The researchers studied the seasonal changes in phytoplankton communities in the Bohai Sea and Yellow Sea. They went on three research cruises in April, June, and October 2019. They found that diatoms were the most common phytoplankton group throughout the seasons. Some types of dinoflagellates were found in warmer, saltier, and nutrient-poor waters. The diversity of phytoplankton was highest in autumn and lowest in summer. The phytoplankton communities in the two seas were divided into three ecological provinces based on their characteristics. Overall, phytoplankton diversity was highest in autumn, followed by spring, and lowest in summer.