Environ. Sci. Technol. 2008, 42, 5118–5124
Mercury in Southeastern U.S. Riverine Fish Populations Linked to Water Body Type ANDREW L. RYPEL,† D. ALBREY ARRINGTON,‡ AND R O B E R T H . F I N D L A Y * ,† Department of Biological Sciences, University of Alabama, Box 870206, Tuscaloosa, Alabama 35487, and Loxahatchee River District, 2500 Jupiter Park Drive, Jupiter, Florida 33458
Received January 21, 2008. Revised manuscript received April 10, 2008. Accepted April 17, 2008.
We compared Hg concentrations in fishes from the regulated Black Warrior River and the unregulated Sipsey River in west Alabama whose neighboring watersheds receive equivalent atmospheric Hg deposition. Average fish fillet Hg concentrations were 3-fold higher in the unregulated river compared to the regulated river. Between river differences in Hg fish concentrations likely originate from structural (e.g., species composition) and functional (e.g., energy flow pathways) differences between the two ecosystems. We tested the generality of these findings by comparing largemouth bass Hg concentrations among unregulated rivers (n ) 6) and reservoirs (n ) 11) throughout the southern Coastal Plain geologic region. ANCOVA revealed that at a given bass length, Hg concentrations were approximately 1.75 times higher in unregulated rivers compared to regulated rivers. Aerial deposition of Hg was not correlated to largemouth bass Hg concentrations. We suggest that the link between atmospheric Hg deposition and fish Hg concentrations is significantly modulated by the structure and function of aquatic ecosystems and this accounts for much of the variation in fish Hg concentrations among systems. Unregulated floodplain-rivers in the south have some of the highest fish Hg concentrations on record and should be intensely monitored to establish human consumption risks.
Introduction Mercury (Hg) contamination of fishes in aquatic ecosystems is of rising concern globally and can have serious human health consequences if contaminated fish are regularly consumed (1). Although Hg is a naturally occurring heavy metal, its concentration in the atmosphere and deposition rates have increased by an order of magnitude since the industrial revolution, in part, due to increased fossil fuel combustion (2, 3). Methylation of Hg into methylmercury (MeHg, the form that readily bioaccumulates in food webs) is tightly coupled to landscape-level processes such that Hg methylation rates and consequently Hg concentrations in organisms can be highly variable among ecosystems (4–6). Although a number of physiologically diverse microbes are capable of Hg methylation in pure culture, Hg methylation in natural environments has been tied primarily to sulfate* Corresponding author phone:
[email protected]. † University of Alabama. ‡ Loxahatchee River District. 5118
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(205)
348-4167;
e-mail:
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 14, 2008
reducing activity by bacteria in anaerobic sediments (7–11). More specifically, microbially catalyzed MeHg production in anaerobic sediments is the major mechanism responsible for MeHg in natural waters. Exceptions to this are in areas where high MeHg concentrations occur in precipitation (e.g., Ontario or Sweden 12, 13) or through natural geology (14). MeHg is quickly assimilated by aquatic organisms and accumulates within food webs reaching greater concentrations at successively higher trophic levels (15–17). Thus MeHg concentrations tend to be highest in predatory fishes, such as largemouth bass, which are commonly consumed by humans. As of 2007, human fish consumption advisories specifically due to Hg were in effect in 48 of 50 U.S. states. Following a common limnology trend (18), most studies on Hg bioaccumulation have focused on pelagic food webs, where much of the seminal work on Hg bioaccumulation originated (19–21). This is in spite of estimates that benthic food webs support 50-65% of higher trophic level production in northern, glacial lakes (22). Southern free-flowing rivers, where benthic or strongly coupled benthic-pelagic food webs dominate the trophic basis for secondary production, have received considerably less attention. Unregulated rivers in the southeastern U.S. are dynamic ecosystems that flow through and interact with extensive marshes, floodplain and oxbow lakes, bottomland hardwood floodplains, and tupelo-cypress wetlands (23). Floods that inundate the riparian landscape typically occur during the wet season, but can also occur during the dry season due to large, regional-scale storms. These ecosystems have substantial quantities of snag habitat, high inputs of labile and recalcitrant allochthonous carbon, and extensive microbial decomposition that drive core ecosystem functions, such as nutrient cycling (23–25). Periods of prolonged floodplain inundation likely encourage biogeochemical conditions conducive to microbially catalyzed MeHg production. However, throughout the world, suppression of the natural flow regime has excised floodplain connectivity in many rivers (26, 27). This inevitably alters the structural and functional properties of rivers (28–31), and may also reduce net Hg methylation in sediments and subsequent bioaccumulation within constituent organisms. This study compares Hg concentrations in fishes from an unregulated and a regulated river that neighbor one another in the southeastern U.S. and uses published fish Hg concentration data and Hg deposition rates to explore whether river regulation is a proximate cause for the observed differences.
