Seasonality, rather than estuarine gradient or particle suspension/sinking dynamics, determines estuarine carbon distributions.

Estuaries are important components of the global carbon cycle; exchanging carbon between aquatic, atmospheric, and terrestrial environments, representing important loci for blue carbon storage and greenhouse gas emissions. However, how estuarine gradients affect sinking/suspended particles, and dissolved organic matter dynamic interactions remains unexplored. We fractionated suspended/sinking particles to assess and characterise carbon fate differences. We investigated bacterial colonisation (SYBR Green I) and exopolymer concentrations (TEP/CSP) with microscopy staining techniques. C/H/N and dry weight analysis identified particle composition differences. Meanwhile, nutrient and carbon analysis, and excitation and emission matrix evaluations with a subsequent parallel factor (PARAFAC) analysis characterised dissolved organic matter. The lack of clear salinity driven patterns in our study are presumably due to strong mixing forces and high particle heterogeneity along the estuary, with only density differences between suspended and sinking particles. Elbe estuary particles' organic portion is made up of marine-like (sinking) and terrestrial-like (suspended) signatures. Salinity did not have a significant role in microbial degradation and carbon composition, although brackish estuary portions were more biologically active. Indicative of increased degradation rates, leading to decreased greenhouse gas emissions, which are especially relevant for estuaries, with their disproportionate greenhouse gas emissions. Bacterial colonisation decreased seawards, indicative of decreased degradation, and shifts in microbial community composition and functions. Our findings span diverse strands of research, concerning steady carbon contributions from both marine and terrestrial sources, carbon aromaticity, humification index, and bioavailability. Their integration highlights the importance of the Elbe estuary as a model system, providing robust information for future policy decisions affecting dissolved and particulate matter dynamics within the Elbe Estuary.

Low discharge intensifies nitrogen retention in rivers - A case study in the Elbe River.

Eutrophication due to excessive nutrient inputs is a major threat to coastal ecosystems worldwide, causing harmful algae blooms, seagrass loss and hypoxia. Decisions to combat eutrophication in the North Sea were made in the 1980s. Despite significant improvements during recent decades, high nitrogen loads and resulting eutrophication problems remain. In this study, long-term changes in nitrogen inputs to the Elbe Estuary (Germany) were characterized based on nitrogen data provided by the Elbe River Basin Community from 1985 to 2019. Additionally, surface water samples were taken at the weir separating the river from the estuary from 2011 to 2021 to characterize dissolved inorganic nitrogen concentrations and nitrate stable isotope composition. The findings suggest a close coupling of river discharge with the riverine nitrogen cycle. Nitrogen loads decreased disproportionately with decreasing discharge. This decrease is due to intensified nitrogen retention in the Elbe catchment, which can double nitrogen retention compared to average discharge conditions. Phytoplankton growth was enhanced by long residence times and high light availability at low water levels. This suggests that the recent decreases in nitrogen loads in the Elbe River were not only a result of management measures in the catchment but were also amplified by a recent long-lasting drought in the catchment. Based on projections from the Intergovernmental Panel on Climate Change, more frequent and extensive droughts are anticipated, which may lead to future seasonal shifts to nitrate limitation in the lower Elbe River.

Nutrient pathways and their susceptibility to past and future change in the Eurasian Arctic Ocean.

Climate change is altering nutrient cycling within the Arctic Ocean, having knock-on effects to Arctic ecosystems. Primary production in the Arctic is principally nitrogen-limited, particularly in the western Pacific-dominated regions where denitrification exacerbates nitrogen loss. The nutrient status of the eastern Eurasian Arctic remains under debate. In the Barents Sea, primary production has increased by 88% since 1998. To support this rapid increase in productivity, either the standing stock of nutrients has been depleted, or the external nutrient supply has increased. Atlantic water inflow, enhanced mixing, benthic nitrogen cycling, and land-ocean interaction have the potential to alter the nutrient supply through addition, dilution or removal. Here we use new datasets from the Changing Arctic Ocean program alongside historical datasets to assess how nitrate and phosphate concentrations may be changing in response to these processes. We highlight how nutrient dynamics may continue to change, why this is important for regional and international policy-making and suggest relevant research priorities for the future.

Seasonal nitrogen fluxes of the Lena River Delta.

The Arctic is nutrient limited, particularly by nitrogen, and is impacted by anthropogenic global warming which occurs approximately twice as fast compared to the global average. Arctic warming intensifies thawing of permafrost-affected soils releasing their large organic nitrogen reservoir. This organic nitrogen reaches hydrological systems, is remineralized to reactive inorganic nitrogen, and is transported to the Arctic Ocean via large rivers. We estimate the load of nitrogen supplied from terrestrial sources into the Arctic Ocean by sampling in the Lena River and its Delta. We took water samples along one of the major deltaic channels in winter and summer in 2019 and sampling station in the central delta over a one-year cycle. Additionally, we investigate the potential release of reactive nitrogen, including nitrous oxide from soils in the Delta. We found that the Lena transported nitrogen as dissolved organic nitrogen to the coastal Arctic Ocean and that eroded soils are sources of reactive inorganic nitrogen such as ammonium and nitrate. The Lena and the Deltaic region apparently are considerable sources of nitrogen to nearshore coastal zone. The potential higher availability of inorganic nitrogen might be a source to enhance nitrous oxide emissions from terrestrial and aquatic sources to the atmosphere.

Tropical Beaches Attenuate Groundwater Nitrogen Pollution Flowing to the Ocean.

