Abstract
In the course of the ongoing global intensification and diversification of human pressures, the study of variation patterns of biological traits along environmental gradients can provide relevant information on the performance of species under shifting conditions. The pronounced salinity gradient, co‐occurrence of multiple stressors, and accelerated rates of change make the Baltic Sea and its transition to North Sea a suitable region for this type of study. Focusing on the bladderwrack Fucus vesiculosus, one of the main foundation species on hard‐bottoms of the Baltic Sea, we analyzed the phenotypic variation among populations occurring along 2,000 km of coasts subjected to salinities from 4 to >30 and a variety of other stressors. Morphological and biochemical traits, including palatability for grazers, were recorded at 20 stations along the Baltic Sea and four stations in the North Sea. We evaluated in a common modeling framework the relative contribution of multiple environmental drivers to the observed trait patterns. Salinity was the main and, in some cases, the only environmental driver of the geographic trait variation in F. vesiculosus. The decrease in salinity from North Sea to Baltic Sea stations was accompanied by a decline in thallus size, photosynthetic pigments, and energy storage compounds, and affected the interaction of the alga with herbivores and epibiota. For some traits, drivers that vary locally such as wave exposure, light availability or nutrient enrichment were also important. The strong genetic population structure in this macroalgae might play a role in the generation and maintenance of phenotypic patterns across geographic scales. In light of our results, the desalination process projected for the Baltic Sea could have detrimental impacts on F. vesiculosus in areas close to its tolerance limit, affecting ecosystem functions such as habitat formation, primary production, and food supply.
Highlights
In the course of global change, the shift in environmental conditions is faster than ever and the distributional ranges of species are or will be on the move (Nicastro et al, 2013; Wernberg et al, 2011)—unless populations can adapt fast enough
In a Baltic‐wide assessment, we investigated the variation of multiple traits including morphology, light‐harvesting pigments, chemical composition, and interactions of F. vesiculosus with con‐ sumers and epibiota along the Baltic Sea‐North Sea salinity gra‐ dient
Our study presents an integrative overview of the geographic vari‐ ation of traits in F. vesiculosus and the putative underlying drivers
Summary
In the course of global change, the shift in environmental conditions is faster than ever and the distributional ranges of species are or will be on the move (Nicastro et al, 2013; Wernberg et al, 2011)—unless populations can adapt fast enough. Numerous studies described negative direct and indirect effects of decreasing salinity, nutrient enrichment and increasing wave exposure on the morphology, growth, photosynthesis, palatability, and epibiosis— among others—of F. vesiculosus (e.g., Bergström, Berger, & Kautsky, 2003; Hemmi, Mäkinen, Jormalainen, & Honkanen, 2005; Kalvas & Kautsky, 1993; Kersen, Kotta, Bučas, Kolesova, & Deķere, 2011; Takolander, Leskinen, & Cabeza, 2017) In light of this evidence, we expected that the effects of the Baltic Sea–North Sea salinity gra‐ dient together with other environmental variables of local relevance would result in (a) biochemical and morphological differences among F. vesiculosus populations, and (b) an overall decrease in the perfor‐ mance of the species with decreasing salinity. The effort to relate this variation with environmental conditions within the sampled habitats may provide new insights into the capacity of F. vesiculosus to respond to future changes
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