Experimental Section Study systems. The Sipsey River is a 175 km long, sixth order tributary to the upper Tombigbee River that drains a 2,044 km2 watershed. Although its headwaters originate in the lower Cumberland Plateau physiographic province, the Sipsey meanders mostly through eastern Coastal Plains. Streamflow throughout the entirety of the river is unregulated helping support one of the most diverse fish and mussel faunas in the southeastern U.S. (23). Average annual discharge ) 18.42 m3 s-1, average annual wet season (months December through February) discharge ) 29.90 m3 s-1, and average annual dry season (months June through September) discharge ) 6.49 m3 s-1. Natural floods inundate and hydrologically connect the main channel to riparian bottomland hardwood forests, and seasonally disjunct, perennial and intermittent floodplain lakes and beaver ponds. The Black Warrior River is a 286 km long, sixth order stream that drains a 16 250 km2 watershed. This watershed is located directly adjacent to and southeast of the Sipsey River 10.1021/es8001772 CCC: $40.75
2008 American Chemical Society
Published on Web 06/06/2008
watershed. Sampling stations on the Sipsey River and Demopolis Reservoir are 48.3 km apart from one another and have equivalent estimated annual mercury wet deposition (http://nadp.sws.uiuc.edu/mdn/maps/2005/05MDNdepo.pdf). The Black Warrior River is formed by the confluence of the Locust and Mulberry Forks roughly 40 km west of metropolitan Birmingham, AL and flows south-southeast through Coastal Plains to its confluence with the Tombigbee River at Demopolis, AL. In the late 19th century, the U.S. government began construction on a series of locks and dams to promote a booming coal industry in the region and later, hydropower. This resulted in a total of 17 locks and dams, as well as, the Tennessee-Tombigbee waterway that artificially connected the Mobile, Tennessee, and Mississippi River drainages for shipping. Demopolis Lock and Dam (32°31′N-87°52′W) impounds both the Tombigbee and Black Warrior Rivers, creating Demopolis Reservoir, which is the second largest impoundment on the Tombigbee and Black Warrior Rivers extending 77 km upriver. As a result of lock and dam operations, Demopolis reservoir has a remarkably stable hydrograph with an average annual discharge of 23.79 m3 s-1, average annual wet season discharge of 23.73 m3 s-1, and average annual dry season discharge of 23.74 m3 s-1. These stabilized water levels result in floodplain areas in downstream reaches of Demopolis Reservoir being continually inundated while floodplain areas in upstream reaches of the reservoir are almost never inundated (32). Fish Sampling. We captured a total of 42 individual fish representing eight species from the Sipsey River main channel near Elrod, AL (n ) 18) and the Black Warrior River segment of the Demopolis Reservoir (n ) 24) during October, 2003 using a boat mounted Smithroot electrofisher. In each river fish were captured from one large reach on the order of a kilometer in length. The species and number of fish captured and analyzed reflect natural abundances of large-bodied fishes in the river on the sampling dates. Fishes analyzed from the Sipsey River were largemouth bass (Micropterus salmoides), channel catfish (Ictalurus punctatus), bowfin (Amia calva), blacktail redhorse (Moxostoma poecilurum), and smallmouth buffalo (Ictiobus bubalus). Fishes captured from Demopolis Reservoir were largemouth bass, channel catfish, gizzard shad (Dorosoma cepedianum), bluegill (Lepomis macrochirus), and black crappie (Pomoxis nigromaculatus). Captured fish were placed under ice in a clean cooler for transport to a University of Alabama laboratory. Each fish was identified with a unique number and the weight (g) and