Etude pilote pour une évaluation de l’état des macrophytes des lagunes méditerranéennes oligo et méso-halines - Convention 2010 - Action 2

The aim of this project was to develop and test a protocol based on submerged macrophyte beds for the assessment of the ecological status of oligo- and meso-haline lagoons (as defined in the Water Framework Directive). This protocol needs to be compatible with those developed by IFREMER and CEMAGREF-Bordeaux respectively on saline lagoons and freshwater bodies. This project was implemented in the framework of a contract involving IFREMER and CEMAGREF and coordinated by Agence de l’Eau.The protocol was tested on 4 lagoons along the French Mediterranean coast covering a wide range of situations, notably of anthropogenic pressure: Scamandre and Charnier in the Gard Department, the Grand Palun on La Palissade estate (Arles) and Bolmon (Berre). The protocol includes 2 types of monitoring sites: open water sites and shore sites. The open water monitoring sites were distributed in a systematic way according to a grid (1 monitoring site per 100ha for large lagoons, i.e above 1000ha and 1 monitoring site / 50ha for smaller lagoons). The location of the shore monitoring sites was randomly selected (one per type of shore, thus maximum 3 shore monitoring sites). The vegetation was described on each site by the abundance (in a semi-quantitative way) of species on 30 sampling points per site using a rake fitted on a telescopic handle. The helophytes and wetland plants species that were present on 1m strip beyond water edge were listed and their abundance estimated. The type of substrate, the salinity and transparency of water were also measured along with the maximum water depth at which macrophytes were present on each monitoring sites. The cover and the species richness of the vegetation were low in all the studied lagoons. The values of diversity and equitability had little meaning as a result of the low species richness. The higher abundances of macrophytes beds for open water sites were found on the Charnier and the lowest on the Scamandre lagoons. Considering shore sites, the higher abundances were found on Grand Palun and the lowest on Bolmon. The dominant species in the 4 lagoons are characteristic of mesotrophic to eutrophic waters and tolerate high levels of pollutions (e.g. Potamogeton pectinatus, Myriophyllum spicatum). They are well adapted to the harsh ecological conditions met in these lagoons, especially fluctuating salinity and low transparency of water. They show a low indicative value for water quality but the other species present in the communities bring additional information. Similarly, the shore vegetation of the lagoons do not show an indicative value for the status of the lagoon but contributes to the assessment of the anthropogenic pressure through man transformation of the shore. The results show that the characteristic of the macrophyte beds are not fully consistent with the level of human pressure on the lagoon or with water quality. Light is probably an important factor with controls the distribution of species. The water turbidity in oligo-halines lagoons is often naturally high as a result of small grain size of sediment and its physical behavior in waters at low salinity. This is reinforced by the shallow depth of water bodies and the wind regime. This naturally high turbidity reduces the indicative value of macrophyte beds.