Fractionation of phosphate in sediments of four representative mangrove stages (French Guiana)

Abstract

Phosphate was fractionated in Guianese mangrove sediments. Fe(OOH)≈P was extracted using a Ca-EDTA + Na-dithionite solution buffered at pH 8. CaCO₃≈P was extracted using Na₂-EDTA solution at pH 4.5. Next, Acid Soluble Organic Phosphate (ASOP) was extracted by H₂SO₄ 0.5 N. Finally, Residual Organic Phosphate (ROP) was digested with H₂SO₄ + H₂O₂. Four representative mangrove stages have been studied: sea edge pioneer mangroves, mature coastal mangroves, mixed riverine mangroves, and declining to dead mangroves. The sum of the P-fractions varied between 638 to 804 μg g^-1 in pioneer and mixed mangroves respectively. In all the stages, the percentage of inorganic phosphate was larger than 50% of the total P. Fe(OOH)≈P varied between 221 (pioneer mangrove) to 426 μg g^-1 (dead mangrove). CaCO₃≈P varied between 75 to 102 μg g^-1 in mixed, dead or mature mangroves and attained 125 μg g^-1 in pioneer mangrove. The sum of the concentrations of organic phosphate (ASOP + ROP) increased markedly from the dead mangrove (189 μg g^-1) to the mixed mangrove (380 μg g^-1). Guianese mangroves, are relatively rich in total phosphate, possibly because they are narrowly related to the 'Amazon dispersal system′. Each mangrove stage can be characterised by a prevailing form of phosphate. The concentrations of these different forms were ascribed to the marked relations with the seawater which controls import or export of suspended matters and to the wave action which controls the resuspension of the sediments and subsequently exchange of phosphate between the suspended matter and the water column.