Influence of environmental parameters on marine plankton diversity in the southern coastal waters of Korea: Emphasis on thermal stratification.

Plankton are fundamental to aquatic ecosystems, forming the base of the marine food web and playing a crucial role in the global carbon cycle. While the impact of environmental factors on marine plankton ecosystems has been widely studied, the seasonal variability in plankton diversity has received comparatively less attention. Here, we investigated the complexity and distribution of plankton species in Korean coastal waters in relation to seasonal environmental changes, particularly those linked to water mass characteristics and the presence of a thermocline, using alpha and beta diversity indices. A thermocline was observed at all stations in July, coinciding with the lowest diversity and richness, but was absent in September, when diversity indices peaked. In the upper layer above the thermocline, diatom species such as Pseudo-nitzschia delicatissima and Skeletonema costatum were dominant. Analysis of the Shannon index suggested that planktonic ecosystems in the South Sea of Korea from May to September 2021 were well-balanced. These findings enhance our understanding of the potential effects of environmental variability and thermal stratification on plankton biodiversity and community dynamics in Korean coastal waters.