Seston deposition

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. 

Paleolimnological Conditions. Because of the interplay between primary production, oxygen content of bottom waters, and the sulfur content and speciation of sediments, sediment profiles of S probably preserve records of paleolimnological conditions. Several studies (23-25, 205) point to increased S content of sediments as a result of eutrophication. Mechanisms involve both rates of S supply to sediments (seston deposition and diffusive gradients) and rates of S reduction and oxidation. The relative S enrichment... 

Internal cycling was examined by measuring rates of accumulation in sediment cores, seston deposition rates, and diffusive fluxes to the sediments. 

Seston Deposition. Sulfur is a minor but essential nutrient for algal production, accounting for 0.15-1.96% of dry weight (35). Sulfur is present as proteins, sulfolipids, ester sulfates, and free sulfate (18.19.36-38) that occur in varying proportions depending upon species and environmental conditions. 

Relative Importance of Seston Deposition and Dissimilatory Reduction In-I-alce Sulfate Sinks... 

In little Rock Lake, seston deposition appears to be a more important sulfate sink than does dissimilatory reduction. Several previous studies (2.41 have concluded that dissimilatoiy reduction is the major mechanism for sulfate retention, and Cook et al. (2) concluded that seston deposition was a minor sulfate sink in experimentally acidified Lake 223. The C/S ratio calculations discussed above snow that approximately 29% of the total S in recent sediments at SB-5 is excess-S derived from dissimilatory reduction and the remaining 71% originated from seston deposition. 

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 addition to limitations on sulfate reduction, seston deposition of S is limited by algal-S content and primary productivity. The relationship between primary productivity and lake acidification is unclear 1621. but limited evidence suggests that primary productivity is not particularly sensitive to moderate lake acidification. Further, there is little evidence to indicate that the S content of seston changes much with acidification (Table III). Hence, within a given lake, the loss of sulfate from the water column from seston deposition probably changes little during the acidification process. 

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. 

Figure 6. Recycling of seston sulfur and carbon at SB-5. Recycling rates were calculated from the difference between measured sediment trap deposition and measured sediment accumulation of seston-S. See text for approach used to calculate seston-S accumulation in sediments.

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