[Causes of succession of planktonic algae in Shennong bay of Three Gorges Reservoir in spring in 2014].

Objective: Algal blooms occurred in some sections of Shennong bay after impounding of Three Gorges Reservoir.

Methods: Related environmental and hydrodynamic factors were monitored during the period of algal blooming season in 2014 (March 20, April 13, May 23) in Shennong bay, Three Gorges Reservoir. To study succession of planktonic algae, water stable coefficient, euphotic depth and mixed layer depth were used to analyze stratification and hydrodynamic characteristics.

Results: We identified 6 phyla, 38 species (genera) planktonic algae. The sensitive area of algal bloom was at SN05 (677.677×105 cells/L) and SN06 (716.761×105 cells/L), and the planktonic algae biomass during this period was significantly different (ANOVA, p<0.05). Moderate water temperature, adequate nutrients, weak stratification and poor mixing promoted the rapid growth and breakout of the diatom bloom with Cyclotella spp. as the dominant species in March. Further increase of water temperature, stronger stratification and decrease of dissolved silicate and mixing layer restricted the diatom growth. Chlorella spp. and Chlamydomonas spp. grew better in shallow mixed layer with rich nutrients and evident stratification. Then Chlorophycean bloom broke out with Chlorella spp. as the dominant species and Chlamydomonas spp. the next-dominant species. High biomass maintained in April. In May, algal bloom gradually vanished due to sharp fluctuation of water level and increase of velocity. Monitored maximum water velocity was 0.1141 m/s at 2 m depth, exceeded an optimal flow rate perfect for growth of planktonic algae.

Conclusion: Stratification and hydrodynamic characteristics had important effect on planktonic algae under the condition of adequate nutrients. Velocity became the main factor that inhibited the growth of algae in Shennong bay in pre-flood falling stage of the Three Gorges Reservoir.