| The harmful cyanobacterial blooms resulted from eutrophication in lakes have become a tough headache of China’s environmental department. And in the context of global climate change, the responses of planktonic ecosystem in spring have attracted much attention. In spring, with the increase of temperature, succession of phytoplankton community in general are developed from diatom domination to green algae and ultimately replaced by cyanobacteia. Temperature increase caused by global change lead to the specific temperature node achieved in advance, but the angle of the solar radiation does not reach the angle of the same temperature in history which means weaker light intensity and shorter exposure time. In result, a "mismatch" of high temperature and low light formed. While higher temperature and lower light radiation are conductive to cyanobacteria establishing their domination in lakes and forming blooms, so the mismatch may be an important factor influencing phytoplankton community composition and succession in spring.This paper focused in cyanobacteria in eutrophiclakes and studied the impacts of the mismatch of elevated temperatures and weakened available light resulted from global change on the progress of cyanobacteria establishing its domination. Though field monitoring, laboratory simulation and field in situ experiments, we researched in detail the responses of the recovery progress of cyanobacteria in spring to the mismatch environment which would explain the mechanism of advanced formation of cyanobacteria blooms in spring. The main results were as follows:(1)The field monitoring studies indicated that the environmental changes of elevated temperature and weakened illumination might have a great effect on phytoplankton growth and succession of Chaohu Lake in spring which could contribute to their recovery, thus speeding up their domination establishment in spring.The results showed that in Chaohu Lake in spring increased temperature could speed up the recovery of algae, but was more beneficial to the growth of cyanobacteria. When the temperature rised to 16.9℃, the growth of cyanobacteria became significant. And with the increasment of temperature, the cyanobacteria recovered gradually. In a certain range, enhanced light radiation could promote the growth of cyanobacteria in recovery. While when the light intensity exceeded 15.17 MJ/m2, it would begin to inhibited cyanobacteria growth in recovery.(2)Pure culture experiments in the laboratory indicated that elevated temperature and weakened light in spring promoted the growth and physiological activity of Microcystis aeruginosa, and the growth response of M. aeruginosa to low light intensity were more sensitive to elevated temperature, which were conductive to M. aeruginosa establishing its domination in lakes. In addition, the temperature rise at the same time, less intensity of patina of algal photosynthetic activity has certain enhancement effect. Temperature can improve the patina of algal esterase activity, and have less effect to the Chlorella pyrenoidosa. At low temperatures, reduced lighting can improve esterase activity of M. aeruginosa, and effects on Chl. pyrenoidosa esterase activity was not significant.The results showed that the growth rate of M. aeruginosa reached its maximum with the cell density of 2.99×106 cells/mL when the temperature was 18℃ and light intensity was 100μmolm-2 s"1. At the same time, its photosynthetic activity was much higher than other groups(P<0.05), which Fv/Fm values reached a maximum of 0.39. While the growth rates of Chl. pyrenoidosa at the temperature of 16℃ and 18℃ were similar, but significantly higher than growth rate at 14℃(P<0.05), and its photosynthetic activity kept fluctuating throughout the experiment.(3)Weakened light radiation played an important role in the progress of cyanobacteria establishing their domination in the phytoplankton community in spring through speeding up the succession progress of phytoplankton community in spring and facilitating cyanobacteria domination happened earlier. Three levels of light radiation were designed with natural light radiation level as the control group and 35% and 15% light reduction as the treatment groups. At the start of the experiments, the algae community began to recovery with temperature at only 12.3℃. As the experiments progressed, the temperature increased gradually to be above 20℃ on day 29 and reached maximum of 26℃ on day 43. Algal biomass of the control group was significantly higher than that of the light treatment groups. However, the succession progress of diatom-green algae-cyanobacteia happened apparently earlier in the treatment groups than in the control groups. During the increasment of the temperature and the recovery of phytoplankton community, decreased light radiation could contribute to restore photosynthetic activity of cyanobacteria but diminish the photosynthetic activity of diatoms and green algae. |
