Sulfur and carbon cycling in a stratifying freshwater lake

Abstract

This thesis is a synopsis of a ten-years research on the anaerobic breakdown of organic matter in the stratifying Lake Vechten. Special attention has been paid to the sulfur cycle and how this cycle interacts with carbon mineralization. The sediment plays an important role in the breakdown of organic matter in freshwater ecosystems. Until a few years ago, there was no insight in how mineralization processes were spatial organized in the sediment. New analytical developments have enabled to measure the steep concentration profiles in flocculent sediment layers of a few cm. These techniques are discussed in the second chapter of this thesis. Chapter 3 is more focused on field data collection and evaluation and Chapter 4 is discussing perspectives for future research.The study in Lake Vechten demonstrated that the penetration depth of O 2 , NO 3-, and SO 42-into the sediment can be limited to a few mm till a few cm in freshwaters. This is a crucial difference with marine systems where SO 42-usually penetrates much deeper.This difference makes it also expectable that microbial kinetics in freshwaters differ from those in marine sediments.The ability to measure concentration profiles across narrow depth intervals has open new ways to estimate electron acceptor consumptions by mathematical modeling. To support modeling, diffusion coefficients, adsorption effects, and kinetics were determined independently. The uptake of sulfate and nitrate in sediment in batches collected form the respiring horizon followed a first order kinetics. This indicates that electron limitation occurs in the top layer of the sediment. Assuming first order kinetics and using the measured concentration profiles, sulfate and nitrate consumption rates were estimated by the model of Berner. The estimated rates for sulfate reduction were twice lower that the values obtained from the batch experiments. A notable result of the batch experiments was the fast uptake rate of nitrate and sulfate in the batches collected from the methanogenic horizon (3-7 cm). Under in situ conditions, nitrate and sulfate will not penetrate till this horizon. The absence of a 'lag time' indicated the presence of a vital sulfate and nitrate reducing community in sediment of approximately 6-14 years old.In the second stage in this study we examined if the accumulated total sulfur in the sediment could give an indication on sulfate reduction rates integrated over a longer period. By comparison sulfate reduction rates and total sulfur sedimentation rates with the actual amount of sulfur present in the sediment, it appears that only about 15% of the annual sulfur input is permanently buried in the sediment. Thus, more than 80% of the sulfur is released again into the lake water column. This idea was supported by the concentration profiles of sulfur species seen in the lake-water column. The lake-water measurements also revealed that substantial more sulfur bearing species were present in the lake water column as could be explained by the sulfate, FeS, and ΣH 2 S pool only.In the last stage of the study we have related the sulfur and carbon cycle by comparing the kinetics of the 'key metabolites' acetate and sulfate. This comparison indicated that the role of sulfate as electron acceptor in the oxidation of acetate in freshwaters is dubious. Accumulation of acetate after addition of molybdate has several times been used as evidence that sulfidogens uses mainly acetate as electron donor in marine systems. Comparable experiments in Lake Vechten indicated however that molybdate also affected other acetate consuming processes besides sulfate reduction, making conclusions from this type of inhibition experiments tentative. In conclusion, sulfate reduction does not play an important role in acetate consumption in freshwater like it does in marine systems.Acetate uptake rates are relative high if compared with sulfate reduction rates and methane production rates. This indicated the existence of other quantitative important acetate consuming processes in the anaerobic part of the sediment. The acetate uptake rates did however not exceed total carbon sedimentation rates as in marine systems