THE IMPACT OF DESICCATION OF A FRESH-WATER MARSH (GARCINES NORD, CAMARGUE, FRANCE) ON SEDIMENT WATER VEGETATION INTERACTIONS .3. THE FRACTIONAL COMPOSITION AND THE PHOSPHATE ADSORPTION CHARACTERISTICS OF THE SEDIMENT

Changes in the fractional P-composition and the P-adsorption capacity of the Garcines (Camargue, France) a freshwater marsh sediment were studied. Desiccation was found to cause considerable shifts in the fractional composition of the sediment layers studied. In the surface layer an increase in Fe(OOH) almost-equal-to P and a slight decrease in CaCO3 almost-equal-to P occurred. A specific mineralization of Acid Soluble Organic Phosphate (ASOP) occurred: about 40% (about 70 mug g-1) of Acid Soluble Organic Phosphate was mineralized in the surface sediment and about 60 % (about 200 mug g-1) in a deeper layer. Therefore, Acid Soluble Organic Phosphate may be considered to be of ecological importance as it seems to be the main active Org-P fraction, under the conditions tested. Desiccation was found to have a positive effect on the phosphate adsorption capacity of the sediment, due to the formation of Fe(OOH) from FeS. Approximately 100% of o-P adsorbed was recovered in the inorg-P-fractions. Since the inactivation of bacteria was found to have no effect on the adsorption capacity of the sediment, bacteria seem not to play an active role in the P adsorption by the sediment. In chloroform treated sediments less residual organic phosphate occurred than in the none treated, indicating that P(bact) is recovered in that fraction.