THE IMPACT OF DESICCATION OF A FRESH-WATER MARSH (GARCINES NORD, CAMARGUE, FRANCE) ON SEDIMENT WATER VEGETATION INTERACTIONS .1. THE SEDIMENT CHEMISTRY

The impact of desiccation on a marsh sediment was studied both in the laboratory and in the field. Changes in the sediment chemistry of a ho mogenized sediment suspension during desiccation were studied in the l aboratory. FeS was oxidized completely. A considerable mineralization of organic phosphate took place, from both the acid soluble organic ph osphate fraction and from the residual organic phosphate fraction, but no significant mineralization of organic matter was observed. The o-P formed during the mineralization was recovered partly in the Fe(OOH) almost-equal-to P fraction and partly in the CaCO3 almost-equal-to P f raction. An upward flux was found. During spring and summer 1990 the w ater inlet to a shallow permanent freshwater marsh with a surface of a bout 1.5 hectares was blocked, in order to desiccate the marsh by evap oration. The sediments initially consisted of a black anoxic organic t op layer and a less organic anoxic gray layer. During the desiccation of the sediment a brown oxic surface layer was formed from the black l ayer and an increase of pH and Eh occurred. Subsequent rainfall made t he Eh increase further but caused a decrease in pH indicating an incre ase in bacterial activity. A progressive oxidation of FeS was observed . An increase in Tot-P in the surface layer and a decrease in the gray and the black layer of the sediment occurred, probably due to a capil lary upward flux. A mineralization of organic matter was observed in t he two deeper layers. In the upper brown layer this mineralization was less evident, probably because it has been masked by the capillary mo vement. A net C loss of 40% was calculated to have occurred in the lay er 0-40 cm. In the deeper layers a decrease in Tot-N was observed, whe reas no important increase occurred in the top layer. Over a sediment layer of 40 cm a N loss of 50% was calculated. C- and N losses occurre d simultaneously, suggesting the importance of mineralization as a sou rce of inorg-N for denitrification. The chemical and physical changes in the sediment during desiccation affected layers down to 40 cm. This means that not only the top layer of a sediment but also deeper layer s are active in systems of which part of the sediment dries occasional ly. Fractionation of the surface sediment phosphate showed an increase of Fe(OOH) almost-equal-to P in the top layer due to the oxidation of FeS to Fe(OOH), enlarging the P-adsorption capacity of the sediment. A mineralization of about 50% of acid soluble organic phosphate occurr ed. After rainfall, a net increase in residual organic phosphate occur red presumably due to an increase of bacterial activity. Drying may th erefor be utilized as a tool, in wetland management, to eliminate orga nic nitrogen and carbon from the sediment. In rice culture, it may be used to make part of the organic nitrogen available to the rice.