Biodiversity of North Sea Native Oyster Reefs: Notes From The Netherlands

Historical records of native oyster associated biodiversity in the North Sea

Historical maps assembled by the Norwegian-British fishery scientist Ole Olsen¹ and published in 1883, show that in the 19^th century shellfish reefs, formed by the shells of live and dead native oysters, occurred in 30% of the of the Dutch part of the North Sea. Now, only the name of this area, the Central Oyster Grounds, reminds us of these extensive natural hard substrate occurrences in an area dominated by soft sediments. Insights into the historical biological community associated with oyster reefs are rarer still. Descriptions of associated biota are largely limited to studies by Möbius² in the German Wadden Sea, and Hinderbanks study along the Belgian Coast (records of G Gilson in Houziaux 2007³). In both cases, only dwindling stocks of oysters remained at the time of their investigations. Yet these glimpses from the past suggests that a rich biodiversity was associated with the native oyster reefs. 

In addition to associated biodiversity and higher abundances of commercially important fish and crustaceans, the filtration capacity of these reefs would have had a major impact on visibility, water quality and carbon fluxes. The North Sea ecosystem would have differed substantially from that of today, however, since the reefs disappeared before the onset of modern ecological research, knowledge on this reef ecosystem is non-existent. Overfishing, disease and habitat destruction have caused these reefs to disappear almost completely in the 20^th century, except for a tiny population in Lake Grevelingen, a closed sea arm in the Dutch delta area. 

What locations have we studied?

In 2016 marine biologist divers of Bureau Waardenburg in co-operation with and funded by ARK/WWF discovered a 40 ha mixed shellfish bed with Ostrea edulis near the Dutch coast in the ‘Voordelta’ (Christianen et al. 2018)⁴. Since its discovery marine biologists, shellfish specialists, NGO’s, offshore companies and community members have become enthusiastic about native oyster reefs and the potential for oyster restoration in the North Sea. An important part of the research is looking into biodiversity and ecosystem services that native oyster reefs provide.

In 2018 oysters were installed at the ARK WWF Borkum Reef Ground oyster pilot⁵.  Annual monitoring with professional divers has also included biodiversity monitoring.

What methods did we use?

From 2015-2018, fieldwork undertaken at the recently discovered shellfish reef explored the native oyster substrate preferences and epibethic species richness.

The initial measurements in the Voordelta were conducted by scuba divers in 75 quadrats that were spread across the oyster bed along 15 transect lines. Along a transect line of 25 meters, 5 quadrats (50cm x 50cm) were placed 5 meters apart (see Figure 1). All transects were positioned perpendicular to the coast and were randomly setup across the oyster reef within the extent of the reef. 

All measurements were conducted simultaneously in each quadrat by two observers using underwater visual census and scuba. One observer measured species richness, while the other observed the substrate preference.

The substrate preference for O. edulis was investigated by careful visual inspection of all individuals in the quadrats. The (shell) material to which each live oyster was attached was identified to the species level. Specifically, the importance of invasive oyster shell fragments as a settlement substrate for native oyster was investigated. 

To assess the species richness, epibenthic invertebrates (> 5 mm) were identified within the quadrat in the field. Species richness was collected from patches with native oysters and bare sand patches within the reef. Additional observations on fish and epibenthic species presence were collected between 2015 and 2018 using (video-) monitoring. 

Biodiversity associated with native oyster reefs

A diver transect study of the oyster reef in the Voordelta shows that the native oyster benefits from the presence of Pacific oyster shells. In the majority of cases, a Pacific oyster shell was identified as the attachment substrate. 74 epibenthic taxa were found on the reef including 14 species with a conservation status (Table 1 ⁴). The species richness of the oyster beds was 60% higher than in the sandy areas around reef⁴ (Figure 2)

Figure 2: Comparison of the average species richness of epibenthic species on the shellfish reef (>5 oysters m⁻²) per quadrat compared with adjacent bare soft sediment areas in the North Sea Voordelta area, P < 0.001.  (Source: Christianen et al. 2018⁴)

Table 1. Species with special conservation status, which were found on the shellfish reef with native oysters (0. edulis) between 2015-2017, in the Dutch Voordelta area of the North Sea. Conservation status ‘H1110A’; typical species of Sundseth and Creed (2008) habitat type subtidal sandbanks. German Red List scores; ‘2’ highly endangered; ‘3’ endangered; ‘R’ extremely rare; ‘G’ unknown. EU Red List of habitats ‘A3’; critically endangered habitat for both the EU 28 and EU 28+. Table (Source: Christianen et al. 2018⁴)

Organisms observed around the shellfish reefs include arthropods, bryozoans, tunicates, fish, anemones, molluscs, and sponges. In 2018 thornback ray (Raja clavata) (near threatened species IUCN, 2018) was observed on the oyster reef during monitoring as well as European lobster Homarus gammarus (Red list Germany – score 2). Combining monitoring and observations of recreational divers of ANEMOON and RAVON the total list observed on the shellfish reef included 160 taxa by the end of 2018⁶. 

