Reproduction, recruitment and geographical distribution of Fucus serratus L. in the Baltic Sea

This paper describes the large-scale geographical distribution, population structure, and reproductive season of the brown algae Fucus serratus L. in the Baltic proper. We have also experimentally tested fertilisation success and germination of F. serratus zygotes in a salinity gradient with the aim of finding out whether an adaptation to reduced salinity has occurred. Fuc us serratus was found to be belt-forming along the shores of southeastern S weden and scattered populations were found up to the middle of the Baltic p roper. The ratio of non-reproductive (juvenile) plants per reproductive (ad ult) plant decreased significantly from 37 +/- 8 S. E. in the south to 0.4 +/- 0.2 in the north. Fucus serratus was reproductive during October to. No vember on the mainland coast, while the populations on the southeast coast of Oland were reproductive during June to July. Laboratory experiments show ed that successful fertilisation of Baltic F. serratus zygotes decreased dr astically from 86.9 +/- 1.0% S. E. at 9 psu to 5.5 +/- 0.5% S. E. at 6 psu. Fucus serratus zygotes from the western coast of Sweden needed a significa ntly higher salinity for successful fertilisation than the Baltic gametes d id, and at 8 psu, only a few eggs were fertilised (5.5 +/- 0.5 % S. E.). Si nce a relatively high proportion (35%) of the Baltic F. serratus zygotes ge rminated in 7 psu, (the salinity at the northernmost populations) we discus s other factors such as ice-scraping and eutrophication as possible factors limiting the geographical and vertical distribution of the species in the Baltic Sea.