The amount of mercury deposition from the atmosphere has increased since the 19th century with the burning of fossils fuels, trash incineration, mining operations, and manufacturing of paper (Benoit, 1994; Swain, 1992). Mercury is neither stationary nor static, as it can move about as a gas or change from  inorganic Hg to the more harmful MeHg during bacterial respiration. Mercury as methylmercury (MeHg) causes more fish consumption advisories than any other contaminant or heavy metal (Gilmour & Riedel, 1999). 

(after Rob Mason & Jani Benoit)

Our project was to examine the mercury content of freshwater fish in the three geological regions of Maryland. If we found differences in those fish, could we ascertain the reasons for those differences? Possibilities included the geography itself, pH, concentration of sulfates and organic content, and the land use type. 

Sixteen large mouth bass (Micropterus salmoides) from Lake Lariat, Lake Clopper, and Big Piney Reservoir in Maryland were obtained from Dr. Robert Mason of the Chesapeake Biological Laboratory. Samples of 0.44g to 0.81g were removed from the previously frozen and filleted fish collected during May and June, 2001. The samples were digested in 5 ml 5 parts HNO₃ : 2 parts H₂SO₄ solution in acid washed glass containers with marble tops at temperatures around 90 degrees Celsius. After cooling the samples were brought up to about 50 ml and weighed to the closest 0.05g. Three days later total Hg analysis was performed by cold-vapor atomic fluorescence (CVAF) with preconcentration of digested samples on gold traps (after Bloom, 1992). The analysis, as shown below, centers on the fact that Hg will form an amalgam with gold, and then can be driven off by heat.

During the length of the digestions, three blanks and six replicates were performed. The blanks ranged from 685 to 746 and had an average of 721 pg Hg, meaning the detection limits were less than 1 ng/g. The replicates, performed on samples from 2 locales, had a replicate standard deviation (RSD) of 14 %. A standard curve was prepared using six Hg samples ranging from 50 to 500 pg, see Figure 2.

Figure 2

Results

            The predigested fish ranged from 312g to 1918g, with lengths from 311mm to 521mm. There was a strong correlation between the length and the weight of fish, both by lake and in all three lakes (see Figs 2 and 3).

Fig.2

Fig.3

            The three lakes have both differing Hg deposition rates and different amount of Hg in the fish muscles; however, the relationship is counterintuitive. 

Lake Clopper is in the urban Baltimore-D.C. area and has a wet deposition rate of about 33 micrograms Hg/m2/yr. The Big Piney reservoir, which is in the western (Piedmont) part of the state, and Lake Lariat which lies in the coastal plain, receive around 14 micrograms Hg/m2/yr (Mason,http://www.umces.edu/annualreport2000/mason.pdf). 

Mercury was strongly correlated with both length and weight (data not shown), but slightly more with length. By correlation analysis length can account for 75% of the variation in the Hg data, while weight can account for 70%. In order to make comparisons among lakes, the tissue concentrations were normalized to a 400mm fish (see fig 4). The fish both in Lake Lariat and the Big Piney Reservoir had about the same amount of Hg in their tissues. Lake Lariat fish averaged 0.496 micrograms/g (+ 0.12 ), and the fish in Big Piney Reservoir averaged 0.501 micrograms/g (+ 0.11). The fish in Clopper Lake had much less mercury, about 0.18 micrograms/g (+ 0.08) (Figure 4).

Fig.4

            A t-test between Lake Lariat and Clopper Lake with 4 degrees of freedom showed a highly significant difference ( P < 0.005) and the test between Lake Clopper and Big Piney was very highly significant (P<0.0005), with 6df.. Clearly Lake Clopper has fish with less Hg than either of the others.

Discussion

           The forms of mercury (Hg) and their conversions are shown in the following graphic,with elemental on top, then inorganic, then organic methylmercury. (by J.M. Benoit).

Most of the mercury in the atmosphere is elemental or inorganic mercury. While distribution occurs naturally, deposition has been increased greatly since the burning of fossil fuels began. Solid waste incineration and fossil fuel combustion facilities contribute about 87 % of the emissions in the United States (EPA 2001). Bodies of water can accumulate mercury from either point or non-point sources, including deposition prior to impoundment of lakes. The water column in a lake usually contains one-millionth of the the amount of mercury found in fish, especially the top predators in a food chain (Fitzgerald, 1993). Because of this bioaccumulation through the food chain, top predators are more affected than other organisms. In our study of 16 largemouth bass sampled from three different regions of Maryland, we found the fish with the least amount of mercury came from a lake that has the highest deposition of mercury. Our lakes from the coastal plain (Lake Lariat) and from the Appalachian plateau (Big Piney Reservoir) had mercury concentrations of about 0.5 ppm which matches the national mean of 0.52 (N=90,000) (EPA 2001). What reasons could be postulated then to account for the 0.18 ppm in bass from Clopper Lake, a lake on the Piedmont close to Washington DC?

