Description: Marine biological invasions can have alarming and devastating ecological and economic, impacts on biodiversity, ecosystem balance, fisheries and tourism. Due to global change, the number of bioinvasions has severely increased over the last decades. Although, range expansion and the invasion of non-native habitats by marine and terrestrial species have occurred naturally since the existence of life, the rate of human-mediated translocations of species as a consequence of expanded worldwide trade, shipping and aquaculture activities has, however, never been larger and will further massively increase in the near future. For the marine environment, it is estimated that, only in ballast water of ships, approximately 10,000 species per day are transported around the globe. Most of these potential invaders fail, but a few survive, establish and spread in their new environment. To control and prevent invasions, it is of high importance to understand the mechanisms and traits determining the success of invasive species. One of the crucial factors, especially in the early stages of an invasion process, is stress tolerance i.e. the ability to maintain fitness under adverse conditions. Studies comparing the performance of invasive species in their invasive range to native species showed that the invasive species performed better in the majority of cases. In this case, stress tolerance can be considered a species-specific trait. Additionally, some studies comparing the performance of native and invasive populations of the same species were conducted with terrestrial plants and revealed higher competiveness in the invasive plant populations. The marine environment however seems to be nearly unexplored with regard to worldwide intraspecific comparisons of stress tolerance. The aim of this study is to compare the performance of native and invasive populations of successful marine invaders under different abiotic stressors. Laboratory experiments with three marine benthic cosmopolitan invaders in four different biogeographic regions were conducted to detect possible differences in stress tolerance between native and invasive populations. Native and invasive populations of adults and juvenile Pacific oysters, Crassostrea gigas, were exposed to hypoxia and heat stress. Juveniles were additionally exposed to hyposalinity stress. Furthermore, a native and invasive population of the vase tunicate, Ciona intestinalis, were exposed to hypoxia and heat stress and native as well as invasive populations of the green alga, Codium fragile, were exposed to heat and hyposalinity stress. Survival (C. gigas, C. intestinalis) or the 6 maximum quantum yield (C. fragile) under stressful conditions was measured and the native and invasive populations of each species were compared. A second experiment attempted to detect effects of stress history. It was tested whether previously stressed groups of Crassostrea gigas or Ciona intestinalis were more stress tolerant towards a subsequent stressor of another quality than previously unstressed groups. The stress history experiment revealed no significant differences between previously stressed and previously unstressed individuals within both species. Invasive populations of adult Crassostrea gigas from the German Wadden Sea as well as juveniles from Guernsey were significantly more tolerant to hypoxia stress (1 mg l-1) than the native population from Japan. An invasive Japanese population of Ciona intestinalis was significantly more tolerant towards hypoxia stress when compared to the native population from Wales. The same pattern was found for Codium fragile comparing invasive (Chile) and native populations (Japan) under heat stress while under hyposalinity stress an inverse pattern was revealed. The native population (Japan) of juvenile of C. gigas was also more tolerant towards heat stress than its invasive population from Guernsey. Insignificant bidirectional differences in stress tolerance were found between the native (Japan) and invasive (USA) populations of juvenile C. gigas in hyposaline and hypoxic conditions. This study shows, in four out of six significant comparisons, higher stress tolerance is found in invasive populations compared to native populations of the same species. Hence, stress tolerance should not only be considered species-specific but also a population-specific trait. These population-specific differences can be either a consequence of selection of stress-tolerant genotypes during transport or of interactions with the biotic and abiotic environment in the target area after introduction.

Description: Increasing human activities and CO2 release to the atmosphere cause global ocean warming and acidification, as well as local oxygen depletion and eutrophication. Laboratory experiments on single species and single stressors show variable responses within and between species and different combinations of stressors can have synergistic, additive or antagonistic effects. Thus, only large-scale multi-species and multi-stressor experiments can predict future community responses. For this purpose, a new benthic mesocosm facility was build at the AWI Wadden Sea Station - Sylt. Each of 12 1,800 l benthocosms serves as independent experimental unit with current and tide simulations as well as multi-parameter control systems to simulate multi-factorial future climate change scenarios such as warming, ocean acidification, eutrophication and extreme weather events. Temperature, pH, oxygen, and salinity are monitored continuously and logged separately for each benthocosm. Additional variables such as total alkalinity, light availability, Chl a, nutrients, DOC or POC are monitored manually. We are investigating the responses of North Sea F. vesiculosus, and its associated community to warming and ocean acidification as well as to the combination of these stressors. Fucus communities were to date incubated in two experimental approaches for 2-3 months under the interactive effects of warming and elevated pCO2 in autumn 2013 and spring 2014. Warming reduced the overall biomass of F. vesiculosus in the system but favoured the reproduction and survival of herbivores (mainly Gammarus sp.). In contrast, elevated pCO2 increased Fucus growth, while the interactive effects of warming and acidification had antagonistic effects leading to no difference compared to the ambient treatment. We furthermore examine physiological responses of F. vesiculosus and its epibionts as well as its consumers. Compared to North Sea ecosystem responses, experiments conducted within the benthocosms facilities at the GEOMAR - Kiel with Baltic Sea communities, show a controversial outcome, where CO2 related impacts on the macrophyte community were much weaker, as were the interactive impacts of warming and elevated pCO2. This leads to the assumption that ocean acidification seems to have a higher impact on the Wadden Sea ecosystem as can be predicted for Western Baltic Sea communities, which already today experience natural high fluctuating pCO2.