[Influence of Rainfall on the in situ Growth of Dominant Algae Species in Xiangxi River]

To understand the influence of rainfall on the in situ growth (in a culture cage) of dominant algae species in the Xiangxi River tributary of the Three Gorges Reservoir, culture experiments were carried out to measure the biomass of Microcystis aeruginosa, Chlorella vulgaris, and Chlorella aeruginosa before and after rainfall. The results showed that ① during the study period (October 4-18, 2017), there were significant differences in hydrodynamic conditions between the rainfall period and the non-rainfall period (ANOVA, P<0.05). Total Chl-a and the specific growth rate of the three main algae during rainfall period were significantly lower than during the non-rainfall period, which inhibited algae growth to some extent. The results of correlation analysis showed that the four hydrodynamic parameters characterizing vertical mixing had a highly significant negative correlation with the specific growth rate of the three dominant algae species. Meanwhile, the changes of shear force τ, the vertical turbulent viscosity coefficient V_r, and the vertical turbulent diffusion coefficient V_t were the key factors leading to the rapid decline of algae; ② the depth of the mixed layer was lower (1-2 m) before rainfall, but increased (>5 m) markedly after rainfall (October 10-18). At the same time, the concentration of Chl-a during the rainfall period was significantly lower than that during the non-rainfall period. The results of the correlation analysis showed that there was a highly significant positive correlation between the ratio of eutrophic depth to mixing depth (Z_eu/Z_mix) and the specific growth rate of the three dominant algae species. This indicated that the vertical disturbance of water was enhanced by rainfall, and mixing layer expanded continuously, which reduced the water temperature stratification and thus inhibited the growth and proliferation of algae; ③ there were significant differences in rainfall, water temperature, light intensity, total nitrogen, and dissolved total nitrogen between the rainfall period and non-rainfall period (ANOVA, P<0.05). Correlation analysis showed that changes in rainfall, water temperature, light intensity, total nitrogen, and dissolved total nitrogen caused by rainfall were the key environmental parameters affecting the in situ growth rate of three dominant algae.