Dissolved oxygen stratification changes nitrogen speciation and transformation in a stratified lake.

Dissolved oxygen (DO) stratification is a natural phenomenon in lakes, which potentially influences nitrogen (N) biogeochemical cycle. However, the specific effects of DO stratification on N speciation and transformation behaviors in different water layers are still poorly understood. Here, we reported that DO stratification remarkably influenced N species and transformation pathways in different water columns by high frequency sampling during summers in Longjing Lake, China. Results showed that DO stratification in the lake created three water layers: epilimnion (1-3 m), oxycline (4-11 m), and hypolimnion (12-20 m). In the epilimnion, N speciation was mainly controlled by phytoplankton assimilation and organic N dominated in this layer. Oxycline was the major place for N transformations and had the most notable N removal capacity (714 kg N from June to August). In the hypolimnion, [Formula: see text] was the major N species, and sediment release contributed nearly 85% hypolimnetic [Formula: see text]. Furthermore, approximately 8 kg of dissolved N2O was also accumulated in the hypolimnion, contributing ~ 70% of N2O in the whole lake. Overall, our results indicated that DO stratification caused the shifts in N speciation and transformation behaviors among different water columns, which may have a great implication for lake managements for providing separated protection strategies from different water depths.