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In this report, the effect of seed mussel fishery activities on sediment composition is analysed. The hypothesis is that dredging for mussels brings fine silt in suspension. Tidal currents move silt away from the fishing site and a more course sediment is left behind. For PRODUS experimental plots of each 400 x 200 m have been marked out on sites were natural mussel seed occurred: on one half of the plots fishing was prohibited and on the other half fishing was allowed. Per plot 12 boxcorer samples were taken from which sediment samples were collected before and after fishing. Most plots have also been sampled later during the research, to test for increasing similarity of fished and control sites over time.

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 and results from 1992 even suggested a net removal of organic matter. This shows that sediment organic matter accumulation is variable and is governed by a delicate balance between input and outputs. Predicting sediment organic matter dynamics will be difficult in these type of ecosystems, since it depends to a large extent on episodic and variable sediment erosion during storms.The remainder of the thesis deals with the initial decomposition of the two main reedlitter polysaccharides cellulose and arabino-xylan in sediments and the enzymes involved in this process. A method was developed to measure extracellular enzyme activities involved in cellulose decomposition by using artificial, fluorochrome labelled substrates (Chapter 4). Inhibition experiments with known substrates and inhibitors of cellulolytic enzymes were used to characterise the enzyme activities as measure in the assays. Results of the inhibition experiment suggested that the measured activity was of bacterial origin in the sediment used. This method was further extended to other enzymes and was used to study temporal and spatial variability of enzyme activities during two years in the littoral sediments (Chapter 5). Absolute activities were mostly determined by organic matter input or content.β-Glucosidase activities were among the highest ever detected in sediments, which is probably the result of the high litter cellulose input to the sediment. Patterns of enzyme activities showed a distinct change in a gradient through the reed bed, which was consistent with the polymeric composition of dominant sedimentary carbon sources (Chapter 2). This suggests that patterns of enzyme activities may be used to study biologically available carbon sources in natural systems.Although the enzyme assays are easy to use and allow information from a large number of samples to be gathered and compared, they give little information on the actual in situ hydrolysis rates of natural occurring polysaccharides in sediments. This is mainly caused by the use of an artificial substrate that may not be representative of the form and availability of the natural substrates. Therefore, a new method was developed to measure hydrolysis rates of naturally occurring polysaccharides in sediments. This new method is based on the selective inhibition of microbial uptake of hydrolysis products by 3% toluene without affecting the extracellular decomposition of polysaccharides. This approach was thoroughly tested (Chapter 6) and subsequently used to study initial decomposition of individual polysaccharides in reed litter and sediments (Chapter 7). The accumulation of hydrolysis products was followed over time by high-performance liquid chromatography, which resulted in a sensitive method with a high degree of resolution for the products formed.A similar pattern of accumulating carbohydrates was found for litter and sediments (Chapter 7). Ratios between glucose and xylose accumulation rates suggested that arabino- xylan was degraded more slowly that cellulose, which was not in agreement with apparent rates of glucose and xylose removal from litter. Accumulation of heterogenic oligomeric compounds besides xylose during the decomposition of arabino-xylan may explain this discrepancy. If direct uptake of these hetero-oligomers can be shown, it would have implications for carbon flow and the associated microbial populations. The turnover time of particulate glucose was estimated at 85 ± 14 d in the top centimetre of the sediment, and showed a three to four fold increase with depth. Comparison between glucose accumulation rates as a measure of cellulose decomposition and total carbon mineralization rates suggested that cellulose decomposition was a major process in the mineralization of organic matter in littoral sediments.Finally, implications of these studies are discussed in view of general functioning of littoral zones and the mechanisms of polymer decomposition in natural systems (Chapter 8).

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 and results from 1992 even suggested a net removal of organic matter. This shows that sediment organic matter accumulation is variable and is governed by a delicate balance between input and outputs. Predicting sediment organic matter dynamics will be difficult in these type of ecosystems, since it depends to a large extent on episodic and variable sediment erosion during storms.The remainder of the thesis deals with the initial decomposition of the two main reedlitter polysaccharides cellulose and arabino-xylan in sediments and the enzymes involved in this process. A method was developed to measure extracellular enzyme activities involved in cellulose decomposition by using artificial, fluorochrome labelled substrates (Chapter 4). Inhibition experiments with known substrates and inhibitors of cellulolytic enzymes were used to characterise the enzyme activities as measure in the assays. Results of the inhibition experiment suggested that the measured activity was of bacterial origin in the sediment used. This method was further extended to other enzymes and was used to study temporal and spatial variability of enzyme activities during two years in the littoral sediments (Chapter 5). Absolute activities were mostly determined by organic matter input or content.β-Glucosidase activities were among the highest ever detected in sediments, which is probably the result of the high litter cellulose input to the sediment. Patterns of enzyme activities showed a distinct change in a gradient through the reed bed, which was consistent with the polymeric composition of dominant sedimentary carbon sources (Chapter 2). This suggests that patterns of enzyme activities may be used to study biologically available carbon sources in natural systems.Although the enzyme assays are easy to use and allow information from a large number of samples to be gathered and compared, they give little information on the actual in situ hydrolysis rates of natural occurring polysaccharides in sediments. This is mainly caused by the use of an artificial substrate that may not be representative of the form and availability of the natural substrates. Therefore, a new method was developed to measure hydrolysis rates of naturally occurring polysaccharides in sediments. This new method is based on the selective inhibition of microbial uptake of hydrolysis products by 3% toluene without affecting the extracellular decomposition of polysaccharides. This approach was thoroughly tested (Chapter 6) and subsequently used to study initial decomposition of individual polysaccharides in reed litter and sediments (Chapter 7). The accumulation of hydrolysis products was followed over time by high-performance liquid chromatography, which resulted in a sensitive method with a high degree of resolution for the products formed.A similar pattern of accumulating carbohydrates was found for litter and sediments (Chapter 7). Ratios between glucose and xylose accumulation rates suggested that arabino- xylan was degraded more slowly that cellulose, which was not in agreement with apparent rates of glucose and xylose removal from litter. Accumulation of heterogenic oligomeric compounds besides xylose during the decomposition of arabino-xylan may explain this discrepancy. If direct uptake of these hetero-oligomers can be shown, it would have implications for carbon flow and the associated microbial populations. The turnover time of particulate glucose was estimated at 85 ± 14 d in the top centimetre of the sediment, and showed a three to four fold increase with depth. Comparison between glucose accumulation rates as a measure of cellulose decomposition and total carbon mineralization rates suggested that cellulose decomposition was a major process in the mineralization of organic matter in littoral sediments.Finally, implications of these studies are discussed in view of general functioning of littoral zones and the mechanisms of polymer decomposition in natural systems (Chapter 8).

