Seasonal dynamics in mixed eelgrass beds, Zostera marina L. and Z. noltii Hornem., in a Mediterranean coastal lagoon (Thau lagoon, France)

The seasonal dynamics of two eelgrass species, Zostera marina L. and Z. noltii Hornem., were examined for 2 years in a Mediterranean lagoon (Thau, south France), within mixed stand in two contrasting sites. A number of environmental parameters (water temperature, light, nutrients in water and in sediment) were also monitored during the study period. The similarity in seasonal conditions and the mixed nature of stands allowed to tackle species specific effect and those imposed by habitat characteristics on seasonal dynamics of these two species. The examination of ecological conditions at the two sites showed that the pattern of dominance arose from differences in physical climate and nutrient regime: Z. noltii prevailed at the south site (SS) which experienced frequent wind generated waves and currents, and Z. marina at the north site (NS) which was more sheltered and permanently supplied with nutrients (reared shellfishes and watershed). Despite apparent species-specific responses to seasonal forcing (biomass of Z. marina being controlled by solely temperature while that of Z. noltii varied with both temperature and light) and species-specific branching potential (max: Z. noltii: 32–50 10 −3 day −1, and Z. marina: 5–7 10 −3 day −1), the two species, when they were dominant, exhibited comparable seasonal pattern with annual biomass maxima (215–226 and 157–287 g DW m −2 in Z. noltii and Z. marina, respectively) and shoot density maxima (2764–2944 and 152–296 shoots m −2 in Z. noltii and Z. marina, respectively) in July and August. When they were not dominant, the two species displayed more patchy distribution and consistent reproductive effort (23–39% of generative shoots) which affected their seasonal biomass pattern (April–June). Shoot size (length and weight) and the way population increased their standing crop appeared to be habitat dependent. The two species built up their biomass through shoot density in exposed area (SS) and through shoot size increase in sheltered area (NS). These results are discussed with regards to the coexistence of these two species within the same stand.