Abstract
Short lived filamentous algae are a major component of the rocky-shore macroalgal vegetation of eutrophic waters in the Baltic Sea. They show considerable variation in abundance both seasonally and from year to year. In this study the seasonal pattern of growth and reproduction is documented in six species to outline their life strategies. Five of the species studied were reproductive in summer or autumn, just after a period of active growth. Pilayella littoralis was the only exception. It grew fast in the spring but reproduction was successful only in winter, when only the basal parts of plants were left. The amount of filamentous algae was very low in winter. The only species which survived the winter period as macroscopic filaments was Ceramium tenuicorne. At least three species, Cladophora glomerata, Dictyosiphonfoeniculaceus and Ectocarpus siliculosus, relied on microscopic stages to survive the unfavourable winter period. Responses of these filamentous species to variations in their physical environment are discussed.