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Prof. Han Olff: 'Biobuilding organisms essential for the ecological recovery of the Wadden Sea'

10 March 2009
Over the next ten years, the traditional fishing of seed mussel in the Wadden Sea will come to an end. This was decided in October 2008, when fishermen, environmental organizations and the Dutch agriculture minister Verburg agreed to a covenant. Last week, the details of the covenant, the so-called ‘Implementation Plan’, were announced. According to the plan, a big environmental recovery programme for the Wadden Sea will start this year. However, Han Olff, Professor of Community and Conservation Ecology at the University of Groningen, fears that the ecological recovery of the Wadden Sea will prove to be difficult. ‘There is a chance that even if we discontinue seed mussel fishing and other seabed-disturbing activities, the Wadden Sea will not return to its former glory.’
According to Olff, the Wadden Sea is currently far from its natural state. ‘The Wadden mud flats seem natural when uncovered by the tide, because you don’t see any buildings. However, the current state of the Wadden Sea is a far cry from the past.´ Olff takes the absence of seagrass as an example. ‘Seagrasses are a variety of marine plants which grow on the seabed. In the past, the Wadden Sea was full of fields of two different species of seagrass. However, the construction of the Afsluitdijk in the 1930s caused the water to become murky. As a consequence, the seagrass was starved of light. A subsequent disease then caused the seagrass to disappear completely. Due to dredging activities, large-scale seabed-disturbing fishing and the reclamation of tidal marshland, the water stayed murky and the seagrass could never grow back.‘

Biological builders

Olff considers seagrass a fine example of a so-called biobuilding organism. A biobuilding organism is a species which reacts not only to its environment, but also changes it and thereby influences many other species. ‘Seagrass fields slow down the current, causing fine silt to be deposited on the seabed.’ The mussel is another example of a biobuilding organism living in the Wadden Sea. ‘Mussels clump together in mussel beds, preventing them from being washed away by the current. This enables mussels to filter nutrients from the water even with high currents. Mussel banks also provide a habitat for species like small crabs and algae.’ Seabed-disturbing fishing has taken a heavy toll on the mussel banks in the Wadden Sea.

Wadden Sea foundations undermined

Olff believes the disappearance of these biobuilding organisms has had a serious impact on the Wadden Sea ecosystem. ‘The foundation of the Wadden Sea has been undermined. The seagrass fields and the mussel banks provided a safe haven for other species. They were a breeding ground from where fish migrated to the North Sea.’ As a consequence, a lot of fish species have disappeared from the Wadden Sea and the North Sea.


The disappearance of the seagrass and the mussels has had another harmful effect, says Olff. ‘We believe there to be two categories of biobuilding organisms. The first stimulates sedimentary deposits and increases the stability of the sediment. Seagrass and mussels belong to this category. The second category has the reverse effect. Take for example the lugworm, which digs burrows in the seabed, causing sediment to spread more easily in the water, making it murky. Due to all the disturbance of the Wadden Sea ecosystem, biobuilding organisms of the second category have become quite dominant.’ The problem is that both category of organisms sustain their own existence. This could mean that the first group of biobuilding organisms will not reappear spontaneously, even if the seabed is left undisturbed from now on. ‘There is a chance that even if we discontinue seed mussel fishing and other seabed-disturbing activities, the Wadden Sea will not return to its former state.’

Research programme

The University of Groningen recently launched a research programme which will examine the role of biobuilding organisms in the Wadden Sea. The project will be carried out in collaboration with the Royal Netherlands Institute for Sea Research on Texel and the Centre for Estuarine and Marine Ecology in Yerseke. ‘If our hypotheses on biobuilding organisms prove to be correct, we will then investigate how to recover these species. We might be able to support these species. We could perhaps stimulate the formation of mussel banks by installing nets on the seabed, increasing the stability of the mussel banks.’

Rising sea levels

The return of mussel banks and seagrass will hopefully once again increase the biodiversity of the Wadden Sea. ‘You will see more scaups and eider ducks. Perhaps sharks and rays will reappear. All sorts of fish species will return. In the past, the Wadden Sea was a very rich sea, abundant with fish species. Today, we mainly see flatfish and shrimps, which are low in the food chain.´ Olff also expects the biobuilding organisms of the first category to help us in our battle against rising sea levels. ´These species enhance the deposit of sediment. This will raise the level of the Wadden mud flats, making them function as breakwaters.´

Curriculum Vitae

Han Olff (1962) studied biology and was awarded his PhD by the University of Groningen in 1992, specializing in ecology. He then worked as a researcher at the University of Wageningen and was appointed Professor of Community and Conservation Ecology at the University of Groningen in 2002. He studies biodiversity and nature conservation both in the Netherlands and the tropics.
On 17 March, Han Olff will give a lecture at Studium Generale Groningen, entitled ‘The ecological richness of the Wadden.’ For more information, visit: Studium Generale

More information

H. Olff, tel. (050) 363 2214, e-mail h.olff

Last modified:25 October 2019 2.42 p.m.
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