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Moderate disturbances intrinsic to the intertidal habitat appear to facilitate seed recruitment by gap formation. Pollen dispersal distances are sufficient to maintain outcrossing and high clonal diversity.

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. KEY WORDS: Seed bank · Seagrass · Persistence · Germination · Zostera noltii · German Wadden Sea Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 380: [73][74][75][76][77][78][79][80] 2009 1930s, dwarf eelgrass Z. noltii has emerged as the primary seagrass species inhabiting the Wadden Sea, particularly in the northern regions (Reise & Kohlus 2008). Dwarf eelgrass is usually confined to the upper intertidal zone of sheltered sandy and/or muddy European coastlines, but sometimes it is found in the shallow subtidal zone (den Hartog 1970).Like most seagrasses, Zostera noltii propagates vegetatively by rhizomatous growth and sexually through seeds. It is a protogynous hermaphrodite, with 4 to 6 female and 4 to 6 male flowers grouped in a single floral unit (spathe). Within a single spathe, flowers mature first to avoid self-fertilization, although asynchronous maturation of several spathes from the same genet (sensu Harper 1977) may result in self-fertilization (geitonogamous selfing) (Reusch 2001). Fertilization relies on sub-aquatic pollen transport to receptive stigmas. Mature seeds are negatively buoyant and drop to the sediment surface after release. While some seeds enter the sediment, the vast majority are dispersed away, either as bare seeds or in the spathes of floating leaf wrack (Loques et al. 1988). Nevertheless, seeds that are buried may or may not provide a seed bank and, likewise, seeds that are transported may or may not be lost.Several studies have directly examined the reproductive ecology of Zoste...

Genotypic structure and temporal dynamics of the dwarf seagrass, Zostera noltii, were studied in an intertidal meadow that has persisted since prior to 1936 near the Wadden Sea island of Sylt. Samples were collected from two 10 × 10 m plots separated by 250 m from May 2002 to June 2005 and from four 1 × 1 m plots from June 2003 to September 2004. All the samples were genotyped with nine microsatellite loci. No genotypes were shared between the plots separated by 250 m. Genetic diversity was higher in the Wadden Sea than in the other regions of its geographic range. The average clone size (genets) (SD) in the two plots was 1.38 (0.26) and 1.46 (0.4) m², respectively, with a range up to 9 m² and <20% persisted for >4 years. A high genetic and genotypic diversity was maintained by annual recruitment of seedlings despite a dramatic decrease in ramet density that coincided with the severe heat stress event of 2003. Fine-scale (1 m²) analysis suggested that extensive loss of seagrass cover precluded space competition among the genets, while a persistent seed bank prevented local extinction. Long-term persistence of Z. noltii meadows in the intertidal Wadden Sea was achieved by high genet turnover and frequent seedling recruitment from a seed bank, in contrast to the low diversity observed in large and long-living clones of Z. noltii and other seagrasses in subtidal habitats.

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