Documenting success stories of management of phosphorus emissions at catchment scale: an example from the pilot river Odense, Denmark.

Documentation of the effects of different mitigation measures adopted at different scales to reduce phosphorus (P) loadings to surface waters is needed to help catchment managers select the best management practices. Water quality monitoring data from the outlets of two paired catchments (the river Odense catchment versus a neighbouring control catchment) on the island of Funen, Denmark, showed significantly different trends in annual flow-weighted P concentrations during the period 2000-2013. A significant downward trend in flow-weighted particulate P (PP) concentrations (0.051 mg P L-1) and loss (0.155 kg P ha-1) was detected for the river Odense catchment, whereas a similar trend did not emerge in the control catchment. The observed differences in PP reductions may be due to wetlands acting as P sinks since wetland restoration activities have been much more comprehensive in the river Odense catchment (1.8 ha wetlands km-2) than in the control catchment (0.5 ha wetland km-2). The excess downward trend in total P and PP in the river Odense catchment (5,600 kg P and 3,700 kg P) is corroborated by extrapolating the results from a mass-balance study and 10 years of in situ measurements of P storage (3,700 kg P and 15,000 kg P).

Successful reduction of diffuse nitrogen emissions at catchment scale: example from the pilot River Odense, Denmark.

Land-based total nitrogen (N) loadings to Danish coastal waters have been markedly reduced since 2000. This has been achieved by general measures reducing discharges from all point sources and N leaching from farmed land supplemented with more local and targeted mitigation measures such as restoration of wetlands to increase the catchment-specific N retention. In the catchment of River Odense, restoration of wetlands has been extensive. Thus, in the major gauged catchment (485 km(2)) eleven wetlands (860 ha) have been restored since 2000. A comparison of data on N concentrations and loss from a gauging station in the River Odense with data from a control catchment (772 km(2)), in which a significantly less intensive wetland restoration programme has been undertaken, showed an excess downward trend in N, amounting to 124 t N yr(-1), which can be ascribed to the intensive wetland restoration programme carried out in the River Odense catchment. In total, the N load in the River Odense has been reduced by 377 t N yr(-1) (39%) since 2000. The observed downward trend is supported by monitoring data from two wetlands restored in 2001 and 2004 in the River Odense catchment.