North Sea ReViFES – Natural Reefs

North Sea ReViFES - Natural Reefs

Over the twentieth century, large areas of natural reef habitat disappeared from the North Sea due to bottom-disturbing activities and overfishing. Although reef restoration is increasingly promoted as a way to enhance biodiversity and ecosystem functioning, empirical evidence demonstrating the ecological benefits of temperate offshore reefs in the North Sea remained limited. This project was established to address that knowledge gap and provide a scientific foundation for future reef restoration efforts.

The project investigated the ecological role of natural reefs across multiple North Sea locations. Researchers studied rocky reefs, shellfish reefs and tubeworm reefs to better understand how these habitats contribute to ecosystem functioning and the wider health of the North Sea. By examining existing reef systems, the project aimed to build a stronger evidence base for future reef restoration and conservation initiatives.

The project demonstrates that natural North Sea reefs provide substantial ecological benefits and play an important role in supporting marine life and ecosystem processes. The findings provide valuable insight into the functions that restored reefs could deliver and contribute to the growing body of knowledge needed to guide effective restoration efforts.

The findings are particularly relevant for offshore wind farms. As bottom-disturbing activities are generally excluded within wind farm areas, these sites offer unique opportunities for reef recovery and active restoration. By improving our understanding of the ecological value of natural reefs, the project helps inform nature-inclusive design and restoration strategies for future offshore developments.

The objective of this project was to quantify the ecological functions and ecosystem services of natural North Sea reefs. Through research at multiple reef locations, the project aimed to provide empirical evidence that can support reef restoration, marine conservation and nature enhancement initiatives in the North Sea.

2020-2025

Project dates

North Sea (Borkum Reef Grounds in Germany/NL, Voordelta in NL, Noss Head in Scotland, Wadden Sea in NL)

Location

NIOZ Royal Netherlands Institute for Sea Research (North Sea ReViFES Consortium), NIOZ Royal Netherlands Institute for Sea Research, Wageningen University & Research (WUR), University of Groningen, Utrecht University, TU Delft, Van Oord, Waardenburg Ecology

Project lead and partners

Benthic invertebrates, fish, bi-valve reef forming species

Target species

Methods

This project combined three field studies to investigate the ecological functions and ecosystem services of natural North Sea reefs. Natural reef systems were used as reference habitats to better understand how reefs influence biodiversity, food webs, fish communities and sediment processes. The studies were conducted across multiple locations in the North Sea, including the Borkum Reef Grounds, Voordelta, Noss Head and the Wadden Sea. Together, the studies examined reef effects from the seabed upwards, covering benthic communities, vertebrate biodiversity and sediment characteristics.

Benthic biodiversity and food webs

Sessile and mobile benthic communities were sampled at rocky reefs, shellfish reefs and sand mason worm reefs, and compared with nearby sandy seabeds without reef habitat. Sampling took place at the Borkum Reef Grounds and the Voordelta. Sessile benthic organisms were collected using box corers and Hamon grabs, while baited traps were used to sample mobile species. Four to eight sampling stations were surveyed within each habitat type. Species diversity, abundance and food web complexity were analysed to evaluate the ecological role of different reef habitats.

Fish communities and vertebrate biodiversity

Fish and other vertebrate communities were studied within and around two shellfish reef systems: a mixed reef of blue mussels, Pacific oysters and native European flat oysters in the Voordelta, and a horse mussel reef at Noss Head in the Scottish North Sea. Surveys were conducted during expeditions using the research vessels RV Pelagia and RV Navicula between 2021 and 2023. Two complementary non-invasive monitoring techniques were applied at reef locations and at distances ranging from 25 metres to 10 kilometres from the reef. Environmental DNA (eDNA) samples were collected from the water column to identify vertebrate species, while Baited Remote Underwater Video systems (BRUVs) were used to record fish communities and observe both juvenile and adult life stages.

Sediment enrichment and carbon storage

The influence of bivalve reefs on surrounding sediments was investigated at twelve intertidal reef locations in the Wadden Sea. Sediment samples were collected inside reefs and at distances up to 400 metres from the reef edge. Sampling was conducted at three sediment depths (0–1 cm, 9–11 cm and 27–30 cm). Measurements included total organic matter content, sediment grain size and silt content. Sampling locations 350–400 metres from reefs were used as reference sites without reef influence. Statistical models were applied to assess relationships between sediment properties, reef characteristics such as size and density, distance to reefs, and environmental variables including flow and orbital velocity.

North_Sea_ReViFES

Results

This project provides empirical evidence that natural reefs in the North Sea deliver significant ecological benefits. Across all three studies, reefs consistently supported higher biodiversity, more complex ecological interactions, enhanced fish habitat and enriched sediment conditions compared to surrounding sandy seabeds. Together, the results strengthen the scientific basis for reef conservation, restoration and nature enhancement initiatives in the North Sea.