total length (mm) recorded. All fish were filleted (with no bones), skinned, packaged in ziplock bags, and stored frozen (-80 °C). Fillets were chosen for analysis because this represents the primary consumable portion of the fish and because this is the protocol typically employed by U.S. state monitoring agencies (33). Laboratory Methods. Frozen, boneless, skinless fillets were shipped overnight to ACZ Laboratories, Steamboat Springs, CO for total Hg analysis using approved U.S. Environmental Protection Agency (EPA) protocols; 600/481-055, M7471A CVAA. Mercury concentrations were reported in mg kg-1 wet weight. Any samples that fell below the procedural detection limit (200 km in length and not have any major physical alterations to be considered “unregulated”. For regulated rivers, we limited site selection to major reservoirs within the Coastal Plain physiographic province such that watershed size would be comparable to the unregulated rivers. Smaller lentic or lotic ecosystems were not analyzed (e.g., no ponds, creeks, or small impoundments). Rivers with a known mercury source (36), aside from atmospheric deposition, were removed from analysis. This eliminated 5 of 13 unregulated Coastal Plain streams and one of 12 reservoirs. We collected additional site information for each regulated and unregulated river including an estimate of average annual air deposition of Hg at the latitude-longitude coordinates specified for each sampling site (TableTable 1, 2, data estimated from National Atmospheric Deposition Program Mercury Deposition Network deposition maps, http://nadp.sws.uiuc.edu/mdn/maps/). We used ANCOVA to test for significant differences in Hg concentrations of largemouth bass between unregulated and regulated rivers. Fish size (e.g., length) heavily influenced Hg concentrations in these largemouth bass; therefore we used fish length as the independent variable, Ln Hg concentrations as the dependent variable and ecosystem type (unregulated, 0; regulated, 1) and average annual air deposition as covariates. VOL. 42, NO. 14, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Hg Concentrations of Eight Fish Species Captured from the Sipsey River and Black Warrior River at Demopolis Reservoir, October, 2003. a Hg range (mg kg-1)
Hg mean (mg kg-1)
SD
N
size range (SL, mm)
U.S. EPA national avg. (mg kg-1)b
0.10 0.06 0.13 0.15
5 3 4 5 1 18
205-230 343-463 455-495 275-300 330-330 205-495
0.43 0.35 0.96 0.43 0.18 0.35
7 6 5 2 4 24
184-388 71-155 146-254 275-294 160-173 71 -388
0.43 0.15 0.19 0.18 0.09 0.35
Sipsey River
largemouth bass smallmouth buffalo bowfin blacktail redhorse channel catfish all fish
0.58-0.82 0.55-0.67 0.47-0.73 0.42-0.79 0.11-0.11 0.11-0.82
0.70 0.62 0.60 0.57 0.11 0.59
Demopolis Reservoir largemouth bass bluegill black crappie channel catfish gizzard shad all fish
0.12-0.35 0.05-0.14 nd-0.21 nd-nd nd-nd nd-0.35
0.26 0.09 0.08 nd nd 0.12
0.17 0.08 0.03 0.09 0.12
nd Indicates measurements in which Hg concentrations were below the detection limit of 0.04 mg kg-1. from the U.S. EPA national mercury database, http://www.epa.gov/waterscience/fish/mercurydata.html. a
Results and Discussion We found measurable Hg concentrations in the majority of fishes examined from both the Sipsey River and Demopolis Reservoir. Mean Hg concentrations of fishes collected from the Sipsey River were significantly higher compared to fishes captured from Demopolis Reservoir (Figure 1a, t test, t ) 10.43, P < 0.0001). With the exception of channel catfish, all species analyzed from the Sipsey River had mean Hg concentrations >0.55 mg kg-1; individual largemouth bass, blacktail redhorse, and bowfin showed fillet Hg concentrations in excess of 0.70 mg kg-1. The single channel catfish captured showed fillet Hg concentrations