Phosphorus, organic matter and nitrogen distribution characteristics of the surface sediments in Nansi Lake, China

Nansi Lake is the largest and most important freshwater reservoir for the South-North Water Diversion Project located in Shandong Province, China. The characteristics of the nutrient compositions and distribution in the lake sediment may significantly influence the upper-level water quality, which has not been well studied. In this study, the distribution characteristics of the total phosphorus (TP), total organic matter (OM) and total nitrogen (TN) contents in the shallow sediments of Nansi Lake were investigated. The experimental results showed that the sedimentary TP, OM, and TN levels of the entire Nansi Lake (expressed as dry weight percentage) were (0.030 ± 0.003)–(0.129 ± 0.018) %, (1.14 ± 0.18)–(10.60 ± 1.30) % and (0.105 ± 0.021)–(0.71 ± 0.08) %, respectively. The three nutrient indicators appeared to be higher in the upstream lake than in the downstream lake. Concentrations of TN and OM were both particularly higher in aquaculture zones of Nansi Lake, where excessive fish feed may largely contribute to the high TN and OM in the sediment. Furthermore, there was a significantly positive correlation (n = 28, R² = 0.7870) between TN and OM. According to the pollution index (P_i) and enrichment factor, there is moderate enrichment of TN and OM in the sediment, while minor enrichment of TP in Nansi Lake. Calculations of the OM index and organic nitrogen index suggested that the surface sediment of the entire Nansi Lake has been contaminated by OM and organic nitrogen. Therefore, there is a pressing need to further investigate the release characteristics of these nutrient contaminants from the sediment of Nansi Lake and potential impacts on the surface water quality.