The interactive effects of water flow and reproductive strategies on seed and seedling dispersal along the substrate in two sub-tropical seagrass species

We quantified the effects of water flow on secondary seed and seedling dispersal for two seagrass species with different reproductive strategies: turtle grass (Thalassia testudinum) whose large seeds (15.1 ± 0.8 mm tall) have the potential for long distance dispersal by current-mediated transport of buoyant fruits, and shoal grass (Halodule wrightii), whose small seeds (2.1 ± 0.1 mm tall) are released adjacent to the parent plant and create a persistent seed bank. Results from field dispersal experiments in Texas indicate that under normal flow conditions (mean water velocity < 5 cm s− 1), turtle grass seedling movement is greater over bare sand than within seagrass beds and seedlings have the potential to move up to 2.1 m d− 1. Fine hairs on the seedling base trap sand grains, which likely leads to final seedling establishment after a few days and a potential secondary dispersal distance along the substrate of < 20 m from the point of release. Under normal flow conditions, shoal grass seeds have the potential to move up to 1.1 m d− 1, but seed entrapment in sediment ripples likely limits the total secondary dispersal distance to < 10 m from the parent plant. Secondary dispersal dynamics are species-specific, related to seed morphology and tightly coupled to each species' reproductive strategy. This phase of seed dispersal has the potential to shape plant population structure and aid in colonization of unvegetated habitats.