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The distribution of total mercury and methyl mercury in a shallow hypereutrophic lake (Lake Taihu) in two seasons
Applied Geochemistry 27 (2012) 343–351
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The distribution of total mercury and methyl mercury in a shallow hypereutrophic lake (Lake Taihu) in two seasons Shaofeng Wang, Denghua Xing, Yonfeng Jia ⇑, Biao Li, Kuanling Wang Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 15 June 2011 Accepted 27 September 2011 Available online 4 October 2011 Editorial handling by J.E. Gray
a b s t r a c t To understand the geochemical cycle of Hg in hypereutrophic freshwater lake, two sampling campaigns were conducted in Lake Taihu in China during May and September of 2009. The concentrations of unfiltered total Hg (unfTHg) were in the range of 6.8–83 ng L1 (28 ± 18 ng L1) in the lake water and total Hg in the sediment was 12–470 ng g1, both of which are higher than in other background lakes. The concentration of unfTHg in 11% of the lake water samples exceeded the second class of the Chinese environmental standards for surface water of 50 ng L1 (GB 3838-2002), indicating that a high ecological risk is posed by the Hg in Lake Taihu. However, the concentrations of unfiltered total MeHg (unfMeHg) were relatively low in the lake water (0.14 ± 0.05 ng L1, excluding two samples with 0.81 and 1.0 ng L1). Lake sediment MeHg varied from 0.2–0.96 ng g1, with generally low ratios of MeHg/THg of <1%. The low concentrations of TMeHg in the lake water may have resulted from a strong uptake by the high primary productivity and the demethylation of MeHg in oxic conditions. In addition, contrary to the results of previous research conducted in deep-water lakes and reservoirs, the low concentrations of MeHg and low ratio of MeHg/THg in the lake sediment indicates that the net methylation of Hg was not accelerated by the elevated organic matter load created by the eutrophication of Lake Taihu. The results also showed that sediments were a source of THg and MeHg in the water. Higher diffusion fluxes of THg and MeHg may be partly responsible for the higher concentrations of THg in the lake water in May, 2009. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Mercury pollution in the environment is a primary concern because inorganic Hg can transform into the more toxic species, methylmercury (MeHg), which humans and wildlife can be exposed to through the food chain (Lebel et al., 1997; Legrand et al., 2007). MeHg can cause damage to the central nervous system in humans (Clarkson, 1993). In aquatic ecosystems, the primary source of MeHg is the methylation of inorganic Hg; this process is mediated by SO4-reducing bacteria (SRB) (Compeau and Bartha, 1985; Macalady et al., 2000) and/or Fe-reducing bacteria (Fleming et al., 2006) in anoxic sediments. The production of MeHg in aquatic habitats is influenced by a variety of environmental factors, such as the organic material content, pH, redox potential, temperature, available SO4 and bacterial species (Clarkson, 1993; Compeau and Bartha, 1985; Lambertsson and Nilsson, 2006). Lake eutrophication, another environmental concern around the world, can alter the environmental parameters significantly (Fukushima et al., 1996; Karlson et al., 2002; Scott et al., 2005; Sweerts et al., 1991), influencing the Hg cycle in lake ecosystems. ⇑ Corresponding author. Tel.: +86 24 83970503; fax: +86 24 83970436. E-mail address: [email protected] (Y. Jia). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.09.029
Recently, studies have investigated the impact of eutrophication on the production and bioaccumulation of MeHg in eutrophic lakes and/or reservoirs. For example, Gray and Hines (2009) observed a high rate of Hg methylation in the range of 2.3–17%/day in sediments; this rate was related to the eutrophication of the reservoir. Macalady et al. (2000) predicted that further eutrophication of Clear Lake would increase Hg methylation, either by altering sediment microbial communities or by increasing the organic C load in the sediment. He et al. (2008a,b) also found that the formation of MeHg in a reservoir could be accelerated by eutrophication. These works concluded that changes in the environmental parameters of eutrophic water bodies are likely to promote Hg methylation and to enhance MeHg accumulation in aquatic organisms. However, the acceleration of net MeHg formation was found mainly in deep-water reservoirs (e.g., Gray and Hines, 2009; He et al., 2008b; Macalady et al., 2000); in shallow, eutrophic lakes, the geochemical cycle of Hg is far from clear. For instance, Vaithiyanathan et al. (1996) determined that the net Hg methylation rate in the eutrophic soil of the everglades was not high, although there was a relatively high potential for Hg methylation and MeHg demethylation. Lake Taihu, a large, shallow, freshwater lake in China, is an important resource of drinking water, fishing and tourism in Eastern China (Sun and Mao, 2008). After decades of intensive input
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of nutrients from industrial and agricultural sources, Lake Taihu has become hypereutrophic, and algal blooms last from May to October annually (Qin et al., 2007). In late May of 2007, millions of people in Wuxi, in the Jiangsu Province of China, were impacted by a drinking water crisis following a massive phytoplankton bloom in Lake Taihu (Qin et al., 2010). In addition to the serious eutrophication, Lake Taihu has suffered Hg contamination. Concentrations of total Hg (THg) in the range of 0.063–0.99 mg/kg with an average of 0.091 ± 0.19 (n = 290) mg/kg were reported in surface sediments from Lake Taihu; the highest concentration was approximately one order of magnitude greater than that in the background lakes (Fan and Zhang, 2009). However, little is known about the Hg cycle under the particular conditions of Lake Taihu. In this study, water and sediment samples were collected from Lake Taihu during two different seasons to explore the speciation and distribution of Hg in this hypereutrophic, shallow, freshwater lake.
with a single-use syringe. After rinsing the borosilicate bottles 3 times with the samples, the unfiltered and filtered water samples were preserved by adding ultra-pure HCl to a final concentration of 0.4%, placed in sealed sample bottles and stored at 4–8 °C in a cooler box with ice bags. Sediment cores were collected using a stainless steel sampler with a plexiglass tube and sectioned into 1–2 cm intervals in the field in a glovebag filled with N2. The sectioned sediment core samples were placed in 50-mL centrifuge tubes, sealed with Parafilm to avoid cross contamination and stored in a sample box with dry ice. In the laboratory, the porewater samples were immediately separated by centrifugation at 4000 rpm for 30 min and then filtered through a 0.45-lm PVDF membrane (Millipore) in an anaerobic chamber (Bactron II, USA). The filtrate was preserved with 0.4% ultra-pure HCl and stored in sealed borosilicate glass bottles for further analysis.
2.3. Determination of THg and MeHg 2. Methods and materials 2.1. Area description Lake Taihu (30°550 4000 –31°320 5800 N, 119°520 320 –120°360 1000 E), the third largest freshwater lake in China, is located in the southern part of Jiangsu province. The lake is shallow and flat-bottomed with an average depth of 1.9 m and maximum depth of 3.0 m. The total area of Lake Taihu is 2, 427 km2, and its volume is 4.43 billion m3 (Sun and Mao, 2008). The climate in the Lake Taihu area is controlled by subtropical monsoons, and the water temperature ranges from 1.5–32.5 °C, averaging 17.6 °C between 1991 and 1999 and 18.2 °C during 2005 and 2006 (Zhu et al., 2008). Traditionally, Lake Taihu is divided into 4 sub-basins based on geographical location and sources of nutrients and metals: north Taihu, central Taihu, east Taihu and west Taihu (Fig. 1). Presently, Lake Taihu as a whole is at a medium to high eutrophication level. However, the eutrophication level varies throughout the lake. Zhu (2009) reported that the average concentrations of total N, total P and Chllorophyll a in Lake Taihu decreased from 2006– 2008 in the following order: North Taihu > Central Taihu = West Taihu > East Taihu. According to previous investigations, North Taihu is at a hypereutrophic level, Central and West Taihu are at an eutrophic level, and east Taihu is at a light to medium eutrophication level. In addition, despite similar hydrology and climate conditions, Lake Taihu has developed two contrasting water types; North, Central and West Taihu resemble algal lakes, while East Taihu is characterized by macrophytic waters (Zhang, 2006). 2.2. Sampling Two sampling campaigns were conducted from May 10–15, 2009, when the biomass began to increase, and from September 1–10, 2009, when the phytoplankton bloom had lasted for approximately 3 months. Surface water, lake-bottom water, interfacial water and sediment cores were collected randomly in each sub-basin during each season (Fig. 1, Table 1). Prior to collecting the water samples, borosilicate bottles (125-mL) were cleaned with an alkali detergent, soaked overnight in dilute HNO3, and heated in a muffle furnace at 450 °C for at least 4 h; this pretreatment process removed any organic matter and trace Hg that was adsorbed on the vessels. Surface and lake-bottom water samples were gathered at a depth of 0.5 m and 1.5 m, respectively, using a self-designed water sampler. An aliquot of water was filtered through a singleuse 0.45-lm polyvinylidene fluoride (PVDF) membrane (Millipore)
To determine the concentration of THg in water, the samples were pre-concentrated and then measured using CVAFS (Model III, Brooksrand, USA) according to EPA method 1631e (USEPA, 2002). The THg in the sediment was measured using hydride generation-atomic fluorescence spectrometry (HG-AFS) (AFS-2202E, Haiguang, China) after digestion by hot HNO3 plus HClO4. The MeHg in water was determined using GC-CVAFS (Model III, Brooksrand, USA) after a distillation and ethylation procedure in accordance with EPA method 1630 (USEPA, 2001). The MeHg in sediment was determined based on the procedure developed by Liang et al. (2004), in which the samples are processed by acid digestion (3 M HNO3), solvent extraction (CH2Cl2), reverse extraction by water, ethylation and detection by GC-CVAFS. Quality assurance and quality control (QA/QC) of the analyses were carried out with duplicates, method blanks, matrix spikes, and certified reference material (IAEA-433). The detection limits of the method were 0.01 ng g1 for THg and 0.005 ng g1 for MeHg in sediment and 0.01 ng L1 for THg and 0.01 ng L1 for MeHg in water. A mean MeHg concentration of 0.17 ± 0.05 ng g1 (n = 4) was obtained for IAEA-433, which has a certified value of 0.17 ± 0.07 ng g1. Recoveries on the spiked MeHg in water samples ranged from 85 to 114%. The relative percentage difference was <6.5% for the duplicate samples of THg and MeHg in sediment and water.
