Major Changes in Macroalgae Community Composition Affectthe Food and Habitat Preference ofIdotea baltica

Abstract

Followed by the massive increase in the biomass of Pilayella littoralis, Fucus vesiculosus has almost disappeared in Koiguste Bay, north-eastern Baltic Sea during 1995‐1998. Consequently, Idotea baltica, which previously fed on Fucus, switched to new food items ‐ Furcellaria lumbricalis and Pilayella littoralis. Idotea grazed more Pilayella than Furcellaria whereas Furcellaria was highly preferred as a habitat. Maximum 2.2% and 4.7% of the production of Furcellaria and Pilayella were removed by grazing. We conclude that the grazing pressure by Idotea is not sufficient to control the outbreak of Pilayella. However, Idotea has an advantageous effect on the persistence of Furcellaria by preferentially removing fast growing epiphyte Pilayella.