Description: Five sites across the Kiel Fjord in the Baltic Sea (GEOMAR Pier: 54.330383 N 10.150683 E, Kiel Canal Entry: 54.361167 N, 10.162533 E, Falckenstein: 54.390217 N, 10.194983 E, Laboe: 54.405633 N, 10.209750 E, and Strande: 54.428267 N, 10.209267 E) were monitored biweekly from June 2013 until October 2020 (with some discontinuities in winter). Depth profiles were taken with a CTD (Sea & Sun Technology, Trappenkamp, CTD 60) recording temperature, salinity, pH, oxygen concentration as well as oxygen saturation. Oxygen and pH were corrected for temperature and salinity based on Standard Data Acquisition SSDA (C)opyright by SST 1999-2006. To avoid showing wrong measurements of the CTD probe, thresholds for the measured parameters were set (Temperature ≤ 25 °C, 8 ≤ Salinity ≥ 23, 5 ≤ pH ≥10, 0 mg L-1 ≤ Oxygen concentration ≥ 12 mg L-1 and 0% ≤ Oxygen Saturation ≥ 120%). Only those values were kept in the data set that did not exceed or fell below these thresholds. Furthermore, station specific depth limits were set (GEOMAR Pier ≤ 19 m, Kiel Canal Entry ≤ 13 m, Falckenstein ≤ 16 m, Laboe ≤ 17 m and Strande ≤ 17 m) and date exceeding of falling below these limits were excluded from the data set.

Description: Impact of warming and upwelling on macroalgal growth (expressed as anomaly or z-growth).

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: Ecological impact of global change is generated by multiple synchronous or asynchronous drivers which interact with each other and with intraspecific variability of sensitivities. In three near-natural experiments, we explored response correlations of full-sibling germling families of the seaweed Fucus vesiculosus towards four global change drivers: elevated CO2 (ocean acidification, OA), ocean warming (OW), combined OA and warming (OAW), nutrient enrichment and hypoxic upwelling. Among families, performance responses to OA and OW as well as to OAW and nutrient enrichment correlated positively whereas performance responses to OAW and hypoxia anti-correlated. This indicates (i) that families robust to one of the three drivers (OA, OW, nutrients) will also not suffer from the two other shifts, and vice versa and (ii) families benefitting from OAW will more easily succumb to hypoxia. Our results may imply that selection under either OA, OW or eutrophication would enhance performance under the other two drivers but simultaneously render the population more susceptible to hypoxia. We conclude that intraspecific response correlations have a high potential to boost or hinder adaptation to multifactorial global change scenarios.