Tropical urbanized coastal regions are hotspots for the discharge of nutrient-enriched groundwater, which can affect sensitive coastal ecosystems. Here, we investigated how a beach modifies groundwater nutrient loads in southern India (Varkala Beach), using flux measurements and stable isotopes. Fresh groundwater was highly enriched in NO3 from sewage or manure. Submarine groundwater discharge and nearshore groundwater discharge were equally important contributors to coastal NO3 fluxes with 303 mmol NO3 m-1 day-1 in submarine and 334 mmol NO3 m-1 day-1 in nearshore groundwater discharge. However, N/P ratios in nearshore groundwater discharge were up to 3 orders of magnitude greater than that in submarine groundwater, which can promote harmful algae blooms. As groundwater flowed through the beach, N/P ratios decreased toward Redfield ratios due to the removal of 30-50% of NO3 due to denitrification and production of PO4 due to mineralization of organic matter. Overall, tropical beaches can be important natural biogeochemical reactors that attenuate nitrogen pollution and modify N/P ratios in submarine groundwater discharge.

Nitrate sources and the effect of land cover on the isotopic composition of nitrate in the catchment of the Rhône River.

The Rhône River originates in the high Alps and drains an intensely cultivated and industrialised catchment before it discharges to the Gulf of Lion. We investigated the interaction of catchment geomorphology with nitrate sources (atmosphere, agriculture, and nitrification of soil organic matter) and removal processes in large and diverse watersheds on the basis of dual nitrate isotope signatures in river water.In March 2015, we took surface water samples along the Rhône River, including its main tributaries, and measured nutrient concentrations and the stable isotopic composition of nitrate (δ15N, δ18O and Δ17O), and water (δ18O-H2O).Results show that high altitude regions are dominated by nitrate from nitrification in pristine soils and atmospheric deposition, while nitrate in the downstream Rhône River originates mainly from nitrification of agricultural/urban sources. Parallel increases in δ15N and δ18O reflect the influence of primary production. Previous studies suggested robust correlations between land use and [Formula: see text]. Based on our observation that nitrate δ15N values at higher altitudes are lower than expected, we assume that lower nitrate δ15N values likely reflect limited nitrate consumption and lower soil nitrogen turnover rates. We propose that correlation between land use and nitrate δ15N is sensitive to slope and geomorphology.

Anthropogenic changes of nitrogen loads in a small river: external nutrient sources vs. internal turnover processes.

Anthropogenic nutrient inputs increase the N-load in many aquatic systems, leading to eutrophication and potential changes of biological N-retention capacity. In this study, nitrate inputs in a small river were investigated along a gradient of anthropogenic influence. We aimed to determine changes in nitrate load and isotope signatures in the water column and to identify the anthropogenic influence on biological nitrogen assimilation and nitrification or denitrification in sediments. In seasonal sampling campaigns, we analysed dissolved inorganic nitrogen concentrations, and stable isotopes of nitrate. To differentiate rates of nitrate production and consumption in the pristine vs. agricultural river section, intact sediment cores were incubated with 15N-labelled nitrate. δ15N values of nitrate in the pristine river section were low, reflecting natural sources, but, as expected, increased with nitrate concentration in all seasons along the gradient. In general, nitrate retention and consumption were higher in the anthropogenically impacted than in the pristine river section, and nitrate consumption exceeded production. In addition to our measurements, modelled results also show that even in a small river, the anthropogenically enhanced consumption capacity is overwhelmed by surplus N-inputs, and nitrate consumption cannot increase in turn with external loads.

Bacterial communities potentially involved in iron-cycling in Baltic Sea and North Sea sediments revealed by pyrosequencing.

To gain insight into the bacterial communities involved in iron-(Fe) cycling under marine conditions, we analysed sediments with Fe-contents (0.5-1.5 wt %) from the suboxic zone at a marine site in the Skagerrak (SK) and a brackish site in the Bothnian Bay (BB) using 16S rRNA gene pyrosequencing. Several bacterial families, including Desulfobulbaceae, Desulfuromonadaceae and Pelobacteraceae and genera, includingDesulfobacterandGeobacter, known to reduce Fe were detected and showed highest abundance near the Fe(III)/Fe(II) redox boundary. Additional genera with microorganisms capable of coupling fermentation to Fe-reduction, includingClostridiumandBacillus, were observed. Also, the Fe-oxidizing families Mariprofundaceae and Gallionellaceae occurred at the SK and BB sites, respectively, supporting Fe-cycling. In contrast, the sulphate (SO4 (2-)) reducing bacteriaDesulfococcusandDesulfobacteriumwere more abundant at greater depths concurring with a decrease in Fe-reducing activity. The communities revealed by pyrosequencing, thus, match the redox stratification indicated by the geochemistry, with the known Fe-reducers coinciding with the zone of Fe-reduction. Not the intensely studied model organisms, such asGeobacterspp., but rather versatile microorganisms, including sulphate reducers and possibly unknown groups appear to be important for Fe-reduction in these marine suboxic sediments.

Changes in atmospheric nitrate deposition in Germany--an isotopic perspective.

We investigated the isotopic composition of atmospheric NO3(-) deposition at a moderately polluted site in Western Europe over an annual cycle from December 2011 to November 2012. On average, we measured load-weighted δ(15)N values of +0.1 and +3.0‰ in wet and dry deposition, respectively. A comparison to source-specific N emission trends and to isotope data from the 1980s reveals distinct changes in δ(15)N-NO3(-) values: In contrast to the increasing relative importance of isotopically depleted natural NOx sources, we find an increase of isotope values in comparison to historical data. We explore the role of land-based N sources, because backward trajectories reveal a correlation of higher δ(15)N to air mass origin from industrialized areas. Nowadays isotopically enriched NOx of coal-fired power plants using selective catalytic converters and land-based vehicle emissions, which use same technology, are apparently the main driver of rising δ(15)N values in nitrate deposition.