Controlling factors of large-scale harmful algal blooms with Karenia selliformis after record-breaking marine heatwaves

Unprecedented, large-scale harmful algal blooms (HABs) dominated by Karenia selliformis occurred off the southeastern coast of Hokkaido, Japan, from late September to early November 2021, about a month after intense and extensive marine heatwaves (MHWs) had subsided. The aims present study were understand mechanism development, maintenance, decay HABs as well investigate effect MHWs on HABs. We developed one-dimensional, lower trophic-level ecosystem model (NEMURO + ) simulate successfully simulated 2021 indicated that their controlled primarily changes water temperature. Nitrate supply subsurface layers seasonal vertical diffusion in autumn also helped maintain Vertical following contributed long duration preferred temperature for K. occurrence pre-bloom , resulting preconditioning accelerating However, simulations normal years (i.e., climatological mean during 2003–2018) showed could have occurred, even absence MHWs. massive other phytoplankton species (e.g., diatoms) would not bloom. implication was species-specific responses . proposed peculiar our area differed previously reported blooms. Specifically, clearly than ; thus, physiological characteristics bloomed those strains. These discoveries provide first evidence explain how affect HABs, inter-regional dissimilarities can lead large-scale, devastating outbreaks under different oceanographic conditions.