Little Rock Lake

Hrabik TR, Watras CJ. 2002. Recent declines in mercury concentration in a freshwater fishery isolating the effects of de-acidification and decreased atmospheric mercury deposition in Little Rock Lake. Sci Total Environ 297 229-237. 

Frost TM, Montz PK, Kratz TK, Badillo T, Brezonik PL, Gonzalez MJ, Rada RG, Watras CJ, Webster KE, Wiener JG, Williamson CE, Morris DP. 1999. Multiple stresses from a single agent diverse responses to the experimental acidification of Little Rock Lake, Wisconsin. Limnol Oceanogr 44(3, part 2) 784-794. 

Watras CJ, Morrison KA, Kratz TK. 2002. Seasonal emichment and depletion of Hg and SO4 in Little Rock Lake relatiorrship to seasonal changes in atmospheric deposition. Can J Fish Aquat Sci 59 1660-1667. 

The example of calculated heavy metal speciation in Little Rock Lake, Wisconsin, USA is shown below (Table 13). 

For Little Rock Lake, a single core from each of its two basins was analyzed and dated in stratigraphic detail. The remaining cores were analyzed for Hg content in three coarse intervals as described, but none of these profiles was actually dated. Instead the sedimentation rates were inferred from a series of five nearby cores that had been dated by 210Pb for other purposes (16). The mean sedimentation rates from dated cores collected at similar depth in the same basin were used to calculate Hg accumulation for each undated profile. 

Figure 4. Plots of sediment-accumulation rate versus age for the cores in Figure 3. Error bars represent one standard deviation propagated from counting uncertainty. The apparent increase in recent sediment accumulation in Little Rock Lake is atypical of other cores from this site.

Figure 3. Locations of the lakes sampled in Vilas County, WI. Trout Lake was not sampled. Little Rock Lake has been artificially divided thus, it was sampled as two different lakes (Little Rock North and Little Rock South).

Site Description. Little Rock Lake (LRL) is a soft-water oligotrophic seepage lake located in the Northern Highland Lake District in north-central Wisconsin (Vilas County 45°59 55"N, 89°42 15"W). Public access to the site, which is in the Northern Highlands State Forest, has been restricted since the study began. 

Figure 1. Bathymetric map of Little Rock Lake, WL Contours are given in meters. Treatment-basin surface area was 9.8 ha mean depth was 3.8 m and maximum depth was 10.3 m. Reference-basin surface area was 8.1 ha mean depth was 3.1 m and maximum depth was 6.5 m.

Little Rock Lake. Net IAG in LRL takes place primarily in or near the sediment. It can be evaluated by measurements of pore-water chemistry, comparison of hypolimnetic and epilimnetic chemistry, and calculation of ion budgets. An example of each approach follows. 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Hydrolysis of sulfate esters also cannot supply the quantity of sulfate required for sulfate reduction. Hydrolysis of sulfate esters has not been measured directly in any lakes (cf. 73, 83), but the annual supply of sulfate esters is less than annual rates of sulfate reduction. In Wintergreen Lake the annual supply of ester sulfate to the sediments is only 4% of annual sulfate reduction (73). Similarly, in Little Rock Lake the supply of ester sulfate is less than 1% of the rate of sulfate reduction (72). In both lakes, hydrolysis of sulfate esters is estimated to be less than half of the rate of supply to the sediments. 

A preliminary mass balance revealed the following interesting insights into Hg cycling in Little Rock Lake (18, 19). 

A major aspect of this study was assessment of the role of groundwater transport in the overall Hg cycle. However, during the study period (1988-1990) Little Rock Lake was mounded (no groundwater inflow), but Pallette Lake had both groundwater inflow and outflow. Therefore, for the purposes of evaluating the importance of groundwater inflow and outflow on Hg transport, we extended our study to Pallette Lake. 

Sedimentation Traps. Sediment traps (25) were installed in the hy-polimnion of Little Rock Lake to estimate the downward flux of Hg to the sediment surface. Traps were constructed of acrylic and Teflon following the design of Shafer (26). No metal components were used to avoid possible contamination artifacts. Traps were placed at 9 m at the 10-m-deep holesampling site. Traps were suspended from surface floats to prevent disturbing bottom sediments during deployment and retrieval. 

Hg Concentrations in Sediments A typical Hg concentration profile in profundal sediment cores from Little Rock Lake Treatment Basin is shown in Figure 3. Mercury concentrations range from about 50-185 ng/g (dry weight). Similar concentrations were observed by Rada et al. (35) in Little Rock Reference Basin (6-205 ng/g for surface grabs across the lake, including sandy sediments) and by R. Rada (University of Wisconsin, LaCrosse, personal communication) for Little Rock Treatment Basin (3-220 ng/g for similarly retrieved surface grabs). The decrease in concentration toward the top... 

Figure 2. Water-column profiles from Little Rock Lake Treatment Basin in 1989. A, temperature B, dissolved oxygen C, dissolved Hg and D, particulate Hg. (Adapted with permission from reference 21. Copyright 1991...

Figure 3. Sediment core from Little Rock Lake (3 m) depicting dry bulk particle (A) and pore-water (A) HgT concentration. Sediments were dated by 13rCs and 210Pb. Inset Coal use in the United States according to a 1986 report to the National Academy of Sciences.

Figure 6. Distribution coefficients (K d) for Hg in Little Rock Lake Treatment Basin water and sediments.

Figure 7. Deposition to the sediment surface of Little Rock Lake in 1989 as measured by sedimentation traps. A, mass flux B, carbon flux bars represent fluxes, lines are particle concentrations of carbon (percent) and C, Hgflux bars represent flux, lines are particle concentrations of Hg in nanograms per gram. (Adapted with permission from reference 21. Copyright 1991 D. Reidel...

Net sedimentation is defined as the flux of material incorporated into the permanent sediment record. 210Pb and 137Cs geochronologies indicate a mass sedimentation rate of 103 g/m2 per year for profundal sediments in Little Rock Lake. By using the mean Hg concentration (118 ng/g) in the top 1-cm slice of our bulk sediment profile, we estimated an annual net sedimentation of 12 xg of HgT/m2 per year. This net accumulation rate is similar to the calculated atmospheric input rate of about 10 xg/m2 per year (18, 19). Additionally, gross deposition rates (from sediment traps) exceeded these estimates by about a factor of 3 this rate suggests substantial internal recycling of material deposited at the sediment-water interface in this lake. 

Table III. Hypolimnetic Hg, Depositional Fluxes, and Calculated Diffusion from Bottom Sediments in Little Rock Lake in 1989...

Processes and mechanisms responsible for recycling at the sediment-water interface cannot be explained by a single process, but are most likely a combination of many biogeochemical processes. Although pore-water HgT concentrations were higher than in lake waters, direct release of pore waters below about 2 cm could not totally account for the observed buildup in the hypolimnion of Little Rock Lake. Remineralization of recently deposited biogenic particulate matter and release of particle-bound Hg from this source most likely accounted for the observed water-column buildup. 

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