Abstract: By using inductively coupled plasma mass spectrometry (ICP-MS), the exposure levels of the typical heavy metals cadmium (Cd) and chromium (Cr) in the water body of Taihu Lake were monitored in September 2010. Based on the analysis of the exposure levels of the Cd and Cr and their chronic toxic effects on the aquatic organisms, the ecological risks of the two heavy metals were assessed by the method of safety threshold. Both Cd and Cr were detected in the Taihu Lake water body, and the average exposure concentrations of Cd and Cr were 0.85 μg·L^-1 and 40.04 μg·L^-1, respectively. Biological evidences and previous researches documented that aquatic organisms were more sensitive to Cd than to Cr, and the safety threshold of Cr was greater than 1 whereas that of Cd was slightly less than 1. According to the safety thresholds,  Cr in Taihu Lake had produced definite ecological risk to the aquatic organisms, whereas Cd didn’t produce obvious risk. Thereby, enough attention should be paid to the relatively low toxicity Cr because of its greater ecological risk at high exposure concentration.

Key words: Microtus fortis, Population, Three Gorge Engineering, Dongting Lake