Coastal eutrophication and trend reversal: A Danish case study

In the past 2 decades significant measures have been taken to reduce nitrogen and phosphorus discharges from Denmark by 50% and 80%, respectively. These nutrient reduction targets now appear within reach after several consecutive reduction measures are fully implemented, particularly toward diffuse discharges, and reduced nutrient concentrations are beginning to be observed in estuaries and the Danish straits. Phosphorus concentrations have declined by 22% to 57% from the early 1990s, mainly owing to improved treatment of urban and industrial wastewater. Changes in nitrogen concentrations, following reduction measures toward diffuse sources, were more recent and partly masked by large interannual variations in freshwater discharge. The response in marine nitrogen concentrations was delayed relative to the decline in riverine concentrations, most likely owing to large internal loading from the sediments. Two consecutive dry years appeared to be the triggering mechanism for nitrogen concentrations to decline. In the last 5 yr, nitrogen levels in estuaries and coastal waters have decreased up to 44% when interannual variations in freshwater discharge were accounted for. These first signs of environmental recovery were most pronounced in estuaries and coastal waters but also were apparent in open waters of the Kattegat, the Sound, and the Belt Sea. This case study is the first to document significant decreases in nutrient concentrations on a large regional scale resulting from an active management strategy to reduce nutrients from both diffuse and point sources. Eutrophication of coastal ecosystems from nutrient overenrichment is widespread (Nixon 1995), with the effects manifested in a myriad of direct and indirect responses (Cloern 2001). Although the sources and pathways of nutrient inputs to aquatic ecosystems can be estimated with reasonable certainty, it has been difficult to achieve reductions in the different sources (Boesch 2002). However, some coastal ecosystems have experienced reductions in inputs of phosphorus and nitrogen primarily through improvement in 1 Corresponding author ([email protected]). 2 Present address: DHI Water & Environment, Agern Allé 5, DK2970 Hørsholm, Denmark.