Characteristics Of Heavy Metals Pollution Of Fish From Three Gorges Reservoir During The Initial Impoundment

Three Gorges Reservoir (TGR) is located in the lower section of the upper reaches of the Yangtze River, China. It is a giant canyon-shape reservoir with great depth, and an important aquatic ecosystem to the lower reaches of the Yangtze River. Due to its large scale, the environmental effect of TGR raises great social concern. Reservoir has been believed as sensitive ecosystem to heavy metal pollution. The "Reservoir Effect" after impoundment can increase methylmercury production in the base of food web and lead to substantial increase in the predator fish, posing a risk to humans. Even before its construction, there were predictions of sharply increased heavy metal levels, e.g. mercury, in fish bodies from TGR after impoundment.This study tried to investigate the heavy metal concentrations in fish and its biomagnification characters along food chains in TGR after175m impoundment. Eleven specie of fish were collected from the main stem and tributaries of TGR, and the contents of mercury (Hg), chromium (Cr), cadmium (Cd), arsenic (As), lead (Pb), nickel (Ni), copper (Cu) and zinc (Zn) in muscle tissue of all fish samples were determined. The levels, spatial distributions and factors of heavy metals in fish from TGR were analyzed based on these data. More over, typical areas were selected to make intensive study, food sources and nutrition levels of the samples were evaluated by using stable isotope (δ13C and δ15N). The characteristic of heavy metal transfer and biomagnification along food chain were investigated. And the Target Hazard Quotient (THQ) was used to assess the potential health risks to local citizens. Result shows:1. The average concentrations of Hg、Cr、Cu、Pb、Ni、Zn、As and Cd in twelve fish species were0.56,0.28,0.23,0.19,0.60,6.1,0.46mg/kg and5.6μg/kg. All heavy metals concentrations in carps were lower than the tolerance limits in the related national standards. Significant positive correlations between Hg、Pb and fish sizes were observed. Heavy metal concentrations of fish related with its feeding type and habitats.2. There was no significant difference between concentrations in fish from main stem and tributary. However, due to strengthening sedimentation along reservoir flow, an obvious decreasing trend of these heavy metals in carp was observed from upstream to middle area of TGR in main stem. In typical tributary, there was no significant difference between the heavy metal concentrations in fishes from backwater and river mouth. But the heavy metal contents in demersal fishes from backwater were significantly higher than those in corresponding species from river mouth.3. Stable isotope results indicate that the food sources of fishes in stem and tributary of TGR could be different:fishes in tributary rely more on pelagic primary production while those in main stem tend to take allochthonous materials carried by runoff. In typical tributary, fishes in backwater have a wide range of food sources including phytoplankton, detritus and insect larvae in sediment, while fishes in river mouth mainly rely on suspended particle organic matter in water. Compared with other fishes, the demersal fishes in backwater water area have more contact with sediment, and this may be an important reason for its higher heavy metal levels.4. The log concentrations of Hg、Cr、Zn、Pb、Cd and As in fish increases through815N increments in TGR, while the log concentrations of Ni and Cu decrease through815N increments. Significant positive correlations in the regressions of log metal content of the biota versus815N was obtained for mercury, indicating that an overall biomagnification of Hg. However, the increase of Hg concentration per trophic level was lower than in previous studies.5. While Log-transformed mercury contents were consistently correlated with δ15N yalues for the fish food web in TGR, the regression slope was significant higher in tributary than that in main stem. This indicates that biomagnification power of mercury is greater in tributary than in main stem of TGR. In typical tributary, the biomagnification power was greater for pelagic species compared with the benthic species, but higher concentration of Hg was found in the base of benthic food chains (intercept) compared with the pelagic food chain.6. The target hazard quotients (THQ) of Hg、Cr、Cu、Zn、Pb、Cd and As was less than1, and the estimated weekly intake (EWI) was far below the provisional tolerable weekly intake (PTWI), which indicates the risks for the general population consumption is low in TGR.