Description: Benthic nitrogen cycling in the Mauritanian upwelling region (NW Africa) was studied in June 2014 from the shelf to the upper slope where minimum bottom water O 2 concentrations of 25 µM were recorded. Benthic incubation chambers were deployed at 9 stations to measure fluxes of O 2 , dissolved inorganic carbon (DIC) and nutrients (NO 3 - , NO 2 - , NH 4 + , PO 4 3- , H 4 SiO 4 ) along with the N and O isotopic composition of nitrate (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) and ammonium (δ 15 N-NH 4 + ). O 2 and DIC fluxes were similar to those measured during a previous campaign in 2011 whereas NH 4 + and PO 4 3- fluxes on the shelf were 2 – 3 times higher and possibly linked to a long-term decline in bottom water O 2 concentrations. The mean isotopic fractionation of NO 3 - uptake on the margin, inferred from the loss of NO 3 - inside the chambers, was 1.5 ± 0.4 ‰ for 15/14 N ( 15 ϵ app ) and 2.0 ± 0.5 ‰ for 18/16 O ( 18 ϵ app ). The mean 18 ϵ app : 15 ϵ app ratio on the shelf (〈 100 m) was 2.1 ± 0.3, and higher than the value of 1 expected for microbial NO 3 - reduction. The 15 ϵ app are similar to previously reported isotope effects for NO 3 - respiration in marine sediments but lower than determined in 2011 at a same site on the shelf. The sediments were also a source of 15 N-enriched NH 4 + (9.0 ± 0.7 ‰). A numerical model tuned to the benthic flux data and that specifically accounts for the efflux of 15 N-enriched NH 4 + from the seafloor, predicted a net benthic isotope effect of N loss ( 15 ϵ sed ) of 3.6 ‰; far above the more widely considered value of ~0‰. This result is further evidence that the assumption of a universally low or negligible benthic N isotope effect is not applicable to oxygen-deficient settings. The model further suggests that 18 ϵ app : 15 ϵ app trajectories > 1 in the benthic chambers are most likely due to aerobic ammonium oxidation and nitrite oxidation in surface sediments rather than anammox, in agreement with published observations in the water column of oxygen deficient regions.

Description: Nitrous oxide (N2O) is a greenhouse gas, with a global warming potential 298 times that of carbon dioxide. Estuaries can be sources of N2O, but their emission estimates have significant uncertainties due to limited data availability and high spatiotemporal variability. We investigated the spatial and seasonal variability of dissolved N2O and its emissions along the Elbe Estuary (Germany), a well-mixed temperate estuary with high nutrient loading from agriculture. During nine research cruises performed between 2017 and 2022, we measured dissolved N2O concentrations, as well as dissolved nutrient and oxygen concentrations along the estuary, and calculated N2O saturations, flux densities, and emissions. We found that the estuary was a year-round source of N2O, with the highest emissions in winter when dissolved inorganic nitrogen (DIN) loads and wind speeds are high. However, in spring and summer, N2O saturations and emissions did not decrease alongside lower riverine nitrogen loads, suggesting that estuarine in situ N2O production is an important source of N2O. We identified two hotspot areas of N2O production: the Port of Hamburg, a major port region, and the mesohaline estuary near the maximum turbidity zone (MTZ). N2O production was fueled by the decomposition of riverine organic matter in the Hamburg Port and by marine organic matter in the MTZ. A comparison with previous measurements in the Elbe Estuary revealed that N2O saturation did not decrease alongside the decrease in DIN concentrations after a significant improvement of water quality in the 1990s that allowed for phytoplankton growth to re-establish in the river and estuary. The overarching control of phytoplankton growth on organic matter and, subsequently, on N2O production highlights the fact that eutrophication and elevated agricultural nutrient input can increase N2O emissions in estuaries.

Description: We investigated nutrient input and retention in the Elbe River (Germany) at the river/estuarine transition with high agricultural loads of nitrogen. Surface water samples were taken at the weir Geesthacht (stream kilometre 585, 53°25'31''N, 10°20'10''E) from 2011 to 2021. In these samples, we analyzed nutrient concentrations, nitrate dual stable isotopes and suspended particulate matter composition. Usually, samples were taken once or twice per month. Aims of the study were to investigate 1) nitrate retention in the Elbe River and catchment, 2) seasonal dynamic of nitrate stable isotopes and 3) key nitrogen turnover processes and their respective controls over a ten year period.

