Characterization of nitrogen and phosphorus at the ice-water-sediment interface and the effect of their migration on overlying water quality in Daihai Lake (China) during the freezing period

Approximately one-third of the year, Daihai Lake experiences freezing conditions. During this period, the primary mechanisms that impact the quality of the lake water quality include the freezing of nutrients by the ice sheet and the migration of nutrients between the ice, water, and sediment. The present investigation involved the collection of samples of ice, water, and sediment, followed by the utilization of the thin film gradient diffusion (DGT) technique to and explore the distribution and migration of diverse nitrogen (N) and phosphorus (P) forms at the interface of ice, water and sediment. The findings indicate that the freezing process led to the precipitation of ice crystals, which in turn caused the migration of a significant proportion (28-64 %) of nutrients to the subglacial water. The predominant constituents of N and P in subglacial water, were NO3--N and PO43--P, which constituted 62.5-72.5 % of total N (TN) and 53.7-69.4 % of total P(TP). Respectively, with increasing depth, the TN and TP of sediment interstitial water increased. The sediment in the lake acted as a source of PO43--P and NO3--N while acting as a sink or NH4+-N. Soluble reactive P (SRP) flux and NO3--N flux were responsible for 76.5 % and 2.5 % of the P and N present in the overlying water. Additionally, it was observed that 60.5 % of the NH4+-N flux in the overlying water was absorbed and subsequently deposited in the sediment. The presence of soluble and active P in the ice sheet could play a crucial role in the regulation of sediment release of both SRP and NH4+-N. Additionally, the presence of high nutritional salts and the concentration of nitrate nitrogen in the overlying water would certainly increase the pressure of the water environment. Endogenous contamination must be urgently controlled.