Monitoring the progress of attempts to reduce nutrient load and inputs of certain compounds in the north sea by 50%

During the winter period an inverse linear relation is found between the concentration of dissolved nutrients (phosphorus and nitrogen) and salinity in the Dutch coastal zone. This indicates a conservative behavior of these compounds from the river, through the estuary to the sea. During summer this relation is much more scattered because of biological processes. The physical and statistical properties of the relationship between salinity and the concentration of dissolved inorganic phosphorus and nitrogen are used to calculate when, where, and how many samples have to be taken in order to monitor a reduction of a compound accurately. It appeared that at any given salinity in the estuary and in the sea, the winter period is the most suitable season to detect a reduction of a given dissolved compound. The higher the salinity in the estuary, the more samples are required to prove the reduction significantly. A reduction of only 10% cannot even be demonstrated by field measurements during summer at salinities above 25. It is concluded that one cruise from the river to the sea, covering the salinities from 0 to 35 during the winter period, aimed at establishing the relationship between the concentration and salinity by taking samples at a salinity interval of for instance 1%, is sufficient to monitor a wide-ranging reduction of 10%–50% in both the fresh water and marine water. This program must be combined with a sampling at a salinity of 0, directed to determine the riverine temporal variability.