Sedimentation reduces recruitment success of Fucus vesiculosus in the Baltic Sea

During the last few decades, Fucus vesiculosus has decreased throughout the Baltic Sea, where it is the dominant canopy-forming macroalga of the sublittoral zone. Concurrently, the organic sedimentation originating from primary production has increased substantially due to a large-scale eutrophication. We conducted a field experiment to investigate the effect of sedimentation on the recruitment success of F. vesiculosus. We manipulated the depositional environment during the reproductive period of Fucus by regularly removing accumulated sediment on artificial substrate in the lower part of the depth distribution of the Fucus vegetation. The natural sediment load significantly reduced the density of F. vesiculosus juveniles on the artificial substrate, compared with the sediment removal treatment. It was mainly juveniles smaller than 5 mm that were affected. Our results also showed that F. vesiculosus has the potential to recruit below the lower depth limit of adult Fucus plants if clean surfaces are present. Although it has frequently been suggested that small-scale variation in the sediment load has a significant influence on the establishment of F. vesiculosus in field conditions, this paper presents the first experimental evidence for this. We conclude that the increased organic sedimentation in the Baltic Sea may have a limiting effect on the abundance and distribution of F. vesiculosus.