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This dataset contains calculated carbonate chemistry parameters, including the concentrations of dissolved CO2, HCO3-, CO32- and dissolved inorganic carbon, the fugacity and partial pressure of CO2 and the saturation states of aragonite (Ωarag) and calcite (Ωcalc), for the alkalization mesocosms. Values were calculated from pH, total alkalinity, salinity and temperature measured between 17 April and 19 June 2024, using the seacarb package (Orr et al., 2018) in R (V. 4.3.2). Measured pH values, which refer to the NBS scale, were first converted to the total scale using the total proton activity coefficient fH of Takahashi et al. (1982), lowering pH by 0.12 to 0.13 units across the measured salinity and temperature range. Calculations used the carbonic acid dissociation constants of Lueker et al. (2000), the bisulfate dissociation constant of Dickson (1990), the hydrogen fluoride constant of Perez & Fraga (1987), total boron after Uppström (1974) and the aragonite and calcite solubility products of Mucci (1983), all evaluated at in situ temperature and salinity. The conversion from the NBS to the total scale carries a residual uncertainty of about 0.01 to 0.02 pH units, because the liquid junction potential between the dilute calibration buffers and seawater is not fully captured by fH. This propagates to an uncertainty of roughly 5 to 15 % in calculated pCO2 and a correspondingly smaller uncertainty in the saturation states.
This dataset contains oyster mortality records and the main physicochemical parameters monitored in the exposure mesocosms, the tanks holding 60 oysters per treatment (unamended control, dissolved alkalinity addition with NaOH and CaCl2 at +250 or +500 µmol/kg, and olivine-based coastal enhanced silicate weathering at +250 or +500 µmol/kg). Mortality is reported as the number of oysters alive, the interval and cumulative number of oysters found dead, and the relative cumulative mortality (a proportion of the initial 60 individuals per tank). Physicochemical parameters include water temperature, dissolved oxygen, conductivity, salinity, total chlorophyll a, suspended particulate matter and ammonia concentration. Measurements span 17 April to 10 July 2024, covering the alkalinization period and the subsequent 63-day chronic exposure. Water temperature, oxygen and conductivity/salinity were recorded with a multi-probe (Xylem Analytics Germany). Total chlorophyll a was measured by fluorescence on unfiltered seawater with a bbe fluorimeter. Suspended particulate matter was determined gravimetrically from filtered seawater. Ammonia was quantified fluorometrically with a microplate reader assay kit.
This dataset contains species-resolved phytoplankton and microzooplankton abundance and biomass data from the KOSMOS 2023 Helgoland mesocosm experiment conducted in the coastal waters off Helgoland Island in the North Sea, Germany. The experiment investigated the effects of Ocean Alkalinity Enhancement (OAE) on plankton communities during the development of a natural spring bloom. Six mesocosms (~6 m³) were subjected to an immediate dilution OAE scenario, establishing an alkalinity enhancement gradient ranging from ΔTA 0 to 1250 µmol kg⁻¹ in increments of 250 µmol kg⁻¹ through calcium-based alkalinity addition. The dataset includes taxonomically resolved phytoplankton and microzooplankton abundance (cells mL⁻¹), carbon biomass (µg C L⁻¹), functional group classifications, and World Register of Marine Species (WoRMS) taxonomic identifiers. Associated carbonate chemistry measurements, including total alkalinity (TA), dissolved inorganic carbon (DIC), pH, fugacity of carbon dioxide (fCO₂), salinity, temperature, and calculated ΔTA values are provided for each sampling event.
We conducted a mesocosm study to investigate ecosystem responses to ocean alkalinity enhancement (OAE) in the temperate waters of the German North Sea on Helgoland in spring 2023. We simulated non-CO2-equilibrated OAE via calcium hydroxide through the addition of calcium chloride and sodium hydroxide. Twelve 6 m³ mesocosms were used to simulate two scenarios: in six mesocosms we established a gradient of added alkalinity from 0 to 1250 µmol/kg in increments of 250 µmol/kg, simulating immediate (imm) dilution of alkalised waters. For the second set of six mesocosms, alkalinity was added only to the top 1 m of each mesocosm, doubling the target added alkalinity. The top layer was mixed with the untreated bottom layer after 48 hours, simulating delayed dilution of alkalised waters (del) and ultimately leading to the same alkalinity gradient as the immediate dilution treatment. This data contains water column biogeochemical variables, including dissolved inorganic nutrient concentrations (µmol/L), chlorophyll a concentrations (µg/L) and suspended particulate matter (biogenic silica, particulate organic carbon, nitrogen and phosphate; µmol/L). Nutrients, particulate organic phosphate and biogenic silica were measured spectrophotometrically (Unicam UV 300, Thermo Spectronic, USA). High-performance liquid chromatography (Thermo Scientific HPLC Ultimate 3000) was performed for chlorophyll a determination and particulate organic carbon and nitrogen were measured using an elemental analyser (Flash EA, Thermo Fisher).
