| In recent years, frequent blooms in Lake Taihu do harm to the society.Many researchers devote to studying the mechanism of bloom formation in Lake Taihu. Nitrogen (N), considered as one of factors that stimulate blooms, is the essential element for phytoplankton, so it has great significance to study the role of N in bloom formation and maintenance of Lake Taihu.Through a laboratory study combined with field monitoring, the different growth and photosynthesis characteristic of Microcystis flos-aquae (M. flos-aquae) on different N source was compared, the competition characteristic between M. flos-aquae and Melosira varians (M. varians) on different N source concentration and NH4+/NO3-was investigated and the variations of water quality of algae-dominated regions (Zhushan Bay, Meiliang Bay, river flowing site, lake center) and a macrophyte-dominated region (southern Gonghu Bay) were surveyed. Finally based on previous studies, the roles of N and Temperature (T) in bloom formation were investigated and the different roles of different N species in bloom maintenance in Lake Taihu were analyzed.The main conclusions of this study are as follows:1. The increase of N source concentration goes against the competition abiliy of Microcystis, while the increase of the percentage of ammonium (NH4+) in N source has the contrary effect. The concentrations of NH4+and dissolved inorganic nitrogen (DIN) in Zhushan Bay were higher that those in the other three zones while ammonium/nitrate (NH4+/NO3-) didn’t have obvious difference between the four zones, making the competition ability of the Microcystis in Zhushan Bay is lower than that in the other three zones, which leads the results that the percentage of Microcystis in the total phytoplankton of Zhushan Bay is less than those of the other three zones while the percentage of Chlorophyta in the total phytoplankton of Zhushan Bay is higher.2. hi Lake Taihu, the appropriate N concentration and NH4+/NO3-which avails the cyanobacteria’s competition with the other algae play a role in the bloom formation.3. When M. flos-aquae growed on NH4+, they had higher growth and photosynthesis ability than those on the other N species, but NH4+concentation above2mg/L would inhibit their growth. The large quantity of regenerated NH4+makes the phytoplankton of Lake Taihu can mainly rely on NH4+to grow, while the low NH4+concentration avoid the growth inhibition of the phytoplankton, which promotes the growth and photosynthesis of the phytoplankton, leading the vigorous growth of Microcystis and maintenance of serious bloom of Lake Taihu.4. The high phytoplankton concentration (PC) in Lake Taihu and the preferential uptake of NH4+by phytoplankton make NH4+concentration was low all over the year while NO3-concentration was high in spring and winter when PC was lower nonetheless low in summer and autumn when PC was higher, leading the higher correlation coefficient between NO3-concentration and phytoplankton and Microcystis concentration than the correlation coefficient between NH4+concentration and phytoplankton and Microcystis concentration,which causes the illusion that the bloom of Lake Taihu is limited by NO3-.5. When the relationship between different algae species is discussed, the correlation analysis results between the percentages of different algae species in the total phytopalnkton. In Lake Taihu, Microcystis compete with Chlorophyta and Bacillariophyta while the relatiobnship between Chlorophyta and Bacillariophyta may be symbiosis.6. The increase of water T only enhances the superiority of the Microcystis in the competition and accelerate the metabolization and growth rate of the phytoplankton, not playing the key role in cyanobacteria bloom formation, so the high T is unlikely the mechanism for the bloom formation of Lake Taihu.7. Microcystis flos-aquae (M. flos-aquae) can use many kinds of N species except L-lysine which may inhibit the growth of M. flos-aquae and only after they metabolize L-lysine to NH4+can they grow. |
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