Relationship between Fucus coverage and algal diversity in the northern Baltic Sea

The canopy-forming brown macroalgae Fucus vesiculosus and Fucus radicans (Fucus) are the largest habitat-building species on rocky shores of the northern Baltic Sea. While their role on the diversity of invertebrate fauna is well documented, there is little information on their importance for the diversity of associated macroalgal species. We used comprehensive mapping data from the Finnish marine area (divided into four sea areas) to test the relationship between Fucus coverage and algal biodiversity (number of taxonomic units, Shannon-Wiener diversity index H′ and Pielou's evenness J'). To account for environmental variation, depth, wave exposure, Secchi depth and salinity were included as covariates. Examination of the algal communities highlighted a similar pool of taxonomic units across sea areas in shallow (0–4 m) rocky shores, with the two filamentous algal species Pylaiella littoralis and Ectocarpus siliculosus combined as one unit being the most common taxon. The Shannon-Wiener diversity index was highest in sites with Fucus coverage between 25% and 75%, while evenness increased with Fucus coverage. We did not detect any effects of Fucus coverage on the number of taxonomic units, which was rather affected by environmental parameters (depth, Secchi depth and salinity). The results show that Fucus, depending on its coverage, both competes with and facilitates the macroalgal community. Based on extensive data, we show how Fucus coverage relates to the diversity of the surrounding algal communities, further improving our understanding of factors that control biodiversity on shallow rocky shores in the northern Baltic Sea.