The effects of a newly created mussel bed and hydrodynamic conditions on the biodiversity and functioning of macrobenthic communities

إنجليزي. In the Belgian Part of the North Sea, the Coastbusters projects aim to develop nature-based coastal protection by favouring mussel bed establishment through dropper lines in situ installation. This was tested in a first finished project, and a mussel bed was formed during summer-autumn. In the second ongoing project, different hydrodynamic conditions were tested (sheltered area versus exposed area), as the successful development of this type of biogenic reef is dependent on the environmental conditions. Mussel beds are known to influence the structure and functioning of underlying macrofaunal communities. Therefore, the aim of this thesis was to determine the effects of the newly created mussel bed and differences in hydrodynamic conditions on the biodiversity and functioning of macrobenthic communities. To detect these effects, a BACI (Before/After Control/Impact) approach was followed. Macrobenthic community structure was characterized by looking at density, biomass, and diversity. Functioning was assessed using secondary production estimates, biological trait analysis and functional diversity indices. Linear mixed models were used to determine statistical influences of the mussel bed and hydrodynamic conditions on those parameters. The mussel bed had no significant effect on the secondary production of associated macrofauna but minor shifts in lifespan, feeding behaviour and development mode were indicated by the biological trait analysis. The functional diversity indices suggested a better utilization of the different niches and higher productivity in the mussel bed compared to bare sediments but also lower traits differentiation, probably due to the higher abundance of oligochaetes in the mussel bed. As changes in associated communities are triggered by changes in mussel density, patch size and bed thickness (Koivisto et al., 2011), the results of this thesis cannot be extended to a year-round persistent mussel bed. Further studies are needed to understand the influence of newly created mussel beds on structural and functional characteristics of associated benthic communities. It is also important to encompass other components of biodiversity, such as epifauna and fish, to have a global overview on the impacts of the mussel bed on the surrounding biodiversity. The baseline characterization of the two Coastbusters 2.0 areas under different hydrodynamic conditions showed higher values of density, species richness and biomass in the sheltered area and high dissimilarity in community composition between the two areas. Important differences in feeding, mobility, development mode and bioturbation were found between the two hydrodynamic conditions in response to different flow and food flux conditions. The functional diversity indices revealed a better resource-use efficiency in the sheltered area, indicating a more valuable ecosystem functioning. Although no mussel bed developed in 2020, this baseline characterisation assessed the potential differences between sites. No differences between control and impacts sites could be found for any of the structural and functional characteristics investigated previously. These results are reassuring because the absence of initial differences between the two types of sites will make it possible to better determine the influence of the mussel bed on the benthos in comparison with a control site not influenced by the latter.

Coastbusters