Aarhus University Seal / Aarhus Universitets segl

No. 344: Nature restoration of boulder reefs in Roskilde Fjord. Basic survey of three selected sites

Dahl K, Al-Hamdani Z, Rasmussen MB, Svendsen JC & Bennike O. 2019. Naturgenopretning af stenrev i Roskilde Fjord. Basisundersøgelse af tre udvalgte lokaliteter. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 53 s. - Videnskabelig rapport nr. 344. http://dce2.au.dk/pub/SR344.pdf 

Summary

The National Park Skjoldungernes Land has a goal to restore boulder reefs within the park area. The present project has investigated the suitability of three predefined areas for reef restoration.

The areas have been investigated in a mapping exercise joining image analysis of photos collected by a drone on very shallow water with underwater acoustics from a water depth of 2 m and deeper. The investigations have been supplemented by visual ground truth work along transects and in selected points.

The investigation has resulted in a description of the seabed surface sediment as well as sub-seabed sediment layers to justify their suitability for carrying the weight of the proposed boulder reef from a geological point of view. Furthermore, key biological elements as eelgrass and seagrass cover and blue mussel beds have been mapped.

Two sub areas have been identified inside the project areas as potential areas for restoration of boulder reefs. Both were located at Nørrerev. Furthermore, a potential area for establishing an ‘information reef’ was also identified having easy access for the public. The information reef is located south-east of Veddelev Harbour and next to a camp site.

The two identified potential restoration areas both have till deposits close to the seabed being a natural source of boulders. The two potential restoration areas, as well as the potential information reef area, contain no mussel beds and only minor eelgrass and other flowering plants reducing the negative impact of a restoration project.