Natural reefs enhance biodiversity and food web complexity

The results show that all reef types significantly increase the diversity and abundance of benthic communities compared to nearby sandy habitats. Both geogenic reefs, such as rocky reefs, and biogenic reefs, including shellfish and tubeworm reefs, supported richer benthic assemblages and more complex food webs.

Although rocky reefs are generally more structurally stable over time, biogenic reefs produced comparable ecological benefits. The findings show that shellfish reefs and tubeworm reefs function as biodiversity hotspots and contribute substantially to ecosystem functioning. In particular, the study demonstrated that tubeworm reefs support benthic communities that are as diverse and abundant as those found on shellfish reefs and rocky reefs.

Natural reefs provide essential habitat for fish and other vertebrates

Fish species diversity was consistently higher within shellfish reef habitats than in surrounding sandy areas. Reef habitats supported a wide range of species, including commercially important fish such as cod, pollock, and ling, as well as smaller reef-associated species including gobies, clingfish, and rocklings.

The results also indicate that shellfish reefs can function as nursery habitat for juvenile fish. Fish diversity declined with increasing distance from the reef, highlighting the importance of these habitats for local fish communities.

The combination of eDNA and BRUV surveys revealed more than 100 vertebrate taxa across the two reef systems. Several notable species were detected, including common skate and Atlantic bluefin tuna. The study showed that both monitoring techniques provide complementary information, with eDNA detecting a broader range of species and BRUV surveys providing valuable information on life stages and habitat use.

Natural reefs enrich sediments and support carbon storage

Bivalve reefs were found to significantly alter the properties of surrounding sediments. Organic matter concentrations in surface sediments were approximately 5% higher within reef habitats, while silt content increased by approximately 30% compared to reference locations without reefs.

Importantly, these effects extended beyond the reef itself. Elevated levels of organic matter and silt were detected up to 50 metres from reef boundaries in surface sediments and up to 200 metres from reefs in deeper sediment layers. Larger and denser reefs generated stronger and more extensive enrichment effects.

The results show that shellfish reefs function as natural sediment traps by reducing water movement and promoting the deposition of fine sediment and organic material. These findings highlight an additional ecosystem service provided by reefs and demonstrate their contribution to sediment carbon enrichment and carbon cycling within the North Sea.

Implications for reef restoration

A key outcome of the project is the confirmation that natural reefs play an important ecological role in the North Sea. The studies demonstrate that reefs support biodiversity, provide habitat for fish and influence sediment processes across a wider area than the physical reef footprint itself.

These findings provide a robust evidence base for future reef restoration projects and strengthen the case for incorporating reef enhancement measures into nature-inclusive offshore wind farm developments. By quantifying the ecological benefits of natural reefs, the project helps reduce uncertainty around expected restoration outcomes and supports investment in large-scale reef recovery initiatives.

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Tips & Tricks

Do not overlook tubeworm reefs.
Sand mason worm reefs support highly diverse benthic communities and can provide ecological benefits comparable to shellfish reefs and rocky reefs. Consider including tubeworm reef development as part of reef restoration strategies.
Both geogenic and biogenic reefs matter.
Rocky reefs, shellfish reefs and tubeworm reefs all contribute to biodiversity enhancement and ecosystem functioning. Restoration efforts do not need to focus exclusively on oyster reefs to achieve ecological benefits.
Use non-invasive monitoring techniques when possible.
Methods such as BRUVs and eDNA provide valuable information on fish and vertebrate communities without disturbing sensitive reef habitats.
Combine monitoring methods for a more complete picture.
eDNA can identify a broad range of species, while BRUVs provide information on fish behaviour, habitat use and life stages. Combining both techniques improves biodiversity assessments.
Consider the wider ecological footprint of reefs.
The ecological influence of reefs extends beyond the physical reef boundary. Fish diversity, food web effects and sediment enrichment can occur at considerable distances from the reef itself.
Sample multiple sediment depths.
When assessing sediment enrichment or carbon storage, include both surface and deeper sediment layers. Reef-related effects can extend much further in deeper sediments than at the surface.
Reef size and density influence ecological impact.
Larger and denser reefs create stronger effects on biodiversity, habitat quality and sediment enrichment. Scale should therefore be considered during restoration design.
Account for carbon storage as an ecosystem service.
Reefs not only support biodiversity but also influence carbon-rich sediment accumulation. This additional ecosystem service can strengthen the case for conservation and restoration projects.

Let's talk

Would you like to know more about this project or get involved? Send an email to the project lead.

k.didderen@waardenburg.eco