Insights into plankton communities in a Margalefidinium polykrikoides (Gymnodiniales, Dinophyceae) bloom and adjacent non-bloom waters

During August 2022, a persistent harmful algal bloom (HAB) caused by the dinoflagellate<i> Margalefidinium polykrikoides</i> (aka <i>Cochlodinium polykrikoides</i>) occurred in the coastal waters of Yeosu, South Korea. This study aimed to char acterize the bloom by comparing planktonic communities between a high-intensity-bloom site (H-BS) and a nearby non-bloom site (N-BS) and identify the associated <i>M. polykrikoides</i> strain. Metazoan abundance and species richness were notably higher at H-BS, suggesting that metazoans like copepods and cladocerans may act as predators or exhibit greater resilience to bloom-induced stress. Conversely, protist species richness was greater at N-BS, indicating that <i>M. polykrikoides</i> may inhibit other protists through allelopathy and / or outcompete them through resource competition. Distinct community structures at H-BS and N-BS suggest a complex interplay of competition, predation, and parasitism driving bloom dynamics. A phylogenetic analysis showed that the bloom-causing strain of <i>M. polykrikoides</i> belonged to a major clade of the East Asian ribotype, indicating that the bloom’s outbreak was likely originated from shared regional waters rather than the intrusion of ocean currents. In conclusion, understanding the interactions between <i>M. polykrikoi</i> des and co-occurring planktonic species is crucial for predicting and managing future HABs in coastal ecosystems. This study further highlights the complex ecological processes underpinning <i>M. polykrikoides</i> bloom dynamics by simultane ously examining the roles of both protist and metazoan communities.