Dissolved organic matter at the fluvial–marine transition in the Laptev Sea using in situ data and ocean colour remote sensing

Abstract

River water is the main source of dissolved organic carbon (DOC) in the Arctic Ocean. DOC plays an important role in the Arctic carbon cycle, and its export from land to sea is expected to increase as ongoing climate change accelerates permafrost thaw. However, transport pathways and transformation of DOC in the land-to-ocean transition are mostly unknown. We collected DOC and a CDOM (λ) samples from 11 expeditions to river, coastal and offshore waters and present a new DOC-a CDOM (λ) model for the fluvial-marine transition zone in the Laptev Sea. The a CDOM (λ) characteristics revealed that the dissolved organic matter (DOM) in samples of this dataset are primarily of terrigenous origin. Observed changes in a CDOM (443) and its spectral slopes indicate that DOM is modified by microbial and photodegradation. Ocean colour remote sensing (OCRS) provides the absorption coefficient of coloured dissolved organic matter (a CDOM (λ) sat ) at λ = 440 or 443 nm, which can be used to estimate DOC concentration at high temporal and spatial resolution over large regions. We tested the statistical performance of five OCRS algorithms and evaluated the plausibility of the spatial distribution of derived a CDOM (λ) sat . The OLCI (Sentinel-3 Ocean and Land Colour Instrument) neural network swarm (ONNS) algorithm showed the best performance compared to in situ a CDOM (440) (r 2 = 0.72). Additionally, we found ONNS-derived a CDOM (440), in contrast to other algorithms, to be partly independent of sediment concentration, making ONNS the most suitable a CDOM (λ) sat algorithm for the Laptev Sea region. The DOC-a CDOM (λ) model was applied to ONNS-derived a CDOM (440), and retrieved DOC concentration maps showed moderate agreement to in situ data (r 2 = 0.53). The in situ and satelliteretrieved data were offset by up to several days, which may partly explain the weak correlation for this dynamic region. Satellite-derived surface water DOC concentration maps from Medium Resolution Imaging Spectrometer (MERIS) satellite data demonstrate rapid removal of DOC within short time periods in coastal waters of the Laptev Sea, which is likely caused by physical mixing and different types of degradation processes. Using samples from all occurring water types leads to a more robust DOC-a CDOM (λ) model for the retrievals of DOC in Arctic shelf and river waters.

Terrestrial Dissolved Organic Matter Mobilized From Eroding Permafrost Controls Microbial Community Composition and Growth in Arctic Coastal Zones

“…However, since our media has been kept in dark during the whole experiment, the S r values in this study relate to the composition of CDOM rather than processing. In addition, it has been shown that lower spectral slopes in the UVA area for Arctic coastal water is not correlated with photodegradation (Juhls et al, 2019), and that the lower spectral slope for UVA instead likely reflects lability of DOM (Matsuoka et al, 2015). Therefore, we conclude that the significant differences in S r values between the three glacial deposit types underline differences in the DOM composition.…”

Amplified Arctic Surface Warming and Sea Ice Loss Due to Phytoplankton and Colored Dissolved Material

“…Note that we do not include additional terrigenous sources of CDOM and suspended material. Depending on the region (Stedmon et al, 2011) and the time of the year (Juhls et al, 2020), we may underestimate CDOM absorption locally by up to an order of 10 times, compared to measured values (Heim et al, 2014; Juhls et al, 2019; Soppa et al, 2019). By accounting for the loading of major Arctic rivers, its effect is expected to be larger than shown here.…”

Identifying Drivers of Seasonality in Lena River Biogeochemistry and Dissolved Organic Matter Fluxes

“…Hydrological flow paths will shift toward increased groundwater flow and affect the source and composition of dissolved organic matter (DOM) (Amon et al, 2012). While particulate material is mostly deposited in shelf sea sediments to become mineralised or buried (Charkin et al, 2011;Wegner et al, 2013), dissolved organic matter is exported offshore into the open Arctic Ocean (i.e., Juhls et al, 2019) and approximately 50% is thought to be exported onwards to the Atlantic (Granskog et al, 2012). Most of the DOM transported by the Lena River originates from leaching of surface soils of dominantly boreal forest vegetation (Amon et al, 2012;Kaiser et al, 2017).…”