Environmental drivers modulate host-parasitoid coupling between the marine diatom Chaetoceros and the cercozoan...

Diatoms are major contributors to marine primary production and biogeochemical cycling; however, the role of parasitoid-mediated control in shaping diatom community dynamics remains poorly understood, particularly under variable eutrophic coastal conditions. This study investigated the seasonal dynamics of diatom populations and host-parasitoid interactions in a eutrophic coastal ecosystem subject to strong environmental gradients. We conducted a year-long field study in Yongho Harbor, Busan, Republic of Korea, combining an ecological niche analysis with targeted observations of parasitoid infections. Our results revealed that parasitoid-mediated control influenced the population dynamics of the dominant species Chaetoceros constrictus, driving shifts in diatom community structure. Parasitoid infections were confined to a narrow thermal window (15.9-21.8 °C) and closely coupled to host dominance, with infection probability increasing sharply when C. constrictus dominance exceeded 23.2 %. In contrast, C. curvisetus/pseudocurvisetus largely escaped infection, likely due to a mismatch between its field dominance at higher temperatures and the thermal niche of Pseudopirsonia chaetoceri. This pattern illustrates how bottom-up processes, such as nutrient enrichment and light availability, interact with top-down regulation by parasitoids to shape host-parasitoid dynamics, highlighting the combined influence of both ecological mechanisms. These findings show the importance of host-parasitoid coupling in diatom-dominated phytoplankton communities and suggest that climate-driven shifts in environmental gradients may destabilize these interactions, with cascading impacts on coastal ecosystem functioning.