Experimental assessment of copepod survival in response to the harmful dinoflagellate Karenia selliformis from the southeastern coast of Hokkaido, Japan

In the autumn of 2021, a large-scale harmful algal bloom (HAB) emerged along the southeast coast of Hokkaido, Japan, and this was predominantly composed of the dinoflagellate Karenia selliformis. The emergence of K. selliformis-dominated HABs was the first observation in Japan; therefore, no previous reports exist on the impact of HABs on zooplankton in the region. This study investigated the effects of K. selliformis on the survival of copepods—a critical component of the zooplankton community. The results indicated that the survival rates of all six copepod species examined were markedly reduced, particularly at elevated K. selliformis concentrations or during extended exposure. The copepod survival rate decline occurred even in the absence of direct contact or ingestion of K. selliformis, thus implying that K. selliformis may excrete harmful substances extracellularly. Feeding experiments revealed that Neocalanus plumchrus consumes K. selliformis cells; however, the ingestion rate diminished with increased concentrations of K. selliformis cells. The results suggest that larger copepods with smaller surface-to-volume ratios may possess greater resilience to the harmful substances compared to smaller species. Further, the HAB in southeast Hokkaido likely exerted deleterious effects on lower trophic levels within the marine ecosystem by disrupting copepod survival and feeding activity.