The role of hydrodynamics on seed dispersal in seagrasses

L. Ruiz-Montoya, R. J. Lowe, K. P. Van Niel and G. A. Kendrick

Limnol. Oceanogr., 57(5), 2012, 1257-1265 | DOI: 10.4319/lo.2012.57.5.1257

ABSTRACT: Two widely distributed seagrasses in Western Australia with contrasting dispersal strategies were studied in terms of their physical characteristics and morphology to understand how physical processes (wind, waves, and currents) drive dispersal. Posidonia australis releases floating fruit that contain a single negatively buoyant seed that lacks dormancy. Halophila ovalis produces fruit and dormant seeds that sit on the sediment surface. The floating stage of P. australis was assessed in situ by tracking the movement of the fruit directly on the ocean surface, together with drifter devices to differentiate between the transport induced by surface currents and wind. The dehiscence time of P. australis fruit was evaluated in seawater tanks, and the associated viability of the seeds was assessed by growth after dehiscence. The settling velocities of P. australis seeds and H. ovalis fruit and seeds were quantified in settling tubes with image-processing techniques to track the fall trajectories. The re-suspension thresholds of the seeds were calculated based on the critical bed shear stresses required to transport the seeds in a unidirectional flow flume. P. australis can travel long distances at the air–sea interface (∼ 55 km), due to wind alone, during its floating stage. The settling velocities of P. australis and H. ovalis seeds (ws = 10.6 ± 0.4 cm s−1 and 4.7 ± 0.1 cm s−1, respectively) and their re-suspension thresholds (τ = 107 ± 4 mPa and 66 ± 1 mPa, respectively) suggest that secondary movement is restricted, but likely to be produced by stronger wave-induced shear stress events.

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