that were considerably lower than other species, however, as only a single fish was captured, any conclusion about this population would be premature. Fishes from the Sipsey River showed Hg concentrations in excess of, and in some cases more than double, the U.S. EPA 0.3 mg kg-1 Hg recommended action level for a consumption advisory. The Sipsey River is not under a consumption advisory and should be evaluated by the appropriate state and federal agencies to verify our findings and establish any necessary advisories. Mercury was detected from the majority (67%) of fish captured from Demopolis Reservoir, although in lower concentrations. No detectable concentrations of Hg were found in any gizzard shad (n ) 4) or channel catfish (n ) 2) collected from Demopolis Reservoir. Mercury concentrations in bluegill and black crappie were low with concentrations in these fishes ranging from 0.05 to 0.14 and below detection to 0.21 mg kg–1, respectively. Largemouth bass had the highest Hg concentrations in any of the fishes examined from Demopolis Reservoir (mean ) 0.26 mg kg-1); however, the average concentration was below the recommended action level for consumption advisories and much lower than Hg concentrations observed in largemouth bass from the Sipsey River, though Demopolis largemouth bass were larger (P ) 0.04). ANCOVA revealed that fish size (measured as total length) was a significant covariate, and after removal of the fish size effect there was a significant difference (Figure 1b, ANCOVA, df ) 1, F ) 16.9, P ) 0.004) in Hg concentrations of largemouth bass between rivers, with largemouth bass from the Sipsey River having significantly higher Hg concentrations for a given fish size. While our results suggest that largemouth bass from the Demopolis Reservoir are currently below the recommended action level for consumption advisories, a previous study (6) (samples collected between 1999 and 2001) found mean largemouth bass Hg 5120
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b
Data accessed
concentrations of 0.29 and 0.59 at two different main channel sites within Demopolis Reservoir. We measured δ15N values for all fishes captured from the Sipsey River and Demopolis Reservoir, as there is typically a link between trophic position (as assessed by δ15N values) and contaminant load (19, 37) and food is often the major exposure route of aquatic animals to mercury (38). For example in northern, pelagic systems, predators from ecosystems with longer food chains, as well as, predators that forage higher in a food web typically show higher Hg concentrations (19). In the Sipsey River, nitrogen stable isotope values ranged from 9.35 to 11.98, whereas δ15N values for fishes from Demopolis Reservoir ranged from 9.06 to 17.31 (Figure 2). Assuming consumers show an approximate 3‰ enrichment in δ15N relative to their diet (39), fishes from the Sipsey River forage within a highly compressed food web, a pattern that was also shown in an unregulated tropical floodplain river (40), and deserves further study. Fishes from the Demopolis Reservoir span approximately 3 trophic levels: shad at the lowest trophic level (δ15N 9-12), bluegill at an intermediate trophic level (δ15N 12-15) and bass and catfish at the highest trophic levels (δ15N 16-19). Regression analysis showed significant positive relationships between Hg fish concentrations to δ15N values for both systems, although neither fit the expected pattern. Ultimately, we are unsure why these systems failed to show typical dietary habit/ contaminant concentration patterns, but these data clearly indicate significant differences exist in the trophic structure of these two ecosystems. A similar atypical dietary habit/ contaminant concentration pattern was found by a previous