The influence of current velocity, tidal height and the lugworm Arenicola marina on two species of seagrass, Zostera marina L. and Z. noltii Hornemann

For this doctorate study several experiments with seagrasses were planned and conducted on the German North Sea island Sylt from 2002 till 2004. The results show that seedlings from both seagrass species grew better at weaker current velocities than at stronger velocities, with Zostera noltii being able to withstand higher velocities longer than Z. marina. The lugworm Arenicola marina negatively influenced Z. noltii, but did not influence Z. marina. The combination of higher current velocities and lugworm presence was particularly bad for Z. noltii. Both seagrass species grew better in the upper eulittoral compared with the lower eulittoral. In the donor population, Z. marina seedlings emerged 3-4 weeks earlier at the sediment surface than Z. noltii seedlings. Despite this Z. noltii seedlings had nearly the same shoot length by late June 2004. Neither seagrass species transferred carbon or nitrogen from the leaves to the roots/rhizomes with the onset of winter. The phosphate concentration in the porewater of the seagrass meadows might be limiting seagrass growth but ammonium and silicate concentrations are not limiting.