A COMPARISON BETWEEN THE BIOMASS PRODUCTION OF POTAMOGETON-PECTINATUSL AND MYRIOPHYLLUM-SPICATUM L IN THE CAMARGUE (SOUTHERN FRANCE) IN RELATION TO SALINITY AND SEDIMENT CHARACTERISTICS

Nomenclature: follows Flora Europea (Tutin et al. 1964-1980). Since th e extension of the irrigation system, the water regime of most of the permanent marshes of the Camargue (southern France) have been intensiv ely controlled. Considerable quantities of nutrient rich Rhone water a re pumped into these marshes, leading to lower salinities and a higher biomass production and consequently an increasing organic matter conc entration of the sediments. Myriophyllum spicatum has become abundant in these permanent marshes since large quantities of freshwater entere d these systems. It has displaced Potamogeton pectinatus in several of these marshes. The different factors likely to influence the distribu tion of P. pectinatus and M. spicatum were investigated experimentally . The impact of Cl- concentrations between 0 and 6 g l(-1) on the biom ass production of both species was tested. P. pectinatus appears to be more salt tolerant than M. spicatum. The influence of sediment qualit y on the biomass production of both species was investigated using six sediments differing in organic matter concentration. Compared to P. p ectinatus, M. spicatum had a lower total biomass production when grown on sediments with low organic matter concentration (2-4% organic matt er) and a higher biomass production on sediments with relatively high organic matter concentration (9-13% organic matter). Nitrogen addition to the sediments yielded an increased biomass production of P. pectin atus and M. spicatum. On some sediments M. spicatum needed higher conc entrations of nitrogen than P. pectinatus to increase its biomass prod uction.The creation of freshwater marshes by the introduction of irrig ation water, resulting in lower salinities and an increase in sediment organic matter concentration, stimulates the biomass production of M. spicatum. As M. spicatum grows less well on poor sediments and at hig her salinities it seems to be unable to displace P. pectinatus in more natural systems in the Camargue.