This study examines the effects of bottom-up (physical-chemical) and top-down (predation) factors on the phytoplankton functional groups in a eutrophic lake. The study revealed that the bottom-up effects of physical-chemical variables on some phytoplankton functional groups was stronger than the top-down effects of zooplankton. The study further showed that zooplankton functional groups were positively correlated with phytoplankton biomass implying that the top-down control of phytoplankton by the zooplankton in Lake Hulun is not strong enough to produce a negative effect. These finding serve as a basis for identifying how phytoplankton dynamics are influenced, which have implications for developing sustainable management strategies and conserving services of Lake Hulun and other eutrophic fresh water lakes.