W&M ScholarWorks W&M ScholarWorks Seed Dispersal In A Marine Macrophyte - Implications For Seed Dispersal In A Marine Macrophyte - Implications For Colonization And Restoration Colonization And Restoration

Seagrasses rely on both vegetative (rhizome elongation) and sexual (seeds) propagation for maintenance of existing beds and colonization of new areas. Yet mechanisms of seed dispersal and survival of seeds in new areas remain poorly described. We conducted seed dispersal experiments in the field and laboratory to better describe seed dispersal characteristics in one species, Zostera marina L. (eelgrass), the dominant seagrass species in the temperate zone of the United States, Japan, and Europe. Seeds were broadcast by hand into unvegetated 5 m diameter plots at three locations over 3 yr (1989-1991) in the York River, Virginia (Chesapeake Bay). These sites had been previously vegetated but were devoid of any vegetation prior to (since 1972) and during the course of the experiments. Resultant seedling distributions closely matched broadcast patterns, with 80% of all seedlings found within the 5 m diameter plots, despite the fact that geophysical processes would appear sufficient to transport seeds greater distances. Wind records for the 2-mo period between seed broadcasting and germination revealed time-averaged wind speeds in excess of 40 km/h on 12 d in each of the 3 yr and gale-force winds (72 km/h) in 2 of 3 yr. A three-dimensional hydrographic computer simulation model of the York River provided instantaneous current velocity estimates from which maximum bottom shear velocities (u.) in the study area were approximated (flood tide: 1.26 cm/s, ebb tide: 1.20 cm/s). These estimates exceeded the critical erosion threshold (ucrit = 0.7 cm/s) for Z. marina seeds determined from laboratory flume experiments. We postulate that small-scale topographic features on the bottom (burrows, pits, mounds, ripples) shield the seeds from the flow. Our results suggest that seeds settle rapidly, dispersing only up to a few metres under the influence of currents and become rapidly incorporated into the sediment. The limited dispersal capabilities of seeds underscore the need to address restoration goals and questions of seagrass ecology in the context of landscape-scale distributional patterns and metapopulation analyses. lack of evidence for seed redistribution in the field after initially striking the bottom suggests that topographic armoring/shielding of seeds from the flow that was observed in the flume is likely acting in these field sites as well. Within natural grass beds current velocities are significantly dampened from drag exerted by the grass (Fonseca et al. 1982, Fonseca and Fisher 1986, Gambi et al. 1990) effectively armoring much of the bed. Thus transport of seeds from within grass beds is expected to be reduced beyond that observed here. Wind records indicate several periods in all years with significant wind events, with gale-force winds occurring during 2 of the 3 yr. We lack data on sediment resuspension via wind-induced waves and currents at this site, but nevertheless find it is surprising that sig- nificant redistribution is not evident as a result of these events. The magnitude of maximum near-bottom currents predicted by the hydrographic model (which does not incorporate wind forcing) and our estimations of shear velocities provided further evidence that geophysical forces should be adequate to redistribute seeds.