Valuable biotope in the Wadden Sea

Thanks to modern technical aids, subtidal mussel beds in the Wadden Sea are now easier to trace than before. There are, however, certain puzzles yet to be solved.

Mussels have been loyal inhabitants of the Wadden Sea from time immemorial. "This is a good thing, too," said coastal geoscientist Dr Klaus Ricklefs of the Research and Technology Centre, West Coast at Kiel University. According to the expert, the mussel beds play a "prominent role in the functioning of the ecosystem and in biodiversity" and so in-depth knowledge about them is more than simply desirable. The problem is that while exposed mussel beds in the Wadden Sea are easily examined twice a day thanks to the ebb and flow of the tides, this is not as simple with subtidal colonies. Unlike in the clear water of the Baltic Sea, mussels living in the usually murky waters of the Wadden Sea are nearly always hidden. For a long time, information on their possible occurrence mainly came from the fishing industry. Then, with their mechanical devices, scientists went on the hunt for these mussels, literally fishing in the dark, with varying degrees of success.

Bottom grabs, special drift nets and dredges are still used today, but in a much more targeted way than before. Comprehensive and detailed images of the seabed are possible thanks to hydroacoustic measuring devices, such as side-scan sonars and multibeam echo sounders, said Ricklefs. Although mussel beds cannot be "detected in this way", the expert can very reliably identify from certain sonar signatures areas with lots of mussel shells and also rule out other areas of the seabed from the outset. "This makes the work considerably easier," explained Ricklefs, who studied geology in Kiel and has been researching the Wadden Sea from the Research and Technology Centre in Büsum since 1991.

It is not all that easy, however, as this type of biotope comprising empty shells, living mussels and a wide variety of other small organisms has become rarer in the North Frisian Wadden Sea. While established mussel beds, which play a similarly prominent role in the Wadden Sea as coral reefs in tropical seas, were very widespread up into the 1920s, populations shrank to their lowest level in the 1950s, returning almost to their 1920 level around the end of the 1980s.

According to Ricklefs, there is a series of hypotheses but hardly any real evidence to explain these considerable fluctuations. The decline around 1950 could be seen, first and foremost, as the result of intensive fishing in the austere times during and after the Second World War. The experts then ascribe the recovery after this to, among other things, the strong eutrophication (enrichment with nutrients) of the Wadden Sea. Waste water from urban areas was often introduced without being treated first, detergents were not yet phosphate-free, fertilisers were washed off from farmland, too, so that by the end of the 1980s the nutrient content in the Wadden Sea had risen sharply. The result of this was a mass proliferation of microalgae suspended in the water. These are one of the mussels’ favourite foods and so they benefited from what was actually anything but desirable over-fertilisation.

Much has improved since the Wadden Sea became a national park and the federal state, the Federal Government and the EU set strict conditions for the protection of the water. As a result, although the Wadden Sea may no longer be a paradise for mussels, they have not gone hungry for a long time now. The marked decline in the human threat to mussel beds is also good news. Fishing from wild mussel beds on tidal flats has been totally prohibited for several years now and fishing of young seed mussels is only allowed in a few tidal channels.

As the EU’s national park administration has to produce a report on the condition of the Wadden Sea every six years, Ricklefs and his small team from the Research and Technology Centre at Kiel University have regularly traced the mussel beds with the aid of the research catamaran EGIDORA, which is equipped with all sorts of measurement instrumentation, since 2008. According to their latest findings, the number of subtidal mussel beds in the Wadden Sea in Schleswig-Holstein that have been established for several years can be counted on one hand. On top of that, the number of mussels on these beds is declining, while, by contrast, the colony of Pacific cupped oysters is continually growing. These oysters, which were artificially introduced to the Wadden Sea as farmed oysters (for companies like Sylter Royal), are flourishing in their new environment, although the fishing industry was not confident they would survive at local latitudes.

At that time, scientists feared that the Pacific cupped oysters could displace the mussels, but this did not happen. Instead, the well-known Wadden Sea ecologist Professor Karsten Reise recently described his observation that the two species of mollusc seemed to have come to an arrangement in their common environment. The mussels are happy to live under or between the oysters with their strong shells. Though there is less nourishment to filter from the water, the "original inhabitants", which are naturally not that robust, are better protected from predators like starfish or birds.

Author: Martin Geist