Sabellaria as ecosystem engineer

 

The honeycomb worm (Sabellaria alveolata) and the ross worm (Sabellaria spinulosa) are two closely related bristle worms that have similar effects on the ecosystem and similar requirements for healthy development. We therefore treat them together here. In the Belgian part of the North Sea, only the ross worm occurs, further south (e.g. in French waters) the honeycomb worm also occurs. Both species can form large reef structures on mixed or hard substrates that can be real hotspots of biodiversity. Sabellaria reefs are less known to us since (presumably due to disturbance) they do not currently form extensive reefs in the Belgian part of the North Sea. However, Sabellaria is mentioned among others in the conservation objectives of the habitat directive area of the Flemish Banks (IHD 10.4) and in the Good Environmental Status of the Marine Strategy Framework Directive. Sabellaria reefs are also included in the OSPAR list of threatened and/or declining habitats and species. Restoration would thus certainly contribute to Belgium's commitments under OSPAR.


Sabellaria reef in the Netherlands © Oceana 
 

Disturbance of Sabellaria reefs

Sabellaria reefs are fairly common in the UK, Germany and France. The largest reef in Europe is located in Mont-Saint-Michel Bay and covers more than 100 ha. In the southern part of the North Sea (Belgium and the Netherlands), reefs are currently rather rare. Occasional sightings in intertidal or gravel zones are still made in the Belgian part of the North Sea. Both species mainly need sufficient currents so that they are provided with food and sediment material to build their tubular structures, making conditions in our North Sea relatively favourable. However, initial indications suggest that seabed disturbance by fishing makes it difficult for the reefs to develop. Exclusion of bottom-disturbing activities could therefore provide an important boost to the recovery of these reefs.

Enhancement of Sabellaria reefs

Situation in our North Sea and surrounding waters

Although Sabbelaria reefs can be part of a healthy and well-functioning ecosystem, there are currently few measures for their conservation and protection. Since these biogenic reefs are classified under habitat type 1070, they should be protected under the Habitats Directive. Sabellaria reefs are also included in the OSPAR list and thus also deserve protection under this convention.

In practice, few member states have taken actions to protect Sabellaria reefs. Experiments on culturing Sabellaria alveolata under laboratory conditions have already been conducted in the UK. Initial results proved cautiously positive. The introduction of cultured Sabellaria into the North Sea proved to be a lot more difficult due to a very high mortality after reintroduction (see bibliography).

Outside the UK, some regional or local measures have been taken in France to inform stakeholders and protect Sabellaria reefs. The Netherlands has carried out preparatory research on the possibilities and options for redeveloping Sabellaria reefs in the North Sea (see bibliography).

Objectives of upgrading Sabellaria reefs

Main objectives:


Restore mature Sabellaria reefs by creating favourable conditions for natural processes to recover and increase the size of current Sabellaria aggregations. The physical structure of mature Sabellaria reefs is restored, along with the typical, long-lived species assemblages associated with them.

 Intermediate objectives: 

  1. Substantially reduce pressure from bottom-dwelling activities on Sabellaria reefs > passive recovery

    • Reduce/exclude bottom disturbance by taking measures to protect bottom integrity in areas suitable for Sabellaria reef recovery, possibly after taking certain recovery measures.

  2. Increase our knowledge of Sabellaria reefs and potential recovery measures

    • Research into the conditions required for successful development of Sabellaria reefs, including based on data from areas where bottom disturbance is already limited, such as the already existing wind farms.

    • Study suitable locations where possible restoration of Sabellaria reefs could take place, taking into account geomorphological conditions and seabed use.

    • Start-up of a monitoring project to monitor natural recovery of Sabellaria reefs and associated fauna. Good T0 monitoring is essential here.

    • Based on the monitoring results, investigate whether there is a need for active recovery of Sabellaria reefs and/or associated fauna. If it turns out that some form of active recovery is needed, it should be examined whether the target species (being Sabellaria or one or more species of the associated fauna) can be reared in the lab and/or whether environmental measures, such as the release of suitable substrate, are necessary.

  3. Upgrading Sabellaria reefs and associated long-lived fauna > active restoration

    • Depending on the results from the preparatory study (see intermediate objective 2), active restoration measures and/or environmental enhancement measures will be applied if necessary.

  4. Raise awareness among stakeholders and the general public on the importance of healthy Sabellaria reefs

Implementation

The selection of suitable sites with sufficiently strong currents, with high concentrations of suspended sediment (fine sediment in the water column), without bottom disturbance and without large migrations of sand waves will be an important first step in the recovery of Sabellaria reefs (see bibliography) . The occurrence of hard substrate and any associated Sabellaria aggregations are also an important facilitator for Sabellaria reef recovery. After excluding bottom disturbance, further monitoring should indicate whether there is passive recovery of the reefs and associated fauna, and whether any additional environmental measures are needed.