Ocean colour remote sensing in the southern Laptev Sea: evaluation and applications

Abstract

Abstract. Enhanced permafrost warming and increased Arctic river discharges have heightened concern about the input of terrigenous matter into Arctic coastal waters. We used optical operational satellite data from the ocean colour sensor MERIS (Medium-Resolution Imaging Spectrometer) aboard the ENVISAT satellite mission for synoptic monitoring of the pathways of terrigenous matter on the shallow Laptev Sea shelf. Despite the high cloud coverage in summer that is inherent to this Arctic region, time series from MERIS satellite data from 2006 on to 2011 could be acquired and were processed using the Case-2 Regional Processor (C2R) for optically complex surface waters installed in the open-source software ESA BEAM-VISAT.Since optical remote sensing using ocean colour satellite data has seen little application in Siberian Arctic coastal and shelf waters, we assess the applicability of the calculated MERIS C2R parameters with surface water sampling data from the Russian-German ship expeditions LENA2008, LENA2010 and TRANSDRIFT-XVII taking place in August 2008 and August and September 2010 in the southern Laptev Sea. The shallow Siberian shelf waters are optically not comparable to the deeper, more transparent waters of the Arctic Ocean. The inner-shelf waters are characterized by low transparencies, due to turbid river water input, terrestrial input by coastal erosion, resuspension events and, therefore, high background concentrations of suspended particulate matter and coloured dissolved organic matter.We compared the field-based measurements with the satellite data that are closest in time. The match-up analyses related to LENA2008 and LENA2010 expedition data show the technical limits of matching in optically highly heterogeneous and dynamic shallow inner-shelf waters. The matchup analyses using the data from the marine TRANSDRIFT expedition were constrained by several days' difference between a match-up pair of satellite-derived and in situ parameters but are also based on the more stable hydrodynamic conditions of the deeper inner-and the outer-shelf waters. The relationship of satellite-derived turbidity-related parameters versus in situ suspended matter from TRANSDRIFT data shows that the backscattering coefficient C2R_bb_spm can be used to derive a Laptev-Sea-adapted SPM algorithm. Satellite-derived Chl a estimates are highly overestimated by a minimum factor of 10 if applied to the inner-shelf region due to elevated concentrations of terrestrial organic matter.To evaluate the applicability of ocean colour remote sensing, we include the visual analysis of lateral hydrographical features. The mapped turbidity-related MERIS C2R parameters show that the Laptev Sea is dominated by resuspension above submarine shallow banks and by frontal instabilities such as frontal meanders with amplitudes up to 30 km and eddies and filaments with horizontal scales up to 100 km that prevail throughout the sea-ice-free season. The widespread turbidity above submarine shallow banks indicates innershelf vertical mixing that seems ...

Methane distribution and oxidation around the Lena Delta in summer 2013

“…In contrast to our more diverse riverine population, the methanotrophic population in the Lena River proper was characterised by a rather homogeneous community (Osudar et al, 2016). However, the classical concept of the r-and k-strategists has today been replaced by the competitor-stress toleratorruderal functional classification framework (Ho et al, 2013). Thus, the type Ia MOB found in the present study, which respond rapidly to substrate availability and are the predominant active community in many environments, can also be classified as competitors (C) and competitor-ruderals (C-Rs) (Ho et al, 2013).…”

“…Methane concentration and nitrogen availability are strong driving forces that shape MOB community composition and activity (Ho et al, 2013). Interactions with other heterotrophic bacteria can further influence the features of the methanotrophic community .…”

“…The bulk of the freshwater from the Lena River stays in the eastern Laptev during the summer season (Fofonova et al, 2015). However, changing atmospheric conditions render the freshwater content in the Laptev sea shelf highly time dependent and turbulent (Heim et al, 2014). The simulations performed by Wahlström and Meier (2014) revealed the importance of the methane oxidation rate constant and the crucial necessity of obtaining an in situ measurement of it.…”

From Fresh to Marine Waters: Characterization and Fate of Dissolved Organic Matter in the Lena River Delta Region, Siberia

“…The Lena Delta region and Laptev Sea have high DOC concentrations (>500 µM) and high CDOM associated with low salinity waters (Alling et al, 2010;Stedmon et al, 2011;Semiletov et al, 2013;Walker et al, 2013;Heim et al, 2014;Dubinenkov et al, 2015a), decreasing toward higher salinities through conservative mixing (Cauwet and Sidorov, 1996;Kattner et al, 1999). This is a characteristic also thought to be shared by other Arctic rivers (Dittmar and Kattner, 2003).…”

“…It contributes approximately 20% to the total fresh water discharge into the Arctic Ocean through its delta into the Laptev Sea (Cauwet and Sidorov, 1996). The Lena Delta and the Laptev Sea inner shelf encompass a large, shallow environment characterized by pronounced physical-chemical gradients (Bauch et al, 2009;Fofonova et al, 2014) and considerable amounts of sediments, dissolved, and particulate organic matter over the water column (Semiletov et al, 2011;Vonk et al, 2012Vonk et al, , 2014Wegner et al, 2013;Heim et al, 2014;Sánchez-García et al, 2014). Eastern Siberia (including the Lena River and its delta) is known to be affected by global warming with a thawing permafrost (Yang et al, 2002;Schuur et al, 2008), which subsequently affects the fresh water discharge, the production of DOM in river catchments and the riverine transport of organic material input into the shelf seas (Frey and McClelland, 2009;Lyon and Destouni, 2010;Semiletov et al, 2012Semiletov et al, , 2013Vonk et al, 2012Vonk et al, , 2014Sánchez-García et al, 2014;Fedorova et al, 2015).…”