Sulfate reduction in lakes

Figure 2. Rates of sulfate reduction in lake sediments reported in the literature range over 3 orders of magnitude and are not correlated with lake sulfate concentrations. All measurements were made with l5S in intact cores or core sections. References are given in Table I.

The studies cited do not clarify what factors determine rates of sulfate reduction in lake sediments. The absence of seasonal trends in reduction rates suggests that temperature is not a limiting factor. Rates of sulfate reduction are not proportional to such crude estimates of carbon availability as sediment carbon content or carbon sedimentation rate, although net reduction and storage of reduced sulfur in sediments often does increase with increasing sediment carbon content. Measured rates of sulfate reduction are not proportional to lake sulfate concentrations, and the relative rates of sulfate reduction and methanogenesis in a variety of lakes do not indicate that sulfate diffusion becomes limiting in eutrophic lakes. Direct comparison of diffusion and reduction rates indicates that diffusion of sulfate into sediments cannot supply sulfate at the rates at which it is reduced. Neither hydrolysis of sulfate... 

Sulfate Reduction in Lake Sediments A Biofilm Model... 

Bacterial sulfate reduction in lakes and seas of the U.S.S.R. (After Ivanov, 1964)... 

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. 

Much confusion exists over the effects of sulfate concentration and carbon availability on rates of sulfate reduction (cf. 1, 4, 5, 72, 85, 106). Sulfate reducers in lake sediments exhibit low half-saturation constants for sulfate (10-70 xmol/L 4, 72, 78, 85) as well as for acetate and hydrogen (4, 13, 87). The low half-saturation constants allow them to outcompete methano-gens for these substrates until sulfate is largely consumed within pore waters (4, 90). Low concentrations of sulfate in lakes confine the zone of sulfate reduction to within a few centimeters of the sediment surface (e.g., 4, 90, 98). The comparability of rates of sulfate reduction in freshwater and marine... 

Relative rates of sulfate reduction and methanogenesis in lakes of varying trophic status are claimed to indicate that sulfate reduction rates are limited by the supply of sulfate (4, 5, 13). According to this hypothesis, at high rates of carbon sedimentation, rates of sulfate reduction are limited by rates of sulfate diffusion into sediments, and methanogenesis exceeds sulfate reduction. In less productive lakes, rates of sulfate diffusion should more nearly equal rates of formation of low-molecular-weight substrates, and sulfate reduction should account for a larger proportion of anaerobic carbon oxidation. Field data do not support this hypothesis (Table II). There is no relationship between trophic status, an index of carbon availability, and rates of anaerobic... 

Existing data lend mixed support to the hypothesis that sulfate reduction is limited by availability of electron donors. Laboratory studies have shown that sulfate reduction in sediments can be stimulated by addition of carbon substrates or hydrogen (e.g., 85, 86). Increases in storage of reduced sulfur in sediments caused by or associated with addition of organic matter (108, 109) also have been interpreted as an indication that sulfate reduction is carbon-limited. Addition of nutrients to Lake 227 in the Experimental Lakes Area resulted in increased primary production and increased storage of sulfur in sediments (110, 111). Natural eutrophication has been observed to cause the same effect (23, 24, 112). Small or negligible decreases in sulfate concentrations in pore waters of ultra-oligotrophic lakes have been interpreted... 

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. 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

Several whole-lake ion budgets have shown that internal alkalinity generation (IAG) is important in regulating the alkalinity of groundwater recharge lakes and that sulfate retention processes are the dominant source of IAG (3-5)1 and synoptic studies (6-9) have shown that sulfate reduction occurs in sediments from a wide variety of softwater lakes. Baker et al. (10) showed that net sulfate retention in lakes can be modeled as a first-order process with respect to sulfate concentration and several "whole ecosystem" models of lake acidification recently have been modified to include in-lake processes (11). 

Mass balance calculations clearly show that sulfate is removed from the water column by in-lake processes. Three processes are potentially important 1) diffusion of sulfate into sediments and subsequent reduction, 2) sedimentation of seston, and 3) dissimilatory sulfate reduction in the hypolimnion. 

Table III. Endproducts of Sulfate Reduction in Little Rock Lake Gyttja in Short-Term Lab Incubations With 35SC>42 Under Various Conditions...

