DYNAMICS OF PHOSPHORUS-COMPOUNDS IN A LOWLAND RIVER SYSTEM - IMPORTANCE OF RETENTION AND NONPOINT SOURCES

Intensive measurements of the fluxes of phosphorus (P) and of P retention were carried out in a Danish lowland watershed (Gjern Angstrom) during two hydrological years (June 1987 to May 1989). Seasonal and short-term variations of P concentrations were measured by intensive automatic sampling covering P fluxes from the entire Gjern Angstrom watershed and two subcatchments: the Gelbaek and Lake Sobygard. Moreover, infrequent sampling was performed in all major tributaries of the river system. Transport of P compounds (dissolved and particulate P) exposed a seasonal pattern which was highly affected by flow conditions, instream processes, as well as point and non-point sources. Stormflow P transport during the two study years (1987-8 and 1988-9) constituted 56 and 61% in the Gelbaek, respectively, compared with 25 and 23% in the Gjern Angstrom, respectively. Particulate P transport in the Gelbaek constituted 56 and 75% of annual total P transport during the two years compared with 77 and 75% in the Gjern Angstrom. The magnitude of monthly baseflow and stormflow total P loads was significantly correlated with the mean monthly discharge during baseflow and stormflow in the Gelbaek, whereas only stormflow loads were correlated to discharge in the Gjern Angstrom. In situ measurements of retention showed that the maximum amount of P stored in the Gelbaek during summer (June to August) was 22 g P m(-2) (1988) compared with 27 g P m(-2) (1987) in the Gjern A. Lake Sobygard was a P source during summer but a P sink during winter and on an annual basis the net P released was 810 and 1420 kg, which corresponded to 7 and 20% of the annual P export from the watershed, respectively. Retention constituted about 30% of gross P transport in the Gelbaek during summer compared with 20% in the Gjern Angstrom. Resuspension of retained P during stormflows in September 1987 constituted 94% of the stormflow P transport and 54% of the total P transport. Monthly total P and dissolved reactive P (DRP) mass balances for the main channel of the Gjern Angstrom revealed a significant DRP retention over 17 months (p < 0.05) caused by P adsorption on sediments and biological uptake. On an annual basis the main channel was a sink of DRP (1220 and 1660 kg P) but a source of total P (3440 and 1000 kg P), implying that the channel is an important source of particulate P. Bank erosion proved to be a significant P source in the main channel of Gjern Angstrom, whereas P delivery from soil erosion was possibly of minor importance. The annual non-point P export from two intensively sampled catchments was significantly higher (0.89 and 0.45 kg P ha(-1) yr(-1)) than from the five extensively sampled subbasins (0.43 and 0.17 kg P ha(-1) yr(-1)). No significant relationship could be established between non-point P export acid the proportion of arable land and soil type. Only for the wet year of 1987-8 was a significant relationship established between non-point P export and the P load from scattered dwellings outside sewage areas in five subbasins.