Inorganic phosphate in exposed sediments of the River Garonne

Abstract

Fractionation of inorganic phosphates in sediments of the River Garonne was carried out during a period of low water discharge. Sediments were collected under 5 cm of water (LS), in the dried river bank (MS) and in the riparian forest (RS). Sediments were sampled at two dates, during a period in which the water level fell gradually, causing sediment LS to be air exposed.

Sediments were analysed for total phosphate, iron bound phosphate (Fe(OOH)∼P) using Ca-NTA and Ca bound phosphate (CaCO₃∼P) using Na-EDTA.

Total-P varies from 552 (RS at date 1) to 2072 µg g⁻¹ (LS at date 2). There are significant differences between sediments and a significant increase from date 1 to date 2 in sediment LS only (1825 to 2072 µg g⁻¹). Fe(OOH)∼P varies from 186 (RS at date 1) to 874 µg g⁻¹ (LS at date 2). The highest values correspond to sediment LS. Moreover, Fe(OOH)∼P increased significantly between date 1 and 2 in LS (560 to 874 µg g⁻¹) as well as in sediment MS (248 to 432 µg g⁻¹). Ca bound P concentrations differed significantly between sediments (75, 112, 235 µg g⁻¹ for sediments RS, MS and LS respectively) but not between sampling dates.

These differences are attributed to the conditions of deposition of the sediments (such as morphology and hydrology of the river) and to the changes in chemical composition during the drying out of the sediments.