Size-Specific Phytoplankton Pigment Characteristics in Jaran and Hansan Bays Based on HPLC Analysis

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.