RESUMEN
OBJECTIVE: To investigate the reasons of natural extinction of Oncomelania hupensis snails by comparing the difference of the water chemical properties between the snail-breeding marshland and non-snail marshland in Eastern Dongting Lake areas. METHODS: Three adjoining marshlands in Eastern Dongting Lake areas were selected, one was a snail-breeding marshland, the second was a non-snail marshland, and the third was a boundary zone between them. During the periods of water-rising season, wet season and water-falling season, the water samples from the bottom of 0.5 m were collected by the systematic random sampling method (200 m × 200 m grid method). In addition, the water samples below the surface of 0.5 m were also collected in the wet season. The physicochemical indexes of the sampling water were detected. RESULTS: According to the Surface Water Environmental Quality Standard (GB 3838-2002), the water quality indicators which were less than the detection limit were Cr, Cd, Zn, Hg, Cu, Sï¼ and CNï¼ during the three sampling seasons. The pollutants which were slightly higher than Grade III but lower than Grade IV (Hg ≤ 1 µg/L, TP ≤ 0.1 mg/L) were Hg and TP. The pollutants below the Grade III included As and Fï¼. The temperature and pH values were within the Grade III. The oxygen consumption indicators covering DO, COD and BOD in the water-rising season and wet season were within the Grade III, while the value of BOD was beyond the scope of Grade III but was between Grade â £ (≤ 6 mg/L) and Grade V (≤ 10 mg/L) in the water-falling season. The differences of the water pollution indexes between the snail-breeding marshland and non-snail marshland were statistically significant, and these indexes included Fï¼, As and pH in the water-rising season (P < 0.05); pH, BOD and Fat the surface layer in the wet season (P < 0.05); Fï¼ at the bottom in the wet season (P < 0.05); TP and Fï¼ in the water-falling season. Moreover, the above-mentioned indexes in the non-snail marshland of Qianliang Lake were higher than those in the snail-breeding marshland of Junshan Park. Thus, Fï¼ was the only index which had statistical differences in every sampling season, and the concentration of Fï¼ at the non-snail marshland was also higher than that at the snail-breeding marshland. In the water-rising season, the pH value at the non-snail marshland exceeded the suitable range for the growth of snails (6.8 to 7.8). CONCLUSIONS: In the Eastern Dongting Lake area, the high concentration of Fï¼ and pH of water may be responsible for the natural extinction of O. hupensis snails.
