Seagrass recovery in the Northern Wadden Sea?

Abstract

Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km2 or 11% of intertidal flats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when flying during low tide exposure 300 to 500 m above ground. When viewed from the air, difficulties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudflats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives confidence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand flats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion. © Springer-Verlag and AWI 2007.