MEPS 289:131-139 (2005)  -  doi:10.3354/meps289131

ABSTRACT: With increasing eutrophication in coastal areas, mass developments of annual filamentous algae, such as Pilayella littoralis and Enteromorpha spp., have increased. Simultaneously, the perennial macroalga Fucus vesiculosus has declined in many areas in the Baltic Sea. To test the effects of P. littoralis on F. vesiculosus we performed a number of laboratory and field studies. The most severe negative effect observed was the reduced number of settled germlings (95%) when P. littoralis was present on the substrate prior to seeding of F. vesiculosus eggs. A low concentration (0.1%) of exudates from P. littoralis reduced the attachment rate of F. vesiculosus during the initial 12 h, while higher concentrations of exudates (5 to 10%) negatively affected germination and rhizoid development. These results could explain for the negative correlations that have been observed between F. vesiculosus and annual filamentous algae in the Baltic Sea. Our field data showed that the biomass of filamentous algae was higher in the summer than in the autumn, suggesting that recruits from the summer-reproducing F. vesiculosus may encounter more competition from P. littoralis and other filamentous algae than do recruits from the autumn-reproducing populations. The summer- and autumn-reproducing F. vesiculosus host similar abundance and biomass of their associated flora and fauna; hence, a potential shift from summer- to autumn-reproducing F. vesiculosus would not change the overall community composition in this ecosystem.

KEY WORDS: Summer-reproducing Fucus vesiculosus · Autumn-reproducing Fucus vesiculosus · Pilayella littoralis · Post-settlement survival · Community composition · Filamentous algae · Attachment · Interference competition · Allelopathy