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 st udied. Desiccation was found to cause considerable shifts in the fract ional composition of the sediment layers studied. In the surface layer an increase in Fe(OOH) almost-equal-to P and a slight decrease in CaC O3 almost-equal-to P occurred. A specific mineralization of Acid Solub le 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 impor tance as it seems to be the main active Org-P fraction, under the cond itions 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 t o 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. I n chloroform treated sediments less residual organic phosphate occurre d than in the none treated, indicating that P(bact) is recovered in th at fraction.