2.4. Determination of the total organic carbon (TOC), Fe and Mn in sediment The total organic C (TOC) content of the sediment was determined by titration with FeSO4 after oxidation at 90 °C for 15 min in a mixture of potassium dichromate (VI) and concentrated H2SO4 (Walkley and Armstrong Black, 1934). The amount of Fe and Mn in sediment samples was measured using flame atomic absorbance spectrometry (FAAS, Varian-AA240, USA) after digestion by hot HNO3–HClO4–HF. The concentrations of elements in sediments are displayed on the basis of dry weight (dw). The certified reference material (marine sediment, GBW 07314) was determined for quality control, and the recoveries for TOC, Fe, Mn and S were 98%, 95%, 97% and 98%, respectively.
2.5. Determination of water quality parameters Dissolved O2 (DO), pH, Eh, water temperature (T) and conductivity of the lake water were measured in the field, using portable multi-meters (Jenco, Shanghai).
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Fig. 1. A sketch map of the sampling locations in Lake Taihu in May and September, 2009.
Table 1 Locations of the samples collected from the 4 regions of Lake Taihu in May and September 2009. Region
Month
Surface water
Lake-bottom water
Sediment core
Central Taihu
May September May September May September May September
C1, C3 C1, C2, C3, C4 E1, E2, E3 E1, E2, E3, E4 N1, N2, N3, N4 N1, N2, N3, N4, N5 W1, W2, W4 W1, W2, W3
C1, C3 C1, C2, C3, C4 E1, E2, E3 E1, E2, E3, E4 N1, N2, N3, N4 N1, N2, N3, N5 W1, W2, W3, W4 W1, W2, W3
C3 C3 E2 E2 N1, N3 N1, N3 W4 W2
East Taihu North Taihu West Taihu
2.6. Calculation of the Hg diffusion flux from the sediment The diffusion fluxes of THg and MeHg from the sediment into lake water in the two seasons were calculated using Fick’s first law, as has been described previously (e.g., Feng et al., 2009):
F ¼ ðuDw =h2 ÞdC=dx where F is the flux of THg or MeHg, C is the concentration at a depth x, u is the sediment porosity, Dw is the diffusion coefficient of THg and MeHg in water in the absence of a sediment matrix, and h is the tortuosity that was calculated according to h = 1 ln(u2). The diffusion coefficients of THg and MeHg in water at 25 °C were assumed to be 9.5 106 and 1.2 105 cm2 s1, respectively, as previously
reported (Covelli et al., 2008; Gill et al., 1999). To calculate the dC, the concentrations of THg and MeHg in the filtered interfacial water and the porewater were used. Because the average temperature of the lake water (24.3–28.8 °C, Table 2) was close to 25 °C, a no temperature correction was made for the diffusion coefficients. 3. Results and discussion 3.1. Water quality parameters No significant difference in the water quality parameters was observed between the surface and lake-bottom water at each sampling site. Seasonally, the average water temperature was
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Table 2 Summary of water parameters in Lake Taihu in May and September 2009. Region
Month
DO (mg L1)
T (°C)
pH
Central Taihu
May September May September May September May September
5.9 ± 0.43 (5.1–6.2)
24.3 ± 0..64 (23.8–25.3) 27.6 ± 0..35 (27–28) 25.0 ± 0.72 (24.5–26.3) 29.1 ± 1.3 (27.5–30.5) 27.0 ± 2.0 (24.3–30.8) 27.5 ± 0.50 (26.8–28.2) 25.5 ± 0.26 (25–25.8) 28.8 ± 0.55 (28–29.5)
7.4 ± 0.54 8.5 ± 0.09 7.6 ± 0.40 8.4 ± 0.23 8.3 ± 0.18 8.3 ± 0.57 7.2 ± 0.49 8.7 ± 0.26
East Taihu North Taihu West Taihu
5.8 ± 0.33 (5.5–6.2) 6.0 ± 0.21 (5.9–6.5) 5.9 ± 0.18 (5.7–6.2)
25.6 ± 1.5 °C and 28.3 ± 1.5 °C in May and September, respectively (Table 2). The fluctuations in water temperature were relatively small during each sampling period; however, there was a significant difference in temperature between the two seasons (p < 0.001, ANOVA). Environmental monitoring at 32 sites in 2009 showed that the average concentration of chlorophyll a was much lower in May (8.5 lg L1) than in September (25 lg L1) (Zhu, 2009, pers. comm.), and this change was consistent with the level of chlorophyll a calculated from remote sensing images collected during the two months (Kuang et al., 2010).This indicates that the algal bloom was at different stages in the two sampling periods. Algal blooms have a strong impact on water parameters, especially the pH and Eh. The lake water showed neutral to alkaline conditions; the pH in May was in the range of 6.5–8.0, which is distinctly lower than the range of 7.3–9.0 measured in September (p < 0.01, ANOVA) (Table 2). In north and central Taihu, which exhibit a stronger algal bloom, the redox potential of the lake water (Eh) was significantly lower in September than in May (ANOVA, p < 0.01). In east and west Taihu, where the influence of the algal bloom is weak, there was no significant difference in Eh between the two seasons (ANOVA, p > 0.01). Because Lake Taihu is a large shallow water body with relatively strong disturbance, the lake water is oxic (DO: 5.1–6.5 mg L1 and Eh: 120–340 mV). 3.2. Fe, Mn and TOC in sediment cores Concentrations of TOC, Fe and Mn varied from 0.96% to 17%, 10 to 41 g kg1 and 180 to 2700 mg kg1, respectively, in sediments from Lake Taihu (Fig. 2). The results show a large elevation in the concentration of TOC in the sediments, implying that long-term eutrophication has significantly increased the organic load in the sediment. The concentration of TOC decreased with depth, and the highest TOC occurred in the top 5 cm of sediment at all sampling sites, which is consistent with a previous study (Zhao et al., 2007). An exception is site E-2, where the highest concentration of TOC in May was at a depth of 10 cm (Fig. 2). Like TOC, Fe and Mn also decreased with depth in the sediment profiles at most sampling sites (Fig. 2). 3.3. Hg in lake water 3.3.1. THg The concentration of unfTHg varied from 6.8 to 83 ng L1 with an arithmetic mean of 28 ± 18 ng L1; the average concentration of unfTHg in the surface water (30 ng L1) was almost identical to that in the lake-bottom water (27 ng L1) (t-test, p = 0.91). The concentration of THg in filtered water (fTHg) was in the range of 4.2–46 ng L1 with an average of 17 ± 11 ng L1, which accounted for about 56 ± 19% (14–94%) of the unfTHg in the lake water. Spatially, the average concentrations of unfTHg and fTHg were higher in east and north Taihu than in central and west Taihu (Table 3). The concentrations of unfTHg and fTHg collected in May were much higher than in September at all sampling sites (Table 3).
(6.5–7.9) (8.4–8.7) (7.2–8.0) (8.3–8.8) (8.0–8.5) (7.3–9.0) (6.9–8.0) (8.5–9.0)
Eh (mV)
Conductivity (ms cm1)
291 ± 45 (200–320) 210 ± 74 (130–290) 280 ± 55 (210–340) 250 ± 36 (220–300) 310 ± 23 (290–340) 170 ± 65 (120–290) 290 ± 50 (220–340) 280 ± 6.6 (270–280)
0.58 ± 0 (0.57–0.59) 0.53 ± 0.03 (0.5–0.57) 0.80 ± 0.09 (0.69–0.93) 0.50 ± 0.03 (0.47–0.55)
In the study, the concentration of THg in Lake Taihu was significantly lower than that found in a previous investigation conducted by Yang et al. (1996), who reported that the THg concentration in the lake water varied from 140–760 ng L1. However, the present results show that the THg concentration in water from Lake Taihu is much greater than in background lakes in the USA (Gray and Hines, 2009; Hines et al., 2004), Sweden (e.g., Regnell et al., 1997) and even reservoirs in geologically Hg-enriched areas of China (Feng et al., 2009; He et al., 2008a,b). Eleven percent of the unfiltered water samples exceeded the second class Hg standard of the Chinese environmental standards for surface water (50 ng L1) (GB 3838-2002). The data indicate that there was a significant difference in the THg concentration between the two seasons. A similar seasonal variation was observed for As; Wei et al. (in press) reported that the concentration of As in water from Lake Taihu was higher in May (10–60 lg L1) than in September (1–9 lg L1). These measurements were recorded on the same boat and during the same period as the samples for the present study. The higher chlorophyll a concentration in September indicates that the primary productivity was higher in this season. Therefore, the increasing quantity of algae in the lake water in September may lead to lower concentrations of THg and As because phytoplankton take up a portion of the inorganic Hg and/or As from the aqueous phase (Pickhardt and Fisher, 2007).
