Algal Decomposition Accelerates Denitrification as Evidenced by the High-Resolution Distribution of Nitrogen Fractions in the Sediment-Water Interface of Eutrophic Lakes

This study investigates the decomposition process of algal blooms (ABs) in eutrophic lakes and its impact on the labile endogenous nitrogen (N) cycle. In situ techniques such as diffusive gradients in thin films (DGT) and high-resolution dialysis (HR-Peeper) were employed to decipher the vertical distribution of N fractions within the sediment-water interface (SWI) in Taihu, China. Additionally, an annular flume was used to simulate regional differences in lake conditions and understand labile nitrogen transformation during AB decomposition. This study reveals that the NH4+-N fraction exuded from algae is subsequently converted into NO3-N and NO2-N through nitrification, resulting in a significant increase in the concentrations of NO3--N and NO2--N at the SWI. The decomposition of algae also induces a significant increase in dissolved organic matter (DOM) concentration, referring to humic acid and humus-like components; a seven-millimeter decrease in dissolved oxygen (DO) penetration depth; as well as a significant decrease in the pH value near the SWI, which consequently promotes denitrification processes in the sediment. Moreover, the decomposition process influences nitrogen distribution patterns and the role conversion of sediments between a "source" and a "sink" of nitrogen. This investigation provides evidence on the migration and/or transformation of N fractions and offers insights into the dynamic processes across the SWI in eutrophic lakes.