Response of Size-Fractionated Phytoplankton to Environmental Variables in Gwangyang Bay Focusing on the Role of Small Phytoplankton

Responses of size-fractionated phytoplankton to environmental variables—particularly temperature, euphotic depth, and ammonium—were investigated in Gwangyang Bay to identify the phytoplankton groups responsible for estuarine blooms. Carbon-normalized biomass clearly showed the dominance of nanoplankton during summer and microplankton during winter. A combination of microscopy and flow cytometry analyses revealed that cryptophytes dominated the summer bloom, whereas diatoms dominated the winter bloom. Polynomial regression models indicated that diatom-dominated microplankton biomass declined sharply with increasing temperature and ammonium, while cryptophyte-dominated nanoplankton and cyanobacteria-dominated picoplankton biomass increased with rising temperature and decreased with increasing euphotic depth. These results highlight the substantial role of small phytoplankton (nanoplankton and picoplankton) in the bay ecosystem, where concerns over declining water quality and reduced nitrate availability are ongoing.