Description: Numerical models are a suitable tool to quantify impacts of predicted climate change on complex ecosystems but are rarely used to study effects on benthic macroalgal communities. Fucus vesiculosus L. is a habitat-forming macroalga in the Baltic Sea and alarming shifts from the perennial Fucus community to annual filamentous algae are reported. We developed a box model able to simulate the seasonal growth of the Baltic Fucus-grazer-epiphyte system. This required the implementation of two state variables for Fucus biomass in units of carbon (C) and nitrogen (N). Model equations describe relevant physiological and ecological processes, such as storage of C and N assimilates by Fucus, shading effects of epiphytes or grazing by herbivores on both Fucus and epiphytes, but with species-specific rates and preferences. Parametrizations of the model equations and required initial conditions were based on measured parameters and process rates in the near-natural Kiel Outdoor Benthocosm (KOB) experiments during the Biological Impacts of Ocean Acidification project. To validate the model, we compared simulation results with observations in the KOB experiment that lasted from April 2013 until March 2014 under ambient and climate-change scenarios, that is, increased atmospheric temperature and partial pressure of carbon dioxide. The model reproduced the magnitude and seasonal cycles of Fucus growth and other processes in the KOBs over 1 yr under different scenarios. Now having established the Fucus model, it will be possible to better highlight the actual threat of climate change to the Fucus community in the shallow nearshore waters of the Baltic Sea.

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: 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: Shallow coastal marine ecosystems are exposed to intensive warming events in the last decade, threatening keystone macroalgal species such as the bladder wrack (Fucus vesiculosus, Phaeophyceae) in the Baltic Sea. Herein, we experimentally tested in four consecutive benthic mesocosm experiments, if the single and combined impact of elevated seawater temperature (? + 5◦C) and pCO2 (1100 ppm) under natural irradiance conditions seasonally affected the photophysiological performance (i.e., oxygen production, in vivo chlorophyll a fluorescence, energy dissipation pathways and chlorophyll concentration) of Baltic Sea Fucus. 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. In contrast to the expectation that warmer winter conditions might be beneficial, elevated temperature conditions and sub-optimal low winter light conditions decreased photophysiological performance of Fucus. In summer, western Baltic Sea Fucus already lives close to its upper thermal tolerance limit and future warming of the Baltic Sea during summer may probably become deleterious for this species. However, our results indicate that over most of the year a combination of future ocean warming and increased pCO2 will have slightly positive effects for Fucus photophysiological performance.

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: The impact of variable underwater photosynthetically active radiation (PAR) and photosynthetic parameters on photosynthetic oxygen production of Laminaria hyperborea off the island of Helgoland (North Sea, Germany) was investigated throughout summer 2014. L. hyperborea was sampled along a depth gradient (0.5, 2, 4, 6 m) and discs from three different blade regions (5, 25 and 50 cm above the stipe-blade transition zone) were set into photosynthesis versus irradiance (PI) curves. After cutting and before the oxygen incubation, maximum quantum yield (Fv/Fm) were measured as a health indicator. PI-curve parameters were normalized to either fresh mass or disc area. Additionally, chlorophyll a content was measured in each disc and normalized to the same two parameters as PI parameters. In situ PAR was measured in different depths (1.2, 2.9, 4.4, 6.6 m) to gain daily diffuse vertical attenuation coefficient (Kd). PAR along the vertical depth profile was calculated and together with PI-curve parameters oxygen production was calculated along the vertical depth profile. Leaf area index (Pehlke and Bartsch, 2008) was used to extrapolate oxygen production rates to seafloor and a photosynthetic quotient (PQ) of 1.18 (Miller III et al., 2009) to convert rates into carbon fixation rates. This net primary production (NPP) was given along the vertical depth profile based on different Kd values (daily Kd, mean Kd, minimum Kd, maximum Kd).

Description: Over the whole water column, daily diffuse attenuation coefficient (Kd in 1/m) values are based on in situ photosynthetically active radiation (PAR) measurements performed in different depths (1.2, 2.9, 4.4, 6.6 m) during summer 2014. PAR for the algae collection depths is calculated based on daily Kd values. Daily net primary production (NPP in g C/m² seafloor/day) for each sampling depth is calculated with in situ vertical profiles based on daily Kd, leaf area index (Pehlke and Bartsch, 2008) and a photosynthetic quotient (PQ) of 1.18 (Miller III et al. 2009). For comparative purposes, daily NPP values were also calculated using the measured maximum and minimum daily Kd and the mean Kd, which were derived from all daily Kd over the entire sampling period.

Description: In-situ photosynetically active radiation (PAR) was measured in different depths (1.2, 2.9, 4.4, 6.6 m) every 10-15 min during summer 2014. Odyssey PAR loggers were calibrated against a cosine-corrected planar PAR sensor (LI-190SA quantum sensor, LI-COR Inc., USA) over a 24 h period at 4 m depth in the Helgolandic South harbor. During the Laminaria hyperborea sampling period (seven weeks), incoming PAR was recorded continuously every 15 min at 1.2 and 2.9 m, and every 30 min at 4.4 and 6.6 m near the sampling area of sporophytes. To avoid biofouling of the sensor heads, PAR loggers were cleaned every week (1.2 m) or every second week (all other depths) by SCUBA divers.

Description: Laminaria hyperborea off the island of Helgoland (North Sea, Germany) was sampled along a depth gradient (0.5, 2, 4, 6 m) throughout summer 2014. Stipe length of the sporophyte was measured. In blade discs from three different blade regions (5, 25 and 50 cm above the stipe-blade transition zone) dry mass, fresh mass and dry mass:area ratio were measured.

Description: Laminaria hyperborea off the island of Helgoland (North Sea, Germany) was sampled along a depth gradient (0.5, 2, 4, 6 m) throughout summer 2014. Discs were cut from three different blade regions (5, 25 and 50 cm above the stipe-blade transition zone) and set into photosynthesis versus irradiance (PI) curves. PI curves were fitted by minimizing the sum of differences between the measured oxygen flux and the model proposed by Jassby and Platt (1976). Parameters were normalized to fresh mass and area.