b'Monitoring pitfalls: Aquatic environmentWindholf J11Department of Bioscience, Aarhus University, Vejlsvej 25, 8600 Silkeborg, DenmarkAssessing the state of and impact on the aquatic environment calls for a strategic monitoring programme in order to give a sufficient assessment of the state of the environment in time and space. In addition, and not least, the monitoring has to include reliable data (sampling,laboratory analysis, statistical calculations and models). Especially when the trends in the state of the aquatic environment is analysed, the consistency of the time series of data is crucial. In Denmark, a strategic monitoring programme has been going on for 30 years. During this period, changes in the number of monitoring stations, chemical analysis, sampling frequency, monitoring equipment and statistical calculation procedures have taken place. Some of these changes have led to problems when evaluating the state and trend of the aquatic environment. Examples of such problems will be given and include; changes in monitoring station network, sampling frequency, laboratory methods used for analysing total nitrogen, total phosphorous and as well as changes in gauging methods for calculation of precipitation and discharge in streams. Furthermore, it has become increasingly evident that the method for calculation and assessment of the transport of total phosphorus in minor streams is biased and is not sufficiently taking into account the dynamics of the concentration of phosphorus in such streams.20'