Factors affecting formation of adventitious branches in the seaweeds Fucus vesiculosus and F. radicans

In the brackish Baltic Sea, shedding of adventitiousbranches is central to asexualrecruitment of new thalli in the brown algae Fucus vesiculosusand F. radicans. To test which factors influence the formation of adventitiousbranches in brackish and in more marine conditions, we sampled 29 Fucussites in the Baltic Sea (salinity 3–11) and 18 sites from the Danish straits, Kattegat, Skagerrak, and the North Sea (salinity 15–35). Separately for each area, we used structural equation modelling to determine which of eight predictor factors (phosphate, nitrate, chlorophyll-a (as a proxy for turbidity), temperature, salinity, oxygen, grazing pressure, and thallusarea) best explained observed numbers of adventitiousbranches. In more marine waters, high yearly average values of phosphate, salinity and turbidity had positive effects on the formation of adventitiousbranches. In brackish-waters, however, high numbers of adventitiousbranches were found in areas with low yearly average values of temperature, salinity and oxygen. Grazing intensity had no significant effect in either of the two study areas, contrasting findings from studies in other areas. In areas with both sexually and asexuallyreproducing Fucusindividuals, clones had on average more adventitiousbranches than unique genotypes, although there was strong variation among clonal lineages. This study is the first to investigate multiple potential drivers of formation of adventitiousbranches in natural populations of Fucus. Our results suggest that several different factors synergistically and antagonistically affect the growth of adventitiousbranches in a complex way, and that the same factor (salinity) can have opposing effects in different areas.