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

Abstract

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

Potential of Sea Urchin Mesocentrotus nudus as a Target Catch Species in the Pacific Ocean off Eastern Hokkaido, Japan

“…In 2021, unprecedented large-scale HABs occurred off southeastern Hokkaido, which has rarely experienced HABs because the area is facing the North Pacific Ocean. Approximately one-month prior, marine heatwaves occurred in the northeastern Pacific, and they likely preconditioned and accelerated the blooms [ 42 ]. All specimens of M. nudus collected in the present study were ≥6 years of age, indicating that the sea urchins survived the HABs that occurred in 2021.…”

Gymnodinium catenatum Paralytic Shellfish Toxin Production and Photobiological Responses under Marine Heat Waves

“…The Blob triggered a large bloom of toxin-producing diatoms ( Pseudo-nitzschia australis ) that lead to massive fish kills and a great increase in toxin production [ 11 ], yielding the highest toxin concentration registered on the North American coast [ 10 ]. Besides the impacts on marine communities, this MHW caused widespread fishery closures, leading to overwhelming economic impacts [ 12 ] Another, more recent extreme MHW off the coast of Japan in 2021 led to an extensive HAB, with the toxic dinoflagellate Karenia selliformis being the most abundant in phytoplankton samples [ 13 , 14 ].…”

Gymnodinium catenatum paralytic shellfish toxin production and photobiological responses under marine heat waves

“…The Blob triggered a large bloom of toxin-producing diatoms (Pseudo-nitzschia australis), leading to massive sh kills and a great increase in toxin production [10], yielding the highest toxin concentration registered on the North American coast [9]. Besides the impacts on marine communities, this MHW caused widespread shery closures, leading to overwhelming economic impacts [11] Another, more recent extreme MHW off the coast of Japan in 2021 led to an extensive HAB, with the toxic dino agellate Karenia selliformis being the most abundant in phytoplankton samples [12,13].…”