Map by USGS

We suggest there are several possible reasons that the fish in Big Piney and Lake Lariat have higher mercury levels although they have lower atmospheric deposition rates than Lake Clopper. We gathered the following lake characteristics from Rob Mason.

The levels of methylmercury in a body of water are dependent in large part on the anaerobic bacteria that live there. These bacteria utilize sulfate as an electron acceptor in cellular respiration. As the level of sulfates in the water column increases, the numbers of bacteria and the amount of methylmercury increases up to a point. (Regnell et al 2001). However, a point is reached where the resulting sulfides inhibit production of MeHg (Gilmour et al, 1998) (Mason, http://www.umces.edu/annualreport2000/mason.pdf) (Benoit, personal communication). Data provided by Mason shows that the Big Piney has 12 mg/L of sulfate to Clopper’s 6.2, possibly meaning that Lake Clopper has lower activity of Sulfate-Reducing Bacteria and consequently lower rates of methylmercury production.

pH is another variable that is important in predicting MeHg levels and the speciation of mercury. In a large study done in Florida, Lange (1993) suggested that pH is the variable to which MeHg in fish is most closely correlated. Xun also found more MeHg in low pH lakes in his study (1987). Why a low pH is correlated to high MeHg levels is the subject of much speculation. It may have to do with the binding of Hg to organic material more readily at low pHs, thereby reducing the sedimentation rate (Lange). If the mercury is bound to organic particles, it will stay in the water column longer and be more readily available to the phytoplankton and zooplankton that are the base of the food web. It may be that low pH is an indicator of low alkalinity and accompanying carbonate chemistry. Most lakes on the Piedmont have higher pHs than those of the coastal plain or Appalachian plateau. Perhaps the slightly lower pH at Big Piney (7.2) (as opposed to Lake Clopper’s 7.5) is enough of a difference to exacerbate the mercury bioaccumulation.

Another influence on methylmercury accumulation could be the dissolved organic carbon (DOC). Higher DOC levels in Big Piney may indicate import from the watershed. Methyl mercury produced in forested soils may be transported into the lake along with DOC.

Most of Maryland's lakes are impoundments. Impoundments are created by flooding land, which may have been pastures, grassland, forests, farmland or combinations of those. Mercury accumulates on the ground from deposition from the atmosphere. When flooded, mercury is added to  the water column. Possibly, the age of the lake can help explain the higher mercury content, since Big Piney has had only half the time to deal the consequences of flooding.

Conclusion

We assayed fish from three geologically different regions of Maryland. Fish from an urban area with higher mercury deposition actually had significantly less mercury in their tissues. We feel that parameters such as pH, sulfate concentration, DOC, age, geologic region,  and alkalinity bear significantly on the uptake of mercury and that at least one, but probably more, of these factors can be used to explain our findings. Furthermore, we hope that Maryland will continue to closely monitor the differences in both the water quality and resulting mercury bioaccumulation in all three regions.

Acknowledgements

We wish to thank Dr. Janina Benoit for her time, patience, teaching, and lab.  Dr. Robert Mason of Chesapeake Biological Laboratory kindly provided both data and fish.

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References

EPA 2001 United States Environmental Protection Agency, Office of Water, Fact Sheet EPA-823-F-01-011.

Fitzgerald WF 1993 Mercury as a global pollutant. The World & I, Oct. 1993 pp. 192-199.

Benoit, J.M., W.F. Fitzgerald and A.W.H. Damman. 1994. Historical atmospheric mercury deposition in the mid-continental U.S. as recorded in an ombrotrophic peat bog. In: Mercury Pollution: Integration and Synthesis (C.J. Watras and J.W.Huckabee, Eds.), pp.187-202, Lewis Publishers, Boca Raton, FL.

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 Gilmour, C.C.,  Riedel G.S. (2000) A survey of size-specific mercury concentrations in game fish from Maryland fresh and estuarine waters. Arch. Environ. Contam. Toxicol. 39, 53-59.

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Xun,L., Campbell, N.E.R.,  and Rudd, J.W.M (1987) Measurements of specific rates of net methyl mercury production in the water column and surface sediments of acidified and circumneutral lakes. Can. J Fish Aquat Sci 44:750-757.