Description: Ocean acidification and warming (OAW) are occurring globally. Additionally, at a more local scale the spreading of hypoxic conditions is promoted by eutrophication and warming. In the semi-enclosed brackish Baltic Sea, occasional upwelling in late summer and autumn may expose even shallow-water communities including the macroalga Fucus vesiculosus to particularly acidified, nutrient-rich and oxygen-poor water bodies. During summer 2014 (July–September) sibling groups of early life-stage F. vesiculosus were exposed to OAW in the presence and absence of enhanced nutrient levels and, subsequently to a single upwelling event in a near-natural scenario which included all environmental fluctuations in the Kiel Fjord, southwestern Baltic Sea, Germany (54°27 ´N, 10°11 ´W). We strove to elucidate the single and combined impacts of these potential stressors, and how stress sensitivity varies among genetically different sibling groups. Enhanced by a circumstantial natural heat wave, warming and acidification increased mortalities and reduced growth in F. vesiculosus germlings. This impact, however, was mitigated by enhanced nutrient conditions. Survival under OAW conditions strongly varied among sibling groups hinting at a substantial adaptive potential of the natural Fucus populations in the Western Baltic. A three-day experimental upwelling caused severe mortality of Fucus germlings, which was substantially more severe in those sibling groups which previously had been exposed to OAW. Our results show that global (OAW), regional (nutrient enrichment) and local pressures (upwelling), both alone and co-occurring may have synergistic and antagonistic effects on survival and/or growth of Fucus germlings. This result emphasizes the need to consider combined stress effects.

Description: Ocean warming and acidification may substantially affect the photophysiological performance of keystone species such as Fucus vesiculosus (Phaeophyceae) in shallow coastal waters. In four consecutive benthic mesocosm experiments (Kiel Outdoor Benthocosm, Kiel, Germany, 54°20'N; 10°09'E), we compared the photophysiological performance (i.e., oxygen production, in vivo chlorophyll a fluorescence, energy dissipation pathways and chlorophyll concentration) of Baltic Sea Fucus under the single and combined impact of elevated seawater temperature (Δ + 5°C) and pCO2 (1100 ppm). Fucus specimens were sampled, in each season (spring: April 2, 2013; summer: July 2, 2013; autumn: 8 October; winter: January 14, 2014) from a depth of 0.2–1 m in the Kiel Fjord (Bülk), western Baltic Sea, Germany (54°27'N; 10°11,5'E). Photosynthetic performance was measured with two different methods, one based on in vivo chlorophyll a fluorescence measurements of photosystem II (PSII), the other one based on oxygen production. For each experiment and treatment, three Fucus specimens 15–25 cm long with 91 ± 30 total apices and apparently equal vigor were chosen, each individually growing on a stone (10–15 cm in diameter) from a single holdfast. For details see material and methods in Graiff et al. 2021. Photosynthesis was highest in spring/early summer when water temperature and solar irradiance increases naturally, and was lowest in winter (December to January/February). Temperature had a stronger effect than pCO2 on photosynthetic performance of Fucus in all seasons. Photophysiological responses were generally positive during the cooler spring months, but strongly negatively affected during summer (due to a marine heat-wave). Especially, future summer temperatures exceeded the thermal tolerance threshold of western Baltic Sea Fucus and had a deleterious impact overall. Potential benefits of the combination of future ocean warming and increased pCO2 over most of the year for Fucus photophysiological performance are suggested by our study, but not during summer peak temperatures.

Description: During summer 2014 (mid-July - mid-September 2014), early life-stage Fucus vesiculosus were exposed to combined ocean acidification and warming (OAW) in the presence and absence of enhanced nutrient levels (OAW x N experiment). Subsequently, F. vesiculosus germlings were exposed to a final upwelling disturbance during 3 days (mid-September 2014). Experiments were performed in the near-natural scenario "Kiel Outdoor Benthocosms" including natural fluctuations in the southwestern Baltic Sea, Kiel Fjord, Germany (54°27 'N, 10°11 'W). Genetically different sibling groups and different levels of genetic diversity were employed to test to which extent genetic variation would result in response variation. The data presented here show the phenotypical response (growth and survival) of the different experimental populations of F. vesiculosus under OAW, nutrient enrichment and the upwelling event. Log effect ratios demonstrate the responses to enhanced OAW and nutrient concentrations relative to the ambient conditons. Carbon, nitrogen content (% DW) and C:N ratios were measured after the exposure of ambient and high nutrient levels. Abiotic conditions the OAW x nutrient experiment and the upwelling event, are shown.

Description: Genetic diversity of baltic F. vesiculosus is low compared to other populations which might jeopardize their potential for adaptation to climate change. Especially the early life-stage F. vesiculosus may be threaten by ocean warming and acidification. To test this, we exposed F. vesiculosus germlings to warming and acidification in the near-natural scenario in the "Kiel Outdoor Benthocosms" maintaining the natural variation of the Kiel Fjord, Germany (54°27 'N, 10°11 'W) in all seasons (spring 2013 - 2014). Warming was simulated by using a delta treatment adding 5 °C and by increasing pCO2 at 1000 µatm. Warming positively affected germlings' growth in spring and in summer but decreased non-photochemical quenching in spring and survival in summer. Acidified conditions showed much weaker effects than warming. The high genotypic variation in stress sensitivity as well as the enhanced survival at high diversity levels indicate higher potential for adaptation for genetically diverse populations. We conclude that the combination of stressors and season determines the sensitivity to environmental stress and that genetic variation is crucial for the adaptation to climate change stress.