Sea Stars: Artificial Reefs Combining Trophic Recovery and Coastal Protection in Danish fjords

Why Do the Fjords Need Artificial Reefs?

Danish fjords are suffocating from a lack of oxygen. This oxygen depletion is caused by algae, which grow a little too well due to all the manure coming from agriculture. In Denmark, for decades, we have scraped mussels, dredged stone reefs, and trawled the Danish fjords, and therefore, today, there is a lack of firm structure where species like mussels, oysters, sea anemones—sessile filter feeders—can settle. At the same time, the runoff of manure has contributed to creating violent blooms of microalgae, which cause oxygen depletion in late summer, suffocating the animals and plants that should cover the seabed. While one could simply suggest reducing manure, this project proposes that one can also address the issue from another angle: by promoting the animals that eat the algae. Sessile filter feeders, which can efficiently filter microalgae out of the water, are key. These animals can filter enormous amounts of water: a single blue mussel can filter up to 100 liters of water a day, and oysters up to 180 liters. The project suggests placing structures where these organisms can settle and, in turn, remove algae from the water. These animals can remove large amounts of algae if only they have somewhere to sit, preferably high up in the water column and in the current. The project proposes building structures for this purpose, namely artificial reefs.

A No-Brainer

This project is about why artificial reefs would be smart, where they should be placed, and how they could be built. The project proposes that reef structures be installed in Venø Bay and, over a few years, be colonized by mussels and other so-called sessile filter feeders, which will form a living reef outside the artificial reef. As these organisms grow, they filter large amounts of water for algae, while also helping fish stocks, creating habitat, and generating coastal protection. Under the structure, shells and other debris accumulate, giving the seabed a new structure with increased stability and complexity, allowing it to develop further on its own. The seabed will thus be covered by wild mussel and oyster banks, even after the structure itself has decayed. Besides counteracting oxygen depletion, they also provide the following essential benefits:

1: Provides biodiversity and (much) larger fish stocks
The biogenic reefs are a fantastic habitat for fish fry—they provide both shelter and food for the young, which increases their survival and thus leads to larger fish stocks. Right now, on artificial reefs around the world, it has been observed that the biomass of fish can increase between 10 and 70 times! In addition to mussels and other filter feeders, which form the reefs, the reefs provide food for adult fish. You can thus view the filter feeders on the reef as a link between the microalgae and the higher trophic levels. It would be more accurate to say that they turn algae into cod, mackerel, herring, trout—and many other types of biomass, which we would much rather have than algae. And the structural complexity of the reefs also forms the foundation for higher biodiversity.

2: Creates aquatic plants—a magnet for life
As mentioned, they purify the water. This means, among other things, that the water becomes more transparent, allowing sunlight to penetrate deeper, and making it possible for eelgrass—and other angiosperms that would like to grow—to begin to grow at greater depths, where it was previously too dark. This means these plants can grow at the bottom—where oxygen depletion would otherwise be worst—and help maintain life in that part of the fjord. The meadows of eelgrass and other aquatic plants also provide further habitat for fish fry and adult fish.

3: Creates shell piles—another magnet for life
The piles of shells that gradually accumulate under the reefs are yet another valuable habitat that remains even after the artificial structures have disappeared. They create opportunities for a wealth of species, which support other species, etc.—ecology is rebuilt from the bottom up.

4: Coastal protection
The deposition of shells under the reefs leads to a slow but steady raising of the seabed. The reefs can thus, in themselves, be a relatively significant contribution to creating large areas of coastal protection in situ. If the reefs are not made too high, their structure will be maintained by natural mussel and oyster banks, which can take over where the artificial reefs end. These grow vertically, and if their conditions are good, they can keep up with sea level rise. This is a thought-provoking perspective on how Danish coasts can be protected against erosion, without having to concrete the entire country.

Site Selection: Oddesund and Venø Bay

Artificial reef structures are best suited to the inner fjords.
These are the areas worst affected by oxygen depletion, which the reefs are intended to address, and where there is less wave energy that could otherwise damage the structures. There is also strong potential for these areas to become covered with seagrass beds and natural oyster and mussel banks—a benefit that could become permanent after being initiated by the reef structures.

