Seed dispersal in the seagrass Zostera marina is mostly within the parent bed in a protected bay

Seed production and dispersal are key processes in plant population dynamics and gene flow. However, few quantitative studies have followed these processes in aquatic plants. We investigated the abundance of seeds produced and dispersed by the seagrass Zostera marina L. at a protected site within an enclosed bay. We also examined the buoyancy potential of seed dispersal units (diaspores) in the laboratory. Field observations showed that 31% of the total potentially produced seeds were dispersed as decayed reproductive shoots on the sea bottom of the parent bed, whereas 14% were dispersed in spathes (a component of reproductive shoots; seeds are contained inside) detached from live reproductive shoots. However, more than half of the dispersed spathes were negatively buoyant because of the weight of the ripe seeds they contained. Thus, <6% of potentially produced seeds were dispersed by rafting away from the parent bed. The abundance of ripe seeds dispersed was comparable to that of seeds in the parent bed sediment. The fate of the remaining 54% of total potentially produced seeds was not detected, and they were assumed to be immature or to have been consumed by herbivores. Fewer than 5% of the dispersed seeds had germinated. Our results show that most seeds were dispersed within the parent bed, supporting one of the fitness-related seed-dispersal hypotheses, namely that dispersal mechanisms play a role in bed maintenance and increased genetic diversity.