Eelgrass seed dispersal via floating generative shoots in the Dutch Wadden Sea: a model approach

Potential long-distance dispersal of eelgrass seeds via floating generative shoots from a 256 ha meadow in the Ems Estuary (Wadden Sea) was studied by means of computer-simulation modelling. Flowering shoot density (peaking at 7.8 m(-2)) and total seed production (1094 million seeds yr(-1)) of this meadow was determined in the field and used as model input. Dispersal of floating generative shoots dislodged by increased water turbulence during autumn was modelled using a 3D transport model (Delft3D-WAQ). Transport of eelgrass seeds contained in floating reproductive shoots with the prevailing hydrodynamic regime revealed maximum dispersal distances of up to 130 km from the source of seed production. The effect of wind speed and direction on seed transport was assessed by applying a wind drag function (3 % of wind speed) on surface currents in the model using actual wind data from 10 consecutive years (1993 to 2002). Main transport direction under average autumn wind conditions was northeastward. Easterly winds (occurring at an average frequency of 15 %) were responsible for some westward transport of eelgrass seeds (at densities > 0.2 seeds m(-2)) up to 20 km from the estuary. Westward eelgrass colonization of suitable areas in the Dutch Wadden Sea appears to be limited by seed supply due to transport limitations posed by the prevailing hydrodynamic regime. Extreme events, such as prolonged Eastern storms, might promote seed dispersal further westwards, allowing for natural recovery of eelgrass in this area; this theory is supported by observations of localized eelgrass patches up to 40 km west of the estuary.