Natural background concentrations of phosphorus and nitrogen in the Dutch Wadden Sea

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 assessed as welt as the seasonal cycles of the background phosphorus and nitrogen concentrations in the Dutch Wadden Sea