Abstract: The diurnal migration rhythm and vertical pattern of phytoplankton are one of the important factors that influence the primary productivity of freshwater ecosystems, which is affected by the physiological characteristics of specific phytoplankton and the fluctuating aquatic environment. In order to understand the diurnal vertical migration characteristics of the phytoplankton and its potential constraints, an investigation on the vertical variation of phytoplankton communities was carried out for four times in one day at two sites (deep water and shallow water) of Shidu Reservoir, Beijing, in late April 2015. Physical and chemical features of water samples from the water columns were measured. Meanwhile, phytoplankton species from each layer of the water column were identified. The results showed that the vertical pattern and diurnal migration rhythm of specific algae were different between the two sites. Generally, the phytoplankton community as a whole was under high density and prone to be concentrated in the upper water layer during the daytime. In deep water, the vertical distribution and migration of phytoplankton was mainly affected by temperature, light, dissolved oxygen and nutrients. The dominant genus Microcystis migrated to the surface water at night due to the illumination. Peridinium aggregated in the deep layer, with its vertical migration mainly influenced by temperature and illumination. In contrast, Navicula dominantly concentrated in the surface layer of deep water. In shallow water, the dominant genera of algae fluctuated in a wider range than those in deep water. The relative abundance of Fragilaria was positively correlated with total dissolved solids (TDS). The distribution of Scenedesmus was mainly impacted by the dissolved oxygen and water temperature.

Key words: Machilus pauhoi, nitrogen addition, fine root, drought stress, root order