Environ. Sci. Technol. 1997, 31, 942-947
Mercury in Lake Michigan ROBERT P. MASON* AND KRISTIN A. SULLIVAN Chesapeake Biological Laboratory, University of Maryland System, P.O. Box 38, Solomons, Maryland 20688
Introduction Mercury contamination of aquatic systems is an important worldwide health concern (1, 2). Recent research has demonstrated that many freshwater lakes in North America, Europe, and Asia contain fish with elevated mercury (Hg) concentrations, i.e., concentrations that exceed state, federal, or international health guidelines (3-12). In the United States in the last decade, an ever-increasing number of states have issued health advisories for freshwater fish consumption, based primarily on the elevated Hg concentrations in piscivorous fish (11). In conjunction, the U.S. Environmental Protection Agency (EPA) has lowered the reference dose for Hg in fish, based on EPA’s Integrated Risk Information System, and many states in the United Statessfor example, Minnesota (12)shave also lowered the acceptable criteria for Hg in fish. These regulatory actions have proven to be contentious (11) and have led to a re-evaluation of the sources of Hg to U.S. waters and the factors controlling Hg accumulation in fish, as there are significant gaps in our knowledge of the sources of Hg to aquatic systems and the role of atmospheric deposition and anthropogenic emissions in providing “bioavailable” Hg to aquatic systems (11-15). A number of studies, focused primarily on smaller lakes, have sought to correlate elevated concentrations of mercury in fish with environmental parameters (e.g., refs 7-10 and 15-17) and have demonstrated the importance of anthropogenic inputs to the atmosphere in contributing to mercury contamination of both nearby and remote watersheds (13, 18). There is, however, little published data for Hg in Lake Michigan and the other Great Lakes. A previous study of mercury and other trace metals in the Great Lakes (19) reported average total mercury concentrations ranging from 10 pM (2 ng/L) for Lake Superior to 225 pM for Lake Michigan (data collected in 1980-1983). Our data, collected in 1994 and 1995 from offshore waters (Figure 1) during the EPAsponsored Lake Michigan Mass Balance Study (LMMB; 20), averaged 1.6 pM total Hg and are 2 orders of magnitude lower than these earlier data for similar sites in Lake Michigan. The concentration is more comparable to that of the open ocean (e.g., ref 21). Historical contamination of samples likely accounts for the differences between our values and previous data, as has been found in Lake Michigan for some of the other trace metals, most obviously for lead (22). It has occurred even recently during the analysis of open ocean waters for trace metals (23). Other recent measurements in Lake Michigan (24) found values for mercury ranging from 5 to 50 pM at a station within 6 km of Chicago. These concentrations are elevated as compared to our measurements, but this site likely receives enhanced inputs, both fluvial and from the atmosphere, from Chicago. One of our sites, approximately 20 km offshore, did not however show any enhancement in concentration as compared to sites more remote from urban influence (20). Gill and Bruland (25) found values of 4.5 and 18 pM for samples collected from the shores of Lake Ontario and Lake Erie, respectively, somewhat elevated relative to the open Lake * Corresponding author phone: (410) 326-7387; fax: (410)3267341; e-mail:
[email protected].
942
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 3, 1997
FIGURE 1. Map showing the sampling locations on Lake Michigan. Michigan water. A limited survey of Lake Superior found total Hg concentrations around 5 pM (26). Studies in the Great Lakes have generally focused on fish and the higher trophic levels of the food chain (e.g., ref 27). The National Contaminant Biomonitoring Program (NCBP) has measured total mercury concentrations in pooled samples of whole fish at a number of sites in the Great Lakes between 1976 and 1984 (28), and there has been no significant decrease in fish concentration over the study period. The concentrations of mercury in bloater and perch (0.04-0.07 ppm wet weight) for the three Lake Michigan sites are lower than those found in lake trout (0.2-0.22 ppm), likely reflecting the food preferences of the different species. Fish Hg concentration data for Lake Ontario collected between 1977 and 1988 (29) are similar to those found by the NCBP. We report here the results obtained during the LMMB study. In addition to our charge under the project of measuring total and particulate Hg in the water column, we were able to collect and analyze invertebrate and fish samples for their total and methylmercury (MMHg) concentration. Water samples were also analyzed for MMHg. Here we present the overall data and discuss both a preliminary budget for Hg in Lake Michigan as well as discuss the bioaccumulation of Hg in Lake Michigan organisms. Our water column data will be discussed in further detail elsewhere (20).
