The researchers studied the changes in phytoplankton in Danjiangkou Reservoir over three years to understand how it affects water quality. They found that the density of phytoplankton increased each year and varied by season, with different types dominating at different times. Water temperature was the main factor influencing phytoplankton, followed by oxygen demand and nutrients like ammonia and phosphorus. The reservoir area had more phytoplankton than the tributaries, with Cyanophyta and Chlorophyta being common in the reservoir and Bacillariophyta and Cryptophyta in the tributaries. Overall, the study provides insights into how environmental factors impact phytoplankton communities in the reservoir.