Description: Defense strength as log effect ratio against microfouling by Bacillus sp., Cytophaga sp. and Vibrio sp. (averaged). Effect size 〉 0 indicates an attractive effect of surface-bond metabolites, effect size 〈 0 an inhibitory effect with the strongest defense at lowest values. Responses are detailed with regards to the four treatments: warm (+°C (delta+5)), acidified (-pH (delta +700µatm pCO2)), warm and acidified (+°C -pH), and ambient during four seasons: spring, summer, autumn, winter (n=3). The data set comprised 4 experimental runs: spring experiment (4.4.-10.6.2013), summer experiment 1 (4.7.-17.9.2013), autumn experiment (11.10-16.12.2013), winter experiment (16.1. - 28.3.2014).

Description: Defense strength as log effect ratio against macrofouling by Mytilus edulis and Amphibalanus imrovisus. Effect size 〉 0 indicate an attractive effect of surface-bond metabolites, effect size 〈 0 an inhibitory effect with strongest defense at lowest values. Responses are displayed in regard to the four treatments: warm (+°C (delta+5)), acidified (-pH (delta +700µatm pCO2)), warm and acidified (+°C -pH), and ambient during four seasons: spring, summer, autumn, winter (n=3) The data set comprised 4 experimental runs: spring experiment (4.4.-10.6.2013), summer experiment 1 (4.7.-17.9.2013), autumn experiment (11.10-16.12.2013), winter experiment (16.1. - 28.3.2014).

Description: Palatability as consumption of F. vesiculosus pellets by Idotea baltica (squares consumed in %). Consumption is displayed in regard to the four treatments: warm (+°C (delta+5)), acidified (-pH (delta +700µatm pCO2)), warm and acidified (+°C -pH), and ambient during four seasons: spring, summer, autumn, winter (n=3). The data set comprised 4 experimental runs: spring experiment (4.4.-10.6.2013), summer experiment 1 (4.7.-17.9.2013), autumn experiment (11.10-16.12.2013), winter experiment (16.1. - 28.3.2014).

Description: Human-induced ocean warming and acidification have received increasing attention over the past decade and are considered to have substantial consequences for a broad range of marine species and their interactions. Understanding how these interactions shift in response to climate change is particularly important with regard to foundation species, such as the brown alga Fucus vesiculosus. This macroalga represents the dominant habitat former on coastal rocky substrata of the Baltic Sea, fulfilling functions essential for the entire benthic community. Its ability to withstand extensive fouling and herbivory regulates the associated community and ecosystem dynamics. This study tested the interactive effects of future warming, acidification, and seasonality on the interactions of a marine macroalga with potential foulers and consumers. F. vesiculosus rockweeds were exposed to different combinations of conditions predicted regionally for the year 2100 (+∆5°C, +∆700 μatm CO2) using multifactorial long-term experiments in novel outdoor benthic mesocosms (“Benthocosms”) over 9–12-week periods in four seasons. Possible shifts in the macroalgal susceptibility to fouling and consumption were tested using consecutive bioassays. Algal susceptibility to fouling and grazing varied substantially among seasons and between treatments. In all seasons, warming predominantly affected anti-fouling and anti-herbivory interactions while acidification had a subtle nonsignificant influence. Interestingly, anti-microfouling activity was highest during winter under warming, while anti-macrofouling and anti-herbivory activities were highest in the summer under warming. These contrasting findings indicate that seasonal changes in anti-fouling and anti-herbivory traits may interact with ocean warming in altering F. vesiculosus community composition in the future.

Description: The plea for using more “realistic,” community‐level, investigations to assess the ecological impacts of global change has recently intensified. Such experiments are typically more complex, longer, more expensive, and harder to interpret than simple organism‐level benchtop experiments. Are they worth the extra effort? Using outdoor mesocosms, we investigated the effects of ocean warming (OW) and acidification (OA), their combination (OAW), and their natural fluctuations on coastal communities of the western Baltic Sea during all four seasons. These communities are dominated by the perennial and canopy‐forming macrophyte Fucus vesiculosus—an important ecosystem engineer Baltic‐wide. We, additionally, assessed the direct response of organisms to temperature and pH in benchtop experiments, and examined how well organism‐level responses can predict community‐level responses to the dominant driver, OW. OW affected the mesocosm communities substantially stronger than acidification. OW provoked structural and functional shifts in the community that differed in strength and direction among seasons. The organism‐level response to OW matched well the community‐level response of a given species only under warm and cold thermal stress, that is, in summer and winter. In other seasons, shifts in biotic interactions masked the direct OW effects. The combination of direct OW effects and OW‐driven shifts of biotic interactions is likely to jeopardize the future of the habitat‐forming macroalga F. vesiculosus in the Baltic Sea. Furthermore, we conclude that seasonal mesocosm experiments are essential for our understanding of global change impact because they take into account the important fluctuations of abiotic and biotic pressures.

Description: Seasonal mesocosm community experiments: Four successive runs of ca 10 weeks in 12 outdoor mesocosms (ca 1400 L) with flow-through and a orthogonally crossed warming ("ambient" temperature vs warming by 5°C = "OW") and acidification ("ambient" CO2 vs increase by 700 µatm = "OA" and combined warming and acidification = "OWA" ) treatment. The tanks were started with the same community composition (Fucus vesiculosus; mesograzers Idotea spp., Gammarus spp., Littorina littorea; seastar Asteria rubens, filter feeders Balanus improvisus, Mytilus edulis); details in Wahl et al. 2015. Seasonal responses to the various treatments were (a) mean daily relative Fucus length growth of thallus tips (%), (b) relative Mytilus shell length growth (%), (c) mean daily relative growth of Balanus basal plate, (d-f) the relative population size changes of the three mesograzer species expressed as the log of final divided by initial abundances, (g) the relative survival of Asterias (%).

Description: Response of Fucus photosynthesis (oxygen production) to light intensity (PI-curve) assessed at 19.5°C and at 2 pH levels (8.2 and 7.75) for an irradiation range between 0 and 800 µmol cm-2 sec-1. The oxygen gradient across the diffusuve boundary layer was measured using microsensors and used to calculate the oxygen flux and deduce the oxygen production (nmol cm-2 sec-1).

Description: Fucus gross (GPP) and net (NPP) primary production, respiration (R) and metabolic balance (GPP/2R) responses to temperature in thermobaths: Temperature levels 5, 10, 15, 20, 22, 25°C; period Oct & Nov 2016; replication was 3.

Description: Fucus oxygen production (NPP) under different levels of acidification: pCO2 400, 900, 1400, 2400, 3900 µatm assessed by incubation; period Apr 2015; replication 6.

Description: Fucus reproduction in response to temperature assessed as % change in conceptacles over the experimental duration of 5 weeks: Temperature levels 0, 5,10,15,20, 25°C; period Apr 2013, replucation 4; details and data in Graiff et al 2017.