Why sea water cannot be purified

Mussels clean sea water

The harvest begins every year in late summer. Then fishermen bring masses of mussels ashore from the North Sea. In the Baltic Sea, on the other hand, the breeding of the delicate sea creatures has only been practiced on a small scale again for a few years - the products are sold as a regional specialty. But the benefits of mussels extend far beyond cooking pots.

As natural water filters in the Baltic Sea, they can help to alleviate the effects of over-fertilization. Here, an excessive accumulation of nutrients such as nitrogen and phosphorus, which enter the sea from agriculture, sewage treatment plants and industry, has led to a massive growth of plant plankton. These suspended particles make the water very cloudy, less light reaches the sea floor, which leads to the death of the plant species that have settled there - for example seaweed, the nursery of many fish species. The sensitive ecosystem is thus out of balance.

Mussels contain many nutrients, trace elements and vitamins and have been consumed in Europe and North America for over 8000 years - which has been proven by archaeological finds. Cultivation dates back to the Middle Ages: in the south-west of France, mussels were grown on wooden stakes in the sea as early as the 13th century. France is still one of the most important production countries in Europe today, alongside the Netherlands, Great Britain, Norway, Sweden, Denmark and Ireland. In this country, the mussels in the North Sea are produced on a large scale. Almost 40 percent of the entire EU aquaculture is accounted for by mussels - around 550,000 tonnes are harvested each year, in Germany around 13,000 tonnes annually.

The blue-black sea creatures are able to suck in up to three liters of water within an hour and filter out microscopic algae and organic particles with their slime-covered gills. That's what they feed on. A square meter of mussel bank can mathematically free 140 liters of water from suspended vegetal particles per hour.

This natural cleaning function was examined in more detail in the international research project BONUS OPTIMUS at the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) from 2017 to 2020, funded by the Federal Ministry of Research and the European Union. In two small test farms off Rügen, in Greifswalder and Wieker Bodden, the team of scientists also researched whether mussel farming would work in this part of the Baltic Sea.

For the experiment, a special line system was attached to buoys and brought into the water. The larvae of the mussels that spawn in spring should attach themselves there and grow within a period of around a year. The researchers used measurements to regularly check population density, growth and special environmental parameters.

The first results were sobering

From an economic standpoint, the results were sobering: the mussels were neither of a commercial size, nor had they settled in large numbers on the underwater farm. Nonetheless, the research team gathered important insights into the ecosystem services of mussels and the environmental impacts of aquacultures.

The low yield is not surprising, as the lagoon waters only have a low salt content of 0.5 to 0.7 percent, says project manager Gerald Schernewski. "Mussels grow much faster in salty water." Temperature fluctuations could also act as stress factors for the mussels, which has a negative impact on the populations.

Plankton in the water is locally reduced

The project achieved promising results from an ecological point of view. The experiments showed a decrease in the chlorophyll concentration in the immediate vicinity of the test farm. It was thus proven that the proportion of plankton in the water is reduced at least locally. Mussels also bind nutrients that are withdrawn from the sea during the mussel harvest.

There is also another point that has been investigated: the use of mussel meat for the production of high quality animal feed. "The price for fishmeal has risen sharply in recent years due to stagnating fishery yields and significantly greater demand," explains Schernewski. Here ecologically sustainable mussel meal could serve as an alternative.

The results should help to improve the water quality in the Baltic Sea

According to the coastal researcher, the “total ecological and economic package” is crucial for mussel farming. "Mussel farming should be environmentally friendly and sustainable and must not affect flora and fauna." The project provides important decision-making aids for authorities in this regard. The results are also incorporated into the action plan of the HELCOM commission of the Baltic Sea neighbors to improve the water quality in the Baltic Sea.

The acceptance of aquaculture also plays a major role. "Our surveys show that an initial skepticism has turned into a positive perception," says Nardine Stybel from the association EUCC - The Coastal Union of Germany. Citizens and local politicians rated the project as ultimately a “future idea”. They regretted that the mussel farms had been dismantled again.