Description: The database for nitrate concentrations and nitrate δ15N includes new data and most of the measurements that have been published to date. This database also includes most of the nitrate δ15N measurements in the database of Rafter et al. (2019; Biogeosciences 16, 2617-2633; https://doi.org/10.5194/bg-16-2617-2019). It consists of 944 stations with 15300 measurements of nitrate δ15N. All data are uploaded, except the GOSHIP P2 and P6 sections for which we report average profiles vs. depth. Full data sets for these sections will be included upon publication in a follow-up version.

Description: R/V Meteor cruise M131 carried out a physical oceanography research program with a biogeochemical sampling component in the South Equatorial Atlantic Ocean and eastern boundary upwelling region off Angola and Namibia. The program was part of the EU collaborative project PREFACE (“Enhancing prediction of tropical Atlantic climate and its impacts”) and the sampling for and analysis of nutrient data were linked to BMBF collaborative project GENUS (“Geochemistry and Ecology of the Namibian Upwelling System”). CTD data of the expedition M131 are archived under doi:10.1594/PANGAEA.910994. The bottle files corresponding to water samples analysed for nutrient concentrations and nitrate isotopic composition were provided by Gerd Krahmann (GEOMAR). Depths and data for temperature, salinity, oxygen concentrations labeled “CTD“ in the table are values from calibrated CTD sensors at closure of the bottles, numbers for fluorescence are uncalibrated. Water samples were taken by Maria-Elena Vorrath during M131 and were analyzed after shipment in the laboratory of Kirstin Dähnke at Helmholtz-Zentrum Geesthacht. Nutrient concentrations were measured with an AutoAnalyzer 3 system (Seal Analytics) using standard colorimetric methods by Markus Ankele. Nitrate was determined after reduction to nitrite, followed by a reaction with sulfanilamide to form a red azo dye (Grasshoff and Anderson 1999). Phosphate was measured after formation of a blue antimony-phosphorous colour complex, according to Murphy and Riley (1962). Ammonium was measured fluorometrically based on Holmes et al. (1999). The relative errors of duplicate sample measurements were below 1.5% for NOx and phosphate concentrations, and below 0.3% for ammonium and silicate. The detection limit was 〈0.5 µmol kg−1 for NOx, 〉 0.1 µmol kg−1 for phosphate, 〉0.013 µmol kg−1 for ammonium and 〉0.016 µmol kg−1 for silicate. Delta15N_NO3 and Delta18O_NO3 were determined in the laboratory at Helmholtz-Zentrum Geesthacht (Kirstin Dähnke) with the denitrifier method (Sigman et al. 2001; Casciotti et al. 2002). The isotopic composition was determined with a GasBench II coupled to a Delta Plus XP mass spectrometer (ThermoFinnigan). Replicate measurements were performed, and two international standards (IAEA-N3, Delta15N = 4.7‰, Delta18O=25.6‰ and USGS 34 (Delta15N=-1.8‰, Delta18O=-27.9‰; Böhlke et al. 2003), were measured with each batch of samples. To correct for exchange with oxygen atoms from water, a bracketing correction was applied (Sigman et al. 2009). The standard deviation for standards and samples was 0.2‰ for Delta15N and 0.4‰ for Delta18O.

Description: In situ fluxes of solutes were measured in benthic chambers along an E-W transect at 18.1 oN using autonomous Biogeochemical Observatories (BIGO). A total of nine BIGO deployments were made at nine stations along a latitudinal transect across the shelf/slope covering a horizontal distance of 50 km from ~50 to ~1110 m water depth. The design and implementation of the BIGO landers has been discussed in detail previously (Krahmann et al. 2021). Each BIGO (I and II) contained two circular flux chambers (K1 and K2) with an internal diameter of 28.8 cm where the sediment was incubated for ~30 h. Discrete samples were taken using glass syringes (eight per chamber) at pre-programmed time intervals for chemical analysis. After BIGO recovery, the syringes were immediately transferred to the onboard cool room (4°C) for filtering (0.2 µm) and sub-sampling. Benthic lander samples were analyzed for the isotopic composition (in per mil units, ‰) of nitrate (δ15N-NO3- and δ18O-NO3-) and ammonium (δ15N-NH4+). Most of the samples were analyzed for δ15N-NO3- and δ18O-NO3-. Nitrate dual isotopes were analyzed using the denitrifier method Samples for δ15N-NH4+ were analyzed using the hypobromite/azide-method where NH4+ concentrations were above the detection limit for a reliable isotope analysis (1 µM). In the denitrifier method, NO3- and NO2- are quantitatively converted to N2O by Pseudomonas aureofaciens (ATTC 13985). The hypobromite/azide-method is based on the chemical conversion of NH4+ to N2O by a subsequent addition of a hypobromite and azide solution. For both methods, the sample volume was adjusted to a sample size of 10 nmol of N2O. N2O was extracted from the sample vials by purging with helium and measured with a GasBench II, coupled to an isotope ratio mass spectrometer (Delta Plus, Thermo Fisher Scientific, Germany). All samples were measured with international standards. NO2- comprised on average 2 % of the combined NO2- + NO3- pool and the contribution of NO2- interference to the reported δ15N and δ18O nitrate values was considered negligible. N and O isotope ratios are reported in per mil (‰), relative to the analytical standards (N2 in air for δ15N, and Vienna Standard Mean Ocean Water (VSMOW) for O).

