Temperature is a major driver for the geographical distribution of organisms, such as the foundation kelp species Saccharina latissima. Globally rising sea surface temperatures and intensification of marine heatwaves have already led to local loss of kelp populations. We investigated temporal variations in the thermal susceptibility of S. latissima. Therefore, we assessed the stress responses of field sporophytes sampled from Helgoland (German Bight) to an experimental heat wave scenario in June 2018, August 2018, and August 2019. The experiment in June 2018 was conducted by Diehl et al. (2021a) and the respective dataset (Diehl et al. 2021b, https://doi.org/10.1594/PANGAEA.931637) was re-evaluated for this study. Treatment temperatures (18, 20, 22, 24 °C) were based on 18 °C summer mean sea surface temperature on Helgoland as control, and Δ+2, Δ+4, Δ+6 °C as temperature-amplitude treatments, mimicking marine heatwaves. After a three-days wound healing phase, seven days of temperature acclimation (day 0-7) and seven days of temperature treatment (day 8-14) followed. The survival, growth and maximum photosynthetic quantum yield (Fv/Fm; June 2018/August 2018: ImagingPAM, Walz Imaging PAM Maxi Version M-series; August 2019: Portable Chlorophyll Fluorometer PAM-2100, Heinz Walz GmbH, Effeltrich, Germany) were measured on day 0 and day 14. To highlight changes as response to the experimental heat wave, physiological parameters were shown as percentage of the initial values. Absolute concentrations of pigments were analyzed using a HPLC. Afterwards, accessory pigment (Acc) and xanthophyll cycle pigment (VAZ) concentration, as well as the de-epoxidation state of the xanthophyll cycle (DPS) and ratios were calculated.
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.
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.
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. Chlorophyll a content was measured. Discs were cut from three different blade regions (5, 25 and 50 cm above the stipe-blade transition zone) and normalized to either dry mass, fresh mass or disc area. The samples were freeze-dried for approximately 36 h (Beta 1–8 LDplus, Christ, Osterode am Harz, Germany), finely ground for several minutes using steel grinding balls (3 mm diameter) in a Mikro-Dismembrator U (Braun Biontech International, Melsungen, Germany), cooled centrifuged (Centrifuge 3K10, Sigma, Osterode, Germany), and measured in the spectrophotometer (U-3310, Hitachi High-Tech, Japan) at two wavelengths (347 and 664.5 nm).
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. Incident light was generated by a slide projector (Liesegang Dianfant, Leitz Prado, Germany) equipped with a halogen lamp (Osram Xenophot 400 W/36 V, Germany) and 11 Schott neutral gray filters. Eleven light steps were conducted between 0 and 560 µmol photons m-2 s-1. Dark respiration was measured first for 20 mins followed by increasing light steps in 10 min intervals. Oxygen concentration expressed in % air saturation was logged by the OxyView software (Presens, Regensburg, Germany) and corrected for air pressure, salinity and logged temperature according to Tengberg et al. (2006). During post-processing, the oxygen production rate for each photon flux density (PFD) level was calculated by plotting a linear regression model through all O2-values measured during the time interval, and was normalized to either fresh mass (FM, unit: µmol O2 g–1 h–1) or disc surface area (DA, unit: µmol O2 cm–2 h–1).
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). The maximum quantum yield of photosystem II was measured after cutting and after one night of wound healing before oxygen measurements were conducted using pulse-amplitude-modulated fluorometry (PAM 2100, WALZ, Germany).
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.
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.
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).
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