Seasonal variations in the transport and biogeochemical turnover of mainly dissolved organic nitrogen from the Lena Delta to the nearshore Laptev Sea

Pan-arctic rivers transport a huge amount of nitrogen to the Arctic Ocean. The permafrost-affected soils around the Arctic Ocean containe a large reservoir of organic matter including carbon and nitrogen, which partly reach the river after permafrost thaw and erosion.Our study aims to estimate the load of nitrogen supplied from terrestrial sources into the Arctic Ocean. Therefore, water, suspended particulate matter (SPM) and sediment samples were collected in the Lena Delta along a (~200 km) transect from the center of the Lena Delta to the open Laptev Sea in late winter (April) and in summer (August) 2019. In winter, 21 sample from 13 stations and in summer, 51 samples from 18 stations were taken. 9 of these sampling stations in the outer delta region were sampled in both seasons.We measured organic and inorganic nitrogen and the 15N stable isotopes composition of all three sample types to determine sources, sinks and processes of nitrogen transformation during transport.In winter, the nitrogen transported from the delta to the Laptev Sea were mainly dissolved organic nitrogen (DON) and nitrate, which occur in similar amounts. The load of nitrate increased slightly in the delta, while no changes to the isotope values of DON and nitrate were observe indicating a lack of biological activity in the winter season. However, lateral transport from soils was a likely source. In summer, nitrogen was mainly transported as DON and particulate nitrogen in the SPM fraction, including phytoplankton.The nitrogen stable isotope values of the different nitrogen components ranges between 0.5 and 4.5 &#8240;, and were subsequently enriched from the soils via SPM/sediment and DON to nitrate. This indicates that nitrogen in the soils mainly originates from nitrogen fixation from the atmosphere. During transport and remineralisation, biogeochemical recycling via nitrification and assimilation by phytoplankton led to an isotopic enrichment in summer from organic to inorganic components. In the coastal waters of the Laptev Sea, the river waters are slowly mixed with marine nitrate containing waters from the Arctic Ocean, and a part of the riverine organic nitrogen is buried in the sediments.We assume that the ongoing permafrost thawing and erosion will intensify and increase the transport of reactive nitrogen to coastal waters and will affect the biogeochemical cycling, e.g. the primary production.

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Particulate and dissolved organic carbon composition in the Lena River and its Delta, from Yakutsk to the Arctic Ocean.

10.5194/egusphere-egu21-8005 ◽

2021 ◽

Author(s):

Olga Ogneva ◽

Gesine Mollenhauer ◽

Matthias Fuchs ◽

Juri Palmtag ◽

Tina Sanders ◽

...

Keyword(s):

Organic Carbon ◽

Dissolved Organic Carbon ◽

Arctic Ocean ◽

Main Channel ◽

The Arctic ◽

Laptev Sea ◽

Lena River ◽

The Arctic Ocean ◽

The Laptev Sea ◽

Lena Delta

Rapid climate warming in the Arctic intensifies permafrost thaw, increases active layer depth in summer and enhances riverbank and coastal erosion. All of these cause additional release of organic matter (OM) into streams and rivers. OM will be (1) transformed and modified during transport and subsequently discharged into the Arctic Ocean, or (2) removed from the active cycling by sedimentation. Here, the nearshore zone (which includes deltas, estuaries and coasts) is of great importance, where the major transformation processes of terrestrial material take place. Despite the importance of deltas for the biogeochemical cycle, their functioning is poorly understood. For our study we examined the Lena River nearshore, which represents the world&#8217;s third largest delta and supplies the second highest annual water and sediment discharge into the Arctic Ocean. Running through almost the entirety of East Siberia from Lake Baikal to the Laptev Sea, the Lena River drains an area of &#8764;2,61&#215;106 km2&#160; with approximately 90% underlain by permafrost. Our aims were to investigate the spatial variation of OM concentration and isotopic composition during transit from terrestrial permafrost source to the ocean interface, and to compare riverine and deltaic OM composition. We measured particulate and dissolved organic carbon (POC and DOC) concentrations and their associated &#948;13C and &#8710;14C values in water samples collected along a &#8764;1500 km long Lena River transect from Yakutsk downstream to the river outlet into the Laptev Sea.We find significant qualitative and quantitative differences between the OM composition in the Lena River main channel and its delta. Further, we found suspended matter and POC concentrations decreased during transit from river to the Arctic Ocean.&#160; DOC concentrations in the Lena delta were almost 50% lower than OM from the main channel. We found that deltaic POC is depleted in 13C relative to fluvial POC, and that its 14C signature suggests a modern composition indicating phytoplankton origin. This observation likely reflects the difference in hydrological conditions between the delta and the river main channel, caused by lower flow velocity and average water depth. We propose that deltaic environments provide favorable growth conditions for riverine primary producers such as algae and aquatic plants. Deltaic DOC is depleted in 14C compared to riverine, especially in samples taken from the water surface, which indicates contributions from an additional old carbon stock source, specific for the Lena Delta. We suggest that this C is released from deltaic bank erosion and partly stays floating on the surface. In conclusion, we found a strong impact of deltaic processes on the fate and dominant signatures of OM discharged into the Arctic Ocean.

