ALBERT

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: The dataset includes net and gross primary production, respiration and epiphyte load on the brown algae Fucus vesiculosus under a temperature gradient (5-25 ºC). Under the same temperature gradient, we present physiological responses of Idotea balthica, i.e. respiration, feces production, growth and carbon consumption.

Description: 13 response variable have been measured for Fucus vesiculosus and Zostera marina. Year: 2015 Where: Kiel Outdoor Benthocosm Treatments: - Co (0HW) = ambient treatment with no heatwaves - 1HW = one summer heatwave - 3HWs = three heatwaves, 2 spring/early summer heatwaves After 3HW means end of the experiment.

Description: This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer (December 2015 - August 2016) in the Western Baltic Sea (Kiel Fijord), using outdoor mesocosms. Two treatments were applied: Ambient temperature regime (Ambient) and Ambient + 3.6C (Heat) over the entire course of the experiment. Temperature regimes were compared to the 22-year temperature average in the area. This dataset shows final above and below ground biomass per shoot for each shoot measured. The organization of the data is hierarchical: Treatment (Heat, Ambient), Benthocosms number (6 benthocosms per treatment), Seagrass box number (4 boxes per benthocosm), shoot number (originally 6 shoots per box, some were lost throughout the experiment). Unit of biomass is g dry weight per shoot.

Description: Climate change will not only shift environmental means but will also increase the intensity of extreme events, exerting additional stress on ecosystems. While field observations on the ecological consequences of heat waves are emerging, experimental evidence is rare, and lacking at the community level. Using a novel “near‐natural” outdoor mesocosms approach, this study tested whether marine summer heat waves have detrimental consequences for macrofauna of a temperate coastal community, and whether sequential heat waves provoke an increase or decrease of sensitivity to thermal stress. Three treatments were applied, defined and characterized through a statistical analysis of 15 years of temperature records from the experimental site: (1) no heat wave, (2) two heat waves in June and July followed by a summer heat wave in August and (3) the summer heat wave only. Overall, 50% of the species showed positive, negative or positive/negative responses in either abundance and/or biomass. We highlight four possible ways in which single species responded to either three subsequent heat waves or one summer heat wave: (1) absence of a response (tolerance, 50% of species), (2) negative accumulative effects by three subsequent heat waves (tellinid bivalve), (3) buffering by proceeding heat waves due to acclimation and/or shifts in phenology (spionid polychaete) and (4) an accumulative positive effect by subsequent heat waves (amphipod). The differential responses to single or sequential heat waves at the species level entailed shifts at the community level. Community‐level differences between single and triple heat waves were more pronounced than those between regimes with vs. without heat waves. Detritivory was reduced by the single heat wave while suspension feeding was less common in the triple heat wave regime. Critical extreme events occur already today and will occur more frequently in a changing climate, thus, leading to detrimental impacts on coastal marine systems.

Description: This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer (December 2015 - August 2016) in the Western Baltic Sea (Kiel Fijord), using outdoor mesocosms. Two treatments were applied: Ambient temperature regime (Ambient) and Ambient + 3.6C (Heat) over the entire course of the experiment. Temperature regimes were compared to the 22-year temperature average in the area. A number of seagrass response parameters were measured throughout the experiment including parameters related to shoot mortality and reproduction (shoot abundance, growth of new shoots, abundance of flowering shoots), (ii) shoot growth (growth rate, leaf production, lengths of the 3rd leaf, number of green leaves per shoot), (ii) biomass (above and below ground biomass per shoot) and (iv) pigmentation (chlorophyll-a, beta-carotene).

Description: This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer (December 2015 - August 2016) in the Western Baltic Sea (Kiel Fijord), using outdoor mesocosms. Two treatments were applied: Ambient temperature regime (Ambient) and Ambient + 3.6C (Heat) over the entire course of the experiment. Temperature regimes were compared to the 22-year temperature average in the area. This dataset shows concentrations of NO2+NO3, NH4, PO4 of each treatment (Ambient, Heat) as the mean and confidence interval (CI) across all benthocosms per treatment (n=6).

Description: This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer (December 2015 - August 2016) in the Western Baltic Sea (Kiel Fijord), using outdoor mesocosms. Two treatments were applied: Ambient temperature regime (Ambient) and Ambient + 3.6C (Heat) over the entire course of the experiment. Temperature regimes were compared to the 22-year temperature average in the area. This dataset shows concentration of dissolved oxygen per treatment (Ambient, Heat) as the mean and confidence interval (CI) across all benthcosms per treatment (n=6).

Description: This study simulated a 9-months warming scenario on the common seagrass Zostera marina from winter into summer (December 2015 - August 2016) in the Western Baltic Sea (Kiel Fijord), using outdoor mesocosms. Two treatments were applied: Ambient temperature regime (Ambient) and Ambient + 3.6C (Heat) over the entire course of the experiment. Temperature regimes were compared to the 22-year temperature average in the area. This dataset shows monthly or bi-monthly measurements taken of each shoot: Number of shoots [no. per box], Number of new shoots at counting event [no. per box], number of new shoots per month [no. per box per month], and number of flowering shoots [no. per box]. Derived data in %: Number of shoots [% of original shoots], number of new shoots [% of original shoots], number of flowering shoots [% of original shoots]. The organization of the data is hierarchical: Treatment (Heat, Ambient), Benthocosms number (6 benthocosms per treatment), Seagrass box number (4 boxes per benthocosm), shoot number (originally 6 shoots per box, some were lost throughout the experiment).