3.3.2. MeHg Table 3 shows that the concentration of MeHg in unfiltered lake water (unfMeHg) samples varied from 0.04 to 0.31 ng L1 with an average of 0.14 ± 0.05 ng L1, with the exception of two samples collected in May with an appreciably high concentration of unfMeHg, one of which was collected from surface water at site N-1 (0.81 ng L1) and the other of which was collected from lake-bottom water at site W-2 (1.0 ng L1). The concentrations of MeHg in filtered water (fMeHg) were in the range of 0.04– 0.24 ng L1 with an average of 0.12 ± 0.04 ng L1. The results also show that the ratio of MeHg/THg was small and that only 0.12– 3.5% of Hg was present as MeHg in the water from Lake Taihu (Table 3). No significant difference was observed in the concentrations of MeHg among the 4 sub-basins of Lake Taihu (ANOVA, p > 0.1), although the average concentrations of MeHg were slightly higher in north and east Taihu (Table 3). Although the concentration of THg in Lake Taihu was relatively high, the concentration of MeHg, the more bioavailable and toxic form of Hg, was low. The concentration of unfMeHg (0.14 ± 0.05 ng L1) in Lake Taihu was consistent with Bloom (1989), who reported that MeHg concentrations in natural surface water typically range from 0.02 to 0.1 ng L1. High concentrations of MeHg have been found extensively in lakes and reservoirs all over the world. However, those lakes with high concentrations of MeHg are also polluted with Hg (e.g., Yan et al., 2008), and high MeHg concentrations are found in the hypolimnetic water of deep lakes rather than in surface water (e.g., Feng et al., 2009; Gray and
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0
0
Depth (cm)
5
5
10
10
0
0
5
5
5
10
10
10
15 15
15
15
15 20
20
20
25
20
25
0 10 20 30
400
30 600
400
25
30
E-2, May
7 14 21
20
25
25
C-3, May
30
Depth (cm)
0
N-1, May
N-3, May
W-4, May
30 35 30 600 0 10 20 30 300 600 900 0 10 20 30 400 800 1200 0 10 20 30 300 600 900
0
0
0
0
0
5
5
5
5
5
10
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10
10
10
15
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15
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25 30
25
25
C-3, September
30 0 10 20 30 400 800 1200 0
E-2, September
20
30 40 300 600 900 0
25
25
N-1, September
30 15 30 500 1000 1500 0
N-3, September
20
TOC, Fe and Mn
W-2, September
30 40 1000 2000 3000 0 10 20 30 TOC (%)
-1
Fe (mg g )
800
1200 -1
Mn (mg kg )
Fig. 2. Sediment profiles of total organic C (TOC), Fe and Mn in Lake Taihu in May and September 2009.
Table 3 Summary of total Hg (THg) and methyl mercury (MeHg) in filtered and unfiltered lake water from Lake Taihu in May and September 2009. Region
Site
Surface water (ng L1) THg
May Central Taihu Eastern Taihu
Northern Taihu
Western Taihu
September Central Taihu
Eastern Taihu
Northern Taihu
Western Taihu
n.d. = no data.
Lake-bottom water (ng L1) MeHg
THg
MeHg
Filtered
Unfiltered
Filtered
Unfiltered
Filtered
Unfiltered
Filtered
Unfiltered
C-1 C-3 E-1 E-2 E-3 N-1 N-2 N-3 N-4 W-1 W-2 W-3 W-4
19.4 45.5 14.0 39.6 38.2 19.5 19.5 43.8 10.5 17.4 23.5 n.d. 10.0
21.9 58.4 32.4 60.5 66.1 82.2 25.8 82.9 27.4 25.3 36.0 n.d. 12.2
0.05 0.13 0.13 0.12 0.12 0.12 0.12 0.11 0.24 0.12 0.06 n.d. 0.11
0.10 n.d. 0.13 0.13 0.10 0.81 0.18 0.14 0.31 0.14 0.18 n.d. 0.12
19.1 20.9 13.1 19.3 32.6 20.6 16.5 24.7 15.3 15.3 24.6 15.5 24.6
26.0 48.4 41.1 20.6 48.6 36.8 22.8 36.9 29.2 27.5 42.4 38.7 31.3
0.05 0.11 0.14 0.04 0.12 0.15 0.04 0.07 0.10 0.09 0.15 0.11 0.19
0.25 0.18 0.16 0.15 0.20 0.19 0.18 0.13 0.13 0.12 1.02 0.24 0.25
C-1 C-2 C-3 C-4 E-1 E-2 E-3 E-4 N-1 N-2 N-3 N-4 N-5 W-1 W-2 W-3
7.9 5.3 4.9 12.1 10.1 4.5 7.1 4.2 9.8 6.7 6.8 16.7 11.2 13.6 9.1 9.8
12.0 18.6 18.6 17.0 11.5 7.3 8.8 6.8 20.9 25.0 47.1 21.7 18.0 31.6 16.2 18.8
0.08 0.14 0.11 0.11 0.09 0.16 0.10 0.13 0.07 0.06 0.12 0.09 0.08 0.11 0.12 0.18
0.12 0.17 0.12 0.16 0.14 0.13 0.16 0.11 0.16 0.22 0.20 0.17 0.11 0.19 0.13 0.10
11.5 4.7 10.0 5.7 9.4 5.2 14.7 5.5 11.1 10.4 9.5 n.d. 7.2 15.5 6.8 7.5
18.4 12.8 13.6 14.3 12.1 8.2 29.1 9.8 26.6 15.6 70.3 n.d. 17.2 32.4 16.5 13.6
0.08 0.07 0.13 0.08 0.16 0.12 0.10 0.13 0.05 0.06 0.07 n.d. 0.09 0.08 0.10 0.14
0.11 0.14 0.14 0.09 0.20 0.13 0.14 0.13 0.17 0.19 0.17 n.d. 0.18 0.13 0.13 0.11
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Hines, 2009). This implies that inorganic Hg does not easily transform into MeHg in shallow Lake Taihu. Relatively low concentrations of MeHg in lake water may be caused by two factors. First, the aerobic and high pH conditions seem to favor the degradation of MeHg (Oremland et al., 1991); a number of studies have shown that demethylation is more rapid and extensive under aerobic (high Eh) conditions (e.g., Oremland et al., 1991; Marvin-DiPasquale and Oremland, 1998; Marvin-DiPasquale et al., 2000). As mentioned above, high pH and Eh levels, induced by algal blooms and shallow water, may enhance MeHg demethylation in Lake Taihu. Second, in eutrophic Lake Taihu, the primary productivity increases rapidly under favorable light and temperature conditions. It is well known that the bioavailability of MeHg for organisms is thousands of times that of inorganic Hg (Clarkson, 1993; Stein et al., 1996). Miles et al. (2001) and Moye et al. (2003) observed the uptake of MeHg from water by different species of algae. The present authors observed that the concentration of MeHg in phytoplankton and hydroplankton was in the range of 0.26–2.84 ng g1 (data not shown), which was approximately one to ten thousand times that in the lake water. This implies that the fast growing primary productivity scavenges a large amount of MeHg from the lake water and contributes to the low concentration of MeHg in Lake Taihu. Unfortunately, low MeHg concentrations in Lake Taihu water and sediment do not represent a low risk of human exposure in the area because the uptake of MeHg by phytoplankton is a key step through which it is biomagnified in the food chain (Miles et al., 2001). 3.4. Hg in sediment 3.4.1. THg The concentration of THg varied from 12 to 470 ng g1 in the sediment, the highest concentration being found in the surface sediment at site E-2 in east Taihu. The THg concentrations in Lake Taihu sediment were far lower than those found in some Hg-polluted reservoirs (Yan et al., 2008) but was higher than those in background lakes, such as the Salmon Falls Creek Reservoir in USA (23–83 ng g1, Gray and Hines, 2009) and Lake Levrasjön in Sweden (20–63 ng g1, Regnell et al., 1997). Fan and Zhang (2009) reported that the THg in the surface sediment of Lake Taihu decreases in the order of north Taihu > east Taihu > central Taihu > west Taihu. Similarly, the present data show elevated average THg concentrations in the sediment at site E-2 (200 ± 110 ng g1) in east Taihu and at site N-3 (130 ± 68 ng g1) in north Taihu, indicating a significant accumulation of THg in these two regions. East Taihu and north Taihu are impacted by aquaculture and industrial effluent, respectively. In contrast, the average concentration of THg (55 and 77 ng g1) was relatively low at site C-3, indicating that the effect of anthropogenic Hg emissions on central Taihu has been largely weakened during transportation. The present data showed significant differences for THg in sediment between the two seasons in north and east Taihu. For example, concentrations of THg in sediment cores ranged from 12–82 ng g1 in May and 51–228 ng g1 in September at site N-3. To control and reduce internal emission of nutrients, ecological dredging has been conducted in Lake Taihu, especially in Zhushan Lake and Meiliang Bay (Fig. 1) in north Tahu with heavy eutrophication and east Taihu with intensive aquaculture. Therefore, the significant difference of THg in sediments in these areas probably results from strong disturbance by the dredging activity. The distribution profile in the sediment shows that the THg concentration decreases from top to bottom (Fig. 3). The highest concentration of THg was often observed in the top 5 cm (Fig. 3). The higher THg concentrations in the upper part of the sediment profiles may indicate that Hg emissions from anthropogenic sources surrounding Lake Taihu have increased in past decades
