THE IMPACT OF DESICCATION OF A FRESH-WATER MARSH (GARCINES NORD, CAMARGUE, FRANCE) ON SEDIMENT WATER VEGETATION INTERACTIONS .2. THE SUBMERGED MACROPHYTE VEGETATION

Many temporary marshes in the Camargue (Southern France) are managed a s permanent marshes to create habitats attractive to waterfowl. Potamo geton pectinatus, a common food for waterfowl, is in a bad condition i n several of these marshes. This might be due to the accumulation of t oxic compounds such as ferrous iron and sulphide in the anoxic, organi c rich sediment. We examined whether desiccation of the sediment of su ch a permanent marsh leads to a better growth of P. pectinatus. Field observations showed that a summer drawdown of the permanent marsh Garc ines led to a considerable abundance of annual plants and a decrease i n the biomass production of P. pectinatus in the following spring. We investigated the difference in growth of P. pectinatus when growing on sediment that dried (border) and sediment that did not dry (centre). However, the border sediment did not dry long enough to oxidize all th e reduced constituents and ferrous iron remained present in the inters titial water. No difference in biomass production was found in plants growing on the border or on the centre sediment. The plants from the c entre sediments had very low root to shoot ratios,- which might be cau sed by sulphide. Many tubers were formed; the tuber production was pro bably enhanced by stress due to the presence of toxic compounds in the sediment. Repeated short winter drawdowns might be the best managemen t to increase P. pectinatus production.