Introduction:
Water eutrophication still causes many environmental and social problems, and so better quantification of the amount of nutrient loads discharged into surface waters is required. The basis for eutrophication analysis is data gathered in the frame of State Environmental Monitoring (SEM). In Poland, these measurements are carried out at the frequency of 6-12 times per year and serve as the basis for calculating the amount of nutrient input discharged to the sea. This article presents the results of the analysis of the representativeness of one measurement make two-hourly per day of the variability of total nutrient concentrations. For this purpose three cycles of field studies were carried out in accordance with plant growing season. The measurements were taken in November 2013, February 2014, and June 2014 in the Słupia River, at the site where it flows through the village of Charnowo (river profile). The analysis showed a significant change in the amount of total nutrient concentration during the day (up to 60% for total nitrogen and total phosphorus). This trend was maintained regardless of the stage of plant growth. Therefore, both the date for water sample collection and the time the measurement was taken become important for the subsequent evaluation of nutrient loads discharged into the sea. It is reasonable to assess the level of nutrient loads discharged into the sea not on the basis of a single measurement but based on the continuous registration of nutrients – this allows us to assign a daily average. It is also important from the point of view of the calibration of mathematical models, which are now an essential tool for analysing and forecasting environmental changes, and for which access to detailed data is very important