Changes in sediment phosphate composition of seasonal ponds during filling

Most ponds from the Donana National Park are shallow temporary freshwater b odies on eolian sands. The total phosphate concentration and the fractional P-composition of the sediments from two small ponds were studied before an d inmediately after they were filled (autumn, 1996). Total phosphorus conce ntration was measured in 2 different size fractions: 2-0.1 mm (coarse) and <0.1 mm (fine). In both ponds, the total phosphorus concentration of the fi ne sediment increased by 20% at the beginning of the filling period, wherea s 5 weeks later it did not increase further. The percentage of organic matt er of the fine sediment was relatively high (between 13-18%) and did not ch ange significantly during filling. The concentration of sediment total iron and Fe(OOH) increased significantly in both ponds during filling. The sequ ential P fractionation of the fine sediment included, besides the determina tion of two inorganic fractions (Ca-bound P and Fe-bound P), an organic-P f raction extracted with acid, another extracted with alkali, two organic-P f ractios from the Ca-EDTA/dithionite and the Na-2-EDTA extracts which contai ned high concentrations of humic substances. The pond sediments were rich i n organic P compounds as the sum of all organic-P fractions ranged between 267 and 320 mu g g(-1) (68% and 79% of the sum of all fractions). Significa nt changes (P<0.01) in the fractional P-composition of sediments were found after filling. Acid soluble organic phosphate increased about 30% in both ponds. Iron-bound phosphate increased significantly (about 40%) only in the pond where a higher concentration of Fe(OOH) was measured after filling. A n adsorption experiment was carried out for each sediment to simulate P inp ut during filling. Both the iron-bound phosphate and Ca-bound P increased s ignificantly (P<0.01) suggesting that these two fractions were involved in the P-adsorption during the laboratory experiment.