Seasonal Variations and Potential Mobility of Sediment Phosphorus in the Al Massira Reservoir, Morocco

In the Al Massira reservoir, the temporal distribution of phosphorus compounds in sediment was identified by phosphate fractionation and the phosphate exchange across the sediment-water interface was determined by adsorption-release tests in laboratory. Sediment phosphate was mainly present in inorganic form [o-P + Fe(OOH)-P + CaCO sub(3)-P = 77% of Tot-P] and the concentration of CaCO3-P was higher than that of Fe(OOH)-P. The organic fractions (acid soluble organic phosphate [ASOP] + residual organic phosphate [ROP]) represented only 23% of total phosphorus (Tot-P). The adsorption-release tests showed that the mineral forms were strongly involved in the P exchange across the sediment-water interface, particularly iron-bound phosphate [Fe(OOH)-P]. The significant seasonal variations of Fe(OOH)-P and CaCO3-P throughout an annual cycle explain the patterns of P storage and mobilization in each of these two fractions. Each one of these fractions can store or mobilize about 17% of the Tot-P content. However, phosphate mobility accounts, on the average, for 23% of the Tot-P content. The differences between the mobility considered by fraction and the total mobility are likely the result of internal mobility between the different fractions within the sediment. The environmental conditions (pH, redox potential) play a role as they favour the release of phosphorus by one fraction, but at the same time, its partial adsorption by another one.