Aquatic Ecology 33: 29–40, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 29 Retention of nutrients in river basins Brian Kronvang 1 , Carl Christian Hoffmann 1 , Lars M. Svendsen 1 , Jørgen Windolf 1 , Jens P. Jensen 2 and Jesper Dørge 3 1 National Environmental Research Institute Department of Streams and Riparian Areas Vejlsøvej 25, DK-8600 Silkeborg, Denmark (Fax: +45 89 20 14 14; E-mail: BKR@DMU.DK); 2 National Environmental Research In- stitute Department of Lake and Estuarine Ecology Vejlsøvej 25, DK-8600 Silkeborg, Denmark; 3 Water Quality Institute, Agern All´ e 11, DK-2970 Hørsholm, Denmark Accepted 29 September 1998 Key words: denitrification, nature restoration, nitrogen, phosphorus, riparian areas, sedimentation Abstract In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg N ha −1 and 0.63 kg P ha −1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export. Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m −2 yr −1 and 3.7–8.3 g P m −2 yr −1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m −2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr −1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr −1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m −2 yr −1 and 0.30 g P m −2 yr −1 , respectively, during the period 1989–1995. The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr −1 and that P-retention increased from −0.80 to 0.90 tonnes yr −1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution. Introduction In the past, Danish lowland streams were typically meandering, had close hydrological contact with their floodplains and passed through extensive riparian wet- lands and many lakes (Iversen et al., 1995; Møller, 1995). Today, most of the about 35 000 km previously natural Danish streams have been modified through straightening and channelization in order to improve drainage efficiency. Moreover, considerable areas of riparian wetlands and many lakes have been exten- sively drained to facilitate reclamation of land for agricultural purposes (Brookes, 1987; Iversen et al., 1993). The consequent change in the hydrological contact across the water-land ecotone and the reduc- tion in water transit time has significantly reduced the nutrient retention capacity within ecosystems (Fleis- cher et al., 1991; Vought, 1995; Hoffmann et al., 1997). At the same time, increasingly intensive agricul- tural production has led to an accelerated diffuse nutri- ent loss to ground water and surface waters (Kronvang et al., 1993; Kronvang et al., 1995). Thus, today dif- fuse losses of nitrogen and phosphorus to the Danish