with the development of local industry and agriculture. For example, the increasingly intensive aquaculture of crab in east Taihu has induced a distinct increase in the THg concentration at depths of 0–5 cm in the sediment. 3.4.2. MeHg The concentration of MeHg in the sediment was in the range of 0.2–0.96 ng g1 during both seasons. In contrast to THg, the average MeHg concentrations in sediments from the 4 regions of the lake decreased in the order of east Taihu > west Taihu > central Taihu > north Taihu, indicating that the THg concentration is not the main factor influencing the production of methylated Hg in Lake Taihu. MeHg concentrations decreased from top to bottom in the sediment profiles, and the peak concentration of MeHg occurred at depths of 4–9 cm at all sampling sites (Fig. 3). The peak MeHg concentrations in the sediment cores were all higher in September than in May (Fig. 3). This is may be caused by the higher temperatures in September because higher temperatures can stimulate the metabolism of SO4-reducing bacteria (SRB) and enhance the methylation process of inorganic Hg in sediment (Ullrich et al., 2001). The concentrations of MeHg in the range of 0.2–0.96 ng g1 are generally lower than in other lakes. The ratios of MeHg/THg (0– 0.79%) in sediments are also lower than the 1.0–1.5% typically reported by Ullrich et al. (2001) and much lower than previously reported values from eutrophic reservoirs. For instance, Gray and Hines (2009) reported that in the eutrophic Salmon Falls Creek Reservoir in Idaho, as much as 5% of the THg was methylated in sediments. This indicates that heavy eutrophication does not accelerate net Hg methylation in Lake Taihu sediment. The high pH and anoxic environment should be the primary factors limiting the methylation of Hg in Lake Taihu. Miskimmin et al. (1992) and Xun et al. (1987) found that a reduction in pH from 7.0 to 5.0 had a substantial effect on Hg methylation, resulting in moderate to large increases in the net methylation rate at both low and high concentrations of DOC. In Lake Taihu, intensive algal blooms may consume a high amount of CO2, creating a neutral-alkali water environment (6.5–8.5 in May and 7.3–9.0 in September), which higher pH may inhibit Hg methylation. It is widely accepted that the production of MeHg is mediated primarily by SO4-reducing bacteria (SRB) in anoxic environments (Compeau and Bartha, 1985; Ullrich et al., 2001). Because Lake Taihu is a large, shallow, freshwater lake that is influenced by wind disturbance and the growth of phytoplankton that consume CO2 and release O2, it always maintains an aerobic environment (Eh: 120–340 mV). Therefore, methylation mediated by anaerobic SRB is suppressed in the sediments of Lake Taihu (Callister and Winfrey, 1986). Vaithiyanathan et al. (1996) found that the rates of methylation and demethylation were high in the aerobic, eutrophic soil of the everglades, but the net Hg methylation rate and the concentration of MeHg were low. Hence, the low methylation rate and high demethylation rate induced by aerobic environments may be one of the primary causes of the low MeHg concentration in the sediment in Lake Taihu. 3.4.3. Correlations among THg, MeHg and TOC in the sediment Correlations between THg, MeHg and TOC in the sediment cores from the 4 sub-basins were analyzed (Table 4). A significant correlation was observed between THg and TOC (p < 0.05) at all sampling sites except site E-2 in East Taihu, indicating that the presence of organic material played an important role in the accumulation of Hg in the sediment. Because phytoplankton are a dominant source of organic matter in Lake Taihu sediment, the relationship between THg and TOC indicates that the uptake of Hg from lake water and the atmosphere by phytoplankton is an important mechanism through which Hg enters the sediment.
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Fig. 3. Sediment profiles of total Hg (THg) and methyl Hg (MeHg) in the 4 parts of Lake Taihu in May and September 2009.
3.5. Hg in porewater and the diffusion fluxes Table 4 Correlation coefficients between THg, MeHg and TOC in sediment cores from the 4 sub basins of Lake Taihu. Sub-basin Central Taihu (C-3) East Taihu (E-2) North Taihu (N-1) North Taihu (N-3) West Taihu (W-2, W-4) a b
THg MeHg THg MeHg THg MeHg THg MeHg THg MeHg
MeHg
TOC
0.08
0.69b 0.40 0.27 0.53a 0.81b 0.79b 0.83b 0.84b 0.77b 0.63b
0.60b 0.73b 0.80b 0.68b
The significance is at the 0.05 level (2-tail). The significance is at the 0.01 level (2-tail).
However, in East Taihu, which is a grass lake area and where the main source of Hg is aquaculture of crab, the effect of organic material on the THg in sediment may be limited. Therefore, no significant relationship between TOC and THg was observed in East Taihu. As a primary electron donor and nutrient for heterotrophic bacteria, a high TOC content in sediment can enhance the formation of methylated Hg (Lambertsson and Nilsson, 2006). Gray and Hines (2009) and Macalady et al. (2000) reported that an increase in the organic material load of sediment could increase the production of MeHg in eutrophic lakes and reservoirs. A significant relationship between the TOC and MeHg contents in sediments was observed at most sampling sites (except site C-3) in Lake Taihu (Table 4). However, the relatively low concentrations of MeHg (0.2–0.96 ng g1) and the ratios of MeHg/THg (0–0.79%) in the sediments indicate that the elevated TOC content did not significantly increase the Hg methylation rate in Lake Taihu.
In Lake Taihu, the concentrations of THg and MeHg in porewater were high, ranging from 8.8–260 ng L1 and 0.14–25 ng L1, respectively (Fig. 4). At all sampling sites, the concentrations of THg and MeHg were higher in May than in September, with the exception of site N-1, where the average MeHg concentration in May was comparable with that in September. For example, the highest concentrations in May reached 150 ng L1 THg and 4.2 ng L1 MeHg at site C-3 and 260 ng L1 THg and 25 ng L1 MeHg at site E-2. These concentrations were much greater than those observed in September, which were 49 ng L1 THg and 1.0 ng L1 MeHg at site C-3 and 64.0 ng L1 THg and 1.6 ng L1 MeHg at site E-2. The distribution profile of MeHg in porewater was not clear, though the concentration of MeHg was found to be the highest on the interface between the sediment and the water; this was true at all sampling sites except C-3, E-2 and N-1 in May, where the peak concentrations of MeHg occurred at depths of 9–13 cm (Fig. 4). The ratio of MeHg/ THg varied from 0.7–9.6% with an average of 2.6 ± 1.6% in September and from 0.3–13.7% with an average of 3.3 ± 1.6% in May. Lake sediment is considered to be an important source of inorganic and organic Hg in the water column. The concentration gradient between the porewater and the water above the sediment drives the diffusion of Hg (Feng et al., 2009; Gray and Hines, 2009). Calculation showed that the sediment in Lake Taihu was an important source of THg and MeHg in both seasons, except at site N-1 in May where the THg showed a net deposition flux from water to sediment (Table 5). In general, the diffusion fluxes of THg and MeHg were higher in North Taihu and East Taihu than in West Taihu and Central Taihu. At the same time, the diffusion fluxes of THg and MeHg were obviously greater in May than in September at all sampling sites except N-1 (Table 5). The net depositional flux of THg observed at site N-1 in May could be due to an elevated THg concentration in the interface water caused by disturbances of the sediment and anthropogenic input from a contaminated river. It is
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Table 5 Estimated diffusion fluxes of THg and MeHg (in ng m2 d1) at 5 sampling sites in Lake Taihu in May and September 2009.
May September
THg MeHg THg MeHg
Central Taihu
East Taihu
North Taihu (N-1)
North Taihu (N-3)
West Taihu
360 5.2 6.5 4.2
720 65 130 3.7
100 6.2 340 9.4
960 18 410 7.0
140 5.2 93 1.9
possible that the high Hg diffusion flux contributed to the higher concentration of THg in the lake water in May, but a reasonable explanation for the higher Hg diffusion flux in May is still not forthcoming.
Although the results provided a preliminary description of the Hg cycle in hypereutrophic, shallow Lake Taihu, many questions still remain and further work is required to fully understand the Hg cycle in eutrophic lakes.
4. Conclusions
Acknowledgements
The THg concentrations in Lake Taihu were much higher than those in background lakes, and 11% of the samples exceeded the second class of the Chinese environmental standards for surface water (50 ng L1), implying that there is serious Hg contamination in Lake Taihu. Despite the high THg concentrations, concentrations of MeHg were relatively low in the lake water, possibly resulting from the strong uptake of MeHg by phytoplankton and the high degradation rate of MeHg in oxic environments. In contrast to deep-water lakes and reservoirs, the MeHg concentration of the lake water and sediment was relatively low, despite the elevated THg and TOC loads in the sediment. The elevated pH induced by algal blooms and the aerobic water conditions may be important factors that inhibit the methylation of Hg, indicating that eutrophication does not accelerate Hg methylation in Lake Taihu. The results also showed that sediment was the net source of inorganic Hg and MeHg in the lake water column and that the significantly higher diffusion flux of inorganic Hg may have contributed to the higher THg concentrations in the lake water in May.
