Variation in phosphorus flux during a hydrological season: the River Adour

A section of the River Adour (S.W. France) between Pontonx and Dax has been studied to determine variations in phosphorus flux, both particulate and dissolved, during the course of a hydrological season that included two major flood events. The flux has been measured at twice monthly intervals, increasing to several measurements per day during the flood events. Over the course of the year a total discharge of 2.52×109 m3 of water was measured at Dax allowing the drainage of 1233 t of dissolved phosphorus and 646.2 t of particulate phosphorus from a watershed covering 7840 km2. Two significant flood events occurred during the period studied, i.e. in November 1991 and June 1992. The measured dissolved phosphorus (DP) concentrations are found to be very variable throughout the year with maximum levels being observed during the flood events. This maximum release was observed when flow rates at Pontonx reached 300 m3 s−1, whereas at Dax the peak concentrations were observed with a flow of 430 m3 s−1. Measured concentrations for each flood event were, however, quite different with mean values at Pontonx and Dax of 2.62 mg l−1 and 2.80 mg l−1, respectively, for the November flood, compared with values of 0.64 mg l−1 and 0.36 mg l−1, respectively, in June 1992. Particulate phosphorus (PP) concentrations are limited during summer, but increased levels are observed in spring and particularly during the flood events. The two flood events account for the transport of 98.6% of the total flux of PP. Finally, it was noted that the variation in DP flux may be categorised into phases of mobilisation and retention. The principle phase of mobilisation occurs during autumn/winter with retention occurring during spring/summer.