Description: Permafrost-affected soils around the Arctic Ocean contain a large reservoir of organic matter including nitrogen, which partly reach the river after thawing, degradation and erosion of permafrost. After mobilization, reactive remineralised nitrogen is either used for primary production, microbial processing or is simply transported to coastal waters. We have analyzed soil, suspended matter and dissolved inorganic and organic nitrogen for their contents and 15N stable isotope composition to create a baseline for a nitrogen inventory of the Lena River Delta in 2019/2020. We used samples from two transect cruises through the delta in March and August 2019, a monitoring program at Samoylov Island in the central delta (2019/2020), and different soil type samples from Samoylov Island. Our data shows that the nitrogen transported from the delta to the Laptev Sea were dominated by dissolved organic nitrogen (DON) and nitrate, which occur in similar amounts of approx. 10 μmol/L. DON was available during the whole year. Nitrate showed a clear seasonal pattern: increase from late summer until the spring flood, during summer the nitrate concentration are close to zero. During the spring flood the nitrogen concentration are higher with up to 100 μmol/L. The nitrogen stable isotope values of the different nitrogen components ranges mainly between 0.5 and 4.5‰, and were subsequently enriched from the soils via suspended particulate matter (SPM)/sediment and DON to nitrate. During the spring flood, the stable isotope signature of nitrate suggested a strong source of atmospheric deposition. The 15N values are depleted with appox. -8‰ and the 18O values are enriched up to 60‰. Our data provides a baseline for isoscape analysis and can be used as an endmember signal for modeling approaches.

Description: Permafrost-affected soils around the Arctic Ocean contain a large reservoir of organic matter including nitrogen, which partly reaches the riverine system after thawing, degradation and erosion of permafrost. After mobilization, reactive nitrogen in form of dissolved organic nitrogen (DON) ordissolved inorganic nitrogen (DIN: ammonium and nitrate) is either used for primary production, microbial turnover and/or is transported to coastal waters where it serves as a key source of nutrition for the marine food web. In this study, we have followed the nitrogen released from permafrost soil via the Lena River into the Laptev Sea and used the natural abundance of 15N stable isotopes to identify sources, sinks and processes. Therefore, we have investigated different soil. We present a comprehensive data set from two transect cruises (03/08 2019) through the delta, and the outcome of a monitoring program (2018 - 2021) at Samoylov Island in the central delta. High-frequency monitoring and cruise data shows that the nitrogen transported from the river to the Laptev Sea was dominated by DON and nitrate, which occurred in similar amounts of approx. 10 μmol L–1 in the river water. The nitrate concentration decreased during the early summer and increased from late summer throughout the winter until the spring flood. During the spring flood, the nitrogen concentration was up to ten times higher. Thus, spring floods transport approx. 20 % of the annual load of reactive nitrogen into the Laptev Sea just at the onset of the growing season. The nitrogen stable isotope values of the different nitrogen components ranged mainly between 0.5 and 4.5‰, and were subsequently enriched from the permafrost soils via suspended particulate matter/sediment and DON to nitrate, which indicate an oligotrophic ecosystem. Using a Bayesian mixing model, the stable isotope signature of nitrate suggested a strong source of atmospheric deposition during the spring flood. During the rest of the year, soils are the main source of the reactive nitrogen, which is transported to the marine realm.