Changes in nitrogen loads to estuaries following implementation of governmental action plans in Denmark: A paired catchment and estuary approach for analysing regional responses

Since the late 1980s a wide range of regulatory measures, addressing both point sources and diffuse N pollution from agricultural land, have been implemented in estuary catchments to reduce the land based nitrogen (N) load of the Danish aquatic environment. Monitoring has been undertaken of key agro-environmental N indicators for different parts of the DPSIR concept (Driving forces, Pressures, State, Impact and Responses for N) in 10 catchments (covering 35% of the Danish land area) and their estuaries during the period 1990–2009. The N load to these 10 estuaries has been reduced by 39% in total, mostly (28%) by reducing the N loss from diffuse sources. For all the 10 linked catchments and estuaries decreasing trends have been reported for various indicators, amounting to 40–53% for the catchment N surplus, ca. 30% for the efficiency of use of cattle slurry N and ca. 40% for pig slurry N, 18–55% for the mean flow-weighted total N concentrations in inlet water to estuaries, and 24–62% for total N concentrations in the upper and middle reaches of the estuaries. In eight catchments and their estuaries we found a direct response (<5 years) of the diffuse N loads from catchments to reductions in the agricultural N surplus, the response differing between catchments because of wide variations in N removal in groundwater and surface waters. In only two of the catchments and estuaries a time delay of N in groundwater aquifers was a major factor (decadal), possibly due to oxic groundwater in chalk aquifers. Knowledge of responses to agricultural N regulation is highly valuable for decision makers and catchment managers working with the implementation of EU directives such as the Water Framework Directive. Based on our findings we suggest that targeted and catchment-specific measures are the most cost-efficient way to achieve good chemical quality in estuaries.

► We analysed the outcome of 25 years’ policy of combating nitrogen pollution. ► Nitrogen surplus decreased in each of the 10 study catchments. ► Nitrogen load to and concentrations in estuaries decreased in 8 catchments. ► Only 2 of 10 analysed catchments showed marked delay in responses. ► Cost-efficiencies of different mitigation measures are presented.