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This dataset reports physiological measurements of two bivalve species, Mytilus edulis (blue mussel) and Magallana gigas (Pacific oyster), obtained during a three-month mesocosm experiment conducted in Sylt, Germany, in 2023. Physiological data were collected between 27 April 2023 and 25 June 2023.Twelve mesocosms were used to investigate the effects of temperature on individual-level physiological traits, with treatments including ambient temperature and ambient +3°C. Parameters measured include clearance rate, ingestion rate, and respiration rate. Environmental variables such as water temperature, depth, and sampling time were recorded for each measurement. Individual bivalves were labeled for tracking, and species identification followed WoRMS taxonomy. Measurements were performed using handheld multiparameter instruments and laboratory analyses. The dataset provides high-resolution, individual-based physiological responses of bivalves to moderate warming, supporting research on temperature-dependent feeding, metabolic processes, and energy flux in coastal ecosystems.
In the context of global change, marine organisms are subjected not only to gradual changes in abiotic parameters, but also to an increasing number of extreme events, such as heatwaves. However, we still know little about the influence of heatwaves on the structure of marine communities, and experimental studies are needed to test the impact of heatwaves alone, and in combination with other environmental drivers. Here, we conducted a mesocosm experiment and applied an integrated multiple driver design to assess the potential impact of heatwaves under ambient and future environmental conditions on natural coastal plankton communities. To represent future environmental conditions, temperature and pH were manipulated based on the Representative Concentration Pathway 8.5 proposed by the IPCC for 2100, and dissolved N:P ratios were increased to simulate the conditions expected in European coastal zones. Throughout the experiment, we measured abiotic conditions as well as the abundance of bacterioplankton, phytoplankton, and microzooplankton.
We simulated an experimental summer storm in large-volume (~1200 m³, ~16m depth) enclosures in Lake Stechlin (https://www.lake-lab.de) by mixing deeper water masses from the meta- and hypolimnion into the mixed layer (epilimnion). The mixing included the disturbance of a deep chlorophyll maximum (DCM) which was present at the same time of the experiment in Lake Stechlin and situated in the metalimnion of each enclosure during filling. Size-fractionated Bacterial Protein Production (BPP) of particle associated (PA, >3.0 µm) and free-living bacteria (FL, 0.2-3.0 µm) (14C-Leu incorporation) as well as abundances of PA (microscopy of DAPI stained cells on 3.0 µm polycarbonate filters) and FL heterotrophic prokaryotes and picocyanobacteria (flow cytometry of SYBR green I stained cells) were monitored for 42 days after the experimental disturbance event. Mixing increased bacterial abundance and production about 3 weeks after mixing, which was associated to a mixing-induced stimulation of phytoplankton growth in the mixed enclosures compared to the controls. Simultaneously, decreased abundances of picocyanobacteria could be observed in mixed enclosures.
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.
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).
The file contains data from the Marine Carbon System. It gathered parameters from the inorganic carbon and incorporate the organic alkalinity as a main contributor to the sea surface microlayer (SML) compared to the Underlaying Water (ULW). Data was collected during Mesocosm Study from 18-May to 17-July 2024 in the Sea sURface Facility (SURF), Institute for Chemistry and Biology of the Marine Environment, Wilhelmshaven, Germany. Discrete samples to measure Dissolved Inorganic Carbon (DIC), Total Alkalinity (TA) and Organic Alkalinity (OA) were collected. For SML data, DIC, TA and OA was collected every third day following the glass‑plate technique (Harvey and Burzell, 1972). The ULW data, DIC, TA and OA were collected every day using a suction system to collect the sample from 0.4 m depth. Discrete samples were transported to the laboratory for further analysis; DIC was determined coulometrically (CM5017, UIC, IL, USA), and TA concentration was directly measured by high-precision closed-cell potentiometric titration (916 Ti-Touch, Metrohm, Switzerland). OA concentration was determined directly after TA was measured, using the same sample (from which all carbonate species had been purged), denoted as back titration. The dataset includes quality flags 0-4 with flags 1 and 2 are ready for use. See metadata for more information.
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 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.
This dataset contains plant growth and biomass measurements collected during a controlled mesocosm experiment conducted at the AWI Sylt Outdoor Mesocosm facility, Wadden Sea Station Sylt, Germany, between June and September 2025. The experiment investigated the effects of sediment grain size, microtopography and climate warming on the salt marsh pioneer Salicornia. Plants were grown in PVC tubes filled with sandy, mixed or muddy sediments and exposed to four different climate treatments (ambient, warming, warming-drought, heatwave-drought). Climate treatments were implemented through active control of the water temperature. Experimental microtopographies simulated small-scale elevation differences. Plant height was measured repeatedly to quantify growth dynamics. Above ground biomass was measured at the end of the experiment. The experiment was conducted under controlled mesocosm conditions to quantify the relative importance of sediment properties, topographic variation and warming for pioneer marsh vegetation performance.
This dataset contains continuous water temperature measurements collected during a controlled mesocosm experiment conducted at the AWI Sylt Outdoor Mesocosm facility, Wadden Sea Station Sylt, Germany, between June and September 2025. The experiment investigated the effects of sediment grain size, microtopography and climate warming on the salt marsh pioneer Salicornia. Plants were grown in PVC tubes filled with sandy, mixed or muddy sediments and exposed to four different climate treatments (ambient, warming, warming-drought, heatwave-drought). Climate treatments were implemented through active control of the water temperature. Experimental microtopographies simulated small-scale elevation differences. The experiment was conducted under controlled mesocosm conditions to quantify the relative importance of sediment properties, topographic variation and warming for pioneer marsh vegetation performance.
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