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 organic material and due to direct release of HP0 42-from the algal cells (lysis). A major portion of the algal material remained at the sediment-water interface and this organic layer probably regulated the sediment-water exchange of HP0 42-directly. The activity of the macrofauna was mainly limited to reworking of the sediment. The effect of the macrofauna on the sediment-water exchange Of HP0 42-was negligible. In the boxcosms to which organic material was added only once, the concentration of NaOH-extractable sediment P increased following the addition, especially in the presence of macrofauna.Sorption of P to Fe oxides is the most important short-term process responsible for the retention of P in sediments. Using a combination of differential X-ray diffraction (DXRD) and extraction procedures, the character of the Fe oxides that bind P in 4 North Sea sediments was studied (Chapter 3). The results indicate that poorly crystalline ferrihydrite and akageneite were present in the fine sediment fraction (< 10μm) of surface samples from all locations. Combination of these results with bulk sediment extractions of Fe and P and sorption characteristics for P provides evidence for the dominant role of poorly crystalline Fe oxides for the binding of P in these North Sea sediments. These poorly crystalline Fe oxides are suggested to act as both a temporary and permanent sink for P.The redox conditions in continental margin sediments can vary both seasonally and spatially. To obtain more insight in the redox conditions in North Sea sediments, the Mn and Fe cycle at 15 locations in 4 different sedimentary environments was studied in 2 contrasting seasons (Chapter 4). The quality and quantity of the organic matter deposited in each environment was found to determine whether sediments become sufficiently depleted of 0 2 and NO 3-to allow for (1) Fe and Mn reduction and (2) escape of dissolved Fe 2+and Mn 2+to the overlying water. A steady- state diagenetic model describing solid phase and pore water metal profiles was developed and applied to Mn and Fe data for 11 and 3 stations, respectively. The model results demonstrate that (1) reversible sorption in combination with sediment mixing can enhance diffusive transport of dissolved metals; (2) precipitation of Fe 2+and Mn 2+in the form of reduced authigenic minerals can explain the reversal of the pore water Fe 2+and Mn 2+gradients at depth at many stations, and (3) in most North Sea sediments, Fe and Mn oxides do not play an important role as redox intermediates in organic C oxidation (accounting for < 4 %); only in the depositional environment of the Skagerrak, metal oxide reduction may contribute substantially to organic C oxidation (-20%).Reversible sorptive reactions can both constrain and enhance the flux of HP0 42-from the sediment to the overlying water. The role of sorption in sediment-water exchange of HP0 42-in North Sea sediments was investigated for 15 locations in 2 seasons (Chapter 5). P sorption data, pore water HP0 42-profiles, solid phase results and measured and calculated rates of sediment- water exchange of HP0 42-were combined. Sorption was found to play an important role in controlling sediment-water exchange of HP0 42-during at least part of the year in 3 of the 4 North Sea environments. At most stations, adsorption limits the flux of HP0 42-to the overlying water. At one station in the Skagerrak, however, desorption is responsible for the maintenance of a flux of HP0 42-to the overlying water. A onedimensional reaction-diffusion model describing the sedimentary P cycle was developed and applied to the results for 2 stations, The model results show that both enhanced retention and enhanced release due to sorption can be adequately described when simultaneous equilibrium and first-order reversible sorptive reactions are assumed.P bound in authigenic minerals may not be solubilized again, whereas Fe-bound and organic P can still be released upon deep burial. Therefore, more insight in the extent of authigenic P mineral formation in continental margin sediments is important. A combination of pore water and solid phase analysis was used to determine whether authigenic carbonate fluorapatite (CFA) is currently forming at two locations on a North Atlantic continental platform (Chapter 6). Results of selective extractions suggest that an authigenic P phase is forming at the expense of Fe-bound P at both stations. A steady-state diagenetic model for the P cycle was developed and applied to the data of I station. The model results indicate that CFA formation can account for the observed increase of authigenic P with depth at this station. Furthermore, the results show that an intense cycling of P between Fe-bound P and pore water HP0 42-at the redox interface can create conditions beneficial for CFA formation. This internal P cycle is driven by downward, bioturbational transport of mainly in-situ formed Fe-bound P into the reduced sediment zone. Losses from the internal P cycle due to CFA formation and HP0 42+diffusion are compensated for by sorption of HP0 42-released from organic matter to Fe oxides. Fe bound P thus acts as an intermediate between organic P and CFA. Burial of CFA can account for between 25 and 70% of the total burial flux of reactive P and thus may act as an important sink for P in this low sedimentation, continental margin environment.