Biodiversity patterns among Fucus vesiculosus belts: Factors associated with algal and invertebrate diversity in the Baltic Sea

Macroalgae are foundation species in coastal ecosystems, acting as shelter, breeding grounds or food source for many faunal species. In the northern Baltic Sea, Fucus vesiculosus is the main canopy-forming perennial algae, forming belts on rocky shores, and is often considered one of the most important species for biodiversity. The Baltic Sea is experiencing various human pressures, including eutrophication and increasing effects of climate change, which modify the environment and negatively affect marine biodiversity. To manage the sea areas sustainably, understanding how the different environmental factors structure biodiversity patterns within Fucus belts is of key importance.
In my thesis, I investigated biodiversity patterns of algae and invertebrate fauna within Fucus belts. For algal communities, this was done in relation to Fucus coverage and abiotic factors, namely depth, wave exposure, salinity and Secchi depth. For invertebrate fauna, biodiversity patterns were studied in relation to eutrophication status, exposure to waves and geographic isolation (from 0.5 km to 2 km). The aim was to provide knowledge on the biodiversity patterns in this key habitat over different spatial scales. I used extensive marine inventory data and field samples covering the southern and western parts of the Finnish coastline. The algal and invertebrate diversity in Fucus belts were compared between sites and areas with different environmental conditions.
The findings of this thesis demonstrate that although the species pool of both algal and faunal communities occurring within Fucus remained mainly the same, Fucus coverage, exposure, eutrophication status and isolation all contributed to structuring the biodiversity in Fucus belts. The relationships between environmental factors and both algal and invertebrate communities were scale-dependent. Large-scale differences in eutrophication led to distinct community formations, while local variability in, e.g. exposure or Fucus coverage, modulated the relative proportions of each taxon within these communities.
I showed a significant association between Fucus coverage and the diversity of the algal community. The effect was non-linear, with the highest algal diversity observed at intermediate Fucus coverage values. The taxonomic richness of algae was correlated with abiotic factors, and the community composition differed between sea areas. The local differences in Fucus coverage were associated with the relative coverage of the algal taxa.
The taxonomic and functional diversity of the invertebrate community within Fucus belts were associated with differences in eutrophication and wave exposure. Areas with moderate water quality hosted higher taxonomic diversity, while areas with the best water quality studied supported a higher functional diversity. In the most eutrophied areas, the invertebrate community of Fucus belts had a different taxonomic composition and showed more opportunistic functional traits. Lastly, a key finding was that geographic isolation had a significant relationship with invertebrate abundances, with lower total abundances in remote Fucus belts. The differences were taxa-specific rather than a uniform trend across all taxa.
This thesis provides detailed knowledge of the spatial structure of marine coastal biodiversity within one of the key habitats of the northern Baltic Sea. The thesis also highlights the importance of using multiple biodiversity metrics, as I detected differences in evenness or functional diversity, which would have been missed by using only taxonomic richness. My results contribute to marine ecosystem-based management in the northern Baltic Sea by documenting how changes in environmental factors influence benthic biodiversity.