Heavy Rain, Less Bloom Under Heat: Succession of Size-Structured Phytoplankton Community Without Biomass Increases in a Monsoonal Korean Coastal Ecosystem

Phytoplankton succession across taxonomic groups and size classes is a key feature of coastal ecosystem dynamics but it remains understudied in monsoon-influenced regions. This study examined the effects of monsoonal rainfall on phytoplankton community succession across different size classes and groups in the southern coastal waters of Korea. Weekly surveys were conducted from June to October 2023 in the Yeosu–Goheung area to capture environmental changes and corresponding shifts in phytoplankton community composition through microscopy, UPLC (Ultra Performance Liquid Chromatography) pigment profiling, and flow cytometry. During the peak rainfall period (June 5–July 25), rapid declines in salinity and increased Si(OH)4 favored diatoms and cryptophytes, despite persistent nitrogen and phosphorus limitation. However, overall abundance of the total phytoplankton community remained low (< 4,000 cells/ml) throughout the monsoonal period. After the rainfall period, total phytoplankton biomass did not rebound despite the nutrient recovery. Sustained high temperatures suppressed overall phytoplankton biomass, while heat-tolerant species such as Chaetoceros spp., Pseudo-nitzschia spp., and Cryptomonas spp. became dominant in low biomass communities, suggesting a shift toward thermal-resilient taxa. Synechococcus abundance remained stable throughout the study period. Our findings indicate that in monsoon-driven coastal ecosystems, phytoplankton succession is characterized by shifts in species dominance than by changes in bloom magnitude due to N, P limitations and heat stress. This suggests that while succession can persist despite environmental fluctuations, bloom intensity is constrained by nutrient availability and thermal stress. These insights enhance our understanding of phytoplankton dynamics and support predictions of coastal productivity and ecosystem resilience under changing climate conditions.