Proprietes physico-chimiques et biodisponibilite potentielle du phosphore particulaire selon lʹorigine des sediments dans un bassin versant du Lac Leman (France)Physico-chemical properties and bioavailability of particulate phosphorus of various origin in

The bioavailability of the external loading of phosphorus to lakes is an important management concern (Williams et al., 1980; Boström et al., 1988). This paper examines some physical and chemical properties and the bioavailability of phosphorus associated with the particulate fraction of samples of sediments collected at various locations within a small 33 km2 watershed that is drained by the Redon River into Lake Geneva (Fig. 1). The watershed is urbanized near its exit and the upland areas include cultivated lands, forested areas and marshes (Pilleboue and Dorioz, 1986). Soluble phosphorus enters the river from sewage discharges at downstream locations and particulate phosphorus runs off into the upstream drainage net from cultivated and forested lands. A total of 13 samples were collected once at various locations in the watershed during the winter following a three weeks period of low flow. Three of these samples were taken from river bottom sediment in the downstream reaches (samples 1, 2, 3, Table 1), three were taken from ditches draining forested areas (samples 9, 10, 11; Table 1), three were taken from small ditches draining intensively cropped areas (samples 6, 7, 8; Table 1), one sample was taken from the inlet and one from the outlet of a wetland receiving agricultural runoff (samples 5, 4; Table 1) and two samples were taken from the parent watershed soils (samples 12, 13; Table 1). The physical and chemical properties of the phosphorus associated with the sediment particles were evaluated using a variety of chemical extraction techniques (Hieltjes and Lijklema, 1980; Fares et al., 1974; Olsen et al., 1954). A bioassay method (Fig. 2) was used to evaluate phosphorus bioavailability. An isotopic tracer (32PO3−4) technique was used to determine the capability of the sediment particles to fix and exchange phosphorus with water (Fardeau, 1981; Tran et al., 1988). In this analysis, the fraction of the tracer remaining in solution after 1 min (r1/R) is an index of phosphorus fixation capacity of the sediment (low values of the ratio indicate high fixation capacities) while the amount of isotopically exchangeable phosphorus (mg P.kg−1 exchanged with the water in 1 min) gives an estimate of immediately available phosphorus (E1). One study compared the bioavailability of the sediment phosphorus and the fixation capacity of sediment samples taken at various locations in the drainage network. The particulate phosphorus in the sediment samples taken from the downstream river bottom (1, 2, 3) showed a high bioavailability (Table 2 and Fig. 3). These sediments had a high ability to release phosphorus and a very low phosphorus fixation capacity in comparison to sediment samples taken from ditches draining both the forested or agricultural areas (4, 5, 6, 9). The total phosphorus content of the sediment samples did not appear to be a good indicator of the change in physico-chemical properties of the sediment or in phosphorus bioavailability (Table 3). Another study (Table 2) compared the physico-chemical properties and the bioavailability of particulate phosphorus among (1) the eroded sediments (samples 5, 6, 7, 8) collected from ditches draining agricultural areas; (2) the uneroded soils (samples 12, 13) of the area, and (3) the sediment at the inlet and outlet of the wetland area (Table 4). The sediments taken from the drainage ditches had substantially lower phosphorus bioavailability levels and increased phosphorus fixation capacities when compared to the parent soil of the area. The sediment sample collected at the outlet of the wetland showed the highest fixation capacity of all the samples collected during this research (Table 4, sample 4). The bioassay method used to determine phosphorus bioavailability was found to be very sensitive to the level of dilution of the sediment during the test. For example, in tests on a single sediment where concentrations were set at 30 and 300 mg.l−1, the percentage of total phosphorus extracted from the samples was 45% and 4.2%, respectively (Table 5). The results of the studies described in this paper suggest that the physico-chemical properties and bioavailability of phosphorus associated with sediments vary not only with origin of the sediment, but also with the chemical-physical-biological phenomena that occur during transport through the watershed system. Figure 4 summarizes how these changes may occur as sediment moves through the watershed. Erosion of natural forested areas produces sediments with a high fixation capacity and a low content in total-P and bioavailable phosphorus. The properties of these sediments are strongly modified in the drainage net during low flow periods by interaction with sources of soluble phosphorus (Cahill, 1977; Golterman et al., 1983). The particulate phosphorus from erosion of agricultural areas (although somewhat lower in fixation capacity) may be modified in the drainage net in the same manner as the sediments from natural areas. During high flow events, sediments with high phosphorus levels may be diluted and release soluble phosphorus (Dorioz et al., 1989).