© Pedro Martinez
MEKUN: Protecting the oceans
Reefs in the North Sea and Baltic Sea – hotspots of marine biodiversity
Many people probably think of tropical coral reefs when they hear the word “reef”. Although these do not exist here, the underwater world off our coast also has many surprises in store! The seabed is not a boring sandy desert, but a mosaic of habitats such as seagrass meadows, algae forests - and reefs.
The value of reefs
Reefs are valuable ecosystems. They are based on hard substrates such as stones or living organisms, e.g. mussels. They provide shelter and food for numerous marine creatures, improve water quality by filtering nutrients and pollutants and slow down waves, which protects coasts from erosion. The protection of reefs is crucial for the preservation of biodiversity in the coastal waters of Schleswig-Holstein. These habitats are regularly mapped, sampled and evaluated in the Wadden Sea by the National Park Authority and in the North Sea around Heligoland as well as in the Baltic Sea by the State Office for the Environment.
Out and about in the North Sea
Waiting for the next sample - a team from the national park administration and scientists are out and about in the Schleswig-Holstein Wadden Sea National Park. A network of stations in various habitats is sampled twice a year.
The highlight of each campaign is the sampling of the reefs - the hotspots of biodiversity in the Wadden Sea. One of the largest so-called geogenic reefs can be found in the Hörnum Deep, a tidal basin between Sylt, Amrum and Föhr.
A total of 230 species of invertebrates have already been found, almost half of which are dependent on solid ground.
Host City Local Partner
The Ocean Race Europe 2025
© Wolf Wichmann
Animals without backbones - the invertebrate fauna
The Hörnum Deep is one of the most species-rich areas of the underwater world in the national park. The animal inhabitants (fauna) of the sand gaps, the invertebrate and fish fauna are being investigated. A total of 230 species of invertebrates have already been found, almost half of which are dependent on solid ground. These include bryozoans, tunicates, sponges, isopod spiders, nudibranchs, echinoderms, goby worms and flower animals.
© Uli Kunz
Biogenic vs geogenic reef explained
A reef that is built up by living organisms is called a biogenic reef. Most people think of colourful coral reefs in the tropics. However, there are also reef builders in the German North Sea and the Baltic Sea. Here, for example, mussels, oysters or the sand coral (a tube-building worm) form biogenic structures. In contrast, we speak of geogenic reefs if they are made up of stones and/or boulders.
The little ones and the big ones
The fish fauna is also very species-rich with 43 different species found. Especially in the rocky and shallow areas with old shells of mussels and snails (shill) of the Hörnum Deep, hooknose and shorthorn sculpin find favoured spawning grounds. This is where the females attach their eggs to shill or stones. But the examination of the very small inhabitants also yields exciting findings. Hidden among the grains of sand, almost nothing is known about the sand gap fauna (meiofauna) in the Wadden Sea. Four new species have already been found during the campaigns in the national park and are now being investigated further.
Unexpected discoveries
The sampling staff was thrilled when they unexpectedly discovered unmarked European lobsters - i.e. not from the Heligoland breeding station. Does the national park have its own lobster population hiding in the few reefs? Curiosity was aroused and a separate project was set up to investigate this question: HUSAWA - On the trail of the lobster.
Reefs in the Baltic Sea
There are also reefs in the coastal waters of the Schleswig-Holstein Baltic Sea. These are predominantly extensive rock and boulder fields that form a settlement area and habitat for a variety of organisms.
Unfortunately, there are only a few reefs left compared to the past. This is because stone fishing was practiced in the Baltic Sea until the 1970s. According to estimates, around 3.5 million tons of stone were extracted for construction projects in the Schleswig-Holstein Baltic Sea. This corresponds to an area of approx. 5.6 km², an area of more than 780 soccer pitches, which are now missing as a habitat and substrate for large algae which need stones to anchor themselves against the current.
The state is endeavoring to restore and protect this ecologically unique habitat in our coastal waters in the long term as part of compensation measures. On the one hand, an extensive monitoring program is being carried out for this purpose. On the other hand, the state of Schleswig-Holstein is currently designating three marine nature reserves in the area of reef occurrences as part of the Baltic Sea Protection Action Plan 2030.
© MEKUN
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