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Description of eco-morphodynamic processes in the Dutch deltas., Delft Cluster

Sandy sediments, which cover about 70% of continental shelves and most beaches, are often thought of as geochemical deserts that harbor no life because they are usually poor in organic matter and other reactive substances. Based on the belief that significant reactions and fluxes and a dynamic ecology require large standing stocks of reactants and organic matter, sandy sediments are neglected., Sandy sediments, which cover about 70% of continental shelves and most beaches, are often thought of as geochemical deserts that harbor no life because they are usually poor in organic matter and other reactive substances. Based on the belief that significant reactions and fluxes and a dynamic ecology require large standing stocks of reactants and organic matter, sandy sediments are neglected.

Policy plans and subsequent measures to reduce nutrient concentrations in coastal waters require realistic historical information on nutrient concentrations. The main goal of the present report was to develop and apply a practical, but scientifically sound strategy to assess the background concentrations of nutrients in the Dutch Wadden Sea. Given the changes in the area (e .g. the closing of the former Zuiderzee) and the availabifity of only a restricted set of historical data, we decided to modify this goal in : the development and application of a practical hindcasting method to estimate nutrient concentrations that would have occurred in the Dutch Wadden Sea under the present conditions, given the nutrient concentrations in the rivers and in the adjacent North Sea in the period prior to the early 1930s. Two subtanks were defined : (1) the assessment of the nutrient concentrations in the adjacent North Sea area representing the period before circa 1930, as well as freshwater discharges, nutrient concentrations, and nutrient loads by the fresh water inputs to the Dutch Wadden Sea (2) the estimation of the seasonal fluctuations in the nutrient concentrations in the basins of the Dutch Wadden Sea. Considered were the river Rhine (at Lobith/Spijk), the river IJssel (at Kampen), the IJsselmeer, the Lauwersmeer, the river Ems, the Westerwoldsche Aa and the Eemskanaal. Attention was focused to the nutrients ammonium, nitrate, phosphate, total nitrogen and total phosphorus. Important steps in the calculation procedure were the assessment of freshwater discharge, the estimation of present and past nutrient concentrations in river water, the nutrient retention in between the rivers and the sea, the nutrient loads at freshwater discharge points and the assessment of the concentrations in the Wadden Sea prior to the early 1930s . Based on these calculation, the seasonal cycles in the phosphorus and nitrogen loads and concentrations at Den Oever and Kornwerderzand were assess

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