Finding the optimal reduction to meet all targets-Applying Linear Programming with a nutrient tracer model of the North Sea

This paper describes a method to trace the origin of nutrients in marine areas and subsequently quantify the optimal reductions of nutrient loads in accordance with eutrophication water quality targets. The tracer method is incorporated in a validated Delft-3D model of the North Sea. For this model, nutrient loads are well characterized. Resulting from the tracer model, nutrient composition matrices are created for each OSPAR area of the North Sea for the year 2002. Water quality targets are also based on OSPAR agreements. The optimal reduction is obtained directly via Linear Programming, which is extremely quick and does not need scenario runs of the full model. The optimum reduction scenario obtained was validated in Delft 3D and showed a similar outcome. Results from the optimization method clearly differ from common practise (uniform reduction of all sources). They also show that some targets are inconsistent spatially or with respect to the DIN to DIP ratio. The method provides a quick and useful tool to quantify the necessary riverine reductions to achieve a healthy ecosystem state. It can be extended to include economic costs per river basin or economic sector and also be applied in combination with other eco-hydrodynamic models or for other substances.