Seasonal dynamics of phytoplankton communities and diatom-selective feeding of the pacific oyster (Crassostrea gigas) in Jaran Bay, Korea

Understanding the feeding behavior of filter-feeding bivalves is critical for maintaining sustainable shellfish aquaculture by ensuring adequate phytoplankton food availability and ecological carrying capacity under shifting environmental conditions. This study investigated the seasonal dynamics of phytoplankton communities and their dietary contributions to the Pacific oyster ( Crassostrea gigas ) in Jaran Bay, a major aquaculture area on the southern coast of Korea. From June to November, when C. gigas typically undergoes active somatic growth and reproduction in Korean coastal waters, we conducted pigment-based CHEMTAX analyses to characterize phytoplankton composition in both the water column and oyster gut contents. Despite seasonal shifts in environmental phytoplankton, with diatom dominance increasing in autumn, oyster gut contents consistently remained dominated by diatoms (mean 96.4% of total Chl- a ). Electivity index (EI) analysis revealed strong positive selection for diatoms (0.7 ± 0.3) and negative selection for dinoflagellates and cryptophytes (–0.7 ± 0.5 and –0.6 ± 0.6, respectively), indicating pronounced dietary preference. This preferential utilization pattern in C. gigas , which appeared partially independent of environmental phytoplankton availability, may be associated with differences in particle size and retention efficiency among phytoplankton groups. These findings suggest that shifts in phytoplankton communities, such as those driven by warming sea surface temperatures or nutrient changes, may directly impact oyster nutrition, condition, and aquaculture productivity. Ensuring the long-term viability of oyster farming in Jaran Bay and similar coastal embayments will require adaptive management approaches that explicitly account for the quality and composition of phytoplankton communities, with a focus on sustaining diatom productivity within ecological carrying capacity limits.