RETENTION OF NITROGEN AND PHOSPHORUS IN A DANISH LOWLAND RIVER SYSTEM- IMPLICATIONS FOR THE EXPORT FROM THE WATERSHED

In a Danish lowland river system intensive measurements were made, in four 80 m reaches, of the nitrogen (N) and phosphorus (P) stored in th e stream sediment. The results were used for calculation of the total retention in the river system during two summers (June to August). In addition, the mobilization of nutrients from the stream bottom in autu mn 1987 was compared with the export from the watershed. During the st udy period (June 1987 to September 1988) the amounts of N and P stored in stream reaches were determined fortnightly using a core-sample tec hnique. In reaches dominated by submersed macrophytes, 25-40 g N m-2 a nd 20-30 g p m-2 were stored during two summers, against only 10-15 g N and p m-2 for sandy and gravely reaches. In riparian zones with emer gent macrophytes the retention was even higher than in the submersed m acrophytes. Gross retention exceeded net retention by a factor of two to three. Net retention of P in the river system during the summer of 1987 was equal to the summer export from the watershed. On an annual b asis, retention in the summer constituted 20% of the P export. In cont rast, retention in the summer of 1988 amounted to 60% of the total P e xport during the same period (38% reduction) and 22% in comparison wit h the annual export. The corresponding figures for N were lower, showi ng reductions of 16% and 12% of the export of total N in the two summe r periods, and about 1% of the annual exports. In September 1987 6.4% of the total N export and 65% of the total P export from the watershed consisted of resuspended material. In 1987 the N and P retained durin g the summer was almost completely resuspended during storm events dur ing September to November.