Vertical migration patterns of different phytoplankton species during a summer bloom in Dianchi Lake, China

Vertical migration patterns of different phytoplankton species were examined during a summer bloom period in Dianchi Lake, China. The ratio of the mean crowding to mean density (x*/x) and mean residence depth (MRD) was used to quantitatively evaluate the distribution patterns. The effects of wind velocity and water column temperature differences on the vertical distribution patterns of Microcystis aeruginosa, Aphanizomenonflos-aquae, and total phytoplankton were then investigated. Over 5 days (July 16–20, 2013), abundant of Microcystis aeruginosa (1.10 ± 0.40 × 10⁹ cells/L), Aphanizomenonflos-aquae (5.11 ± 1.38 × 10⁷ cells/L), and total phytoplankton (1.24 ± 0.40 × 10⁹ cells/L, 239.63 ± 79.26 μg/LChl-a, n = 64) were found throughout the water column. Values of x*/x and MRD showed that Microcystis aeruginosa aggregated on the water surface during the calm morning [wind velocity (WV) <2 m/s], and distributed uniformly in the windy afternoon (WV >2–3 m/s). Aphanizomenonflos-aquae tended to be randomly distributed for most of the time. Wind velocity was significantly correlated with the x*/x and MRD of Microcystis aeruginosa (P < 0.05), but not with those of Aphanizomenonflos-aquae. Meanwhile, the effects of thermal differentiation on the vertical distributions of all species were not significant. Therefore, the vertical distributions of Microcystis aeruginosa may be determined by wind velocity rather than thermal differentiation in Dianchi Lake.