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DETERMINATION OF THE SEDIMENT PHOSPHATE COMPOSITION BY THE EDTA METHOD OF FRACTIONATION

The bioavailability of phosphate in sediments is a central issue in the study of the eutrophication of aquatic systems. The determination of the P-fractional composition is an essential tool to separate inorganic and organic P-pools in sediments. Sediments from temporary ponds of the Doiiana National Park (SW, Spain) are rich in organic matter and total P concentrations are relatively high. These ranged from 200 to 464 mg kg-' dw. at different sites in a temporary pond in Doiiana National Park in samples taken in March and September 1999. The EDTA method (Golterman, 1996) has been shown to be a suitable P-extraction procedure for this kind of sediments. Additionally, this procedure allows the determination of the chemical structure of some organic-P substrates by enzymatic methods. With this method it appeared that the sediment phosphate composition was dominated by P-organic fractions (64-92%). The concentration of phytic acid in the sediment ranged from 9 to 31 mg kg-' dw. and was determined by the use ofphytase (at pH 2.5). The presence ofo-P i n the fulvic acids extracted with 1-butanol from the Pfraction bound to CaCO,, suggested that the org-P compounds extracted by the Na,-EDTA may be partly hydrolysed during the Murphy & Riley reaction in these sediments.