This thesis deals with the microbial decomposition of organic matter in littoral sediments of lakes. Special attention was given to the initial step in the decomposition of polysaccharides that form a major component of macrophyte litter produced in these systems. This initial step, an extracellular enzymatic hydrolysis, is generally regarded as the rate limiting step in the decomposition of biopolymers in natural systems. The study site selected was an almost monospecific stand of common reed, Phragmites australis, that covers the upper littoral zone of Lake Gooimeer, The Netherlands.The first two experimental chapters of this thesis deal with several general aspects of sedimentary carbon cycling, which are necessary for an implementation of a detailed study on the initial decomposition of biopolymers. In Lake Gooimeer, we could show that deep inside the reed bed, organic matter cycling was dominated by one source, namely macrophyte litter (Chapter 2). Plant litter is mainly made of lignocellulose, which has a rather simple and distinct polymeric composition. This makes the reed bed a good test system to study initial decomposition of biopolymers under natural conditions. Carbon sources gradually changed to an algal dominance going from inside the bed towards the lake (Chapter 2). This transition already begins inside the reed bed and no macrophyte derived material was found outside the bed, despite the high annual production of common reed.Results of the budget study (Chapter 3) suggest that there are two major processes involved in the removal of sedimentary organic matter. Transport caused by erosion of the sediment during storms explains about 60% of the total carbon and only 30% was removed by mineralization. Mineralization rates were mainly determined by temperature, changes in oxic and anoxic conditions, and by amounts of organic matter present. Less than 5% of the annual production remained in the sediment over a period of 25 years. No short term accumulation could be detected during both years studied ...