Regime shift in phytoplankton bloom dynamics in an anthropogenically polluted semi-enclosed bay

Regime shifts in phytoplankton communities are important due to their cascading effects on the ecosystem functioning, and long-term monitoring is essential to detect these shifts and understand ecosystem responses to environmental alterations. Over a 17-year period from 2007 to 2023, phytoplankton bloom dynamics were investigated in anthropogenically polluted Yeongil Bay, whose compact site makes it suitable for long-term monitoring. While river discharge declined but nutrients recovered since 2017, total phytoplankton abundance declined and showed a slight recovery, but did not recover to previous levels. Notably, in 2017, phytoplankton communities shifted dramatically from a long-standing dominance by Leptocylindrus danicus and Cryptomoans spp. to Pseudo-nitzschia spp. and Chaetoceros spp. Interestingly, after 2017, community stability increased sharply. Leptocylindrus and Cryptomonas returned, indicating resilience, while straight Chaetoceros , previously less abundant, became consistently dominant, suggesting resistance. The shifts were driven by declining NH 4 + , increasing salinity, and altering nutrient stoichiometry. Leptocylindrus was negatively correlated with nutrients, Cryptomonas spp. was positively correlated with salinity during blooms, and straight Chaetoceros exhibited positive correlations with temperature. Collectively, the increased nutrient availability, rising community stability after 2017, but lack of full recovery in total abundance suggest that the Yeongil Bay ecosystem experienced a regime shift in 2017, which may not return to its previous state unless abrupt environmental changes occur. Under future climate scenarios, continued long-term monitoring will provide further insights into phytoplankton bloom dynamics. • Long-term (2007–2023) monitoring revealed phytoplankton regime shift in Yeongil Bay. • Community shifted in 2017 from Leptocylindrus and Cryptomonas to Pseudo-nitzschia and Chaetoceros . • Post-2017, community stability increased despite reduced total abundance. • Shifts driven by declining NH₄ + , higher salinity, and altered nutrient ratios. • Results suggest a persistent regime shift requiring continued monitoring.