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 nolt ii Hornem., were examined for 2 years in a Mediterranean lagoon (Thau, sout h France), within mixed stand in two contrasting sites. A number of environ mental parameters (water temperature, light, nutrients in water and in sedi ment) were also monitored during the study period. The similarity in season al conditions and the mixed nature of stands allowed to tackle species spec ific effect and those imposed by habitat characteristics on seasonal dynami cs of these two species. The examination of ecological conditions at the tw o sites showed that the pattern of dominance arose from differences in phys ical 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 w ith nutrients (reared shellfishes and watershed). Despite apparent species- specific responses to seasonal forcing (biomass of Z marina being controlle d by solely temperature while that of Z noltii varied with both temperature and light) and species-specific branching potential (Inax: Z noltii: 32-50 10(-3) day(-1), and Z marina: 5-7 10(-3) day(-1)), the two species, when t hey were dominant, exhibited comparable seasonal pattern with annual biomas s maxima (215-226 and 157-287 g DW m(-2) in Z noltii and Z marina, respecti vely) and shoot density maxima (2764-2944 and 152-296 shoots m(-2) in Z nol tii and Z I,marina, respectively) in July and August. When they were not do minant, the two species displayed more patchy distribution and consistent r eproductive effort (23-39% of generative shoots) which affected their seaso nal biomass pattern (April-June). Shoot size (length and weight) and the wa y population increased their standing crop appeared to be habitat dependent . The two species built up their biomass through shoot density in exposed a rea (SS) and through shoot size increase in sheltered area (NS). These resu lts an discussed with regards to the coexistence of these two species withi n the same stand. (C) 1999 Elsevier Science B.V. All rights reserved.