Environ. Sci. Technol. 2010, 44, 8030–8037
Tracing Sources and Bioaccumulation of Mercury in Fish of Lake Baikal- Angara River Using Hg Isotopic Composition V I N C E N T P E R R O T , * ,† V L A D I M I R N . E P O V , * ,† MIKHAIL V. PASTUKHOV,‡ VALENTINA I. GREBENSHCHIKOVA,‡ CYRIL ZOUITEN,§ JEROEN E. SONKE,§ SØREN HUSTED,| OLIVIER F. X. DONARD,† AND DAVID AMOUROUX† Laboratoire de Chimie Analytique Bio-Inorganique et Environnement, Institut Pluridisciplinaire de Recherche sur l’Environnement et les Mate´riaux, CNRS-UPPA-UMR-5254, He´lioparc, 2 Avenue du Pre´sident Pierre Angot, Pau, 64053, France, Laboratory of Problems of Geochemical Mapping and Monitoring, Institute of Geochemistry SB RAS, 1A Favorskogo Street, PB-304, Irkutsk, 664033, Russia, Laboratoire des Me´canismes et Transferts en Ge´ologie, Observatoire Midi-Pyre´ne´es CNRS-UMR-5563 Universite´ de Toulouse 3, 14 avenue Edouard Belin, 31400 Toulouse, France, and Department of Agriculture & Ecology, Plant and Soil Science, University of Copenhagen, Thorvaldsensvej, 401871 Frederiksberg C Denmark
Received October 6, 2009. Revised manuscript received September 8, 2010. Accepted September 25, 2010.
This study presents the determination and comparison of isotopic compositions of Hg in sediments, plankton, roach, and perch of two freshwater systems in the Lake Baikal-Angara River aquatic ecosystem: the man-made Bratsk Water Reservoir contaminated by Hg from a chlor-alkali factory and the noncontaminated Lake Baikal. Isotopic ratios of biota exhibit both significant mass-independent fractionation (MIF) (∆199Hg from 0.20 to 1.87‰) and mass-dependent fractionation (MDF) (δ202Hg from -0.97 to -0.16‰), whereas sediments exhibit high MDF (δ202Hg from -1.99 to -0.83‰) but no MIF. δ15N and δ13C are correlated with methylmercury in organisms from both sites, indicating bioaccumulation and biomagnification through food webs of both regions. Combining this with isotopic composition of samples shows that δ202Hg increases with the trophic level of organisms and also with methylmercury in fish from Lake Baikal. This study demonstrates that MIF in fish samples from Bratsk Water Reservoir allow to trace anthropogenic Hg, since fish with the highest levels of Hg in muscle have the same isotopic composition as the sediment in which anthropogenic Hg was deposited. Less contaminated fish do not exhibit this anthropogenic signature accumulating * Address correspondence to either author. E-mail: v.perrot@ etud.univ-pau.fr;
[email protected]; phone: +33 (0) 540 175 035; fax: +33 (0) 559 407 781. † Institut Pluridisciplinaire de Recherche sur l’Environnement et les Mate´riaux. ‡ Institute of Geochemistry SB RAS. § Universite´ de Toulouse. | University of Copenhagen. 8030
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relatively lower Hg amount from the contaminated sediments. This work reveals that Hg isotopic composition can be used to track the contribution of anthropogenic sources in fish from a contaminated lake.
Introduction The toxicity of mercury (Hg) is well-known to be dependent on its molecular speciation, and it has been demonstrated that Hg undergoes multiple biogeochemical transformation pathways in various environmental compartments. In aquatic systems, inorganic Hg (IHg) is known to be methylated mainly by bacteria (1) which subsequently leads to bioaccumulation and biomagnification of methylmercury (MeHg) in aquatic organisms (2-5) in relation to water chemistry (6, 7). Although the geochemistry of Hg in the environment is well described (8, 9), a complete understanding of its biogeochemical cycle is still not available, including information about its source(s) in many environments (10, 11). Measuring precise and accurate isotopic ratios of stable isotopes of heavy elements using multi-collector inductively coupled plasma/mass spectrometer (MC-ICP/MS) has been developed (12-14), thus, the study of Hg isotopes has emerged as a powerful tool to trace sources and chemical transformations of Hg in the environment (15, 16). Several authors have reported mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg in the environment (17-21), and various biogeochemical transformations fractionate Hg isotopes (22-25). Consequently, it has been reported that in the environment, different compartments show distinct Hg MIF signatures of odd isotopes: atmospheric IHg deposition has mainly been reported to have negative MIF anomalies (26, 27), whereas in aquatic ecosystems MeHg displays positive MIF (17, 18, 21, 28, 29). MIF of Hg odd isotopes during these reactions have been related to nuclear volume effect and/or magnetic isotope effects (19, 20, 30). However, most of the studies on MIF production during chemical transformations of Hg have been made in laboratory experiments (25, 31, 32), and the extent and the probability of such reactions under environmental conditions is still unclear. Combining the study of Hg with analysis of carbon (δ13C) and nitrogen (δ15N) stable isotopes in aquatic biota has already been used to study bioaccumulation and biomagnification of Hg in food webs (5, 6, 33). Previous study (18) has suggested that MDF of Hg isotopes traces Hg bioaccumulation and excretion in fish. More recently several studies (17, 28, 34, 35) have shown that Hg isotopic composition can be used in conjunction with δ13C and δ15N analyses to trace Hg patterns in aquatic ecosystems, relative to the trophic levels, observing that both MDF of even isotopes and MIF of odd isotopes of Hg occur in fish tissues. The aim of the present work is to investigate how variations in the natural stable isotopic composition of Hg in sediments, plankton, and fish can trace the sources and bioaccumulation pathways of Hg in the Lake Baikal-Angara River aquatic ecosystem. Two sites impacted with distinct Hg sources are studied to investigate whether tracing Hg contamination with isotopic composition in food webs is feasible. The first site is Lake Baikal (LB), where human impact is minimal and the food web structure in open water is relatively simple (36), hence representing an ideal natural laboratory to study biogeochemical cycles and processes. Atmospheric and river inputs of Hg to LB are predominant (37). In the water column of LB, concentration of total Hg is low (