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Faculty Opinions recommendation of Dissolved organic nitrogen dynamics in the Arctic Ocean.

Faculty Opinions – Post-Publication Peer Review of the Biomedical Literature ◽

10.3410/f.717988876.793472785 ◽

2013 ◽

Author(s):

Robie Macdonald

Keyword(s):

Arctic Ocean ◽

Dissolved Organic Nitrogen ◽

Organic Nitrogen ◽

Nitrogen Dynamics ◽

The Arctic ◽

The Arctic Ocean

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On the biogeochemical signature of the Lena River from its headwaters to the Arctic Ocean

Biogeosciences Discussions ◽

10.5194/bgd-8-2093-2011 ◽

2011 ◽

Vol 8 (2) ◽

pp. 2093-2143 ◽

Cited By ~ 6

Author(s):

I. P. Semiletov ◽

I. I. Pipko ◽

N. E. Shakhova ◽

O. V. Dudarev ◽

S. P. Pugach ◽

...

Keyword(s):

Organic Carbon ◽

Arctic Ocean ◽

River Discharge ◽

Total Carbon ◽

The Arctic ◽

Time Shift ◽

Laptev Sea ◽

Lena River ◽

The Arctic Ocean ◽

The Laptev Sea

Abstract. The Lena River integrates biogeochemical signals from its vast drainage basin and its signal reaches far out over the Arctic Ocean. Transformation of riverine organic carbon into mineral carbon, and mineral carbon into the organic form in the Lena River watershed, can be considered a quasi-equilibrated processes. Increasing the Lena discharge causes opposite effects on total organic (TOC) and inorganic (TCO2) carbon: TOC concentration increases, while TCO2 concentration decreases. Significant inter-annual variability in mean values of TCO2, TOC, and their sum (TC) has been found. This variability is determined by changes in land hydrology which cause differences in the Lena River discharge, because a negative correlation may be found between TC in September and mean discharge in August (a time shift of about one month is required for water to travel from Yakutsk to the Laptev Sea). Total carbon entering the sea with the Lena discharge is estimated to be almost 10 Tg C y−1. The annual Lena River discharge of particulate organic carbon (POC) may be equal to 0.38 Tg (moderate to high estimate). If we instead accept Lisytsin's (1994) statement concerning the precipitation of 85–95% of total particulate matter (PM) (and POC) on the marginal "filter", then only about 0.03–0.04 Tg of POC reaches the Laptev Sea from the Lena River. The Lena's POC export would then be two orders of magnitude less than the annual input of eroded terrestrial carbon onto the shelf of the Laptev and East Siberian seas, which is about 4 Tg. The Lena River is characterized by relatively high concentrations of primary greenhouse gases: CO2 and dissolved CH4. During all seasons the river is supersaturated in CO2 compared to the atmosphere: up to 1.5–2 fold in summer, and 4–5 fold in winter. This results in a narrow zone of significant CO2 supersaturation in the adjacent coastal sea. Spots of dissolved CH4 in the Lena delta channels may reach 100 nM, but the CH4 concentration decreases to 5–20 nM towards the sea, which suggests only a minor role of riverborne export of CH4 for the East Siberian Arctic Shelf (ESAS) CH4 budget in coastal waters. Instead, the seabed appears to be the source that provides most of the CH4 to the Arctic Ocean.

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Structure and dynamics of mesoscale eddies over the Laptev Sea continental slope in the Arctic Ocean

10.5194/os-2018-22 ◽

2018 ◽

Cited By ~ 1

Author(s):

Andrey Pnyushkov ◽

Igor V. Polyakov ◽

Laurie Padman ◽

An T. Nguyen

Keyword(s):