In the ARK WWF Borkum reef Ground oyster pilot diver transects show the seabed consisted of sandy substrate, with patches of gravel, shell substrate, Lanice congregations and native oysters. A total of 28 species were observed on the seabed, and a total of 42 species were observed in the project area. Large quantities of juvenile queen scallop were also present on the seabed⁷.

Although no quantitative abundance data of oyster epifauna were collected from the restoration site, divers noted a large biodiversity on and underneath native oysters. This included the sand mason worm (Lanice conchilega), the solitary individuals of ross worm (Sabellaria spinulosa), queen scallop (Aequipecten opercularis), nudibranchs, nudibranch eggs, different species of sea anemones and the tube worm (Spirobranchus triqueter). Furthermore, fish species such as the goldsinny wrasse (Ctenolabrus rupestris) and goby species were observed hiding underneath oysters ⁷. This indicates that single specimens of native oysters already provide habitat for other reef associated species.

Work in progress

In 2020 North Sea Reef Vitalization For Ecosystem Services (North Sea ReViFES) was started as a 4- year scientific project looking into ecosystem services provided by reefs^{8 .} Annual surveys of the Voordelta reef and Borkum Reef Ground will also continue alongside. These new and ongoing studies will contribute to our knowledge of the biodiversity and ecosystem services provided by native oyster reefs. It is hoped that a deeper understanding of these services will in turn generate further support for the protection and restoration of European native oyster reefs.

Authors and Project affiliation

This information is based on studies and observations of researchers from Bureau Waardenburg, WUR, WMR, Sas consultancy and ARK nature development / World Wide Fund for Nature, as well as volunteers of RAVON and ANEMOON. The full author list reads: Karin Didderen, Joost Bergsma, Marjolijn Christianen, Floor Driessen, Wouter Lengkeek, Hein Sas and Tom van der Have, Karel van de Wijngaard, Emilie Reuchlin-Hugenholtz and Pauline Kamermans. Studies were part-funded by Ark, WWF, Bureau Waardenburg, National Postcode Lottery Droomfonds Haringvliet, Bureau Waardenburg, the Dutch Ministry for Economic Affairs, the Ministry for Infrastructure and Environment, the Province of South Holland, Port of Rotterdam and EU LIFE funding. North Sea ReViFES is part of the TTW‐Open Technology Program (#17671)

References (with direct link to document)

1. Olssen 1883
   http://www.vliz.be/imisdocs/publications/ocrd/213714.pdf
2. Mobius 1877
   https://books.google.nl/books?id=Ug4AAAAAQAAJ&printsec=frontcover&source=gbs_ge_summary_r&redir_esc=y#v=onepage&q&f=false
3. Houziaux et al 2007
   http://www.vliz.be/imisdocs/publications/135511.pdf
4. Christianen et al. 2018
   ‘Return of the native facilitated by the invasive? Population composition, substrate preferences and epibenthic species richness of a recently discovered crustacean reef with native European flat oysters Ostrea edulisin the North Sea.’
   DOI: 10.1080/17451000.2018.1498520
5. Didderen et al 2019a
   https://www.ark.eu/sites/default/files/media/Schelpdierbanken/Report_Borkumse_Stenen.pdf
6. Didderen et al. 2019b
   https://www.ark.eu/sites/default/files/media/Schelpdierbanken/Annual_report_Voordelta_2018.pdf
7. Didderen et al. 2020
   https://www.wwf.nl/globalassets/pdf/wwf-and-ark-borkum-reef-ground-oyster-pilot.pdf
8. Project page North Sea Revifes
   https://www.nwo.nl/onderzoek-en-resultaten/programmas/ttw/open-technologie-programma/2019—17671.html 

For further information you can also see the associated factsheet for shellfish reefs:
https://webwinkel.ark.eu/producten/publications-publicaties/factsheet-schellfish-reefs