis (Ks), marking the first recorded red tide event caused by this species in Japan. In this study we conducted an integrated analysis based on field observations, satellite analyses, numerical experiments, and culture and exposure experiments to clarify the factors responsible for the outbreak of the southeastern Hokkaido bloom, its transport processes, and its impacts on the lower-trophic-level ecosystem. The results showed that, from mid-July 2021 prior to the bloom, one of the strongest marine heatwaves on record developed in the northwestern Pacific and rapidly transitioned to a marine cold spell by mid-August. Such an abrupt shift from extremely warm to cold conditions is far rarer than the occurrence of marine heatwaves or cold spells alone and likely acted as a trigger for the southeastern Hokkaido bloom. Forward and backward particle-tracking experiments indicated that Ks originated from the eastern coast of the Kamchatka Peninsula and was transported to the southeastern Hokkaido region mainly by combination of the East Kamchatka Current and the Oyashio Current. Furthermore, the source population of Ks was likely associated with a large-scale bloom that occurred along the eastern Kamchatka coast in autumn 2020. Forward particle tracking showed an increase in particles arriving in the southeastern Hokkaido region from June 2021 onward—several months before the bloom and immediately prior to the intensive marine heatwaves—which was consistent with the timing of Ks appearance observed in ship-based surveys. High-resolution shelf surveys and satellite analyses during the bloom revealed that Ks was mainly concentrated near the sea surface shallower than 20 m, and that high-density distributions over the shelf were closely associated with submesoscale filamentary and patchy structures on spatial scales of several kilometers. During the development and decay of the bloom, the high-density Ks region migrated from the eastern to the western part of southeastern Hokkaido, a feature also detected by satellite observations and consistent with field observations conducted by the Hokkaido Research Organization. In addition, comparison of the lower-trophic-level ecosystem structure on the southeastern Hokkaido shelf in autumn 2021 with previous years revealed a shift from a diatom-dominated system to a Karenia-dominated system, indicating substantial changes in ecosystem structure, including major nutrient fluxes.