Arctic Ocean ◽

Continental Slope ◽

Mesoscale Eddies ◽

The Arctic ◽

Laptev Sea ◽

Surface Mixed Layer ◽

Fram Strait ◽

Structure And Dynamics ◽

The Arctic Ocean ◽

The Laptev Sea

Abstract. Heat fluxes steered by mesoscale eddies may be a significant (but still not quantified) source of heat to the surface mixed layer and sea ice cover in the Arctic Ocean, as well as a source of nutrients for enhancing seasonal productivity in the near-surface layers. Here we use four years (2007–2011) of velocity and hydrography records from a moored profiler over the Laptev Sea slope, and 15 months (2008–2009) of acoustic Doppler current profiler data from a nearby mooring, to investigate the structure and dynamics of eddies at the continental margin of the eastern Eurasian Basin. Typical eddy scales are radii of order of 10 km, heights of six hundred meters, and maximum velocities of ~ 0.1 m s −1. Eddies are approximately equally divided between cyclonic and anticyclonic polarizations, contrary to prior observations from the deep basins and along the Lomonosov Ridge. Eddies are present in the mooring records about 20–25 % of the time, taking about one week to pass through the mooring at an average frequency of about one eddy per month. We found the eddies observed are formed in two distinct regions–near Fram Strait, where the western branch of Atlantic Water (AW) enters the Arctic Ocean, and near Severnaya Zemlya, where the Fram Strait and Barents Sea branches of the AW inflow merge. These eddies, embedded in the Arctic Circumpolar Boundary Current, carry anomalous water properties along the eastern Arctic continental slope. The enhanced diapycnal mixing that we found within EB eddies suggests a potentially important role for eddies in the vertical redistribution of heat in the Arctic Ocean interior.

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Sea ice circulation in the Laptev Sea and ice export to the Arctic Ocean: Results from satellite remote sensing and numerical modeling

Journal of Geophysical Research Atmospheres ◽

10.1029/2000jc900029 ◽

2000 ◽

Vol 105 (C7) ◽

pp. 17143-17159 ◽

Cited By ~ 27

Author(s):

Vitaly Y. Alexandrov ◽

Thomas Martin ◽

Josef Kolatschek ◽

Hajo Eicken ◽

Martin Kreyscher ◽

...

Keyword(s):

Remote Sensing ◽

Numerical Modeling ◽

Sea Ice ◽

Arctic Ocean ◽

Satellite Remote Sensing ◽

The Arctic ◽

Laptev Sea ◽

The Arctic Ocean ◽

The Laptev Sea

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Mercury and methylmercury along a transect from the Lena river estuary across the Laptev Sea Shelf

10.5194/egusphere-egu2020-13727 ◽

2020 ◽

Author(s):

Van Liem Nguyen ◽

Birgit Wild ◽

Örjan Gustafsson ◽

Igor Semiletov ◽

Oleg Dudarev ◽

...

Keyword(s):

Organic Matter ◽

Continental Shelf ◽

Arctic Ocean ◽

River Delta ◽

The Arctic ◽

Laptev Sea ◽

Terrestrial Organic Matter ◽

Lena River ◽

The Arctic Ocean ◽

The Laptev Sea

Widespread accelerated permafrost thawing is predicted for this century and beyond. This threatens to remobilize the large amounts of Mercury (Hg) currently &#8216;locked&#8217; in Arctic permafrost soils to the Arctic Ocean and thus potentially lead to severe consequences for human and wildlife health. Future risks of Arctic Hg in a warmer climate are, however, poorly understood. One crucial knowledge gap to fill is the fate of Hg once it enters the marine environment on the continental shelves. Arctic rivers are already today suggested to be the main source of Hg into the Arctic Ocean, with dissolved and particulate organic matter (DOM and POM, respectively) identified as important vectors for the land to sea transport.In this study, we have investigated total Hg (HgT) and monomethylmercury (MeHg) concentrations in surface sediments from the East Siberian Arctic Shelf (ESAS) along a transect from the Lena river delta to the Laptev Sea continental slope. The ESAS is the world&#8217;s largest continental shelf and receives large amounts of organic carbon by the great Arctic Russian rivers (e.g., Lena, Indigirka and Kolyma), remobilized from continuous and discontinuous permafrost regions in the river catchments, and from coastal erosion. Data on HgT and MeHg levels in ESAS sediments is however limited. Here, we observed concentrations of Hg ranging from 30 to 96 ng Hg g-1 d.w. of HgT, and 0.03 to 9.5 ng Hg g-1 d.w. of MeHg. Similar concentrations of HgT were observed close to the river delta (54 &#177; 19 ng Hg g-1 d.w.), where >95 % of the organic matter is of terrestrial origin, and the other section of the transect (42 &#177; 7 ng Hg g-1 d.w.) where the terrestrial organic matter is diluted with carbon from marine sources. In contrast, we observed higher concentrations of MeHg close to the river delta (0.72 &#177; 0.71 ng Hg g-1 d.w. as MeHg) than further out on the continental shelf (0.031 &#177; 0.71 ng Hg g-1 d.w. as MeHg). We also observed a positive correlation between the MeHg:Hg ratio and previously characterized molecular markers of terrestrial organic matter (Br&#246;der et al. Biogeosciences (2016) & Nature Com. (2018)). We thus suggest riverine inputs, rather than in situ MeHg formation, to explain observed MeHg trends.

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