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Organic carbon (OC) content, nitrogen content, C/N ratio, carbonate content, δ13C of OC, mean grain size, surface area, calculated fraction, content and loading of terrestrial and marine OC in surface sediments of Helgoland Mud Area in the North Sea

The grain size of sediments was determined with a Cilas 1180 laser-diffraction particle analyzer (range 0.04–2500 μm) and the mean grain size was calculated. The surface area (SA) was calculated from the grain size distribution of the sediments. The OC content of surface sediments was determined using a carbon-sulfur analyzer (CS-125, Leco) after decarbonization with HCl. The total carbon (TC) and nitrogen (TN) contents of the samples were analyzed using a carbon-nitrogen-sulfur analyzer (Elemental III, Vario) and used to calculate carbonate contents (CaCO3= (TC − OC) × 8.333) and the mass ratio of OC to TON content ((C/N)OC), which was corrected for mineral-associated inorganic N. The stable carbon isotope composition of OC in surface sediments (δ13COC) was measured using a Thermo Delta isotope ratio mass spectrometer coupled to a Carlo Erba elemental analyzer. δ13COC values are given in per mil notation relative to the Vienna Pee Dee Belemnite standard. The standard deviation of duplicate analyses ranged from 0.01‰ to 0.18‰, with an average of 0.10‰. See details in related publication (Wei et al., 2025).

Geochemical properties in surface sediments of the Helgoland Mud Area and surrounding sandy areas taken during HEINCKE cruise HE595 in the North Sea

In March 2022, 56 surface sediments were collected from the Helgoland Mud Area and surrounding sandy areas in the North Sea. These surface sediments were analyzed for grain size, organic carbon (OC) content, total nitrogen content (TN), stable carbon isotope of OC, and abundances of source-specific alkanes and fatty acids, in aim to determine and quantify composition and sources of OC, to understand the degradation and sequestration of marine and terrestrial OC in sediments, and to estimate the burial fluxes and burial efficiencies of marine and terrestrial OC in the Helgoland Mud Area. Detailed dataset interpretation can be found in Wei et al. (2024, in preparation).

Continuous water level (Table 5)

The water level was continuously measured at 5 stations along the transect during the entire sampling period to monitor changes in hourly intervals. Please note that individual loggers were did experience technical failure and did not measure the entire time.

Brackish water rewetting of a temperate coastal peatland in NE Germany: Effects on Biogeochemistry, Microorganisms and Greenhouse gas emissions

The rewetting of drained peatlands is a promising measure to mitigate carbon dioxide (CO2) emissions by preventing the further mineralization of the peat soil through aeration. While freshwater rewetted peatlands can be significant methane (CH4) sources in the short-term, in coastal ecosystems the input of sulfate-rich seawater could potentially mitigate these emissions. The purpose of the data collection was to examine whether the presence of sulfate, known as an alternative electron acceptor, can cause lower CH4 production and thus, emissions by favoring the growth of sulfate-reducers, which outcompete methanogens for substrate. We therefore investigated underlying variables such as the methane-cycling microbial community along with CH4 fluxes and set them in context with CO2 fluxes along a transect in a coastal peatland before and directly after rewetting. In this way, a conclusion about the short-term greenhouse gas mitigation potential of brackish water rewetting of coastal peatlands could be drawn. This data collection consists of six data sets, with direct comparisons before and after rewetting of CO2 and CH4 fluxes (Tab. 2) and associated microbial communities (Tab. 1) being the main data. Pore water geochemistry (Tab. 1 and 3) and surface water parameters (Tab. 4) were collected simultaneously to provide potential explanatory variables. The sampling of continuous water level (Tab. 5) within wells and atmospheric weather data (air and soil temperature, relative humidity, photosynthetic photon flux density; Tab. 6) from a weather station was done in addition. Measurements started in June/July/August 2019 after field installation was finalized and were conducted on the drained coastal fen "Polder Drammendorf" on the island of Rügen in North-East Germany. On 26th November 2019, the dike was opened and channeled in order to rewet the peatland with brackish water. Before, the dike separated the peatland from the adjacent bay "Kubitzer Bodden", which is part of a brackish lagoon system connected to the Baltic Sea. Therefore, the peatland was nearly completely flooded and now resembles a shallow lagoon with high fluctuating water levels. We measured along a humidity (pre-rewetting)/water level (post-rewetting) gradient (stations 0-8) towards and across the main North-South oriented drainage ditch, including four stations on the Eastern side of the ditch (1–4), two ditch stations (0, 5) and two stations (6, 7) on the Western side of the ditch. Station 8 was chosen as an additional station farther towards the adjacent bay on the Western side, but was only accessible before rewetting. CH4 and CO2 fluxes (stations 0-7) were calculated from online gas concentrations measurements using laser-based analyzers and manual closed chambers (Livingston, G. P., & Hutchinson, G. (1995). Enclosure-based measurement of trace gas exchange: Applications and sources of error. In P.A. Matson, & R.C. Harriss (Eds.). Biogenic trace gases: Measuring emissions from soil and water (pp. 14–51). Blackwell Science Ltd., Oxford, UK). Soil cores for microbial, dissolved gas concentrations and isotopic analysis were taken using a Russian type peat corer (De Vleeschouwer, F., Chambers, F. M., & Swindles, G. T. (2010). Coring and sub-sampling of peatlands for palaeoenvironmental research. Mires and Peat, 7, 1–10) before and after rewetting. Each time, we took duplicates at stations 1-8 for this rather labor-intensive process and divided the core into four depth sections: surface, 5–20, 20–40 and 40–50 cm. Subsamples for dissolved gases and stable carbon isotope analyses were taken with tip-cut syringes with a distinct volume of 3 ml (Omnifix, Braun, Bad Arolsen, Germany) and immediately placed into NaCl-saturated vials (20 ml, Agilent Technologies, 5182-0837, Santa Clara, USA) leaving no headspace and closed gas-tight using rubber stoppers and metal crimpers (both: diameter 20 mm, Glasgerätebau Ochs, Bovenden, Germany). Absolute abundances of specific functional target genes, including methane- and sulfate-cycling microorganisms, were measured with quantitative PCR (qPCR) after DNA was extracted (GeneMATRIX Soil DNA Purification Kit, Roboklon, Berlin, Germany) and quantified (Qubit 2.0 Fluorometer, ThermoFisher Scientific, Darmstadt, Germany). Surface and pore water parameters were measured in parallel to the gas measurements and soil coring for microbial analyses. Most surface water variables (pH, specific conductivity, salinity, nutrients, oxygen, sulfate and chloride concentrations, DOC/DIC) were measured in-situ using a multiparameter digital water quality meter or taken to the laboratory as water samples for further analysis. Likewise, pore water/soil variables (pH, specific conductivity, nutrients, metals, sulfate and chloride concentrations, CNS) were either measured in-situ or taken to the laboratory as soil samples. While surface water analysis was only conducted in the drainage ditch before rewetting, it was done along the entire transect after rewetting. In contrast, pore water/soil analysis was mostly conducted before rewetting and only repeated occasionally after rewetting where possible.

