Evidence for persistent seed banks in dwarf eelgrass Zostera noltii in the German Wadden Sea

The intertidal dwarf eelgrass Zostera noltii is a dominant species in the Dutch and German Wadden Sea. Although numerous studies of its reproductive ecology have been conducted, few have examined the importance of seeds and seed banks for meadow maintenance. We investigated the contribution of a seed bank (size, genetic potential and persistence) to annual recruitment of dwarf eelgrass in the German Wadden Sea using temporal sampling of seeds from the sediment and genetic assignment tests of seedlings to populations of adult shoots from previous years. Annual sediment seed density (SD) was 487.5 m(-2) (269.4) and 367.3 m(-2) (95.5) in 2004 and 2005, respectively, and distribution of seeds in the sediment was highly aggregated. The proportion of over-wintering seeds that germinated under laboratory conditions was 16 to 25%, and field-germination revealed a 12% survival to the seedling stage. Nearly 20% of all shoots present in May 2004 were seedlings. Using 9 microsatellite loci, seedlings sampled in 2004, 2005 and 2006 were compared with adults sampled in 2002, 2003 and 2004; results revealed that 7 to 33% of seedlings could be assigned to the local adult population in Current or previous years. Although new recruitment plays an important role in the maintenance of these meadows, considerable new recruitment comes from within the meadow., itself. Seeds are viable for at least 3 yr, thereby forming a relatively short-term, but persistent, seed bank.