This study attempted to reveal the effect of solar radiation fluctuation on the dynamics of phytoplankton communities expressed as cell morphology in eutrophic water bodies where the impacts of nutrients could be considered as small. Two morphological descriptors were proposed, cellular projected area (φ_𝑝) and flattening index (f), which were able to he cellular light-harvesting potential and energy requirement, respectively. A model was established to describe the effects of natural light availability on selecting phytoplankton assemblages with underwater field and mixing process in the water column considered. Based on the data collected from the eutrophic Yanghe Reservoir, the model was derived as V=37.92λφ_𝑝 (R²=0.673, P<0.01), where V is bio-volume, λ is a function of solar elevation angle (θ) and mixing/euphotic depth ratio (z𝑚𝑖𝑥/z𝑒𝑢) in water, and φ p. Post-analysis of the model results revealed that species with large φ p and f in general have advantages in spring and winter when underwater light availability is low; by contrast, those with small φpφp and f have advantages in summer. Larger φp and f mean that the cells could harvest more light energy and consumed less, allowing them to be selected under low light availability; and vice versa. We thus concluded that the underwater light field probably the most important factor in selecting phytoplankton morphology in eutrophic water bodies.