Impact of Mussel Beds on the Wadden Sea Much Greater than Expected

01-05-2012   Mussels and Pacific oysters have a significant impact on Wadden Sea biodiversity, not only as food sources, but also because they create a habitat for many other species. In a study, recently published online in the international journal Ecosystems, scientists from the Royal Netherlands Institute for Sea Research (NIOZ) and the University of Groningen (RUG) show that the impact of mussel and oyster beds that are exposed at low tide, may be much greater than their actual size would suggest.

Mussels and Pacific oysters greatly influence their environment, which is why they are sometimes called 'biobuilders'. In the Wadden Sea, they create a solid base in the otherwise sandy seabed. In addition, they filter the water, excreting digested and undigested particles that form a kind of silty mussel excrement that covers the sandy seabed around the mussel beds. These biobuilders do not only create a suitable habitat for themselves, but also for other species. Living on or around the mussels and oysters are many plants and animals, such as seaweed, sea snails, fish and shrimp, which in their turn are eaten by larger fish or waders.

Mussel Excrement
Scientists from the WaddenEngine - a joint project by RUG and NIOZ - have studied the area around mixed mussel and oyster beds on the mudflats south of the island of Schiermonnikoog that are exposed at low tide. Seabed samples show that the percentage of silt (mussel excrement in this case) was high on and in the direct vicinity of the mussel beds, as expected, but they also show that silt, carried by currents, could be found as much as 200 metres away from the beds.

The scientists found many more different species in this silty mussel excrement than in the sand, and cockles, tube worms and sandworms (Alitta virens) were found there in large numbers. Many of these species are important sources of food for waders.

Observation Towers
This is the reason why the scientists have also monitored the distribution of waders foraging at low tide. They used a special technique for this: they plotted the positions of all waders on the mudflats using specially developed equipment that had been mounted on two four-metre-high observation towers. These plottings showed that oystercatchers forage not just on the mussel beds (mussels form an important part of their diet), but also to a large extent in the silty area around the mussel beds. In addition, the plottings indicate that these areas also attract many curlews and bar-tailed godwits, foraging for small crabs and worms. This means that mussel and oyster beds do not just influence the food chain in the immediate vicinity, but also in a much wider area.

Preservation
Mussel and oyster beds only cover a small part of the Wadden Sea (1-5%), but the impact of these biobuilders on the ecology is much greater than expected on the basis of their number alone. It is therefore essential to preserve these species and ensure that they can return to areas where they have disappeared.

Spatially Extended Habitat Modification by Intertidal Reef-Building Bivalves has Implications for Consumer-Resource Interactions, Els M. van der Zee, Tjisse van der Heide, Serena Donadi, Johan S. Eklöf and Britas Klemens Eriksson, Han Olff, Henk W. van der Veer and Theunis Piersma. Ecosystems, Online First™, 5 April 2012.

Links to websites about related research projects (in Dutch):
Waddensleutels
Mega-kokkelverhuizing
Tropisch zusje van de Waddenzee