Effects of Cyanobacterial Blooms on Nitrous Oxide Emission From Taihu Lake

During a cyanobacterial bloom, surface water nitrous oxide (N₂O) concentrations were measured at 12 sites in Taihu Lake, including Meiliang Bay and the open area. It was found that N₂O concentration at the sampling sites of Lake Taihu were significantly different. The mean N₂O concentration at the estuary area was averagely around 26.8 μmol·m^-3, which was much higher than those at the other sites (< 11 μmol·m^-3), among which the northeast area of Meiliang Bay and the open area were averagely only 4.0 μmol·m^-3. N₂O concentrations in the surface water were correlated with chlorophyll a (Chl-a) concentrations (P < 0.05), indicating that cyanobacterial blooms might promote the production of N₂O in Lake Taihu. To test the hypothesis, a microcosm simulation experiment was conducted in laboratory. With sufficient nitrate, N₂O release from the treatment with a small amount of cyanobacteria added (about 80 μg·L^-1 Chl-a) was significantly higher than that from the control group without cyanobacteria added (about 30 μg·L^-1 Chl-a, P < 0.001). However, the N₂O releases decreased significantly (P < 0.001) when Chl-a concentrations increased further. This suggests that when nitrogen is abundant, a certain concentration of cyanobacteria can promote N₂O release, but higher concentrations might inhibit it. In an acetylene inhibition experiment, the N₂O production rate increased markedly with increasing Chl-a, even with high cyanobacteria concentrations. This indicates that high cyanobacteria concentrations in microcosms did not inhibit N₂O production, but rather accelerated the reduction process of N₂O to N₂.