Pond/ditch benthic invertebrate data on d15N/d13C (northeast-Germany, 2020)

Benthic invertebrate samples were taken directly in the field via hand-netting in four ditches and three ponds (defined as water bodies with a maximum extent of one hectare) in October and November 2020. Sampling was carried out in Brandenburg in Germany in the region Havellaendisches Luch. The landscape is characterized by an intensively maintained ditch system that was created for industrial agricultural production during 1980s. Additionally, the landscape is characterized by a large number of small standing water bodies (kettle holes, ponds), which were formed during the last glacial period. A detailed description of the area can be found in Trau & Lorenz, 2024. Invertebrate samples were analysed for the stable isotopes of carbon and nitrogen. The data set includes data from 35 families. The whole animal was used for the analysis of stable isotopes of carbon and nitrogen. Analyses were performed on a Thermo Delta V isotope ratio mass spectrometer (IRMS) interfaced to a NC2500 elemental analyser by the Cornell University Stable Isotope Laboratory (https://cobsil.cornell.edu/). The dataset was used to evaluate isotopic niches of functional feeding groups (collector/gatherer, collector/filterer, grazer/scraper, shredder, predator and omnivore) in the two water body types (pond and ditch), in order to evaluate effects of agriculture (nutrient concentrations and pesticide residues) on the isotopic niches (Trau et al. 2025, under review).

Surface water parameters (pH, specific conductivity, salinity, nutrients, oxygen, sulfate and chloride concentrations, DOC/DIC) (Table 4)

Surface water parameters were measured in parallel to the gas measurements and soil coring for microbial analyses. Most surface water variables (pH, specific conductivity, salinity, nutrients, oxygen, sulfate and chloride concentrations, DOC/DIC) were measured in-situ using a multiparameter digital water quality meter or taken to the laboratory as water samples for further analysis. While surface water analysis was only conducted in the drainage ditch before rewetting, it was done along the entire transect after rewetting.

Pore water and soil variables (pH, specific conductivity, nutrients, metals, sulfate and chloride concentrations, CNS) (Table 3)

Pore water parameters were measured in parallel to the gas measurements and soil coring for microbial analyses. Pore water/soil variables (pH, specific conductivity, nutrients, metals, sulfate and chloride concentrations, CNS) were either measured in-situ or taken to the laboratory as soil samples. Pore water/soil analysis was mostly conducted before rewetting and only repeated occasionally after rewetting where possible.

CO2 and CH4 fluxes before and after rewetting (Table 2)

CH4 and CO2 fluxes (stations 0-7) were calculated from online gas concentrations measurements using laser-based analyzers and manual closed chambers (Livingston, GP, & Hutchinson, G, 1995). Blackwell Science Ltd., Oxford, UK).

Absolute abundances of methane- and sulfate-cycling microorganisms, pore water gas concentrations and stable carbon isotopes (Table 1)

Soil cores for microbial, dissolved gas concentrations and isotopic analysis were taken using a Russian type peat corer (De Vleeschouwer et al. 2010) before and after rewetting. Each time, we took duplicates at stations 1-8 for this rather labor-intensive process and divided the core into four depth sections: surface, 5–20, 20–40 and 40–50 cm. Subsamples for dissolved gases and stable carbon isotope analyses were taken with tip-cut syringes with a distinct volume of 3 ml (Omnifix, Braun, Bad Arolsen, Germany) and immediately placed into NaCl-saturated vials (20 ml, Agilent Technologies, 5182-0837, Santa Clara, USA) leaving no headspace and closed gas-tight using rubber stoppers and metal crimpers (both: diameter 20 mm, Glasgerätebau Ochs, Bovenden, Germany).

Stable isotopes of cultured Gigantidas childressi in deep-sea mussel culture

Stable isotopes (a: δ15N, b: δ13C) of G. childressi that experienced different nutritional regimes (CH4: 'M', algae + CH4: MA', only algae: 'A') in our culture. As reference, t0 animals were sampled upon arrival at GEOMAR after spending 2 months post-capture at Charles Fisher's 'deep-sea lab' aquarium system, Pennsylvania State University

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