Seston-S deposition probably is a more important process than dissimilatory reduction in lakes with low [SO42 ]. As lakewater sulfate concentrations increase, seston deposition reaches a plateau limited by the overall primary production rate and the maximum algal S content, but diffusive fluxes continue to increase in direct proportion to [SO42 ]. Thus, in highly acidic lakes (pH 3 5 [SOjt2 J > 100 peq/L), such as McCloud Lake, Florida and Lake 223, Ontario, dissimilatory sulfate reduction probably is the major sulfate sink. Nriagu and Soon (131 concluded that endproducts of dissimilatory reduction and elevated sediment S content would not be observed below S mg/L (240 / eq/L), but we see clear evidence of dissimilatory reduction in Little Rock Lake at concentrations of approximately SO /teq/L. 

In-lake processes remove approximately half of the sulfate inputs from the water column of Little Rock Lake. Two processes, seston deposition and dissimilatory reduction, are responsible for sulfate retention. For the preacidified lake, seston deposition probably is the dominant sink, accounting for 70% of net retention. Preliminary data and theoretical considerations suggest that the diffusive flux of sulfate to sediments will increase during experimental acidification, and we believe that dissimilatoty reduction is the dominant sulfate sink in lakes with elevated sulfate concentrations. 

Konopka A., Gyure R. A., Doemel W., and Brooks A. (1985) Microbial sulfate reduction in extremely acid lakes. Purdue University Water Resources Research Center, Technical Report 173, West Lafayette, pp. 1—50. 

Bak F. and Pfennig N. (1991) Microbial sulfate reduction in littoral sediment of Lake Constance. FEMS Microbiol. Ecol. 85, 31-42. 

With abundant evidence for sulfate reduction in the hydrosphere, the question arises as to the actual site of the sulfate-reducing activity. This depends upon oxygen input to the system, organic matter concentration, and other factors. Suitable conditions are often encountered at, or just below, the water-sediment interface. Here, the population of sulfate-reducers is highest because of the availability of sulfate and organic matter. In some non-eutrophic lakes a secondary population maximum may arise at depths around 3 m in the sediments (Kuznetsov et al., 1963). Sulfate reduction occurs both within the water column and the sediments of the Black Sea and a number of the lakes examined by Ivanov (1964) and Sorokin (1970). 

It is appropriate to discuss sulfate reduction in ponds, lakes and fjords together because each exhibits similar variations of either a seasonal or epi-... 

Figure 12 Vertical depth profiles of arsenate with rates of respiratory arsenate and sulfate reduction in the water column of meromictic Mono Lake, California, made during October, 1999. Sulfate-reduction profiles from the last period of meromixis (1986) when the lake was 4 m shallower are shown for comparison. (From Ref. 57.)...

PHA synthesis has been advanced as an energy-producing mechanism in anaerobic syntrophic (H2-producing, acetogenic) bacteria [159]. Extracellular PHBV degradation concurrent with sulfate reduction in anoxic lake-sediment samples was seen by Urmeneta et al. [160] as an indication that PHAs could serve as carbon and electron sources. 

Further SO2 emission reductions, estimated to be of the order of 75% beyond current reduction commitments, are required in both Canada and the United States to protect lakes in eastern Canada from sulfate deposition in excess of the critical loads. 

Similar trends were detected in a more limited study conducted by the Adirondack Lakes Survey Commission during the 1990s. The commission found a reduction of 92 percent in sulfate deposition in a selected sample of lakes in the Adirondack Mountains between 1992 and 1999, but an increase of 48 percent in nitrogen deposition in the same lakes. 

Microbes residing in sediment beneath oceans and lakes derive energy by oxidizing organic matter. 02 is available as the oxidant at the sediment-water interface, but it is depleted within millimeters below the interface. Nitrate and Fe(III) oxidants are available in the first few centimeters of sediment. When they are exhausted, sulfate becomes the predominant oxidant for a distance of 1 m. The sulfate reduction product, HS-, is released in millimolar concentrations into solution in the sediment pores. 

Because sulfuric acid was used to acidify the treatment basin, the increase in SO4 2" was expected. However, only about 50% of the sulfate added as sulfuric acid remained in the water column of the treatment basin the remainder was lost to outseepage and in-lake processes. For example, had the added sulfate remained in the water column, [S042- ] at pH 4.7 would have been 257 xequiv/L (versus the measured [S042- ] = 147 ixequiv/ L). The loss of sulfate by reduction may contribute to the generation of alkalinity this possibility is discussed in the Internal Alkalinity section. Chloride showed no significant trend with decreasing pH. 

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