study (6) that compared largemouth bass from the Demopolis Reservoir and an adjacent wetland site; reservoir fish had a mean Hg concentration of ∼0.24 mg Kg1- and mean δ15N values of ∼16.1, whereas wetland fish had a mean Hg concentration of ∼0.40 mg Kg1- and mean δ15N values of ∼13.5. They concluded that the wetland was a separate foodweb module and that significant differences exist in the trophic structure of the two ecosystems. To investigate whether hydrologic regulation might contribute to differences in fish Hg concentrations between the Sipsey and Black Warrior Rivers, we compared Hg concentrations in largemouth bass from regulated and unregulated rivers with no ancillary Hg point sources in the Coastal Plain physiographic province of the southeastern U.S. (Table 2). Using fish length as an independent variable and Hg deposition rate as a covariate, Hg concentrations
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0.36-1.00 0.10-0.50 0.50-0.50 0.16-0.41 0.50-0.50 0.12-0.35 0.11-0.13 0.06-0.17 0.06-1.02
32.48573, -88.79728
31.7525, -88.13583 and 31.95833, -88.07106
32.05619, -87.25483 and 32.11544, -87.39811
32.85, -88.15556 and 33.08333, -88.26431
32.34614, -86.77403 and 32.40178, -86.64622
32.3105, 87.4729 shoreline of main lake, GA,
main lake body, Bibb County, GA n/a
0.62-0.89
34.40789, -89.77564 0.53-1.02
0.25-0.95
32.39416, -90.01759 and 32.42506, -90.07358
33.80884, -89.76254
0.06-0.74
0.10-3.50
n/a
main lake body, GA
0.10-1.75
33.38445, -81.11334 and 33.06445, -80.61278 and 33.35556, -80.88612 and 33.02778, -80.39167 34.05167, -79.75389 34.3133, -79.0386 and 34.31334, -79.03862 at U.S. highway 84, GA 33.1342, -87.4653 near town of Bunker, FL, and near town of Ebro, FL, and near town of Isagora, FL, and near town of Caryville, FL, and ∼1.5 mi north of the AL-FL state line 1.21-2.76 1.17-2.02 0.27-3.50 0.87-1.26 0.58-0.82
range bass Hg (mg kg-1)
0.60 0.69 0.37 0.14 0.39
0.26 ( 0.08 0.12 ( 0.01 0.10 ( 0.06 0.43 ( 0.29
0.34
0.54
0.37
0.67
0.62
0.63
0.46
0.50 ( 0.00
0.25 ( 0.14
0.50 ( 0.00
0.30 ( 0.28
0.69 ( 0.24
0.82 ( 0.24
0.77 ( 0.14
0.65 ( 0.29
0.23
1.39
1.29 ( 0.77 0.29 ( 0.19
1.05
1.67 1.41 1.35 1.74 1.50
0.84 ( 0.34
1.81 ( 0.46 1.48 ( 0.47 1.27 ( 1.05 1.00 ( 0.23 0.70 ( 0.10
mean bass Hg (mg kg-1)c
368 ( 69 411 ( 70
323 ( 66 318 ( 60
379 ( 42
367 ( 52
391 ( 29
377 ( 11
404 ( 82
440 ( 48
428 ( 41
449 ( 81
431 ( 70
381 ( 84
350 ( 76
421 ( 73 412 ( 19 385 ( 90 295 ( 61 269 ( 12
Mean Bass Length (mm)c
15 (3) 191 (51)
7 (7) 15 (3)
12 (3)
18 (3)
12 (2)
12 (2)
20 (5)
10 (4)
13 (3)
22 (6)
42 (18)
80 (71)
25 (20)
18 (18) 3 (3) 22 (22) 15 (3) 5 (5)
Bass sample sizee
13 n/a
15 11
15
15
15
15
17
15
13
13
15
n/a
17
11 11 9 15 15
Air Hg deposition (µg m2yr-1)f
a All data except Sipsey River and Demopolis Reservoir are from the EPA national database. b Localities presented represent the most detailed information available in the EPA national database. c Values are the mean of all samples from a water body with equal weighting given to composite and individual fillet samples, mean ( S.D. d Expected Hg concentration for a bass 400 mm in length. Calculated as the (observed Hg concentration × predicted Hg concentration LSML-1) × (predicted Hg concentration 400 mm) where LSML ) lake specific mean length. Predicted concentrations were generated from the regression lines presented in Figure 3. e The total number of bass sampled followed, in parentheses, by the total number of composite and individual samples included in the mean. f Air deposition values were determined from National Atmospheric Deposition Program Mercury Deposition Network deposition maps, http://nadp.sws.uiuc.edu/mdn/maps. Values were estimated from the sampling locations and determined as the median values from the ranges presented on the map.
