Profundal sediments

This chapter summarizes our results from two northern Wisconsin seepage lakes that were chosen to assess the importance of various processes controlling transport of Hg across the sediment-water interface. Total Hg (HgT) concentrations were determined as a function of depth in the solid and dissolved phases of the water column, and in littoral and profundal sediments. New sampling and analytical procedures allowed for the detection of low (picogram) levels of Hg. Measurements obtained in this phase of the study together with those obtained from previously published data on these lakes were used to make a preliminary examination of the relative importance of factors influencing Hg cycling at the sediment-water interface. 

Profundal Sediments. Sediment cores were collected in precleaned acrylic tubes by scuba divers following similar clean sampling procedures... 

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... 

Hg Concentrations in Pore Waters. Profundal sediment pore-water concentrations varied from 10 to 30 ng/L throughout the profile (Figure 3). 

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. 

The subsurface maximum in pore-water HgT (Figure 3) suggested that diffusion from the profundal sediments to the overlying water column could be important. Fickian diffusive flux calculations (eq 2) were used to estimate Hg loading from pore waters. Diffusion coefficients for mercury in pore waters were not available. However, free-water diffusion coefficients for monovalent anions (see Table I) averaged about 5 X 10"6 cm2/s (53, 55) and... 

Recycling of N and P occurs in the water column and at interfaces between the water and substrata such as profundal sediments. In Lake Calado, regeneration of ammonium and phosphorus is dominated by planktonic heterotrophs less than 53 pm in size (Table 14.7, Lenz et al. 1986, Fisher et al. 1988a, Fisher et al. 1988b, Morrissey and Fisher 1988). Sediment-water exchange is smaller than planktonic processes, but is substantial... 

Hauck S., Benz M., Brune A., and Schink B. (2001) Ferrous iron oxidation by denitrifying bacteria in profundal sediments of a deep lake (Lake Constance). FEMS Microbiol. Ecol. 37, 127-134. 

Schultz S. and Conrad R. (1996) Influence of temperature on pathways to methane production in the permanently cold profundal sediment of Lake Constance. FEMS Microbiol. Ecol. 20, 1-14. 

Schultz S., Matsuyama H., and Conrad R. (1997) Temperature dependence of methane production from different precursors in a profundal sediment (Lake Constance). FEMS Microbiol. Ecol. 22, 207-213. 

D.J. Rowan, J. Kalff, J.B. Rasmussen (1992). Profundal sediment organic matter concentration and physical character do not reflect lake trophic status, but rather reflect inorganic sedimentation and exposure. Can. J. Fish. Aquat. Sci., 49, 1431-1438. 

Sutherland IW (1996) A natural terrestrial biofilm. J Ind Microbiol 17 281-283 Sweerts J-PRA, de Beer D (1989) Microelectrode measurements of nitrate gradients in the littoral and profundal sediments of a meso-eutrophic lake (lake Vechten, The Netherlands). Appl Environ Microbiol 55(3) 754-757 Thaveeshi J, Dafrbnchio D, Liessens B, Vandermeren P, Verstraete W (1995) Granulation and sludge bed stability in UASB reactors in relation to surface thermodynamics. Appl Environ Microbiol 61(10) 3681—3686 Thomas RC (1978) Ion-sensitive intracellular microelectrodes, how to make and use them. Academic, London... 

Davis RB. 1974. Tubificids alter profiles of redox and pH in profundal lake sediment. Limnology and Oceanography 19 342-346. 

For the most part, sediments are also stratigraphically uniform, showing only a few percentage variation in lithologic composition. Cores from Mountain Lake, which consistently show up-core decreases in carbonate content (to about 60% that at depth), are the only exception. A number of shallow-water cores that contain a thin veneer of organic-rich sediments overlying silt and sand were also excluded from analysis. In most locations the spatial boundary between organic-rich profundal-type sediments and littoral deposits of coarse detritus or massive silt was clearly defined. 

Dating and Sediment Accumulation. Stratigraphic Patterns Lead-210 profiles from profundal cores from each study lake show conformable declines in unsupported activity to an asymptote of supported 210Pb typically below 40-60 cm deep (Figure 3). Supported activity, attained at shallower depths in Thrush (28 cm) and Kjostad (36 cm), indicates slower linear rates of sedimentation at these sites. The activity profiles for several lakes, most notably Thrush and Kjostad, are almost perfectly exponential and thus indicate nearly constant sediment accumulation rates. Others, such as Cedar and Mountain, show flat spots and kinks that probably represent shifts in sediment flux. 

Comparison of profundal diffusion rates with observed increases in the hypolimnion (Table III) indicated that pore-water diffusion calculated from these profiles was probably not an important transport mechanism for Hg in this seepage lake. For the June-July period, pore-water diffusion accounted for only 13% of the hypolimnetic increase. For the July-August interval, pore-water diffusion could account for only 7% of the observed increase. Therefore, we can assume that the buildup in the hypolimnion is more likely a result of redissolution of recently fallen particulate matter at the sediment surface than of direct pore-water diffusion. Our present sampling scheme (2-cm intervals) precludes evaluation of dissolution in the uppermost sediments and would require much more detail (<1 cm) in the sediment-water interfacial zone. 

Jones, J.G., 1980. Some differences in the microbiology of profundal and littoral lake sediments. J. Gen. Microbiol., 117 285-292. 

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