Methods Samples were collected during six cruises on Lake Michigan aboard the EPA RV Lake Guardian. Typically, cruises lasted 2-4 weeks with samples being collected from different parts of the lake during that period (Figure 1). Cruises took place in June, August, and October/November 1994 and April, August, and September/October 1995. Our field sampling strategy was based on the techniques and methods developed for the measurement of mercury in open ocean waters (3033) and involved the deployment of Teflon-lined Go-Flo bottles attached to a Kevlar line with a nonmetallic weight for the collection of water samples. The water was immediately decanted on board under cleanroom conditions
S0013-936X(96)00656-6 CCC: $14.00
1997 American Chemical Society
TABLE 1. Average Concentration of Total and Particulate Mercury and Methylmercury for Each of the Cruises in 1994 and 1995a cruise date
total Hg (pM)
n
particle Hg (pM)
n
particle Hg (nmol/g)
log Kd
June 1994 August 1994 October 1994 April 1995 August 1995 October 1995
1.70 ( 0.65 1.45 ( 0.65 1.65 ( 0.55 1.90 ( 0.85 1.75 ( 0.50 1.15 ( 0.30
16 28 20 24 23 21
0.85 ( 0.30 0.70 ( 0.25 0.65 ( 0.20 0.55 ( 0.20 0.50 ( 0.30 0.25 ( 0.10
16 29 19 23 23 21
0.85 0.88 1.09 1.10 0.63 0.37
5.7 5.8 5.8 5.8 5.6 5.5
cruise date
total MMHg (pM)
n
particle MMHg (pM)
n
particle MMHg (nmol/g)
log Kd
August/October 1994
0.063 ( 0.022
26
0.015 ( 0.015
14
0.019
5.7
a
Data condensed from that in ref 20.
into 2-L acid-cleaned Teflon bottles. Water from the Go-Flo bottles (4-8 L) was also filtered through quartz fiber filters, housed in a Teflon holder, for particulate determinations. Samples were sealed, double-bagged, and frozen for shipment back to the University of Maryland. Before analysis, samples were unfrozen in a laminar flow hood. Samples were analyzed for total Hg and for MMHg using standard techniques for Hg analysis at low levels (e.g., refs 34-37). For total Hg, samples were pre-oxidized using 0.5 mL of 0.2 N bromine monochloride solution (34) prior to quantification by the tin chloride reduction-atomic fluorescence technique. This method, outlined in EPA Method 1631 (38), is based on methods developed by Bloom and Crecelius (34) and Fitzgerald’s research group at the University of Connecticut (e.g., refs 30, 35, and 36). Dissolved MMHg was measured on water samples using direct ethylation of samples (37). For particulate samples and biota, distillation techniques were used to separate the MMHg from the sample matrix prior to ethylation using a method that was similar to that of Horvat et al. (39). After distillation, the distillate was analyzed in a similar fashion to the water samples. The detection limit (DL) for total Hg varied between 0.28 and 0.65 pM for the various cruises. Lower DLs were a result of the analysis of larger samples on the latter cruises. Similarly, the DL for particulate Hg was also a function of the volume of water sampled but was typically less than 0.05 pM. For MMHg, the DL was 0.025 and 0.005 pM for dissolved and particulate MMHg, respectively. The data presented here for total Hg have been submitted to the EPA and have been audited by the EPA QA/QC personnel. Invertebrate samples were collected using plankton nets and the plankton benthic tow that was on board the Lake Guardian. Zooplankton, individually picked from the bulk sample using Teflon tweezers, were rinsed in distilleddeionized water and frozen in Teflon vials. All samples collected consisted of only one genus of organism. On different occasions, invertebrates (Mysis, Diporeia, and Diaptomus) were collected. Sculpin were also taken from the benthic sled samples. All invertebrate samples were analyzed for total Hg and MMHg; the limited number of sculpin were analyzed only for total Hg. Samples were pre-digested in a nitric/sulfuric acid mixture for total Hg analysis before being treated in a similar fashion to the water samples (i.e., bromine monochloride oxidation, purge-and-trap after tin chloride reduction, with atomic fluorescence detection).
