Seasonal variation of Cymodocea nodosa in the Ghar El Melh lagoon (Tunisia), with reference to insolation, temperature and salinity effects

Abstract

Two sampling stations were selected in order to assess the response of the seagrass to lagoon conditions. Leaf growth was estimated using Zieman method and rhizome production was evaluated by marking rhizome terminals. The vegetative development of C. nodosa inside the lagoon at the limit of its distribution is different from that observed close to the communication channel with the sea. The growth of C. nodosa show a clear unimodal cycle in the two stations reaching maximum leaf development in summer. Shoot density decreased from 915 to 500 shoot m-2 from the communication channel toward the lagoon. The average total biomass varied between 241.6 plus or minus 34 g DW m-2 and 413.8 plus or minus 46 g DW m-2. Leaf plastochrone interval varied seasonally with an annual average of about 23 and 28 days. The average leaf productivity decreased from 1.42 to 0.15 g DW m-2 d-1 inside the lagoon. As for the annual leaf production, it was approximately ten times higher near to communication channel (518.6 g. DW m-2 year-1) than inside the lagoon (54 g. DW m-2 year-1). Our results show also that the biological characteristics of C. nodosa at this limit of its distribution in the lagoon were more sensitive to the variability of environmental parameters (insolation, water temperature and salinity).