Sang Heon Lee

The Ross Sea, one of the most productive regions of the Southern Ocean, plays a key role in biological carbon sequestration that contributes significantly to the global carbon cycle. In this study, we investigated the vertical distribution of particulate organic carbon (POC) and the biochemical composition of particulate organic matter (POM) in relation to phytoplankton assemblage structure during the austral summer of 2023. Stations were classified as either Phaeocystis antarctica ( P. antarctica )‐dominated or mixed assemblages composed of diatoms and P. antarctica . P. antarctica ‐dominated stations were characterized by carbohydrate (CHO)‐ and transparent exopolymer particle (TEP)‐enriched POM in the euphotic zone, whereas mixed assemblages contained relatively protein (PRT)‐rich material. Concentration‐based vertical attenuation patterns, assessed using power‐law fits to suspended POM profiles, indicated a weaker decline in Non‐biopolymeric carbon (Non‐BPC) with depth under P. antarctica dominance, whereas mixed stations exhibited steeper declines in suspended POC concentrations. These compositional differences in POM influenced the persistence of suspended POM remaining below the euphotic zone. Our findings suggest that phytoplankton assemblage structure, driven by environmental factors such as hydrographic structure, plays an important role in regulating the lability and potential preservation of organic carbon in Antarctic waters. The results highlight how future climate‐driven shifts in assemblage structure could alter the composition and persistence of organic carbon transferred into deeper waters of the Ross Sea.

Expanded polystyrene litter in marine environments harbors diverse and distinct microbial communities, referred to as the plastisphere. This study aimed to investigate the monthly dynamics of bacterial and potentially pathogenic bacterial (PPB) communities on expanded polystyrene over one year. Vibrio species dominated the PPB community, cooccurring at consistently higher abundances on expanded polystyrene than in the surrounding seawater, particularly under higher temperatures and low dissolved organic carbon (DOC) levels. At a temperature threshold of 16 °C, the abundance of zoonotic species, such as Vibrio parahaemolyticus and Vibrio alginolyticus, increased significantly. Some psychrotrophic Vibrio spp. were detected under moderately eutrophic conditions, suggesting that expanded polystyrene may also serve as a dispersal vector facilitating their transport to more favorable habitats. Multivariate analyses, including partial least squares path modeling, revealed temperature and DOC as the primary environmental factors influencing PPB community composition. However, environmental responses varied by taxonomic groups, with different preferences observed under varying eutrophic conditions. In conclusion, these findings demonstrate that expanded polystyrene litter supports a selective and environmentally responsive bacterial population, highlighting the potential role of plastic debris in promoting pathogenic bacterial persistence and spread in marine ecosystems, particularly under conditions associated with climate change, including warming and eutrophication.

This study investigated the spatial and seasonal dynamics of phytoplankton communities in Jaran Bay, inner Hansan Bay, and outer Hansan Bay, with particular emphasis on size structure and pigment-based indicators of productivity and physiological status. Water sampling was conducted during May, August, and October in 2020, 2022, and 2023 and phytoplankton communities were analyzed using size-fractionated chlorophyll a measurements and high-performance liquid chromatography (HPLC) pigment analysis. Chlorophyll a concentrations exhibited pronounced seasonality, with consistently elevated values in August across all bays. Diatoms were predominant throughout the study period; however, their relative contribution declined in outer Hansan Bay during summer, coinciding with increased contributions from cryptophytes and cyanobacteria. Size-fractionated analyses revealed that large-sized phytoplankton (>20 µm) predominantly consisted of diatoms, whereas small-sized phytoplankton (<20 µm) were composed of diatoms and cryptophytes. Comparisons between fluorometric and pigment-based approaches indicated that pigment-based diagnostics overestimated microphytoplankton contributions, attributable to the presence of small-sized diatoms. Pigment indices further revealed that large-sized phytoplankton were characterized by higher photosynthetic carotenoid concentrations and lower photoprotective carotenoid ratios, indicative of enhanced photosynthetic activity and productivity. Overall, these findings highlight the critical role of phytoplankton size structure in regulating productivity and physiological responses in aquaculture-dominated coastal bays.