Reservoirs Lake Seminole, GA Ross Barnett Reservoir, MS Sardis Reservoir, MS Grenada Reservoir, MS Okatibbee Reservoir, MS Coffeeville Reservoir, AL Dannelly Reservoir, AL Gainesville Reservoir, AL Jone’s Bluff Reservoir, AL Demopolis Reservoir, AL Lake Olmstead, GA Lake Tobesofkee, GA all reservoirs
Choctawhatchee River FL, AL All unregulated rivers
Edisto River, SC Lynches River, SC Lumber River, NC Satilla River, GA Sipsey River, AL
Unregulated Rivers
sampling locationb
predicted bass Hg at 400 mm (mg kg-1)d
TABLE 2. Summary Statistics for Hg Concentrations in Largemouth Bass from Unregulated and Regulated Rivers in the Southeastern Coastal Plain Regiona
FIGURE 1. Mean Hg concentrations of (a) all fishes sampled and (b) largemouth bass from the Sipsey River and Demopolis Reservoir, October, 2003. Error bars indicate ( 1 SE and the dashed line denotes the 0.3 mg kg-1 action level recommended by the U.S. EPA for establishment of fish consumption advisories.
FIGURE 2. Relationships between Hg concentrations and δ15N values of individual fish for all individuals and species captured from the Sipsey River (closed circles, solid regression, y ) 0.17x - 1.23, r2 ) 0.45, P ) 0.002) and Demopolis Reservoir (open triangles, dashed regression, y ) 0.02x - 0.22, r2 ) 0.24, P ) 0.01), October 2003. Open squares have been placed around symbols from both systems to indicate individual largemouth bass. The horizontal gray dashed line denotes the 0.3 mg kg-1 action level recommended by U.S. EPA for establishment of fish consumption advisories.
were significantly higher in unregulated rivers compared to regulated rivers (Figure 3, ANCOVA, F )22.23, P < 0.0001). Hg deposition rate was not a significant covariate (F ) 0.02, P ) 0.90), indicating that largemouth bass Hg concentrations were not correlated with atmospheric deposition of Hg in regulated and unregulated rivers of the southeastern Coastal Plain. Our results are consistent with a recent study that documented 4.5-fold differences in Hg concentrations of fishes from eight Isle Royale lakes; this island within Lake Superior is 72 km long × 14 km wide and shows no spatial differences in rates of atmospheric deposition of mercury (41). 5122
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FIGURE 3. Comparison of Hg concentrations by fish size (total length) for largemouth bass from unregulated (circles, solid regression line, y ) 0.006x - 0.92, r2 ) 0.86, P ) 0.001) and regulated (squares, dashed regression line, y ) 0.005x - 1.39, r2 ) 0.66, P ) 0.001) rivers in the southeastern Coastal Plain geologic region, U.S. Error bars indicate ( 1 SE; the open circle and square represent the Sipsey River and Demopolis Reservoir, respectively. Our results were inconsistent with a recent study (35) that concluded methylmercury in freshwater fish was linked to atmospheric mercury deposition and that the relationship was not affected by waterbody type (reservoirs, lakes, or rivers). Recent experimental evidence clearly demonstrates that within-system methylmercury levels in freshwater fish increase with increased atmospheric mercury deposition (42, 43). Our data clearly indicated that variation in largemouth bass methylmercury levels among systems was linked to waterbody type and fish size and not to atmospheric mercury deposition rates. We propose several reasons for the differences in findings. First, the Hammerschmidt and Fitzgerald (35) study accounted for the influence of fish size by limiting comparisons to bass ranging from 300-400 mm total length. However, they did not account for variations in Hg concentrations that occur within this length range. As seen in Figure 3, the smallest and largest fish in this size range can vary more than 2-fold in Hg concentrations. Second, our study included a narrower range of average annual wet atmospheric deposition rates (9-17 µg m-2 y-1) compared to the range used in