Impact of hydrodynamics on development and morphology of intertidal seagrasses in the Wadden Sea

Abstract

Field surveys and in situ experiments were conducted to determine the impact of hydro- dynamics (in particular tidal currents) on the development and structure of intertidal seagrass beds. Field observations in the Sylt-Romo Bight (German Wadden Sea, North Sea) from 1997 to 1999 re- vealed dense seagrass beds with a high biomass in sheltered areas, whereas at exposed sites seagrass beds occurred only sparsely with a comparatively low biomass. In addition, the shoot length and leaf length were distinctly higher at sheltered sites than in exposed beds. Cross transplantation experi- ments and enclosure experiments between sheltered and exposed seagrass sites showed that the den- sity as well as the shoot morphology (leaf number per shoot, shoot length, leaf length and leaf width) were drastically reduced after transplantation into an exposed bed, and were even lower than seagrass values of the adjacent exposed seagrasses after 6 wk. In contrast, the seagrass density as well as the length of shoots and leaves increased distinctly after transplantation into a sheltered seagrass bed. By means of an in situ 'three-current flume', experimentally modifying current flow at the same site, it was shown that increasing tidal current velocities resulted in decreased densities and length values in seagrass shoots, despite growing under the same habitat conditions. These results suggest that strong hydrodynamics directly affect the development and architecture of Zostera noltii beds by reducing seagrass density and shoot morphology, as well as the extent of seagrass beds. Since changes in envi- ronmental conditions are ongoing (e.g. higher storm frequency, sea level rise in combination with man-made protective structures, such as dikes and dams), it is suspected that increasing hydrody- namics might contribute to losses in intertidal seagrass beds.