Unravelling interactions between salt marsh evolution and sedimentary processes in the Wadden Sea (southeastern North Sea)

Salt marshes in the Wadden Sea constitute about 20% of all salt marshes along European coasts. They are of immense importance for coastal protection reasons and as habitat for coastal plant, bird, and invertebrate species. The Wadden Sea is a coastal sedimentary ecosystem in the southeastern North Sea. Besides salt marshes, it is composed of tidal flats, high sands, and sandy shoals, dissected by (sub)tidal channels and located behind barrier islands. Accelerated global sea-level rise (SLR) and changes in storm climate have been identified as possible threats for the persistence of the Wadden Sea ecosystem including its salt marshes. Moreover, it is known that the amount and composition of the sediment available for salt marshes are the most important parameters influencing their ability to adapt to current and future SLR. Assessing these parameters requires a thorough understanding of the sedimentary system of the salt marshes and the adjacent tidal basins. In the present review, we investigate and unravel the interactions of sedimentary processes in the Wadden Sea with the processes taking place on the salt marshes. We identify the most crucial processes and interactions influencing the morphological development of salt marshes in the Wadden Sea. A conceptual model is proposed, intended as a framework for improved understanding of salt marsh development and for incorporation into new salt marsh models. The proposed model may also be applicable to regions other than the Wadden Sea

Wadden Sea mussel beds invaded by oysters and slipper limpets: competition or climate control?

Introduced species are often considered to bea threat to residents, but not all reciprocal trends mayreflect species interaction. In the northern GermanWadden Sea, native mussel Mytilus edulis beds aredeclining and overgrown by introduced Pacific oystersCrassostrea gigas and slipper limpets Crepidula fornicata.We review the population development of thethree species and analyse whether the invading speciesmay be responsible for the decline of native mussels.The Pacific oyster predominately settles on musselbeds in the intertidal and the slipper limpet dominatesaround low water line. We compare the developmentof mussels and invaders in two subregions: musselbeds near the islands of Sylt and Amrum decreasedboth in the presence (Sylt) and absence (Amrum) ofthe two invading species and more detailed investigationscould not confirm a causal relationship betweenthe increasing invaders and decreasing mussel beds.There is evidence that the decline of mussel beds ismainly caused by failing spatfall possibly due to mildwinters, whereas the increase in slipper limpets andoysters is facilitated by mild winters and warm summers,respectively. We conclude that changing speciescomposition is a result of the climatic conditions inthe last decade and that there is no evidence yet thatthe exotic species caused the decline of the natives. Itremains an open question whether the species shift willcontinue and what the consequences for the nativeecosystem will be