Response of microalgae size-class structure to nutrients differences in northern Yellow Sea, China.

Marine phytoplankton size-class structure affects ecological functions and shellfish culture. We use high-throughput sequencing and size-fractioned grading techniques to identify and analyze responses of phytoplankton differences in environmental variables at Donggang, northern Yellow Sea (high inorganic nitrogen (DIN)) and Changhai (low DIN) for 2021. The main environmental variables that correlate with differences in the proportional contributions of pico-, nano-, and microphytoplankton to the total phytoplankton community are inorganic phosphorus (DIP), nitrite to inorganic nitrogen ratio (NO2/dissolved inorganic nitrogen (DIN)), and ammonia nitrogen to inorganic nitrogen ratio (NH4/DIN), respectively. DIN, which contributes most to environmental differences, mainly positively correlates with changes in picophytoplankton biomass in high DIN waters. Nitrite (NO2) correlates mostly with changes in the proportional contribution of microphytoplankton in high DIN waters and nanophytoplankton in low DIN waters, and negatively correlates with changes in the biomass and proportional representation of microphytoplankton in low DIN waters. For near-shore phosphorus-limited waters, an increase in DIN may increase total microalgal biomass, but proportions of microphytoplankton may not increase; for high DIN waters, an increase in DIP may increase proportions of microphytoplankton, while for low DIN waters, an increase in DIP may preferentially increase proportions of picophytoplankton and nanophytoplankton. Picophytoplankton contributed little to the growth of two commercially cultured filter-feeding shellfish, Ruditapes philippinarum and Mizuhopecten yessoensis.