Baltic algae

This special issue is dedicated to Baltic algae. Which to many readers may raise the question “Baltic algae”? Issued in 2018? Isn’t the Baltic the region in which Linnaeus lived? So everything has been known for centuries? Indeed, recent taxonomy for most of the algal groups starts with Linnaeus’s classification and many taxa still bear the epithets given by him. But the Baltic Sea is also a geologically young system with ongoing immigration processes – a “Sea of invaders” is the catchphrase ultimately coined by the perspective article of Leppäkoski et al. (2002). And immigrating into a system characterized by steep gradients of climatic, salinity and eutrophication conditions, species are shaped by strong habitat filters until their acclimation potential is either exhausted or evolution begins to dominate the process, leading to speciation.

Evolution is indeed an ongoing process over billions of years and the basic principles were discovered already more than 150 years ago by Wallace and Darwin (Wallace 1858, Darwin 1859). Examples of recent speciation are rare and whether or not we are able to experimentally evolve new species by the principles discovered by Wallace and Darwin still depends on the species concept applied. So, irrespective of its long history in nature as well as in science, the terms “species” and “speciation” are still raising debates and can turn distinguished scientists into emotional fighters for concepts – which is always big theatre, as probably first seen on stage in 1860 as the still famous “Huxley-Wilberforce debate”.

A number of Linnaean algal species are described by type material from the Baltic Sea – which means that the type represents morphologies from the edge of their distribution ranges. And in phycology we are still botanists, who agree to a species concept based on phenetic characters. By developing molecular tools we really could try to determine the vertical components, developing it to a phylogenetic one and leaving ultimately Linnaeus’s ground. We have not only accepted species with sexual dimorphisms, but also heteromorphic life-cycles. But phenotypic plasticity resulting from acclimation is a completely different issue and, for immigrants with a gene pool shaped by a series of strong habitat filters, the results of acclimation experiments, crossing attempts and molecular tools often just raise confusion. Where, along a phenetic and often also genetic continuum to draw the line between species?

It’s fascinating how many answers will be given immediately before the unavoidable debate starts. And there is still no commonly agreed answer yet; at the very best, the opponents will come to a friendly agreement that they need more material and “objective” tools. So, e.g. friendly physiologists became involved too. And field phycologists were getting annoyed by perpetual changes of taxonomic rank – and especially by the disappearance of species – due to the combination of (ecologically meaningful) morphotypes.

And this is what this special issue is about. The Baltic Sea is kind of an open laboratory for acclimation and adaptation processes and, consequently, it is studied by the many institutions located at its shores. This became very obvious in 2017 when the world community of phycologists met in Szczecin, Poland for the 11th International Phycology Congress. After a well-attended special session on “Baltic algae”, covering two slots of brilliant presentations, the idea of presenting its highlights to a broader community by a special issue of Botanica Marina appeared.

The contributions ranged from cyanobacteria to macroalgae, and from molecular methods to in situ experiments related to climate change. In this special issue, we present seven very interesting research papers on different aspects of primary producers in the Baltic Sea. The members of the genus Fucus act as key species in the Baltic Sea, so it is of no surprise that four papers deal with different species of Fucus: Fucus vesiculosus Linnaeus, the relatively new species Fucus radicans L. Bergström & L. Kautsky, and the introduced species Fucus evanescens C. Agardh. Asexual reproduction in F. radicans was the focus of the contribution by Schagerström and Salo, where temperature and light were shown to have an interactive effect on the re-attachment of fragments, further suggesting that the metabolic balance of the fragment was a crucial feature for a successful re-attachment. Effects of substratum manipulation and algal exudates on recruitment and survival of F. vesiculosus were studied by Kautsky et al. In the same contribution, the authors address transplantation studies where epiphytic load, light, grazing and type of substratum were some of the factors important for a successful restoration of F. vesiculosus. At least 4–5 years were needed to reach reproductive age for F. vesiculosus in the Baltic Sea. Gradual adaptation of the introduced species F. evanescens to low salinity was addressed by Romoth et al. where it seemed that the vegetative stage was not limited by salinity, indicating that this species has the potential to spread further into the Baltic Sea. In the artificial Kiel Canal, Steinhagen et al. explored the potential of the canal as, for example, a stepping stone for introduced species. F. evanescens was detected nearly exclusively inside the canal, while F. vesiculosus dominated outside the sluice gates. Also, three Ulvales species, Ulva linza Linnaeus,Ulva intestinalis Linnaeus and an unknown and possibly introduced species of the genus Blidingia were found. Species distribution and branching patterns (U. intestinalis) were affected by salinity. Within the canal, specific eutrophication status affected thallus size. Another introduced species, the green alga Kornmannia leptoderma (Kjellman) Bliding, raised the interest of Weinberger et al. The authors studied the recent expansion of the species and speculate that maybe the specimens found could represent a newly introduced cryptic species, expanding its temperature range. A different group of green algae, the charophytes, were in focus when Nowak and Schubert studied their genetic variability in the Baltic Sea area. But the Baltic Sea is not only inhabited by macroscopic algae. A microscopic spring bloom community inoculated with filamentous cyanobacteria was studied by Olofsson et al. to reveal if the cyanobacteria could have a competitive advantage over other phytoplankton groups in a future warmer Baltic Proper with increased carbon dioxide concentration. The results indicated a highly efficient resistance towards short-term (12 days) changes in abiotic factors by the natural Baltic Sea spring bloom community.

The guest editors hope that the reader will find the contents interesting and inspiring.