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The distribution of total mercury and methyl mercury in a shallow hypereutrophic lake (Lake Taihu) in two seasons Shaofeng Wang, Denghua Xing, Yonfeng Jia ⇑, Biao Li, Kuanling Wang Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 15 June 2011 Accepted 27 September 2011 Available online 4 October 2011 Editorial handling by J.E. Gray
a b s t r a c t To understand the geochemical cycle of Hg in hypereutrophic freshwater lake, two sampling campaigns were conducted in Lake Taihu in China during May and September of 2009. The concentrations of unfiltered total Hg (unfTHg) were in the range of 6.8–83 ng L1 (28 ± 18 ng L1) in the lake water and total Hg in the sediment was 12–470 ng g1, both of which are higher than in other background lakes. The concentration of unfTHg in 11% of the lake water samples exceeded the second class of the Chinese environmental standards for surface water of 50 ng L1 (GB 3838-2002), indicating that a high ecological risk is posed by the Hg in Lake Taihu. However, the concentrations of unfiltered total MeHg (unfMeHg) were relatively low in the lake water (0.14 ± 0.05 ng L1, excluding two samples with 0.81 and 1.0 ng L1). Lake sediment MeHg varied from 0.2–0.96 ng g1, with generally low ratios of MeHg/THg of <1%. The low concentrations of TMeHg in the lake water may have resulted from a strong uptake by the high primary productivity and the demethylation of MeHg in oxic conditions. In addition, contrary to the results of previous research conducted in deep-water lakes and reservoirs, the low concentrations of MeHg and low ratio of MeHg/THg in the lake sediment indicates that the net methylation of Hg was not accelerated by the elevated organic matter load created by the eutrophication of Lake Taihu. The results also showed that sediments were a source of THg and MeHg in the water. Higher diffusion fluxes of THg and MeHg may be partly responsible for the higher concentrations of THg in the lake water in May, 2009. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Mercury pollution in the environment is a primary concern because inorganic Hg can transform into the more toxic species, methylmercury (MeHg), which humans and wildlife can be exposed to through the food chain (Lebel et al., 1997; Legrand et al., 2007). MeHg can cause damage to the central nervous system in humans (Clarkson, 1993). In aquatic ecosystems, the primary source of MeHg is the methylation of inorganic Hg; this process is mediated by SO4-reducing bacteria (SRB) (Compeau and Bartha, 1985; Macalady et al., 2000) and/or Fe-reducing bacteria (Fleming et al., 2006) in anoxic sediments. The production of MeHg in aquatic habitats is influenced by a variety of environmental factors, such as the organic material content, pH, redox potential, temperature, available SO4 and bacterial species (Clarkson, 1993; Compeau and Bartha, 1985; Lambertsson and Nilsson, 2006). Lake eutrophication, another environmental concern around the world, can alter the environmental parameters significantly (Fukushima et al., 1996; Karlson et al., 2002; Scott et al., 2005; Sweerts et al., 1991), influencing the Hg cycle in lake ecosystems. ⇑ Corresponding author. Tel.: +86 24 83970503; fax: +86 24 83970436. E-mail address: [email protected] (Y. Jia). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.09.029
Recently, studies have investigated the impact of eutrophication on the production and bioaccumulation of MeHg in eutrophic lakes and/or reservoirs. For example, Gray and Hines (2009) observed a high rate of Hg methylation in the range of 2.3–17%/day in sediments; this rate was related to the eutrophication of the reservoir. Macalady et al. (2000) predicted that further eutrophication of Clear Lake would increase Hg methylation, either by altering sediment microbial communities or by increasing the organic C load in the sediment. He et al. (2008a,b) also found that the formation of MeHg in a reservoir could be accelerated by eutrophication. These works concluded that changes in the environmental parameters of eutrophic water bodies are likely to promote Hg methylation and to enhance MeHg accumulation in aquatic organisms. However, the acceleration of net MeHg formation was found mainly in deep-water reservoirs (e.g., Gray and Hines, 2009; He et al., 2008b; Macalady et al., 2000); in shallow, eutrophic lakes, the geochemical cycle of Hg is far from clear. For instance, Vaithiyanathan et al. (1996) determined that the net Hg methylation rate in the eutrophic soil of the everglades was not high, although there was a relatively high potential for Hg methylation and MeHg demethylation. Lake Taihu, a large, shallow, freshwater lake in China, is an important resource of drinking water, fishing and tourism in Eastern China (Sun and Mao, 2008). After decades of intensive input
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of nutrients from industrial and agricultural sources, Lake Taihu has become hypereutrophic, and algal blooms last from May to October annually (Qin et al., 2007). In late May of 2007, millions of people in Wuxi, in the Jiangsu Province of China, were impacted by a drinking water crisis following a massive phytoplankton bloom in Lake Taihu (Qin et al., 2010). In addition to the serious eutrophication, Lake Taihu has suffered Hg contamination. Concentrations of total Hg (THg) in the range of 0.063–0.99 mg/kg with an average of 0.091 ± 0.19 (n = 290) mg/kg were reported in surface sediments from Lake Taihu; the highest concentration was approximately one order of magnitude greater than that in the background lakes (Fan and Zhang, 2009). However, little is known about the Hg cycle under the particular conditions of Lake Taihu. In this study, water and sediment samples were collected from Lake Taihu during two different seasons to explore the speciation and distribution of Hg in this hypereutrophic, shallow, freshwater lake.
with a single-use syringe. After rinsing the borosilicate bottles 3 times with the samples, the unfiltered and filtered water samples were preserved by adding ultra-pure HCl to a final concentration of 0.4%, placed in sealed sample bottles and stored at 4–8 °C in a cooler box with ice bags. Sediment cores were collected using a stainless steel sampler with a plexiglass tube and sectioned into 1–2 cm intervals in the field in a glovebag filled with N2. The sectioned sediment core samples were placed in 50-mL centrifuge tubes, sealed with Parafilm to avoid cross contamination and stored in a sample box with dry ice. In the laboratory, the porewater samples were immediately separated by centrifugation at 4000 rpm for 30 min and then filtered through a 0.45-lm PVDF membrane (Millipore) in an anaerobic chamber (Bactron II, USA). The filtrate was preserved with 0.4% ultra-pure HCl and stored in sealed borosilicate glass bottles for further analysis.
2.3. Determination of THg and MeHg 2. Methods and materials 2.1. Area description Lake Taihu (30°550 4000 –31°320 5800 N, 119°520 320 –120°360 1000 E), the third largest freshwater lake in China, is located in the southern part of Jiangsu province. The lake is shallow and flat-bottomed with an average depth of 1.9 m and maximum depth of 3.0 m. The total area of Lake Taihu is 2, 427 km2, and its volume is 4.43 billion m3 (Sun and Mao, 2008). The climate in the Lake Taihu area is controlled by subtropical monsoons, and the water temperature ranges from 1.5–32.5 °C, averaging 17.6 °C between 1991 and 1999 and 18.2 °C during 2005 and 2006 (Zhu et al., 2008). Traditionally, Lake Taihu is divided into 4 sub-basins based on geographical location and sources of nutrients and metals: north Taihu, central Taihu, east Taihu and west Taihu (Fig. 1). Presently, Lake Taihu as a whole is at a medium to high eutrophication level. However, the eutrophication level varies throughout the lake. Zhu (2009) reported that the average concentrations of total N, total P and Chllorophyll a in Lake Taihu decreased from 2006– 2008 in the following order: North Taihu > Central Taihu = West Taihu > East Taihu. According to previous investigations, North Taihu is at a hypereutrophic level, Central and West Taihu are at an eutrophic level, and east Taihu is at a light to medium eutrophication level. In addition, despite similar hydrology and climate conditions, Lake Taihu has developed two contrasting water types; North, Central and West Taihu resemble algal lakes, while East Taihu is characterized by macrophytic waters (Zhang, 2006). 2.2. Sampling Two sampling campaigns were conducted from May 10–15, 2009, when the biomass began to increase, and from September 1–10, 2009, when the phytoplankton bloom had lasted for approximately 3 months. Surface water, lake-bottom water, interfacial water and sediment cores were collected randomly in each sub-basin during each season (Fig. 1, Table 1). Prior to collecting the water samples, borosilicate bottles (125-mL) were cleaned with an alkali detergent, soaked overnight in dilute HNO3, and heated in a muffle furnace at 450 °C for at least 4 h; this pretreatment process removed any organic matter and trace Hg that was adsorbed on the vessels. Surface and lake-bottom water samples were gathered at a depth of 0.5 m and 1.5 m, respectively, using a self-designed water sampler. An aliquot of water was filtered through a singleuse 0.45-lm polyvinylidene fluoride (PVDF) membrane (Millipore)
To determine the concentration of THg in water, the samples were pre-concentrated and then measured using CVAFS (Model III, Brooksrand, USA) according to EPA method 1631e (USEPA, 2002). The THg in the sediment was measured using hydride generation-atomic fluorescence spectrometry (HG-AFS) (AFS-2202E, Haiguang, China) after digestion by hot HNO3 plus HClO4. The MeHg in water was determined using GC-CVAFS (Model III, Brooksrand, USA) after a distillation and ethylation procedure in accordance with EPA method 1630 (USEPA, 2001). The MeHg in sediment was determined based on the procedure developed by Liang et al. (2004), in which the samples are processed by acid digestion (3 M HNO3), solvent extraction (CH2Cl2), reverse extraction by water, ethylation and detection by GC-CVAFS. Quality assurance and quality control (QA/QC) of the analyses were carried out with duplicates, method blanks, matrix spikes, and certified reference material (IAEA-433). The detection limits of the method were 0.01 ng g1 for THg and 0.005 ng g1 for MeHg in sediment and 0.01 ng L1 for THg and 0.01 ng L1 for MeHg in water. A mean MeHg concentration of 0.17 ± 0.05 ng g1 (n = 4) was obtained for IAEA-433, which has a certified value of 0.17 ± 0.07 ng g1. Recoveries on the spiked MeHg in water samples ranged from 85 to 114%. The relative percentage difference was <6.5% for the duplicate samples of THg and MeHg in sediment and water.