This dataset contains nickel, chromium and cobalt concentrations measured in gill, mantle and hepatopancreas tissue of individual European flat oysters (Ostrea edulis) sampled on day 63 (9 July 2024) at the end of the chronic alkalinity-enhancement exposure experiment. A total of 19 individual oysters were sampled (5 control, 5 olivine 250, 3 olivine 500, 3 dissolved 250 and 3 dissolved 500); not all three tissues could be obtained from every individual due to tissue availability. Ten measurements of the certified reference material ERM-CE278k (mussel tissue) are included across the three tissue batches for quality control. Samples for metal analysis were freeze-dried for 24 h, and dry weight was recorded prior to extraction. Extraction vials were pre-cleaned (p.a. quality) with concentrated nitric acid (65 %, single-distilled) and dried for 48 h. Approximately 2 mL of nitric acid was added to the dried tissues, followed by thermic digestion using a microwave digestion system (5 min heating at 400 W; 5 min heating to 100 °C at 800 W; 20 min heating to 175 °C and 20 min held, 1600 W). The digested solutions were transferred to clean vials and diluted to 25 mL; vials were weighed before and after filling to determine the precise sample weight. Metal concentrations of Ni, Cr and Co were measured on 12 March 2025 at the AWI ICP Facility on a Thermo Scientific iCAP 7000 Series Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Calibration was performed with an in-house standard composed of the respective individual element standards (ICP-Standards ROTI® Star, Carl Roth). The standard deviation of the control standard was 0.1 µg/l for Ni, 0.3 µg/l for Cr and 0.2 µg/l for Co. External reproducibility (RSD) was 0.5 % for Ni, 1.2 % for Cr and 0.7 % for Co. Limits of detection and quantification were 0.13 and 0.7 µg/l for Ni; 0.26 and 1.32 µg/l for Cr; and 0.10 and 0.47 µg/l for Co. Analytical accuracy was verified with the certified reference material ERM-CE278k.
This dataset contains calculated carbonate chemistry parameters, including the concentrations of dissolved CO2, HCO3-, CO32- and dissolved inorganic carbon, the fugacity and partial pressure of CO2 and the saturation states of aragonite (Ωarag) and calcite (Ωcalc), for the exposure mesocosms. Values were calculated from pH, total alkalinity, salinity and temperature measured between 8 May and 10 July 2024, using the seacarb package (Orr et al., 2018) in R (V. 4.3.2). Measured pH values, which refer to the NBS scale, were first converted to the total scale using the total proton activity coefficient fH of Takahashi et al. (1982), lowering pH by 0.12 to 0.13 units across the measured salinity and temperature range. Calculations used the carbonic acid dissociation constants of Lueker et al. (2000), the bisulfate dissociation constant of Dickson (1990), the hydrogen fluoride constant of Perez & Fraga (1987), total boron after Uppström (1974) and the aragonite and calcite solubility products of Mucci (1983), all evaluated at in situ temperature and salinity. The conversion from the NBS to the total scale carries a residual uncertainty of about 0.01 to 0.02 pH units, because the liquid junction potential between the dilute calibration buffers and seawater is not fully captured by fH. This propagates to an uncertainty of roughly 5 to 15 % in calculated pCO2 and a correspondingly smaller uncertainty in the saturation states.
This dataset contains measurements of seawater pH and total alkalinity from a short-term factorial feeding experiment conducted independently of the main chronic alkalinity-enhancement exposure. The experiment tested whether the shellfish diet used to feed the oysters in the main experiment (Reed Mariculture SD1800, a concentrated microalgae feed) contributes to seawater total alkalinity, and whether this contribution depends on the presence of oysters. Four treatment combinations were run in parallel over a 9-day period (12 to 21 November 2024): unamended seawater without oysters or feed (control), seawater with food but without oysters (SD 1800), seawater with oysters but without feed (oysters) and seawater with both oysters and feed (oysters + SD 1800). Oysters in the unfed treatments were left entirely without feed for the full 9 days. Each treatment was sampled on six days (day 0, 1, 3, 5, 7 and 9) in triplicate. Seawater pH was measured with a pH electrode and total alkalinity by open-cell potentiometric titration, both on a Metrohm 855 Robotic Titrosampler.
