Distribution of nitrogen and phosphorus and estimation of nutrient fluxes in the water and sediments of Liangzi Lake, China

Water samples and sediments from Liangzi Lake were taken and used to study the vertical distribution characteristics of nitrogen (N) and phosphorus (P) in both the overlying and pore water. Fluxes of ammonia (NH₄⁺-N) and phosphate (PO₄³⁻-P) were calculated using a one-dimensional transport-reaction model based on Fick's First Law. The results showed that the mean NH₄⁺-N and NO₃⁻-N concentrations in the overlying water of Liangzi Lake were 2.59 and 0.46 mg L⁻¹, respectively. The mean PO₄³⁻-P concentrations were lower than the detection limit. Both N and P displayed peaks at the sediment-water interface. For example, the mean concentration of ammonia in pore water in the surficial layer (0–5 cm) was 4.29 ± 2.74 mg·L⁻¹, which was twice than that of the overlying water. Two PO₄³⁻-P vertical profile regimes were identified; one had a gradually increasing trend, while the other first increased and then decreased. The mean orthophosphate concentration in the pore water of the surface layer (0–5 cm) was 0.01 ± 0.01 mg·L⁻¹. The spatial distribution of ammonia flux values was highly heterogeneous. Using these data, the annual load contribution of autochthonous ammonia was calculated to be 481 t a⁻¹. Studying N and P pollution and fluxes in the lakes of urban drinking water sources facilitates the provision of protection measures.