Seasonal succession of harmful and non-harmful phytoplankton under hydrographic water-mass variability and stratification dynamics

Abstract

Seasonal succession of harmful and non-harmful phytoplankton in coastal ecosystems is closely associated with spatiotemporal variability in water-column stratification. To clarify these mechanisms, monthly surveys based on a one-year observational dataset were conducted at nine stations from the Nakdong River estuary to the Geoje coast between June 2016 and May 2017. Phytoplankton community changes were closely associated with stratification development reflecting density differences and vertical density gradients, and were rapidly influenced by freshwater input, seasonal heating, and intrusion of the Tsushima Warm Current. Stratification intensity was numerically quantified using two complementary indices: Δρ (stratification index) and N2 (water-column stability), enabling statistical evaluation of relationships between water-mass structure and community responses. RDA ordination patterns suggested that Gymnodinium spp. and Chaetoceros spp. were more closely associated with strongly stratified conditions, whereas weakly stratified and well-mixed conditions supported Cryptomonas spp. and Skeletonema spp. The genus-level associations described in this study were interpreted qualitatively from ordination patterns and community-level relationships, and statistical significance was not independently tested for each individual taxon. Monsoon rainfall in July reduced salinity and increased nitrate and silicate concentrations. Strong summer stratification supported harmful dinoflagellates such as Margalefidinium polykrikoides, although abundance declined in August following intrusion of Yangtze River diluted water. During the study period, autumn weakening of stratification and nutrient resupply were associated with increased abundance of Pseudo-nitzschia delicatissima, while winter mixing coincided with lower phytoplankton biomass and greater relative abundance of non-harmful diatoms. Alexandrium spp. became more abundant during spring when stratification gradually redeveloped. The combined use of Δρ and N2 distinguished integrated stratification strength from depth-specific stability, providing a mechanistic framework linking physical structure to biological dynamics. These findings suggest that zone-specific environmental factors contributed to differences in dominant taxa during the study period, and that the stratification index and water-column stability may serve as useful indicators for interpreting seasonal phytoplankton succession.