From Long-Term Trends to Extreme Events : Phytoplankton Community Shifts and HAB Dynamics in the Southern Coastal Waters of Korea = 한국 남해 연안의 식물플랑크톤 군집 변동과 유해성 적조 발생 동태: 장기적 변화와 기후위기 영향

다국어 초록 (Multilingual Abstract)

This study integrates nearly three decades of nationwide harmful algal bloom (HAB) records (1995-2025), 12 years of high-resolution environmental and phytoplankton monitoring (2013-2024), and two contrasting extreme events, a 2024 marine heatwave and a 2025 cold-water intrusion, to examine how long-term climate trends and short-term disturbances jointly shape phytoplankton dynamics in the southern coastal waters of Korea. In Chapter 2 standardized analysis of historical HAB bulletins confirms that Margalefidinium polykrikoides has remained the dominant bloom-forming species, with outbreaks most frequently initiating in the Goheung-Yeosu region and subsequently propagating eastward under variable oceanographic and meteorological conditions. Bloom phenology advanced during the early 2000s in response to gradual warming. However, more recent summers characterized by earlier and more intense high-temperature extremes have, in some years, delayed bloom initiation. In Chapter 3 long-term biweekly observations reveal progressive summer warming and freshening accompanied by pronounced interannual variability. Despite these changes, the phytoplankton community has remained largely diatom-dominated, with only a modest decline in the diatom-to-dinoflagellate ratio. In Chapter 4,thermal response analysis indicates distinct temperature optima for major functional groups, with sharp declines in biomass above ~29°C, suggesting that extreme warming may suppress overall primary productivity. During the 2024 marine heatwave, sustained high temperatures and nutrient depletion reduced surface phytoplankton biomass, inhibited M. polykrikoides persistence, and favored small-sized taxa such as Synechococcus, while deeper layers provided partial thermal and nutritional refugia for diatoms. In Chapter 5, in contrast, the 2025 cold-water intrusion delivered nutrient-rich subsurface waters to the euphotic zone, alleviating heat stress and promoting bloom development, although responses varied spatially depending on the vertical structure and intensity of cooling. Collectively, these results demonstrate that HAB dynamics in the southern coastal waters of Korea are governed not by temperature alone but by the interaction between thermal forcing, stratification, vertical nutrient supply, and episodic physical disturbances. Incorporating these coupled processes into forecasting frameworks is essential for improving early-warning systems, refining region-specific management strategies, and assessing ecosystem resilience under increasing climate variability.

목차 (Table of Contents)

• Content
• List of Figures
• List of tables
• Abstract
• Chapter I. General introduction 1
• Chapter II. Occurrence of Harmful Algal Blooms in Korea 6
• 2.1 Introduction 6
• 2.2 Materials and Methods 8
• 2.3 Results 11
• 2.4 Discussion 20
• 2.5 Conclusion 23
• Chapter III. Long-term Changes in Phytoplankton Communities in the
• Southern Coastal Waters of Korea 24
• 3.1 Introduction 24
• 3.2 Materials and Methods 26
• 3.3 Results 28
• 3.4 Discussion 47
• 3.5 Conclusion 55
• Chapter IV. Changes in Phytoplankton Characteristics and Succession under Unprecedented High-Temperature Conditions in the Southern Coastal Waters of Korea 56
• 4.1 Introduction 56
• 4.2 Materials and Methods 59
• 4.3 Results 63
• 4.4 Discussion 71
• 4.5 Conclusion 81
• Chapter V. From Marine Heatwaves to Cold-Water Intrusion: Divergent Drivers of Margalefidinium Bloom in the Southern Coastal Waters of Korea 83
• 5.1 Introduction 83
• 5.2 Materials and Methods 85
• 5.3 Results 88
• 5.4 Discussion 100
• 5.5 Conclusion 105
• Chapter VI. General conclusion 108
• References 113
• 국문초록 139