the previous study (∼4-18 µg m-2 y-1) and 12 of the 22 states used in their regression fell below the range of deposition rates covered in our study. Third, data from two (GA, SC) of six states used in our study were excluded from the previous study; Hammerschmidt and Fitzgerald (35) excluded GA as a statistical outlier from their established regression line and SC because of the high analytical detection limit for samples (0.25 µg g-1 wet weight). We included data from SC rivers because the fish methylmercury values were 4.5- to 11-fold greater than the analytical detection limit. Finally, the Hammerschmidt and Fitzgerald (35) study was national in scope while ours was limited to the southeastern Coastal Plain. Physiographic provinces may differ in how river regulation impacts ecosystem function and thus mercury accumulation in fishes. This is not the first time that fish Hg concentrations in southern U.S. rivers have failed to conform to a nationally observed trend. The predicted positive correlation between MeHg sediment and water concentrations and largemouth bass Hg concentrations (44) was not borne out in a detailed study of the Mobile Alabama River Basin (6). The authors proposed differences in water and sediment dynamics, as well as, different trophic dynamics as the proximate causes.
Based on our data, one might deem river regulation desirable as it decreases mercury levels in fish, at least over the long-term. The Black Warrior-Tombigbee River system has been regulated for over 100 years as construction of a series of 17 dams and 18 locks began in 1895 and was completed in 1915; modernization of the system, undertaken after World War II, led to the building of the lock and dam that forms the Demopolis Reservoir in 1955 (45). Studies focused on the immediate effects of river regulation find the opposite trend: in the years immediately following dam construction fish mercury levels increased (46, 47). Regardless of the ultimate effect of regulation on fish mercury levels, our findings should not be interpreted as supporting river regulation as regulation induces numerous negative effects including, but not limited to, losses of biodiversity and ecosystem function (48). A comparison of the mercury cycle, in relation to the carbon, nitrogen, oxygen, sulfur, and iron cycles, in relatively new versus older reservoirs should yield useful insights as to the impacts of river regulation on the aquatic mercury cycle. While Hg deposition rates have risen dramatically over the last century (2), the ability to predict fish Hg concentrations based solely on atmospheric Hg deposition (however appealing) has, in our opinion, not been sufficiently demonstrated across fundamentally different landscapes to warrant adoption of this paradigm. We suggest that the link between atmospheric Hg deposition and fish Hg concentrations is significantly modulated by the structure and function of aquatic ecosystems and, as seen is this study, these differences can account for large variations in fish Hg concentrations. These results suggest that unregulated rivers and river reaches that support fisheries leading to human fish consumption may need increased monitoring of fish flesh mercury levels even in areas where atmospheric deposition rates are relatively modest.
Acknowledgments This research was supported, in part, by the University of Alabama, Department of Biological Sciences, the J. Nichole Bishop Endowment (RHF), an Alabama License Tag Fellowship for Conservation (ALR) and a University of Alabama Graduate Council Fellowship (ALR). Jenjit Khudamrongsawat, Jacob Culp, Lori Valentine, Micah Bennett and Nick Brown assisted in field collection of fishes. Note Added after ASAP Publication. There was an error in Figure 3 in the version published ASAP June 6, 2008; the corrected version published ASAP June 19, 2008.
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