Of all the Danish fjords, Limfjorden has been chosen because its ecological potential—thanks to its salinity gradient—is much greater than in other fjords and straits. Limfjorden could become home to up to 2,200 species, compared to just a few hundred in, for example, the Øresund. The reefs will be placed at Oddesund and gradually spread east through Venø Bay, and from there, possibly further into the eastern parts of the fjord. Oddesund is therefore the starting point for potential further expansion. The choice of Oddesund is based on the following factors:

1. Limfjorden’s strongest currents provide perfect growth conditions
for the sessile filter feeders that will colonize the reef. Mussel farms managed by the company “Blå Biomasse” (Blue Biomass) demonstrate that blue mussels grow well and quickly in this area. Historically, there have also been high densities of blue mussel banks in the bay, which further confirms that the basic natural and geographic conditions for biogenic reefs are present.

2. It is located outside Natura 2000 areas
—a classification that, paradoxically, would make it much more difficult, or even impossible, to obtain permits for artificial reefs.

3. The ecological status sought is “poor, but not too poor”
—that is exactly the case here. In the western Limfjord, the ecological status is so bad it almost needs an ambulance, but thanks to the strong water flow, there is not much oxygen depletion in this area. This means the reefs will have maximum effect, while the risk of lethal oxygen depletion—which could wipe out the entire reef—is low. The cleaner water left in the downstream “wake” of the reefs will, over time, improve conditions for bottom-dwelling organisms. After a number of years, conditions may be good enough to expand the reefs further in that direction.

Summary
Oddesund and Venø Bay are located relatively close to the fresh water at Thyborøn, so there is enough oxygen for the reefs to survive. The strong currents allow filter feeders to grow quickly. This is supported by the mussel farms and the historically high densities of mussels in the bay. There is also plenty of space between nature reserves, shipping lanes, and other infrastructure that should be avoided. Finally, the large open areas in the western Limfjord mean there is great potential for the reef structures to create the right conditions for extensive seagrass beds.

The Danish Waterscape 

On this background map, you see the pure water. Groundwater and seawater, water beneath us and the water around us. Water connects everything it touches: Sea and land, nature and culture, the big and the small, past and future. Denmark has 7300 km of coastline and 30 large fjords. 

We Have Hit Rock Bottom 

Bottom-trawling fishing gear, such as trawling, is both ecologically and widely used in Denmark – even inside protected marine areas. The nets are used down to 200 meters depth. Bottom trawling is a problem due to its serious direct effect on seabed communities of organisms. These are long-term consequences for marine life. It is alarming and shocking – what surprises many is how widespread this practice still is in Denmark. On the map, you can see how often/annually an area is visited by bottom-trawling gear.

Stone Fishing 

Stone fishing is the removal of stones from the seabed, which has occurred in Denmark on a large scale since the 19th century. The stones were mainly used for construction, road building, and harbor construction, but also for lime production and, in more recent times, for coastal protection. Stone reefs are important habitats for many marine species, including fish, crustaceans, and algae. The removal of stones from the seabed has therefore had significant negative consequences for biodiversity and the marine environment. In recent years, there has been increasing focus on restoring stone reefs, and several projects have been launched to reestablish reefs in Danish waters. 

Eutrophication 

Eutrophication is the technical term for "too much nutrition." Manure is plant nutrition, but if the ecology is overloaded, the nutrition becomes poison. Nutrient loading from manure in Denmark is a driving force behind algae blooms. An unnatural accumulation of phytoplankton overshadows bottom plants, and when the algae decompose after a bloom or after heavy die-off, all oxygen is used up, and wild organisms suffocate. 

The Water Framework Directive 

Achieving good ecological status in all Danish water bodies, including lakes, streams, coastal waters, and groundwater, is a legal requirement. Denmark should have achieved this goal at the latest by December 2027. The deadline has been extended since 2015, and there are now very limited opportunities for further extensions. If Denmark does not meet the EU's Water Framework Directive requirements for nutrient reductions within the fixed deadline, there is a risk that Denmark will be brought before the EU Court of Justice. The EU Commission has already initiated infringement cases that could result in large fines. 

Hope you enjoyed, please do not hestitate to contact me in case you have any questions about the project:)