Seasonal variations of P compounds and their concentrations in two coastal lagoons (Herault, France)

This article concerns seasonal variations in the phosphate concentrations in two coastal lagoons near Montpellier (Mediterranean coast, France). The o-P concentration in the overlying water is highest during summer. The role of the sediment, particularly that of the different P fractions in the sediment, is discussed. Significant variations, especially in the FeOOH approximately equals P fraction, occur. For both Tot-P sub(sed) and the FeOOH approximately equals P fraction a gradient from surface to bottom is observed, as well as a distinct decrease in the FeOOH approximately equals P fraction in the surface sediments during summer and autumn. Variations in the FeOOH approximately equals P fraction appear to be compensated by variations in the CaCO sub(3) approximately equals P fraction. These variations appear to be determined by the ferric hydroxide concentration. This compound represents only a small part (maximally 15%) of the total iron in the sediments and is related to the dissolved oxygen content of the immediately overlying water. Besides the fractions o-P, Fe(OOH) approximately equals P, a large part of the CaCO sub(3) approximately equals P fraction is potentially bioavailable. A large proportion of the Tot-P sub(sed) is therefore bioavailable.