Nutrients and heavy metals in multi-cores from Zhushan Bay at Taihu Lake, the largest shallow lake in the Yangtze Delta, China

Taihu Lake is the third largest freshwater lake of China. The Taihu Lake drainage basin is an economically developed area, with one of the highest population densities in China. Zhushan Bay was one of the most polluted water bodies in northern Taihu Lake. Ten short sediment cores were collected from Zhusan Bay in 2003 and 2004. Heavy metals and nutrients were analyzed in order to understand their temporal and spatial change in recent sediments. The profiles of heavy metals (Cu, Pb, Zn, Cr and Ni) and nutrients (TOC, TN and TP) were similar showing upcore increasing tendencies in the top 5–10 cm sediments for most cores while the depth of increase for OP was deeper than for other nutrients. TOC, TN, TP and OP averages are, respectively, 8.17, 0.92, 0.37 and 0.015 g/kg and 11.65, 1.16, 0.67, 0.093 g/kg for 10–30 cm and 0–10 cm sediment. The average value of Cu, Pb, Cr, Zn and Ni is 18.4, 24.6, 52.8, 55.6 and 27.8 mg/kg for 10–30 cm sediment, and 30.2, 39.5, 73.9, 100.2 and 41.0 mg/kg for 0–10 cm sediment, respectively. Except for lead, high positive correlations (>0.86, P < 0.001) were found for Cr–Cu, Cr–Zn, Cr–Ni, Cu–Zn, Cu–Ni, Ni–Zn elements indicating the different source of lead pollution. According to Sediment Quality Guidelines, most layers of 0–5 cm sediment were moderately or heavily polluted by Pb, Cu, Zn, Cr and Ni in cores from Zhushan Bay. Most 5–30 cm sediments were unpolluted by Pb, Cu and Zn. However, all sediments from Zhushan cores were polluted by Ni and Cr. Compared with the background level of Ni and Cr elements, high local levels of Ni and Cr for Jiangsu paddy field leads to high concentration of Ni in Zhushan core sediment.