2.4. Determination of the total organic carbon (TOC), Fe and Mn in sediment The total organic C (TOC) content of the sediment was determined by titration with FeSO4 after oxidation at 90 °C for 15 min in a mixture of potassium dichromate (VI) and concentrated H2SO4 (Walkley and Armstrong Black, 1934). The amount of Fe and Mn in sediment samples was measured using flame atomic absorbance spectrometry (FAAS, Varian-AA240, USA) after digestion by hot HNO3–HClO4–HF. The concentrations of elements in sediments are displayed on the basis of dry weight (dw). The certified reference material (marine sediment, GBW 07314) was determined for quality control, and the recoveries for TOC, Fe, Mn and S were 98%, 95%, 97% and 98%, respectively.
2.5. Determination of water quality parameters Dissolved O2 (DO), pH, Eh, water temperature (T) and conductivity of the lake water were measured in the field, using portable multi-meters (Jenco, Shanghai).
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Fig. 1. A sketch map of the sampling locations in Lake Taihu in May and September, 2009.
Table 1 Locations of the samples collected from the 4 regions of Lake Taihu in May and September 2009. Region
Month
Surface water
Lake-bottom water
Sediment core
Central Taihu
May September May September May September May September
C1, C3 C1, C2, C3, C4 E1, E2, E3 E1, E2, E3, E4 N1, N2, N3, N4 N1, N2, N3, N4, N5 W1, W2, W4 W1, W2, W3
C1, C3 C1, C2, C3, C4 E1, E2, E3 E1, E2, E3, E4 N1, N2, N3, N4 N1, N2, N3, N5 W1, W2, W3, W4 W1, W2, W3
C3 C3 E2 E2 N1, N3 N1, N3 W4 W2
East Taihu North Taihu West Taihu
2.6. Calculation of the Hg diffusion flux from the sediment The diffusion fluxes of THg and MeHg from the sediment into lake water in the two seasons were calculated using Fick’s first law, as has been described previously (e.g., Feng et al., 2009):
F ¼ ðuDw =h2 ÞdC=dx where F is the flux of THg or MeHg, C is the concentration at a depth x, u is the sediment porosity, Dw is the diffusion coefficient of THg and MeHg in water in the absence of a sediment matrix, and h is the tortuosity that was calculated according to h = 1 ln(u2). The diffusion coefficients of THg and MeHg in water at 25 °C were assumed to be 9.5 106 and 1.2 105 cm2 s1, respectively, as previously
reported (Covelli et al., 2008; Gill et al., 1999). To calculate the dC, the concentrations of THg and MeHg in the filtered interfacial water and the porewater were used. Because the average temperature of the lake water (24.3–28.8 °C, Table 2) was close to 25 °C, a no temperature correction was made for the diffusion coefficients. 3. Results and discussion 3.1. Water quality parameters No significant difference in the water quality parameters was observed between the surface and lake-bottom water at each sampling site. Seasonally, the average water temperature was
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Table 2 Summary of water parameters in Lake Taihu in May and September 2009. Region
Month
DO (mg L1)
T (°C)
pH
Central Taihu
May September May September May September May September
5.9 ± 0.43 (5.1–6.2)
24.3 ± 0..64 (23.8–25.3) 27.6 ± 0..35 (27–28) 25.0 ± 0.72 (24.5–26.3) 29.1 ± 1.3 (27.5–30.5) 27.0 ± 2.0 (24.3–30.8) 27.5 ± 0.50 (26.8–28.2) 25.5 ± 0.26 (25–25.8) 28.8 ± 0.55 (28–29.5)
7.4 ± 0.54 8.5 ± 0.09 7.6 ± 0.40 8.4 ± 0.23 8.3 ± 0.18 8.3 ± 0.57 7.2 ± 0.49 8.7 ± 0.26
East Taihu North Taihu West Taihu
5.8 ± 0.33 (5.5–6.2) 6.0 ± 0.21 (5.9–6.5) 5.9 ± 0.18 (5.7–6.2)
25.6 ± 1.5 °C and 28.3 ± 1.5 °C in May and September, respectively (Table 2). The fluctuations in water temperature were relatively small during each sampling period; however, there was a significant difference in temperature between the two seasons (p < 0.001, ANOVA). Environmental monitoring at 32 sites in 2009 showed that the average concentration of chlorophyll a was much lower in May (8.5 lg L1) than in September (25 lg L1) (Zhu, 2009, pers. comm.), and this change was consistent with the level of chlorophyll a calculated from remote sensing images collected during the two months (Kuang et al., 2010).This indicates that the algal bloom was at different stages in the two sampling periods. Algal blooms have a strong impact on water parameters, especially the pH and Eh. The lake water showed neutral to alkaline conditions; the pH in May was in the range of 6.5–8.0, which is distinctly lower than the range of 7.3–9.0 measured in September (p < 0.01, ANOVA) (Table 2). In north and central Taihu, which exhibit a stronger algal bloom, the redox potential of the lake water (Eh) was significantly lower in September than in May (ANOVA, p < 0.01). In east and west Taihu, where the influence of the algal bloom is weak, there was no significant difference in Eh between the two seasons (ANOVA, p > 0.01). Because Lake Taihu is a large shallow water body with relatively strong disturbance, the lake water is oxic (DO: 5.1–6.5 mg L1 and Eh: 120–340 mV). 3.2. Fe, Mn and TOC in sediment cores Concentrations of TOC, Fe and Mn varied from 0.96% to 17%, 10 to 41 g kg1 and 180 to 2700 mg kg1, respectively, in sediments from Lake Taihu (Fig. 2). The results show a large elevation in the concentration of TOC in the sediments, implying that long-term eutrophication has significantly increased the organic load in the sediment. The concentration of TOC decreased with depth, and the highest TOC occurred in the top 5 cm of sediment at all sampling sites, which is consistent with a previous study (Zhao et al., 2007). An exception is site E-2, where the highest concentration of TOC in May was at a depth of 10 cm (Fig. 2). Like TOC, Fe and Mn also decreased with depth in the sediment profiles at most sampling sites (Fig. 2). 3.3. Hg in lake water 3.3.1. THg The concentration of unfTHg varied from 6.8 to 83 ng L1 with an arithmetic mean of 28 ± 18 ng L1; the average concentration of unfTHg in the surface water (30 ng L1) was almost identical to that in the lake-bottom water (27 ng L1) (t-test, p = 0.91). The concentration of THg in filtered water (fTHg) was in the range of 4.2–46 ng L1 with an average of 17 ± 11 ng L1, which accounted for about 56 ± 19% (14–94%) of the unfTHg in the lake water. Spatially, the average concentrations of unfTHg and fTHg were higher in east and north Taihu than in central and west Taihu (Table 3). The concentrations of unfTHg and fTHg collected in May were much higher than in September at all sampling sites (Table 3).
(6.5–7.9) (8.4–8.7) (7.2–8.0) (8.3–8.8) (8.0–8.5) (7.3–9.0) (6.9–8.0) (8.5–9.0)
Eh (mV)
Conductivity (ms cm1)
291 ± 45 (200–320) 210 ± 74 (130–290) 280 ± 55 (210–340) 250 ± 36 (220–300) 310 ± 23 (290–340) 170 ± 65 (120–290) 290 ± 50 (220–340) 280 ± 6.6 (270–280)
0.58 ± 0 (0.57–0.59) 0.53 ± 0.03 (0.5–0.57) 0.80 ± 0.09 (0.69–0.93) 0.50 ± 0.03 (0.47–0.55)
In the study, the concentration of THg in Lake Taihu was significantly lower than that found in a previous investigation conducted by Yang et al. (1996), who reported that the THg concentration in the lake water varied from 140–760 ng L1. However, the present results show that the THg concentration in water from Lake Taihu is much greater than in background lakes in the USA (Gray and Hines, 2009; Hines et al., 2004), Sweden (e.g., Regnell et al., 1997) and even reservoirs in geologically Hg-enriched areas of China (Feng et al., 2009; He et al., 2008a,b). Eleven percent of the unfiltered water samples exceeded the second class Hg standard of the Chinese environmental standards for surface water (50 ng L1) (GB 3838-2002). The data indicate that there was a significant difference in the THg concentration between the two seasons. A similar seasonal variation was observed for As; Wei et al. (in press) reported that the concentration of As in water from Lake Taihu was higher in May (10–60 lg L1) than in September (1–9 lg L1). These measurements were recorded on the same boat and during the same period as the samples for the present study. The higher chlorophyll a concentration in September indicates that the primary productivity was higher in this season. Therefore, the increasing quantity of algae in the lake water in September may lead to lower concentrations of THg and As because phytoplankton take up a portion of the inorganic Hg and/or As from the aqueous phase (Pickhardt and Fisher, 2007).
