North Sea reefs : benthic biodiversity of artificial and rocky reefs in the southern North Sea

The objective of the research presented in this thesis was to understand the patterns of benthic biodiversity on reefs in the North Sea. To gain this understanding, I studied which species are present on natural and artificial reefs, what environmental and biotic variables influence the presence and absence of a selection of these species and one of the possible pathways by which these species may colonise the reefs studied; the stepping stone effect. The first goal was to increase the available knowledge on which species are present at reefs. The second goal was to understand the patterns observed in the variation of species at these reefs. The third goal was to evaluate whether Mytilus edulis utilises offshore artificial structures as stepping stone to colonise very far offshore locations. The final goal was to assess the impact artificial reefs have on the benthic biodiversity of the North Sea. To attain these goals, natural and artificial reefs were sampled using diver operated airlift samplers and box corers. To evaluate the effects generalised linear and additive models were created. When available, additional data from other sources were also used. The following conclusions are drawn: Lanice conchilega is an ecosystem engineer creating intermediate sand-reef systems. When rocky reefs are present on a sandy bottom, local biodiversity is doubled. Distributions of the native Caprella linearis and the invasive Caprella mutica showed a significant difference, demonstrating that C. linearis’ habitat preference does not fully overlap with that of C. mutica. Thus, the native and alien Caprellids are likely to be able to co-exist in the North Sea. M. edulis presence increases habitat heterogeneity, which increases species richness. Mytilus edulis uses offshore structures as stepping stones to colonise locations in the North Sea that cannot be reached in a single generation. Depth, location effect and habitat type influence the species composition on North Sea reefs most strongly. The relation between depth and species richness on artificial reefs is non-linear, with a maximum at intermediate depths. Although substrates with mixed surface orientation (i.a. rocks) hold the most species rich communities, biodiversity is also strongly influenced by M. edulis and Psammechinus miliaris. When artificial reefs are to be colonised by communities that are similar to natural reefs, their structures should resemble natural reefs as much as possible.