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Many of the marine species that were introduced to the Baltic Sea during the Littorina stage (c. 8500–3000 years BP), e.g.Fucus vesiculosusandF. serratus, have adapted to the present low salinity. These marine species have gone from marine conditions into lower salinity environments. In this paper we ask why the recently discovered endemic brown algaFucus radicansshows the opposite pattern.Fucus radicansis only present in the northern parts of the Baltic Sea, the low salinity Bothnian Sea (4–6 psu). Potentially, the fitness ofF. radicansmight be reduced in higher salinities if it is better adapted to brackish conditions. We hypothesize, however, that the southern distribution limit ofF. radicansis set by biotic factors, e.g. competition withF. vesiculosusand higher grazing pressure byIdotea balthicaand not by salinity. Our results show that the reproductive output ofF. radicansis limited by low salinity (4 psu) but increases in higher salinities. However, the southern distribution limit, i.e. the northern Baltic Proper, is regulated by biotic factors, where the additive effects from shading by tallerF. vesiculosusthalli and grazing onF. radicansby the isopodI. balthicalimit the biomass production ofF. radicans. We suggest thatF. radicansstill maintains marine traits due to its ability to propagate clonally and is restricted to the Bothnian Sea by interactions withF. vesiculosusandI. balthica. We also propose that increased precipitation due to climate change might affect the northern range limit and that the distribution ofF. radicanscould be expected to shift further south into the Baltic Proper. [ABSTRACT FROM AUTHOR]

Reproduction of attached large brown algae is known to occur only by sexual zygotes. Using microsatellites we show evolution of asexual reproduction in the bladder wrack promoting population persistence in the brackish water Baltic Sea (<6 psu). Here a dwarf morph ofFucus vesiculosusis dominated by a single clone but clonal reproduction is also present in the common form of the species. We describe a possible mechanism for vegetative reproduction of attached algae, and conclude that clonality plays an important role in persistence and dispersal of these marginal populations, in which sexual reproduction is impaired by low salinity. [ABSTRACT FROM AUTHOR]

The reproductive state of Ceramium tenuicorne (Kütz.) Wærn was studied in a salinity and latitudinal gradient near its inner distribution limit in the brackish Baltic Sea, and in laboratory growth experiments on the gametophytes and tetrasporophytes.Along the gradient,a strong reduction in sexual reproduction and an increased tetrasporophyte dominance were observed. The observed changes may not only be explained by salinity constraints. Growth occurred between 1 and 24 PSU and the optimum range for growth corresponded well to the salinity regime of the area studied. Field observations showed that sexual reproduction is at least occasionally possible in salinities around 4 PSU. It is likely that severed climatic conditions constrain sexual reproduction towards the north,but C.tenuicorne remains an ecologically dominant species in large parts of its distributional area.Viable populations are maintained essentially by vegetative reproduction,such as fragmentation and rhizoidal re­attachment.Observations on vegetative propagules were sparse,but these may serve as an important physical escape from temporarily unfavourable conditions in areas where sexual reproduction is unreliable. [ABSTRACT FROM AUTHOR]

Geographical morphological variations in Fucus vesiculosus populations were studied along the coast of Iceland. Principal component analysis (PCA) based on 11 morphological characters clustered the 26 sampling sites into four different morphological types as follows: (1) a morphological type found along the S-SW-W coast; (2) an intermediate form comprising only two populations in the NW; (3) a type found along the N-NE-E-SE coast; and (4) a type found independent of geographical area, in estuaries and at sites influenced by large freshwater outflows. Populations influenced by low salinity had significantly (P ≤ 0·001) shorter thalli, a shorter distance from the holdfast to the oldest dichotomy, smaller fronds, narrower stipes and midrib width compared to the morphology of all other more saline populations. No significant difference in frond width was found between the S-SW-W and the N-NE-E-SE populations. However, significant (P ≤ 0·001) morphological differences between them were observed, the former having shorter thalli, a greater distance from the holdfast to the oldest dichotomy, narrower stipes and smaller midrib width compared to the latter. [ABSTRACT FROM PUBLISHER]

Abstract Background Most species of brown macroalgae recruit exclusively sexually. However, Fucus radicans, a dominant species in the northern Baltic Sea, recruits new attached thalli both sexually and asexually. The level of asexual recruitment varies among populations from complete sexual recruitment to almost (> 90%) monoclonal populations. If phenotypic traits have substantial inherited variation, low levels of sexual activity will decrease population variation in these traits, which may affect function and resilience of the species. We assessed the level of inherited variation in nine phenotypic traits by comparing variation within and among three monoclonal groups and one group of unique multilocus genotypes (MLGs) sampled in the wild. Results Of the nine phenotypic traits, recovery after freezing, recovery after desiccation, and phlorotannin content showed substantial inherited variation, that is, phenotypic variation in these traits were to a large extend genetically determined. In contrast, variation in six other phenotypic traits (growth rate, palatability to isopod grazers, thallus width, distance between dichotomies, water content after desiccation and photochemical yield under ambient conditions) did not show significant signals of genetic variation at the power of analyses used in the study. Averaged over all nine traits, phenotypic variation within monoclonal groups was only 68% of the variation within the group of different MLGs showing that genotype diversity does affect the overall level of phenotypic variation in this species. Conclusions Our result indicates that, in general, phenotypic diversity in populations of Fucus radicans increases with increased multilocus genotype (MLG) diversity, but effects are specific for individual traits. In the light of Fucus radicans being a foundation species of the northern Baltic Sea, we propose that increased MLG diversity (leading to increased trait variation) will promote ecosystem function and resilience in areas where F. radicans is common, but this suggestion needs experimental support.