3.3.2. MeHg Table 3 shows that the concentration of MeHg in unfiltered lake water (unfMeHg) samples varied from 0.04 to 0.31 ng L1 with an average of 0.14 ± 0.05 ng L1, with the exception of two samples collected in May with an appreciably high concentration of unfMeHg, one of which was collected from surface water at site N-1 (0.81 ng L1) and the other of which was collected from lake-bottom water at site W-2 (1.0 ng L1). The concentrations of MeHg in filtered water (fMeHg) were in the range of 0.04– 0.24 ng L1 with an average of 0.12 ± 0.04 ng L1. The results also show that the ratio of MeHg/THg was small and that only 0.12– 3.5% of Hg was present as MeHg in the water from Lake Taihu (Table 3). No significant difference was observed in the concentrations of MeHg among the 4 sub-basins of Lake Taihu (ANOVA, p > 0.1), although the average concentrations of MeHg were slightly higher in north and east Taihu (Table 3). Although the concentration of THg in Lake Taihu was relatively high, the concentration of MeHg, the more bioavailable and toxic form of Hg, was low. The concentration of unfMeHg (0.14 ± 0.05 ng L1) in Lake Taihu was consistent with Bloom (1989), who reported that MeHg concentrations in natural surface water typically range from 0.02 to 0.1 ng L1. High concentrations of MeHg have been found extensively in lakes and reservoirs all over the world. However, those lakes with high concentrations of MeHg are also polluted with Hg (e.g., Yan et al., 2008), and high MeHg concentrations are found in the hypolimnetic water of deep lakes rather than in surface water (e.g., Feng et al., 2009; Gray and
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0
0
Depth (cm)
5
5
10
10
0
0
5
5
5
10
10
10
15 15
15
15
15 20
20
20
25
20
25
0 10 20 30
400
30 600
400
25
30
E-2, May
7 14 21
20
25
25
C-3, May
30
Depth (cm)
0
N-1, May
N-3, May
W-4, May
30 35 30 600 0 10 20 30 300 600 900 0 10 20 30 400 800 1200 0 10 20 30 300 600 900
0
0
0
0
0
5
5
5
5
5
10
10
10
10
10
15
15
15
15
15
20
20
20
20
20
25 30
25
25
C-3, September
30 0 10 20 30 400 800 1200 0
E-2, September
20
30 40 300 600 900 0
25
25
N-1, September
30 15 30 500 1000 1500 0
N-3, September
20
TOC, Fe and Mn
W-2, September
30 40 1000 2000 3000 0 10 20 30 TOC (%)
-1
Fe (mg g )
800
1200 -1
Mn (mg kg )
Fig. 2. Sediment profiles of total organic C (TOC), Fe and Mn in Lake Taihu in May and September 2009.
Table 3 Summary of total Hg (THg) and methyl mercury (MeHg) in filtered and unfiltered lake water from Lake Taihu in May and September 2009. Region
Site
Surface water (ng L1) THg
May Central Taihu Eastern Taihu
Northern Taihu
Western Taihu
September Central Taihu
Eastern Taihu
Northern Taihu
Western Taihu
n.d. = no data.
Lake-bottom water (ng L1) MeHg
THg
MeHg
Filtered
Unfiltered
Filtered
Unfiltered
Filtered
Unfiltered
Filtered
Unfiltered
C-1 C-3 E-1 E-2 E-3 N-1 N-2 N-3 N-4 W-1 W-2 W-3 W-4
19.4 45.5 14.0 39.6 38.2 19.5 19.5 43.8 10.5 17.4 23.5 n.d. 10.0
21.9 58.4 32.4 60.5 66.1 82.2 25.8 82.9 27.4 25.3 36.0 n.d. 12.2
0.05 0.13 0.13 0.12 0.12 0.12 0.12 0.11 0.24 0.12 0.06 n.d. 0.11
0.10 n.d. 0.13 0.13 0.10 0.81 0.18 0.14 0.31 0.14 0.18 n.d. 0.12
19.1 20.9 13.1 19.3 32.6 20.6 16.5 24.7 15.3 15.3 24.6 15.5 24.6
26.0 48.4 41.1 20.6 48.6 36.8 22.8 36.9 29.2 27.5 42.4 38.7 31.3
0.05 0.11 0.14 0.04 0.12 0.15 0.04 0.07 0.10 0.09 0.15 0.11 0.19
0.25 0.18 0.16 0.15 0.20 0.19 0.18 0.13 0.13 0.12 1.02 0.24 0.25
C-1 C-2 C-3 C-4 E-1 E-2 E-3 E-4 N-1 N-2 N-3 N-4 N-5 W-1 W-2 W-3
7.9 5.3 4.9 12.1 10.1 4.5 7.1 4.2 9.8 6.7 6.8 16.7 11.2 13.6 9.1 9.8
12.0 18.6 18.6 17.0 11.5 7.3 8.8 6.8 20.9 25.0 47.1 21.7 18.0 31.6 16.2 18.8
0.08 0.14 0.11 0.11 0.09 0.16 0.10 0.13 0.07 0.06 0.12 0.09 0.08 0.11 0.12 0.18
0.12 0.17 0.12 0.16 0.14 0.13 0.16 0.11 0.16 0.22 0.20 0.17 0.11 0.19 0.13 0.10
11.5 4.7 10.0 5.7 9.4 5.2 14.7 5.5 11.1 10.4 9.5 n.d. 7.2 15.5 6.8 7.5
18.4 12.8 13.6 14.3 12.1 8.2 29.1 9.8 26.6 15.6 70.3 n.d. 17.2 32.4 16.5 13.6
0.08 0.07 0.13 0.08 0.16 0.12 0.10 0.13 0.05 0.06 0.07 n.d. 0.09 0.08 0.10 0.14
0.11 0.14 0.14 0.09 0.20 0.13 0.14 0.13 0.17 0.19 0.17 n.d. 0.18 0.13 0.13 0.11
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Hines, 2009). This implies that inorganic Hg does not easily transform into MeHg in shallow Lake Taihu. Relatively low concentrations of MeHg in lake water may be caused by two factors. First, the aerobic and high pH conditions seem to favor the degradation of MeHg (Oremland et al., 1991); a number of studies have shown that demethylation is more rapid and extensive under aerobic (high Eh) conditions (e.g., Oremland et al., 1991; Marvin-DiPasquale and Oremland, 1998; Marvin-DiPasquale et al., 2000). As mentioned above, high pH and Eh levels, induced by algal blooms and shallow water, may enhance MeHg demethylation in Lake Taihu. Second, in eutrophic Lake Taihu, the primary productivity increases rapidly under favorable light and temperature conditions. It is well known that the bioavailability of MeHg for organisms is thousands of times that of inorganic Hg (Clarkson, 1993; Stein et al., 1996). Miles et al. (2001) and Moye et al. (2003) observed the uptake of MeHg from water by different species of algae. The present authors observed that the concentration of MeHg in phytoplankton and hydroplankton was in the range of 0.26–2.84 ng g1 (data not shown), which was approximately one to ten thousand times that in the lake water. This implies that the fast growing primary productivity scavenges a large amount of MeHg from the lake water and contributes to the low concentration of MeHg in Lake Taihu. Unfortunately, low MeHg concentrations in Lake Taihu water and sediment do not represent a low risk of human exposure in the area because the uptake of MeHg by phytoplankton is a key step through which it is biomagnified in the food chain (Miles et al., 2001). 3.4. Hg in sediment 3.4.1. THg The concentration of THg varied from 12 to 470 ng g1 in the sediment, the highest concentration being found in the surface sediment at site E-2 in east Taihu. The THg concentrations in Lake Taihu sediment were far lower than those found in some Hg-polluted reservoirs (Yan et al., 2008) but was higher than those in background lakes, such as the Salmon Falls Creek Reservoir in USA (23–83 ng g1, Gray and Hines, 2009) and Lake Levrasjön in Sweden (20–63 ng g1, Regnell et al., 1997). Fan and Zhang (2009) reported that the THg in the surface sediment of Lake Taihu decreases in the order of north Taihu > east Taihu > central Taihu > west Taihu. Similarly, the present data show elevated average THg concentrations in the sediment at site E-2 (200 ± 110 ng g1) in east Taihu and at site N-3 (130 ± 68 ng g1) in north Taihu, indicating a significant accumulation of THg in these two regions. East Taihu and north Taihu are impacted by aquaculture and industrial effluent, respectively. In contrast, the average concentration of THg (55 and 77 ng g1) was relatively low at site C-3, indicating that the effect of anthropogenic Hg emissions on central Taihu has been largely weakened during transportation. The present data showed significant differences for THg in sediment between the two seasons in north and east Taihu. For example, concentrations of THg in sediment cores ranged from 12–82 ng g1 in May and 51–228 ng g1 in September at site N-3. To control and reduce internal emission of nutrients, ecological dredging has been conducted in Lake Taihu, especially in Zhushan Lake and Meiliang Bay (Fig. 1) in north Tahu with heavy eutrophication and east Taihu with intensive aquaculture. Therefore, the significant difference of THg in sediments in these areas probably results from strong disturbance by the dredging activity. The distribution profile in the sediment shows that the THg concentration decreases from top to bottom (Fig. 3). The highest concentration of THg was often observed in the top 5 cm (Fig. 3). The higher THg concentrations in the upper part of the sediment profiles may indicate that Hg emissions from anthropogenic sources surrounding Lake Taihu have increased in past decades
