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.
Temporal trends in N & P concentrations and loads in relation to anthropogenic effects and discharge in Odense River 1964–2002
Morten Andreas Dahl Larsen, Henrik Soegaard, Klaus Hinsby; Temporal trends in N & P concentrations and loads in relation to anthropogenic effects and discharge in Odense River 1964–2002. Hydrology Research 1 February 2008; 39 (1): 41–54. doi: https://doi.org/10.2166/nh.2008.028
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