Resilient reefs: Protecting Læsø Trindel & Tønnerberg Banke

Beneath the shimmering waters of the Kattegat, between Denmark and Sweden, lie two vital marine ecosystems: Læsø Trindel and Tønneberg Banke. These underwater landscapes, rich in biodiversity, have served as sanctuaries for marine life for centuries. But in recent decades, human activities have threatened their fragile existence, leading to an urgent call for protection efforts.  

To understand the significance of these marine protected areas (MPAs), we spoke with Karsten Dahl from Aarhus University, a Danish expert in hardbottom ecology and restoration. His insights reveal the resilience of nature and the power of collective action in restoring these vital ecosystems. 

Located off Denmark’s northeastern coast, Læsø Trindel and Tønneberg Banke are unique underwater habitats featuring boulder reefs, sandy and gravelly seabeds, and so-called "bubbling reefs”. 

The bubbling reefs are extremely rare and form when methane gas seeps through the seabed, binding calcium carbonate, sand and small stones into hardened sandstone structures. Over time, erosion exposes these formations, leaving them perched on top of the seabed.  

Shaped over millennia, the boulder reef habitats provide essential refuge for marine life, from small invertebrates to large predatory fish. But their significance extends beyond ecology—they are remnants of the last Ice Age, sculpted by glacial movements and later reshaped by erosion in shallow waters, creating a dynamic and ever-evolving underwater seascape.  

For centuries, the reefs of the Kattegat have played a vital role in maintaining ecological balance, supporting diverse marine life and shaping local fishing traditions. 

Recognizing their ecological significance, Denmark has designated the waters northeast of Læsø as part of the Natura2000 network to protect the boulder reefs and bubbling reefs—the latter officially classified as submarine structures made by leaking gasses in the EU Habitats Directive. These reefs serve as essential habitats for fish, molluscs, and other marine species. Their intricate mosaic of columns, irregular surfaces, and numerous caves provide shelter and rich feeding grounds for a variety of organisms. 

Historically, Læsø Trindel has been a source of raw materials. Since the 1930s, there has been evidence of boulder removal. The reef's top has deepened, and the remaining reef shows clear signs of ongoing erosion. Its vegetation has been influenced by the unstable hard seabed left behind after boulder extraction for harbour construction on Læsø island. In contrast, the deeper Tønneberg Banke is renowned for its kelp forests and rich species diversity. The bubbling reefs, in particular, support a diverse and thriving community with algae vegetation covering the tops and rich fauna dominating the vertical sides.  

The restoration of the boulder reef led to a remarkable boost in biodiversity in 2012, both in terms of species abundance and overall biomass.  This resurgence is particularly striking when viewed against the broader ecological context: around the turn of the millennium, northern horsemussels (Modiolus modiolus) were still found near the base of the reef at approximately 18 metres depth, but they have likely since disappeared—possibly due to oxygen depletion caused by decaying seaweed drifting from the shallower parts of the reef. 

Cod (Gadus morhua) populations have seen a notable increase, especially in the shallow areas around 2-6 metres, where the restored caverns of the reef provide a favourable environment. These shallow regions are particularly beneficial for larger juvenile cod, which thrive on the abundant food supply, primarily composed of amphipods (Gammaridae). This boost in cod numbers is part of a broader trend across the reef, with an overall increase in biomass of 6-8 fold and a 4-6 fold increase in the abundance of individual fauna species. 

Invertebrate populations have also flourished as a result of the reef restoration. The abundance of small crustaceans, snails, mussels, and polychaetes has risen significantly, enriching the food supply for various fish species. Additionally, seaweed species, such as sugar kelp (Saccharina latissima), have thrived, creating vital habitats for invertebrates like dahlia anemones (Urticina felina) and sea stars (Asterias rubens), further enhancing the reef’s biodiversity. 

Wrasse species (Symphodus spp.) have shown a strong preference for the shallow areas of the reef, although the greatest increase in their numbers has been observed in the deeper regions, between 6 and 10 metres. This suggests that while the shallow areas are important for certain species, the restoration has positively affected all depth ranges of the reef. 

However, not all species have benefited equally. Flatfish populations have declined in the shallow reef areas, likely due to the changes in habitat structure. Similarly, lobster (Homarus gammarus) numbers remain low, probably because of their slow growth rate, which limits their ability to take advantage of the restored habitat.  

Despite these challenges, the overall impact of the reef restoration has been overwhelmingly positive. The biodiversity of the reef has been greatly enhanced, with an increase in both the variety of species present and their populations. As the reef continues to develop, further improvements in biodiversity are expected. 

Despite their significance, Læsø Trindel and Tønneberg Banke have faced increasing pressures from human activities. Sand and gravel extraction, as well as bottom trawling very close to the reef structures, disrupt the seabed and potentially displace boulders, damaging habitats that provide vital shelter for marine life. These disturbances undermine the overall health of the ecosystem, making it harder for species to survive and reproduce. 

Eutrophication from agricultural runoff continues to affect marine ecosystems in the region, although the situation has improved over time. Excess nutrients from fertilizers can fuel harmful algal blooms, which in turn may lead to oxygen depletion in deeper waters, creating so-called "dead zones" where marine life struggles to survive. However, in the case of the horse mussels, the observed oxygen deficiency was likely caused by a thick layer of drifting algae—torn from the degraded reef itself—rather than broader eutrophication in the northern Kattegat. At this open-water site, the primary effect of eutrophication is reduced water clarity, which limits light penetration and contributes to a poorer development of seaweed forests on the reef structures. 

Climate change remains a stressor on these ecosystems. Rising sea temperatures and shifting weather patterns exacerbate existing challenges, causing species to migrate or struggle to adapt.  The combined impact of these threats weakens the ecosystem's resilience, making it more vulnerable to future challenges and hindering its ability to recover and thrive. 

In response to these threats, Denmark has implemented extensive conservation measures to restore and protect these marine habitats. The Blue Reef restoration project was one of the most ambitious efforts, involving the careful placement of boulders to recreate the natural reef structures. This project produced ‘codes of conduct’ that serve as a valuable model for restoring natural stone reefs.  

Beyond physical restoration, sustainable fishing regulations have been introduced to mitigate harmful practices. Bottom trawling is now restricted at all reef sites within Natura 2000 areas, extending from the Skagerrak border to Bornholm, while all types of fishing are prohibited at protected "bubbling reef" sites. A 240-metre buffer zone surrounds these protected areas to safeguard their integrity. Ongoing scientific monitoring ensures that biodiversity continues to thrive, although it has taken over 10 years for newly colonized boulders to develop a fully established biological community.