Drastic Changes in a Lower-Trophic-level Ecosystem Attributed to Unprecedented Harmful Algal Outbreaks in 2021 on the Pacific Shelf off Southeast Hokkaido, Japan

Unprecedented, large-scale harmful algal blooms (HABs) occurred in Pacific coastal shelf waters off the south-eastern coast of Hokkaido, Japan, in the autumn of 2021. To understand how these HABs, composed primarily of Karenia selliformis, changed the structures of lower-trophic-level ecosystems in shelf waters, we analyzed high-resolution in situ ship measurements taken during October 2015-2021. We mainly compared physico-biochemical variables averaged over the shelf in 2021 and in 2015-2020. Chlorophyll-a concentrations at a depth of 10 m were approximately 2.3-fold higher in 2021 than in 2015-2020. Chlorophyll-a concentrations were highest at the sea surface in 2021, and vertically integrated concentrations were 1.8-fold higher in 2021 than in 2015-2020. Silicate concentrations at a depth of 10 m were higher in 2021, whereas nitrate and phosphate concentrations were largely the same. Abundances of major diatoms and picophytoplankton (e.g., Synechococcus spp.) at a depth of 10 m declined by 2 orders and 1 order of magnitude in 2021, respectively, whereas Karenia spp. abundance greatly increased. These results indicated that outbreaks of K. selliformis sup-pressed the growth of other algal species and dramatically changed nutrient flows and balances in the coastal shelf ecosystem. The suppression of non-Karenia algal species might have occurred through interspecific competition for nutrients as well as allelopathy and/or mixotrophy by K. selliformis.