Survival and re-establishment of vegetative fragments of eelgrass (Zostera marina)

Vegetative fragmentation of clonal aquatic plants is considered a form of asexual reproduction. Although vegetative fragmentation of eelgrass (Zostera marina L.) is considered a mode of asexual reproduction and dispersal, no data exist to evaluate the potential contribution of fragments (rhizomes with meristems and green leaf shoots) to eelgrass populations. We estimated (i) the size of the fragment population relative to the size of adjacent eelgrass populations, and (ii) the potential for fragments to re-establish in eelgrass beds in Mission Bay, San Diego, Calif. We surveyed the abundance of fragments on adjacent beaches and determined the survival and growth of detached fragments and of fragments re-established in eelgrass beds after varying time spent in the water column. Although vegetative fragments occurred throughout the year, but mostly in winter, they represented a loss of <4% of the leaf shoots in adjacent eelgrass populations. Only 60% of the fragments survived in the water column after 6 weeks, and growth and survivorship of re-established fragments declined with time spent in the water column. Thus, fragmentation represents a small net loss to the eelgrass beds studied and is not a common recruitment mechanism. This result does not support the hypothesis that asexual reproduction in the form of vegetative fragments contributes to eelgrass population growth. Because successful re-establishment is improbable, vegetative fragments also are not likely to contribute substantially to eelgrass dispersal, at least in Mission Bay.