Mercury contents in fish and its biomagnification in the food web in Three Gorges Reservoir after 175m impoundment

Three Gorges Reservoir(TGR) is a giant canyon-shaped reservoir located in the lower section of the upper reaches of the Yangtze River.After impoundment,hydrological signatures,chemical factors and species composition,had been greatly changed in TGR.The new environmental conditions could increase methylmercury(MeHg) production,and induce substantial increase of mercury concentration in the predator fish through food chain biomagnification.Concern about human consumption of fish contaminated with MeHg had been raised even before the construction of TGR.This study tried to investigate the total mercury concentrations in fish and biomagnification characters of mercury along food chains in TGR after 175m impoundment.The stable isotope analysis could be a useful method of checking and evaluating the trophic conditions of aquatic ecology.We collected eleven fish species from three main stem sections and seven typical tributaries of TGR from 2011 to 2012.Carp(Cyprinus carpio) was selected as a typical species to investigate the spatial distribution of mercury level in fish from TGR.Total mercury concentrations and stable isotope ratios(δ13C and δ15N) of all fish samples were determined.Result showed that mercury concentrations in fishes from TGR had not obviously increased after impoundment.The carp mercury concentrations in TGR were comparable to values reported for other areas of Yangtze River Basin.But spatial difference was found among different sections of main stem.Due to the enhanced sediment deposition along reservoir,there was a decreasing trend of mercury concentration in fish from upper stream to lower stream within TGR.Mercury concentrations in the fishes from Luoqi in up area of reservoir(88.0μg/kg,average) was significantly higher than those from Wushan in middle area of reservoir(43.1μg/kg,average) and tributaries(57.1μg/kg,average).The mercury concentrations in fish from tributaries were comparable to those from main stem,and there were no significant difference among most tributaries.Based on the carbon and nitrogen stable isotope ratios and total mercury contents in eleven species fish,the biomagnification power of mercury in two sections and one tributary were evaluated.While log-transformed mercury contents were consistently correlated with δ15N values for the fish food web in all sampling sites,the slope of the relationship with δ15N(biomagnification power value) was significant higher in Shennong River than that in Wanzhou and Wushan section.This indicated that biomagnification power of mercury is greater in tributary than in main stem of TGR.Carbon and nitrogen stable isotope analysis also indicate a difference in food source of fishes between tributary and main stem.In tributary(Shennong River),the values of fish samples were-22.08‰-28.92‰ for δ13C and 8.26‰-13.15‰ for δ15N;In main stem,the values of fish samples were-23.11‰-26.87‰ for δ13C and 8.95‰-16.98‰ for δ15N.This indicate fishes in tributary rely more on pelagic primary production while those in main stem tend to take allochthonous materials carried by runoff.In conclusion,the body type of tributary and main stem after impoundment could be different,and this may be an important reason of different mercury bioaccumulation between tributary and main stem within TGR.