Influence of Region-Specific Marine Environments on Phytoplankton and Bacterial Communities in the Korean Coastal Waters in Winter 2021

We assessed the influence of the distinct characteristics of the three seas surrounding the Korean coast (the Yellow Sea, YS; the South Sea, SS; and the East Sea, ES) on the phytoplankton and bacterial communities by conducting field surveys at 23 stations in February 2021. The average water temperature (WT) of the ES and the SS (9.5 +/- 2.4degree celsius) was significantly higher compared to that of the YS (4 +/- 1.3degree celsius), on account of the Tsushima Warm Current and their relatively deep depth. Additionally, the unique tidal range of the YS, combined with its low WT, led to the highest dissolved oxygen concentration (YS: 11 +/- 1.2 mg L-1. SS: 10 +/- 1.3 mg L-1. ES: 9 +/- 1.0 mg L-1). This difference was related to regional Chl. a concentrations and the dominance of diatoms. Particularly, due to freshwater input, the diatom Thalassiosira rotula proliferated significantly at St. Y2, while strong tides in the YS led to the sub-dominance of benthic diatom Paralia sulcata. In addition, we found a relatively higher number of endemic species (13 species) in intermediate SS region than in other regions (YS: 6 species with strong mixing condition and ES: 3 species with low seed population). Moreover, there were differences in the number of shared species depending on proximity of each region (YS-SS: 10; SS-ES: 4; YS-ES: 2 species). The dominant diatoms, Eucampia zodiacus and Chaetoceros pseudocurvisetus, in the SS exhibited a negative correlation with the concentrations of nutrients in PCA analysis, suggesting that nutrients supplied by water mixing were being utilized for their growth. Despite high nutrient levels, the biomass was low in the deep ES region, averaging Chl. a < 0.3 <mu>g L-1 with Cryptomonas spp. dominating instead of diatoms, indicating a lower presence of the seed population of diatoms in this region. Bacterial communities were dominated by the genus Polaribacter within the order Flavobacteriales and the genus Sulfitobacter within the order Rhodobacterales, showing their association with WT and dominant species of phytoplankton. Additionally, the dominance of specific bacterial species associated with predominant phytoplankton resulted in a decrease in bacterial diversity. These findings suggest that the region-specific oceanographic factors, such as water depth, tidal range, and ocean currents of Korean coastal waters, during winter have a significant impact on the micro-ecosystem, including phytoplankton and bacterial communities.