To investigate the potential impact of marine heatwaves on the intertidal fauna of the German Wadden Sea, a mesocosm experiment was conducted on the island of Sylt, Germany, from June to August 2022. Organisms of the two common bivalve species Cerastoderma edule and Macoma balthica were collected from the upper intertidal of the local Wadden Sea in May 2022. Organisms were then placed into open boxes filled with sieved sediment (1000 µm) from the sampling area, as single-species communities mimicking in-situ observed densities of 119 individuals m-2 (C. edule) and 357 m-2 (M. balthica) and with initial community wet weights corresponding to approximately 35.7 g m-2 (both species) to ensure comparability. The communities were introduced into outdoor land-based mesocosm tanks equipped with tidal simulation, seawater flow through and temperature control on June 1st, 2022. After an adjustment phase of three weeks, four replicate tanks (and communities) were exposed to a single, mild 15-day heatwave with water temperatures of maximum 2.8°C above ambient, entailing nine days at peak temperature and three days onset/ offset each. Another four replicates were exposed to a strong heatwave of the same design but with maximum 4.4°C above ambient, and four more replicates were kept under ambient conditions for control. Two weeks after the heatwave exposure, the mesocosm experiment was terminated and all communities were again weighed and counted for surviving individuals. No significant treatment effects on community survival or biomass were recorded for either model species. However, average survival was lower following the strong heatwave for C. edule, corresponding to observed community biomass losses. In contrast, survival in M. balthica remained high across all treatments, and community biomasses appeared to have increased marginally over the course of the experiment.
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.
This dataset originates from a laboratory mesocosm experiment investigating the effects of simulated marine heatwaves (MHWs) on benthic microbial communities from sublittoral sediments of Spiekeroog (Wadden Sea, Germany). Surface sediments were sieved (1 mm) and incubated under controlled light and temperature conditions before exposure to three temperature treatments (19 °C control, 23 °C, and 25 °C). Following a gradual warming phase, MHW conditions were maintained for six days, followed by a recovery phase at ambient temperature. Sediment samples were collected repeatedly throughout the experiment. Surface sediments (upper 2 cm) were collected using syringe cores. Pooled samples were analyzed for chlorophyll a as a proxy for microphytobenthos biomass using ethanol extraction and spectrophotometric pigment analysis. Extracellular polymeric substances (EPS) were quantified using EDTA extraction followed by phenol–sulfuric acid carbohydrate analysis. DNA was extracted from sediment subsamples using a Qiagen PowerSoil kit. Prokaryotic abundance was estimated by quantitative PCR targeting the 16S rRNA gene (primers 519F/907R), using an Escherichia coli 16S rRNA gene standard curve. The dataset includes chlorophyll a concentrations (µg g⁻¹ dry sediment), EPS carbohydrate concentrations, and prokaryotic 16S rRNA gene copy numbers for all sampling times, elevations, and treatments.
To investigate the potential impact of marine heatwaves on the intertidal fauna of the German Wadden Sea, a mesocosm experiment was conducted on the island of Sylt, Germany, from June to August 2022. Organisms of the two common bivalve species Cerastoderma edule and Macoma balthica were collected from the upper intertidal of the local Wadden Sea in May 2022. Organisms were then placed into open boxes filled half with sieved sediment (1000 µm) from the sampling area, as single-species communities mimicking in-situ observed densities of 119 individuals m-2 (C. edule) and 357 m-2 (M. balthica) and with initial community wet weights corresponding to approximately 35.7 g m-2 (both species) to ensure comparability. The communities were introduced into outdoor land-based mesocosm tanks equipped with tidal simulation, seawater flow through and temperature control on June 1st, 2022. After an adjustment phase of three weeks, four replicate tanks (and communities) were exposed to a single, mild 15-day heatwave with water temperatures of maximum 2.8°C above ambient, entailing nine days at peak temperature and three days onset/ offset each. Another four replicates were exposed to a strong heatwave of the same design but with maximum 4.4°C above ambient, and four more replicates were kept under ambient conditions for control. Before and during heatwave exposure, community filtration rates were measured keeping the communities intact, using their host boxes as incubation chambers within the mesocosms. To initialize measurements, chlorophyll spikes were provoked by adding Rhodomonas salina mono culture to the boxes' water columns, after which water samples were taken with a 10-minute interval and immediately analyzed for the relative change in chlorophyll-a concentration using a bbe© AlgaeLabAnalyser (370 – 610 nm). To obtain absolute chlorophyll-a values, additional water samples were taken after 1, 20, and 60 minutes and filtered through glass microfiber filters. Subsequently, 90% acetone solution was yielded from the filters and measured for light absorbance rates at 750, 663, 645, and 630 nm wave lengths using an UVIKON© XS UV/Vis spectrophotometer. Chlorophyll-a concentrations were then calculated from absorbance rates following Jeffrey and Humphrey (1975). In a next step, 10-minute interval values were interpolated by fitting the results to the relative concentration changes previously determined via the AlgaeLabAnalyser. Community filtration rates in µg chlorophyll-a per hour were calculated for each 10-min interval from the respective concentration changes, normalized to per hour and the incubated water volume. Filtration rates differed significantly between species, with higher community filtration rates for C. edule than M. balthica, the latter resembling the bare sand controls. For C. edule, the mild heatwave induced elevated community filtration rates at high chlorophyll-a concentration compared to communities exposed to no heatwave, the same was not observed for the strong heatwave. Due to the methodology applied, the exact number of alive and actively filtering individuals at the times of measurement is unknown, thus results represent community-level responses.
To investigate the potential impact of marine heatwaves on the intertidal fauna of the German Wadden Sea, a mesocosm experiment was conducted on the island of Sylt, Germany, from June to August 2022. Organisms of the two common bivalve species Cerastoderma edule and Macoma balthica were collected from the upper intertidal of the local Wadden Sea in May 2022. Organisms were then placed into open boxes filled with sieved sediment (1000 µm) from the sampling area, as single-species communities mimicking in-situ observed densities of 119 individuals m-2 (C. edule) and 357 m-2 (M. balthica) and with initial community wet weights corresponding to approximately 35.7 g m-2 (both species) to ensure comparability. The communities were introduced into outdoor land-based mesocosm tanks equipped with tidal simulation, seawater flow through and temperature control on June 1st, 2022. After an adjustment phase of three weeks, four replicate tanks (and communities) were exposed to a single, mild 15-day heatwave with water temperatures of maximum 2.8°C above ambient, entailing nine days at peak temperature and three days onset/ offset each. Another four replicates were exposed to a strong heatwave of the same design but with maximum 4.4°C above ambient, and four more replicates were kept under ambient conditions for control. Two weeks after the heatwave exposure, the mesocosm experiment was terminated and all surviving bivalves were measured for shell size at the widest point and for condition index, determined from the ratio of dry soft tissue mass to dry shell mass. No significant treatment effects on shell size were recorded for either model species. However, condition index appeared lower for both species following exposure to a single, strong heatwave.