This dataset contains concentrations of 27 polar metabolites measured in gill and mantle tissue of individual European flat oysters (Ostrea edulis) sampled before the start of the chronic alkalinity-enhancement exposure (7 May 2024, baseline, n = 10 per tissue) and after 21, 42 and 63 days of exposure (28 May, 18 June and 9 July 2024) to one of five treatments, with 3 to 5 individuals sampled per treatment, tissue and timepoint. Extraction: Frozen gill and mantle samples were wet-weighed (100 to 400 mg WW) and, if necessary, ground to smaller pieces with a mortar and pestle pre-chilled in liquid nitrogen before being transferred to Precellys tubes. Polar metabolites were extracted following the protocol of Tripp-Valdez et al. (2017), adjusted as follows: 400 µL methanol and 125 µL MilliQ water for homogenization and 400 µL chloroform and 400 µL MilliQ water for phase separation. All steps used pre-chilled (0 to 4 °C) instruments and reagents to minimize metabolite degradation. The polar upper layer was transferred to 1.5 mL tubes and dried overnight in a SpeedVac vacuum concentrator at 30 °C (RVC 2-18, Martin Christ Freeze Dryers GmbH, Germany). Dried polar metabolites were resuspended in D2O containing 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (TSP; 0.05 wt %; Sigma Aldrich, St. Louis, USA) as internal standard and chemical shift reference, in a 1:1 ratio of µL D2O per mg tissue WW. Measurement: Untargeted metabolite profiling was performed using an ultra-shielded vertical 9.4 T NMR spectrometer (Avance III HD 400 WB, Bruker-BioSpin GmbH, Germany) equipped with a 1.7 mm TXI MicroProbe. Metabolite spectra were acquired using TopSpin 3.2 (Bruker-BioSpin GmbH, Rheinstetten, Germany) at room temperature, with the following parameters: 1D-Carr-Purcell-Meiboom-Gill (CPMG) pulse train with f1 presaturation (cpmgpr1d), 90° pulse 8.4 µs, TD 70.656, sweep width 8802 Hz (22 ppm), acquisition time 4.01 s, relaxation delay 4 s, 4 dummy scans and 128 scans per sample. ¹H-NMR spectra were processed and analyzed in Chenomx NMR Suite V.12.0 (Chenomx Inc., Canada): automated zero-filling, exponential multiplication (0.3 Hz line broadening), phase correction, baseline correction, shim correction and TSP calibration. Spectra from each tissue type within experimental groups were overlaid in Chenomx Profiler to evaluate spectral quality, comparability, linewidths, shifts and baselines, a step critical for accurate metabolite quantification. The adenylates ATP, ADP and AMP were summed as one metabolite (adenylates), resulting in a total of 27 metabolites.
The data presented herein originates from a mesocosm study conducted as part of the BMBF CDRmare, Retake project (grant agreement no. 03F0895A), aimed at investigating the ecological ramifications of ocean alkalinity enhancement (OAE). Twelve mesocosms were deployed in Helgoland South Harbor, Germany, and systematically sampled using integrated water samplers over the period spanning from March 12th to April 20th, 2023. Six alkalinity levels under two dilution scenarios were established to differentiate between localized and uniform OAE additions. Alkalinity was increased stepwise to ΔTAmax = 1250 μmol kg-1 (250 μmol TA kg-1 increments) using sodium hydroxide (NaOH) with calcium chloride (CaCl2) to simulate cation release during calcium-based mineral dissolution, causing strong carbonate chemistry perturbations (e.g., pHT > 9.25). The dataset encompasses a spectrum of sediment trap particle flux data, water column biogeochemistry including pigment variables, inorganic nutrients, carbonate chemistry parameters. The study and data set offer insights into impacts of alkalinity enhancement on marine ecosystems and their associated biogeochemistry.
This dataset contains repeated measurements of seawater pH and total alkalinity recorded in the alkalinization mesocosms, a set of tanks without oysters that received the same alkalinity treatments as the exposure mesocosms reported in a companion dataset. Total alkalinity was raised in three alkalization cycles to reach target increases of +250 or +500 µmol/kg over unamended seawater, using either dissolved alkalinity addition (NaOH and CaCl2) or olivine-based coastal enhanced silicate weathering. The three cycles ran 17 April to 7 May 2024, 8 May to 27 May 2024 and 30 May to 19 June 2024. Measurements were taken predominantly in triplicate per treatment and sampling day. Seawater pH was measured with a pH electrode and total alkalinity by open-cell potentiometric titration, both on a Metrohm 855 Robotic Titrosampler.
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