Denitrification is the most important nitrogen removal process in lake waters, and is of great significance for mitigating nitrogen pollution and controlling eutrophication in lakes. The outbreak and decline of cyanobacterial blooms may promote denitrification directly in the water column by changing the nitrogen circulation pathway and microenvironment of the water body, and accelerate the removal of nitrogen. In order to verify this hypothesis, the cyanobacteria with different biomass and the NO3--N, PO43--P nutrient for 10 days were taken from Taihu Lake water to simulate the effects of cyanobacteria growth and degradation on denitrification. The dynamic changes of algal biomass and various forms of nitrogen concentration were simultaneously determined by 15N isotope addition culture combined with membrane inlet mass spectrometer (MIMS) for real-time quantitative determination of denitrification rate. The results showed that cyanobacteria absorbed nitrogen into particle nitrogen during the growth period. During the decay period, algae cells released a large amount of NH4+-N by degrading mineralization, which was then converted into NO3--N to provide a substrate for denitrification. That is the key to promoting denitrification in water; the denitrification rate (as N2) reaches (1614.52±301.57)μmol·(m2·h)-1, which is three times higher than the denitrification rate[ (534.45±242.18)μmol·(m2·h)-1]of the lowest concentration cyanobacterial group at the same time. At the end of the experiment, the highest rate of TN removal was highest in the group with the highest initial biomass of cyanobacteria (40.02%), which was 2.26 times of the TN removal rate (17.72%) in the control, indicating that cyanobacterial accumulation can significantly promote the intensity of denitrification and accelerate the removal of nitrogen in water. The rate of denitrification in the decline of cyanobacteria is significantly affected by the concentration of NH4+-N, indicating that the coupling of nitrification-denitrification of microorganisms attached to algae is the main route of nitrogen removal. The results study indicate that the cyanobacteria bloom rapidly during the growth period. Nitrogen is converted into particle nitrogen. The degradation of cyanobacteria is accelerated by coupled nitrification-denitrification, which may be one of the reasons for the decrease of nitrogen concentration in Taihu Lake.
Keywords: 15N isotope addition; Taihu Lake; cyanobacteria concentration; denitrification rate; membrane inlet mass spectrometer(MIMS).