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Most of the organic material in the oceans that reaches the sea floor is deposited on continental margins and not in the deep sea. This organic matter is the principal carrier of phosphorus (P) to sediments. A part of the organic material is buried definitely. The other part decomposes, resulting in a release of dissolved HP0 42-to the pore water. This HP0 42-either returns to the overlying water and becomes available for uptake by phytoplankton, or is retained in the sediment in an organic or inorganic form.Quantification of the P release from and P retention in sediments on relatively short time scales of days to years is necessary for a correct understanding of the nutrient dynamics in regional seas such as, for example, the North Sea. An accurate assessment of the modem global ocean burial flux of reactive P (i.e potentially bioavailable P) and the burial flux in the geological past is important for understanding the global oceanic P cycle. This, in turn, can provide insight in possible controls on organic C burial and atmospheric concentrations of C0 2 and 0 2 , because P may limit oceanic primary production and thus determine the amount of organic material in the oceans on geological time scales.The research presented in this thesis concentrates on the short-term processes controlling sediment P release and retention in temperate, non-upwelling, continental margin environments. The research commenced with a laboratory study on the effect of organic matter deposition and macrofauna on sediment-water exchange and retention of P in Fe oxide-poor, sandy sediments (Chapter 2). A suspension of dead algal cells (Phaeocystis sp.) was applied to sediment in experimental systems (boxcosms), either once or every week during 19 weeks. The results demonstrate that deposition of organic matter on this type of sediment enhances pore water concentrations and sediment-water exchange of HP0 42-. The enhanced HP0 42-release was due to microbially mediated mineralization of the o

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 accumulati