The nitrogen reduction in eutrophic water column driven by Microcystis blooms.

During the bloom seasons, the dissolved inorganic nitrogen declines, which results in the occurrence of nitrogen limitation. It is unclear where the nitrogen goes. Our enclosure experiments and batch tests suggested that Microcystis blooms could significantly reduce the nitrogen in water bodies and the key mechanisms for the nitrogen reduction in different layers were different. The assimilation was the main pathway for nitrogen reduction in the surface layer, while denitrification played an important role both at the sediment-water interface and in the overlying water. Stable nitrogen isotope experiments showed that the nitrate reduction efficiency at sediment-water interface was enhanced by Microcystis, reaching to 76.5∼84.7 %. Dissimilation accounted for 63.8∼67.3 % of the nitrate reduction, and the denitrification rate was 7.4∼8.5 times of DNRA rate. In the water column, the Microcystis bloom facilitated the formation of dark/anoxic condition, which favored the denitrification. The Microcystis aggregates collected from the field showed a great potential in removing nitrogen, and the TN in the overly water was reduced by 3.76∼6.03 mg L-1 within two days. This study provided field evidences and deeper insights into the relationship between Microcystis blooms and nitrogen reduction in the whole water column and gave more details about the enhancing effects of Microcystis on nitrogen reduction.