with the development of local industry and agriculture. For example, the increasingly intensive aquaculture of crab in east Taihu has induced a distinct increase in the THg concentration at depths of 0–5 cm in the sediment. 3.4.2. MeHg The concentration of MeHg in the sediment was in the range of 0.2–0.96 ng g1 during both seasons. In contrast to THg, the average MeHg concentrations in sediments from the 4 regions of the lake decreased in the order of east Taihu > west Taihu > central Taihu > north Taihu, indicating that the THg concentration is not the main factor influencing the production of methylated Hg in Lake Taihu. MeHg concentrations decreased from top to bottom in the sediment profiles, and the peak concentration of MeHg occurred at depths of 4–9 cm at all sampling sites (Fig. 3). The peak MeHg concentrations in the sediment cores were all higher in September than in May (Fig. 3). This is may be caused by the higher temperatures in September because higher temperatures can stimulate the metabolism of SO4-reducing bacteria (SRB) and enhance the methylation process of inorganic Hg in sediment (Ullrich et al., 2001). The concentrations of MeHg in the range of 0.2–0.96 ng g1 are generally lower than in other lakes. The ratios of MeHg/THg (0– 0.79%) in sediments are also lower than the 1.0–1.5% typically reported by Ullrich et al. (2001) and much lower than previously reported values from eutrophic reservoirs. For instance, Gray and Hines (2009) reported that in the eutrophic Salmon Falls Creek Reservoir in Idaho, as much as 5% of the THg was methylated in sediments. This indicates that heavy eutrophication does not accelerate net Hg methylation in Lake Taihu sediment. The high pH and anoxic environment should be the primary factors limiting the methylation of Hg in Lake Taihu. Miskimmin et al. (1992) and Xun et al. (1987) found that a reduction in pH from 7.0 to 5.0 had a substantial effect on Hg methylation, resulting in moderate to large increases in the net methylation rate at both low and high concentrations of DOC. In Lake Taihu, intensive algal blooms may consume a high amount of CO2, creating a neutral-alkali water environment (6.5–8.5 in May and 7.3–9.0 in September), which higher pH may inhibit Hg methylation. It is widely accepted that the production of MeHg is mediated primarily by SO4-reducing bacteria (SRB) in anoxic environments (Compeau and Bartha, 1985; Ullrich et al., 2001). Because Lake Taihu is a large, shallow, freshwater lake that is influenced by wind disturbance and the growth of phytoplankton that consume CO2 and release O2, it always maintains an aerobic environment (Eh: 120–340 mV). Therefore, methylation mediated by anaerobic SRB is suppressed in the sediments of Lake Taihu (Callister and Winfrey, 1986). Vaithiyanathan et al. (1996) found that the rates of methylation and demethylation were high in the aerobic, eutrophic soil of the everglades, but the net Hg methylation rate and the concentration of MeHg were low. Hence, the low methylation rate and high demethylation rate induced by aerobic environments may be one of the primary causes of the low MeHg concentration in the sediment in Lake Taihu. 3.4.3. Correlations among THg, MeHg and TOC in the sediment Correlations between THg, MeHg and TOC in the sediment cores from the 4 sub-basins were analyzed (Table 4). A significant correlation was observed between THg and TOC (p < 0.05) at all sampling sites except site E-2 in East Taihu, indicating that the presence of organic material played an important role in the accumulation of Hg in the sediment. Because phytoplankton are a dominant source of organic matter in Lake Taihu sediment, the relationship between THg and TOC indicates that the uptake of Hg from lake water and the atmosphere by phytoplankton is an important mechanism through which Hg enters the sediment.
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3.5. Hg in porewater and the diffusion fluxes Table 4 Correlation coefficients between THg, MeHg and TOC in sediment cores from the 4 sub basins of Lake Taihu. Sub-basin Central Taihu (C-3) East Taihu (E-2) North Taihu (N-1) North Taihu (N-3) West Taihu (W-2, W-4) a b
THg MeHg THg MeHg THg MeHg THg MeHg THg MeHg
MeHg
TOC
0.08
0.69b 0.40 0.27 0.53a 0.81b 0.79b 0.83b 0.84b 0.77b 0.63b
0.60b 0.73b 0.80b 0.68b
The significance is at the 0.05 level (2-tail). The significance is at the 0.01 level (2-tail).
However, in East Taihu, which is a grass lake area and where the main source of Hg is aquaculture of crab, the effect of organic material on the THg in sediment may be limited. Therefore, no significant relationship between TOC and THg was observed in East Taihu. As a primary electron donor and nutrient for heterotrophic bacteria, a high TOC content in sediment can enhance the formation of methylated Hg (Lambertsson and Nilsson, 2006). Gray and Hines (2009) and Macalady et al. (2000) reported that an increase in the organic material load of sediment could increase the production of MeHg in eutrophic lakes and reservoirs. A significant relationship between the TOC and MeHg contents in sediments was observed at most sampling sites (except site C-3) in Lake Taihu (Table 4). However, the relatively low concentrations of MeHg (0.2–0.96 ng g1) and the ratios of MeHg/THg (0–0.79%) in the sediments indicate that the elevated TOC content did not significantly increase the Hg methylation rate in Lake Taihu.
In Lake Taihu, the concentrations of THg and MeHg in porewater were high, ranging from 8.8–260 ng L1 and 0.14–25 ng L1, respectively (Fig. 4). At all sampling sites, the concentrations of THg and MeHg were higher in May than in September, with the exception of site N-1, where the average MeHg concentration in May was comparable with that in September. For example, the highest concentrations in May reached 150 ng L1 THg and 4.2 ng L1 MeHg at site C-3 and 260 ng L1 THg and 25 ng L1 MeHg at site E-2. These concentrations were much greater than those observed in September, which were 49 ng L1 THg and 1.0 ng L1 MeHg at site C-3 and 64.0 ng L1 THg and 1.6 ng L1 MeHg at site E-2. The distribution profile of MeHg in porewater was not clear, though the concentration of MeHg was found to be the highest on the interface between the sediment and the water; this was true at all sampling sites except C-3, E-2 and N-1 in May, where the peak concentrations of MeHg occurred at depths of 9–13 cm (Fig. 4). The ratio of MeHg/ THg varied from 0.7–9.6% with an average of 2.6 ± 1.6% in September and from 0.3–13.7% with an average of 3.3 ± 1.6% in May. Lake sediment is considered to be an important source of inorganic and organic Hg in the water column. The concentration gradient between the porewater and the water above the sediment drives the diffusion of Hg (Feng et al., 2009; Gray and Hines, 2009). Calculation showed that the sediment in Lake Taihu was an important source of THg and MeHg in both seasons, except at site N-1 in May where the THg showed a net deposition flux from water to sediment (Table 5). In general, the diffusion fluxes of THg and MeHg were higher in North Taihu and East Taihu than in West Taihu and Central Taihu. At the same time, the diffusion fluxes of THg and MeHg were obviously greater in May than in September at all sampling sites except N-1 (Table 5). The net depositional flux of THg observed at site N-1 in May could be due to an elevated THg concentration in the interface water caused by disturbances of the sediment and anthropogenic input from a contaminated river. It is
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Table 5 Estimated diffusion fluxes of THg and MeHg (in ng m2 d1) at 5 sampling sites in Lake Taihu in May and September 2009.
May September
THg MeHg THg MeHg
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East Taihu
North Taihu (N-1)
North Taihu (N-3)
West Taihu
360 5.2 6.5 4.2
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960 18 410 7.0
140 5.2 93 1.9
possible that the high Hg diffusion flux contributed to the higher concentration of THg in the lake water in May, but a reasonable explanation for the higher Hg diffusion flux in May is still not forthcoming.
Although the results provided a preliminary description of the Hg cycle in hypereutrophic, shallow Lake Taihu, many questions still remain and further work is required to fully understand the Hg cycle in eutrophic lakes.
4. Conclusions
Acknowledgements
The THg concentrations in Lake Taihu were much higher than those in background lakes, and 11% of the samples exceeded the second class of the Chinese environmental standards for surface water (50 ng L1), implying that there is serious Hg contamination in Lake Taihu. Despite the high THg concentrations, concentrations of MeHg were relatively low in the lake water, possibly resulting from the strong uptake of MeHg by phytoplankton and the high degradation rate of MeHg in oxic environments. In contrast to deep-water lakes and reservoirs, the MeHg concentration of the lake water and sediment was relatively low, despite the elevated THg and TOC loads in the sediment. The elevated pH induced by algal blooms and the aerobic water conditions may be important factors that inhibit the methylation of Hg, indicating that eutrophication does not accelerate Hg methylation in Lake Taihu. The results also showed that sediment was the net source of inorganic Hg and MeHg in the lake water column and that the significantly higher diffusion flux of inorganic Hg may have contributed to the higher THg concentrations in the lake water in May.
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