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 (PO ₄-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 PO ₄-P concentrations and loads were detected. However, DIN generally showed stronger trends than PO ₄-P and 19 of the 29 tested PO ₄-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 PO ₄-P and discharge changed from significantly negative to mostly significantly positive, a phenomenon most likely due to the change in phosphorus sources.