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).Original Abstract: La production primaire et le cycle de la biomasse de Cymodocea nodosa (Ucria) Ascherson ont ete calcules mensuellement a partir de novembre 2006 a octobre 2007 au niveau de la lagune de Ghar El Melh (NE de la Tunisie). Deux stations d'echantillonnage ont ete selectionnees afin d'evaluer la reponse des herbiers aux conditions lagunaires. La croissance des feuilles a ete estimee en utilisant la methode Zieman et la production du rhizome a ete evaluee par le marquage des rhizomes. Le developpement vegetatif de C. nodosa l'interieur de la lagune a la limite de sa repartition est different de cellui observe a proximite du canal de communication avec la mer. La croissance de C. nodosa montre un cycle unimodal dans les deux stations atteignant un developpement maximal des feuilles en ete. La densite des faisceaux diminue de 915 a 500 faisceaux par m-2 du canal de communication vers l'interieur de la lagune. La biomasse totale varie entre 241,6 plus or minus 34 g sec m-2 et 413,8 plus or minus 46 g sec m-2. Le plastochrone intervalle des feuilles variait de facon saisonniere, avec une moyenne annuelle d'environ 23-28 jours. La productivite moyenne des feuilles diminue de 1,42 a 0,15 g sec m-2 j-1 a l'interieur de la lagune. Quant a la production annuelle de feuilles, elle est dix fois plus importante pres du canal de communication (518,6 g. sec m-2 an-1) qu'a l'interieur de la lagune (54 g. sec m-2 an-1). Nos resultats montrent egalement que les caracteristiques biologiques de C. nodosa a la limite de sa distribution dans la lagune etaient plus sensibles a la variabilite des parametres environnementaux (ensoleillement, la temperature et la salinite).