Results and Discussion Water Column. The average total Hg concentration for Lake Michigan waters during 1994-1995 was 1.6 pM (0.32 ng L-1) while the average particulate Hg was 0.58 pM, about 36% of the total on average. The averages for the various cruises are given in Table 1. Stations were dispersed throughout the lake with one station in Lake Huron (Figure 1). There was no discernible north-south trend in Hg concentration nor was there a distinct difference between samples collected in the mid-hypolimnion and the mid-epilimnion (20). Schottler
and Eisenreich (40) found a similar lack in gradient, either vertically or horizontally, for atrazine in Lake Michigan; a similar lack of vertical gradient was found for trace metals (22). The Hg concentrations found in 1994-1995 for Lake Michigan are similar to those measured in the remote ocean, away from coastal influence (e.g., the North Atlantic; 32). The same is true for other trace metals, which are also present in Lake Michigan at relatively low concentrations (22). The particulate Hg concentration for Lake Michigan ranged from 20 to 50% of the total Hg and was lowest in the fall compared to the spring and summer (Table 1). Concentrations were also lower in 1995 as compared to 1994. Average suspended matter concentrations are lowest in early spring, increasing from an average concentration of 0.5 mg L-1 for the epilimnion in April to 1 mg L-1 in June (41). During summer, concentrations decrease somewhat to around 0.8 mg L-1 in August, with a further decrease in surface waters through the fall. Concentrations increase, however, near the bottom due to fall water column mixing and winter storm events. Diatom blooms occur in the spring, and the particulate mass in the spring and summer is dominated by diatoms. Overall, biogenic particles are the dominant particulate in Lake Michigan (41). From late August to October, there is precipitation of calcite particlessa result of the biologically-mediated pH increase of the surface waterss and these particles dominate the water column during this period. Using the average particulate concentrations, it is possible to estimate dissolved particulate distribution coefficients (Kd) for the different time periods (Table 1). There is a coincident decrease in Kd, although small, associated with the decreased concentration of particulate Hg found in the late summer/ fall of 1995. The reason for this seasonal difference has not been elucidated. The difference could be a function of seasonal differences in particulate loadings due to seasonal differences in both phytoplankton and zooplankton standing stocks and in particulate type (20). Inter-annual differences do occur (e.g., ref 42) and could easily account for the observed differences in Hg distribution. The particulate Hg concentrations, on a per gram basis, are similar to the total Hg concentrations in surface sediments of Lake Michigan (0.3-0.6 nmol g-1 (60-120 ng g-1; 43) and to that of open ocean North Atlantic suspended matter (0.250.5 nmol g-1; 44). The average Kd value for the lake is similar to that of Wisconsin rivers (log Kd ranges from 5.0 to 6.9 for Lake Michigan tributaries; 45) and comparable to that of the open ocean (log Kd ) 5.4; 44). Concentrations of methylmercury (MMHg) were measured on a number of samples from the August and October 1995 cruises (Table 1). As the concentrations were low and near the analytical detection limit, not all samples were analyzed. Dissolved MMHg concentrations were between 25 and 50 fM for most samples while the particulate MMHg concentration was 10-15 fM, about 25% of the total MMHg concentration. The estimated log Kd for MMHg is around 5.7, based on these average concentrations. The particulate MMHg, at around
VOL. 31, NO. 3, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
943
TABLE 2. Mercury and Methylmercury in Organisms (nmol/g Dry Weight)
a
organism
total Hg (nmol/g)
MMHg (nmol/g)
% MMHg
seston (phytoplankton) zooplankton (Dioptamus) Mysis amphipods (Diporeia) sculpin bloater lake trout
0.82 ( 0.28 (n ) 130) 0.32 ( 0.24 (n ) 3) 0.20 ( 0.11 (n ) 8) 0.39 ( 0.26 (n ) 10) 0.45 ( 0.06 (n ) 3)# 1.0b 2.75 ( 1.0a
0.019 0.06 ( 0.04 (n ) 3) 0.24 ( 0.14 (n ) 9) 0.12 ( 0.05 (n ) 10) 0.45 ( 0.06 (n ) 3) 1.0 2.75 ( 1.0
1-3 18 ( 2 103 ( 36 35 ( 12 100a 100 100
All the mercury in fish is assumed to be methylmercury.
b
Data taken from ref 29 and converted to dry weight by assuming 80% water in fish.
10-30 pmol g-1, was about 1-3% of the total particulate Hg concentration. Mercury and Methylmercury in Organisms. The concentration of Hg and MMHg in seston (phytoplankton), zooplankton (Dioptamus), Mysis, amphipods (Diporeia), and sculpin was determined (Table 2). Samples were collected on an opportunistic basis, and thus the data do not cover all the lake nor all the sampling periods. As the particulate matter is largely biogenic and of microbial origin, it is reasonable to use this concentration as an estimate of phytoplankton concentration. This is especially true of the spring and summer when the particulate load of the surface waters is dominated by phytoplankton. Mason et al. (46) have shown that the concentration of MMHg in phytoplankton is a function of the water chemistry that controls the accumulation of Hg and MMHg at the base of the food chain. Applying their model
[Hgpp] (dry weight) ) (9720D0w[MMHg])/Rµ to the waters of Lake Michigan gives a value for MMHg in phytoplankton of 40 pmol g-1 dry weight. In the model, Dow is the overall octanol-water partition coefficient for MMHg at the pH and chloride concentration of the lake (0.07), estimated based on the measurements of ref 46; R is the weighted average cell radius (1 µm) and µ is the average phytoplankton growth rate, taken as 1 day-1. The mean phytoplankton cell radius was estimated from the relative mass of phytoplankton size classes (
