Abstract
The structure of the seed bank (including Chara oospores), in relation to depth within the sediment anddisturbance, was studied in two Rh6ne delta temporary marshes for two years. The seeds of all specieswere concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. Whenan exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site2 was more pronounced than outside the exclosure.In experiment 1, the emergence capacity of seeds from different depths and buried under layers ofsterile equivalent to those in the field was measured. Depending of the species, 22 to 98 ~o of the seedsgerminated from unburied seeds in the top 2 cm. Only 1 ~o of the oospores of Chara (from site 2) at 2to 4 cm depth in the sediment emerged.In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth.The samples contained numerous recent seeds and the emergence percentage reached 41 ~o (for Ruppiamaritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. Theemergence percentage from 2 cm depth or more was always lower than at the surface. These experimentsshowed that both burial and ageing of seeds decrease germination capacity.The majority of the active seeds located at the surface germinate when the marsh is flooded. Seedslocated between 2 and 4 cm can be brought back to the surface by disturbances and play the role of areserve involved in maintenance of populations that go without seed production for one or some years.