Effects of water dynamics on Zostera marina: transplantation experiments in the intertidal Dutch Wadden Sea

Pilot experiments in the tidally dominated Dutch Wadden Sea indicated a negative relationship between Zostera marina L. transplantation success and tidal depths. As light availability was sufficient, we hypothesised that water dynamics (particularly waves) and ensuing sediment mobility (movement or resuspension of the sediment) were the major cause for the loss of transplants at larger depths. Transplantation experiments were carried out at intertidal flats and depths under conditions of normal and reduced water dynamics and sediment mobility, using exclosures that reduced water dynamics and shells that stabilised the sediment. To test bioturbation effects, cages that excluded large biota were used. Without protection, Z. marina plants were successfully established in a belt within the intertidal zone (0 to –0.20 m mean sea level, MSL) at 2 out of 3 transplantation sites during the first growing season. Reduced water dynamics by exclosures prevented the loss of plants in the zone –0.40 to –1.15 m MSL; removal of the exclosures after 1 mo resulted in loss of all plants within a few days. Neither light limitation nor bioturbation could explain these results. We conclude that prevalent water dynamics, particularly the relative period of exposure to wave dynamics, were too high for establishment and maintenance of intertidal Z. marina at depths below –0.20 m MSL at these sites of intermediate exposure. At the third, exposed, transplantation site, water dynamics prevented transplantation success along the entire depth gradient studied. Reduction of sediment mobility by shells had a positive effect on transplant survival, particularly at –0.20 m MSL. Shell armour can therefore be recommended in transplantations near the lower limit of potential intertidal habitats of Z. marina. Anchorage of the plants with pegs had no positive effect. Depth limitation of intertidal Z. marina populations by water dynamics can explain zonation patterns that occur in several tidal systems in northwest Europe.