Temporal trends in N & P concentrations and loads in relation to anthropogenic effects and discharge in Odense River 1964–2002

Using water quality data from 1964 to 2002, concentrations and loads of dissolved inorganic nitrogen (DIN) and phosphate-phosphorus (PO₄-P) in the Odense River were analyzed for trends in periods subsequent to and following the implementation of the Danish Action Plan I for reduction of nutrients in the environment (DAP) in 1987. In periods between 1964 and 1987 statistical significant increasing trends in DIN and PO₄-P concentrations and loads were detected. However, DIN generally showed stronger trends than PO₄-P and 19 of the 29 tested PO₄-P load tests showed non-significant results. In the subsequent periods between 1980 and 2002, significant nutrient reductions were detected at all sites and with the strongest trends for PO₄-P. Also, tests combining data from several sites showed equally strong trends. Significant positive correlations between DIN concentrations and discharge were found throughout the period using the non-parametric Spearman Rank Correlation, suggesting soil leaching to be the main nitrogen source. Conversely, correlations between PO₄-P and discharge changed from significantly negative to mostly significantly positive, a phenomenon most likely due to the change in phosphorus sources.