Means of rapid eelgrass ( Zostera marina L.) recolonisation in former dieback areas

Recolonisation of eelgrass ( Zostera marina L.) was studied in a Danish estuary during summer 2001 following an anoxia event the previous summer. Leaf bundles had detached from the rhizomes while healthy-looking roots and rhizomes remained in the sediment. We hypothesise that the stabilising effect of remaining belowground biomass, the presence of rhizomes with buds, the presence of a large seed bank and the potential surviving shoots from the previous population may stimulate and speed up recovery in previously colonised areas compared to bare areas. A large seed bank containing more than 11,000 seeds m −2 was found in the dieback area. Seeds were found in the upper 14 cm of the sediment but judging from the length of the hypocotyle of the seedlings, only seeds from the upper 5.5 cm of sediment germinated successfully. The upper 5.5 cm represented a seed pool of approximately 1000 seeds m −2. Germination of these seeds was the primary mode of recolonisation in the estuary, since 96% of the plants in the investigated plot were seedlings. Only 4% of the plants were survivors from the previous year. Although densities of seedlings may exceed densities of surviving shoots, we argue that plants surviving oxygen depletion may still contribute considerably to the recolonisation of a former dieback area as these plants have faster elongation and branching rates and lower mortality rates relative to seedlings. There was no indication of recolonisation from dormant buds on rhizomes. This finding was confirmed in laboratory experiments where buds failed to germinate in the absence of the apical shoot. We examined the structure and ageing of buds and found general withering with age, indicating that buds should germinate shortly after the dieback if at all. Our results, therefore, suggest that rhizome buds are not dormant buds but simply side shoots that have failed to grow.