| In the early 1970s, several researchers found that the methylmercury (MeHg) concentration in fish in new impoundments was higher than that of fish in natural lakes, and the MeHg level of some fish even exceeded the food hygiene standards recommended by World Health Organization. From then on, mercury contamination of aquatic ecosystems has become an important worldwide health concern. It has been proved that reservoir is a typical eco-sensitive area due to its special "mercury activation effect", which is an important source of lipophilic MeHg accumulated in the water and fish because of the submerged soils and vegetations. Three Gorges Reservior(TGR) is a distinguished impoundment in terms of its extra-large size and large flooding areas called water-level fluctuation zones(WFZ) formed from March to August every year. Meanwhile, plentiful vegetations in WFZ grow in these large areas due to sufficient rainfall and optimal temperature. Then, the WFZ is submerged gradually from September to February. Thus, the soil in WFZ keeps wet-dry alternatively every year, and the plant renews with a growth-inundation-growth process periodically. Therefore, the periodic change of water level makes TGR seem to be a new reservoir every year.Up to now, researches about mercury biogeochemistry in WFZ in TGR are mainly focused on the mercury concentration investigation in soil, water and fish, the interface chemical fate of mercury, the absorption and resolution of mercury in soil as well as MeHg photodegradation in the surrounding water. However, scarce information is available regarding the release and transformation of mercury in sediment and plant in WFZ, the effect of submerged plant on mercury in water and its accumulation in fish. Thus, the objectives of this study were to investigate the transformation of mercury (particularly methylation) in soil and plant, and to evaluate the bioavailability of mercury. The results showed that:(1) The level of total mercury (THg) in soil decreased with the duration of submerging time while the THg level in the water increased first and then decreased. Repeated submergence made THg level decrease slightly. The level of MeHg in soil increased gradually and the trend was the most obvious during the first 7 days. The MeHg level in soil with original mercury concentration treatment (SO) was 22.49% higher than that of observed in soil with high mercury concentration treatment (SH). After 90 days, the MeHg level in the SOMT and SHMT increased 3.25 and 1.62 times respectively, as compared with background level. After the water was drained, MeHg level in soil decreased, particularly in SHMT while the level of MeHg in water increased first and then decreased. During the first impounding, MeHg levels in the water of SOMT and SHMT were highest in 14 days, with 0.83 ng·L-1 and 1.19 ng·L-1, respectively. Repeated submergence made MeHg level increase slightly.(2) During the first submergence (90 days), the level of THg in soil in soil with original mercury concentration and plant treatment (SP) and SO decreased by 0.53 μg·kg-1·d-1 and 0.43 ug·kg-1·d-1, respectively, but no significant difference were observed between them (p>0.05), which suggested that the submerging plant had little significant effect on the variation of THg level in soil. The level of THg in water in plant treatment (PT), SOPT and SOMT increased to an average level of 17.61 ng·L-1,22.39 ng·L-1 and 16.65 ng·L-1, respectively, which were much higher than control treatment (CT,8.46 ng·L-1), which suggested that soil and plants of WLFZ were both important sources of mercury in the water of reservoir. The submergence decreased the THg level in plant but increased its MeHg level.(3) During the submergence, the level of MeHg in soil in SP increased significantly in the first several days, particularly in the first days and maximum value of 2656.69 ng·kg-1 was observed in soil in SP after 14 days, which was 6.15 times higher than the background level. Then the MeHg level in soil decreased gradually, and it was almost the same with that of SO in 90 days. The plant suffered from a second submergence still facilitated mercury methylation in the soil. Among the first submergence, the level of MeHg in water in plant spiked treatment (PT and SP) increased first, particularly from 7 days to 21 days, and then remained constant after 28 days. Besides, the MeHg level in water was positively correlation with the level of Dissolved Organic Carbon (p<0.05).(4) During either submergence, the levels of THg in the head, viscera and muscle of zebrafish increased in all the treatments (except CT), particularly in the first submergence. The THg level in the muscle was higher than that of observed in head and viscera to some extent. In the first submergence, the average THg level in the head was measured to be:SH (137.63 ng·g-1)>PT (135.96 ng·g-1)>SO (124.96 ng·g-1)>SP (117.75 ng·g-1); and that in viscera with the order of SH (133.79 ng·g-1)>SP (119.42 ng·g-1)>PT (90.67 ng·g-1>SO (88.18 ng·g-1); whereas in the muscle to be:SP (190.35 ng·g-1)>SH (185.77 ng·g-1)>PT (162.55 ng·g-1)>SO (140.62 ng·g-1).(5) During either submergence, the levels of MeHg were accumulated obviously in the head, particularly in the plant spiked treatment (PT and SP) compared to those treated only by soil. In the first submergence, the MeHg levels in the viscera increased obviously in soil and/or plant spiked treatments, particularly in the first 7 days. The MeHg levels in muscle in all treatments increased along the submerging time, which were higher than those of observed in relevant head and viscera. In the first submergence, the average MeHg level in the head was measured to be:SH (92.53 ng·g-1)> PT (89.54 ng·g-1)> SP (88.06 ng·g-1)> SO (74.46 ng·g-1); and that in viscera with the order of SH (113.31 ng·g-1>SP (95.53 ng·g-1)>PT (74.15 ng·g-1)>SO (73.83 ng·g-1); whereas in the muscle to be:SH (151.73 ng·g-1)>SP (96.61 ng·g-1)>PT (71.94 ng·g-1)>SO (66.10 ng·g-1). MeHg levels accumulated in the head, viscera and muscle of zebrafish to different degrees, but each of them positively correlated with the corresponding THg levels. |
PDF Full Text Request