| The potential dispersal of eelgrass seeds from a 256 ha meadow in the Ems Estuary towards other parts of the eastern Dutch Wadden Sea was studied using computer simulation modelling. Flowering shoot density (peaking to 7.8 - 5.4 m-2) and total seed production (1095 million seeds year-1) of this meadow were determined in a field monitoring study and used as model input. Dispersal of floating generative shoots and rafts of plants dislodged by increased water turbulence during autumn was modelled using a 3D-transport model (Delft3DWAQ). Transport of eelgrass seeds contained in floating reproductive shoots with the prevailing hydrodynamic regime revealed maximum dispersal distances of well over 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) to surface currents in the model using actual wind data (hourly averages) from 10 consecutive years (1993-2002). The main direction of transport under average autumn wind conditions was north-eastward. 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 mouth of the estuary. These results indicate that westward eelgrass colonisation of suitable areas in the Dutch Wadden Sea is limited by seed supply due to transport limitations imposed by the prevailing hydrodynamic regime. Extreme events, however, such as prolonged Eastern storms, might promote the transport of eelgrass seeds further westwards allowing for the possible natural recovery of former eelgrass vegetations in this area, which is supported by regular field observations of localised eelgrass patches as far as 40 km west of the estuary. |
