Description: Aim: Numerous regions worldwide are highly impacted by anthropogenic activities and globalization, with climate change and species introductions being among the greatest stressors to biodiversity and ecosystems. A main donor region of non‐indigenous species (NIS) for numerous European water bodies, as well as in the North American Great Lakes is the Ponto‐Caspian region (i.e., Black, Azov and Caspian Seas), with some of those species having significant impact on local communities and ecosystem functioning. Location: Northern European, Ponto‐Caspian and North American regions. Methods: To determine environmental tolerance of native species and related NIS under current and future global warming scenarios of the Baltic Sea, we conducted common garden experiments to test temperature tolerance of three euryhaline gammarid species: one Baltic (Gammarus oceanicus), one Ponto‐Caspian (Pontogammarus maeoticus) and one North American species (Gammarus tigrinus) in two different salinities. Results: Our results determined that mortality of P. maeoticus in all temperature treatments (i.e., increased, control, and decreased) at the end of both experiments (i.e., conducted in salinities of 10 and 16 g/kg) was lower when compared to mortality of G. oceanicus and (c) G. tigrinus. The highest mortality was observed for G. oceanicus, reaching 100% in both experiments in the increased temperature treatment. Main conclusions: Due to the high environmental tolerance of the Ponto‐Caspian species tested in this study, as well as the fact that Ponto‐Caspian species evolved in environmentally variable habitats and currently inhabit warmer waters than species from North America and Northern Europe, we suggest that species from the Ponto‐Caspian region may benefit from global warming when invading new areas. Those new invasions may, in the best case scenario, increase biodiversity of the Baltic Sea. However, if notorious invaders arrive, they may have a significant impact on local communities and ecosystem functioning.

Description: One of the most dominant concepts in invasion ecology is the stage-based invasion model, consisting of transport, introduction, establishment and spread. Many species fail at one of the stages, with propagule pressure (i.e. number of introduced individuals) identified as a principal factor affecting establishment success. Population characteristics such as phenotypic plasticity and beneficial life history traits may facilitate successful transition of species through different stages of the process; however, studies on the latter are not so common and most of those studies focus on terrestrial taxa. In this study, we hypothesized seven life history traits that may be beneficial for invasion success of aquatic species, and determined those traits for established non-indigenous species (NIS) in the North and Baltic Seas (i.e. marine environment) and Great Lakes-St. Lawrence River regions (i.e. freshwater environment). This is the first study that examined certain life history traits of all NIS established in particular regions, as well as compared those traits between marine and freshwater habitats. Our study determined some differences in life history traits between NIS in the marine and freshwater habitats. Those differences were connected to different taxonomic groups that were dominant NIS in these two types of habitats. Furthermore, species originating from different donor regions had also different life history traits. The majority of NIS in both regions were r-strategists. There was a significantly higher number of NIS that were able to reproduce both asexually and sexually and to produce dormant stages in the freshwater than in marine habitat. Finally, as r-strategy, asexual reproduction and dormancy were dominant traits of NIS in the freshwater habitat, freshwater ecosystems may be under greater invasion risk than marine ones, as those traits reduce both demographic and environmental stochasticity during the invasion process.

Description: Due to rising atmospheric carbon dioxide concentrations, the oceans take up more C02 resulting in a decrease of seawater pH. This effect is called ocean acidification. Already today, low pH values can be encountered in Kiel Fjord during summer, when COrenriched bottom water is upwelled to the surface. In the present study, recently settled Mytilus edulis were grown for 7 weeks under experimental conditions. The effect of 4 pC02 treatments (control=380, 1120, 2400, 4000 μatm) in combination with 3 different feeding levels (low, medium, high) of Rhodomonas sp. cells on mussel growth was measured. Low food supply had a larger negative impact on shell length growth than elevated pC02 (hypercapnia), while high nutritional supply could compensate a negative effect of hypercapnia. In the 4000 μatm group, mussels reached a mean length of about 1301 ± 75 μm and 3380 ± 677 μm under low and high feeding conditions, respectively. No differences in the length-dry mass ratio were detected, but high pC02 (4000 μatm) incubated shells contained significantly more organic and less calcium carbonate. The organic content increased to 13-15.6 % compared to 11.5-13 % of shell dry mass in the control treatment. Since the organic part of the shell and the body somatic tissue make up similar fractions of total organic mass, shell formation was associated with high energetic costs. 43-84 % of the energy uptake in the low and 33-46 % in the high feeding group accounted for energy that was used for growth processes (somatic and shell). Mussels of the intermediate feeding group used 23-63% of the energy input for growth, with lowest values under highest hypercapnia. The remaining energy fraction was used in respiration and excretion. These parameters were also measured and displayed high fluctuations, which might be due to methodical problems. The results of the study support the view that Mytilus edulis populations can cope with the near future predicted ocean acidification when enough food is available.

Description: Highlights: • Effect of sea freshening on keystone predator impact, Asterias rubens, assessed. • Three ecologically relevant salinity treatments (18, 15, 12ppt). • Zero consumption occurred at the lowest, “future” treatment. • Consumption found to mirror larval recruitment results. • Likely implications for the structuring and functioning of ecological communities. Abstract: Predicting the myriad effects of climate change on ecological communities is a major challenge for scientists, and to date relatively few studies have focused on the effects of sea freshening on species interactions. In particular, changes in keystone species predatory effects could be pervasive. Here, we assess the consequences of decreasing salinity on the ecological impact exerted by a keystone predatory sea star, Asterias rubens. We quantified sea star functional responses (FRs; per capita predation as a function of prey density) under decreasing salinity treatments aligned with climate change projections (18ppt, 15ppt, 12ppt). Furthermore, we combined FRs with larval recruitment estimates, i.e. ecological “Impact Potential”, to act as an ecological indicator of predator population-level responses under this environmental change. Attack and maximum feeding rates of sea stars were reduced by decreasing salinities, with no instances of predation found at 12ppt. Given that decreasing salinities also reduced larval sea star recruitment, the overall Impact Potential of this keystone predator species was lessened by decreased salinity. Sea freshening projections by the end of this century could thus drive significant decreases in the effects of this keystone predator, with serious implications for the structuring and functioning of ecological communities.