CDOM Optical Properties and DOC Content in the Largest Mixing Zones of the Siberian Shelf Seas

Abstract

Notable changes in the Arctic ecosystem driven by increased atmospheric temperature and ice cover reduction were observed in the last decades. Ongoing environmental shifts affect freshwater discharge to the Arctic Ocean, and alter Arctic land-ocean fluxes. The monitoring of DOC distribution and CDOM optical properties is of great interest both from the point of view of validation of remote sensing models, and for studying organic carbon transformation and dynamics. In this study we report the DOC concentrations and CDOM optical characteristics in the mixing zones of the Ob, Yenisei, Khatanga, Lena, Kolyma, and Indigirka rivers. Water sampling was performed in August–October 2015 and 2017. The DOC was determined by high-temperature combustion, and absorption coefficients and spectroscopic indices were calculated using the seawater absorbance obtained with spectrophotometric measurements. Kara and Laptev mixing zones were characterized by conservative DOC behavior, while the East Siberian sea waters showed nonconservative DOC distribution. Dominant DOM sources are discussed. The absorption coefficient aCDOM (350) in the East Siberian Sea was two-fold lower compared to Kara and Laptev seawaters. For the first time we report the DOC content in the Khatanga River of 802.6 µM based on the DOC in the Khatanga estuary.

Multi-Element Composition of Diatom Chaetoceros spp. from Natural Phytoplankton Assemblages of the Russian Arctic Seas

“…During the sampling period, the thermohaline structure of the water masses of the Kara, Laptev, and East Siberian seas was heterogeneous and was characterized by pronounced vertical stratification [35][36][37]. It was associated with the freshwater runoff of the largest rivers of Western and Eastern Siberia, where almost ubiquitous desalination of the surface layer was observed down to less than 28 practical salinity units (PSU).…”

“…5625 (Laptev Sea) and +3.46-+3.72 • C and 25-28 PSU at station no. 5587/2 (Kara Sea), occupied the surface water layers of 0-9 m and 0-15 m, respectively [35]. It was separated from the lower layers by a sharp halocline, under which colder (down to −1.77 • C) and saltier (30)(31)(32)(33)(34) water masses were located.…”

“…Sampling was performed at stations located at a maximum possible distance from the shore in areas least affected by the freshwater runoff of large rivers of Siberia and characterized by the presence of the diatom microalgae Chaetoceros spp. A summary of the stations is provided in Table 1 [35,38,40,41].…”

“…Sampling was performed at stations located at a maximum possible distance from the shore in areas least affected by the freshwater runoff of large rivers of Siberia and characterized by the presence of the diatom microalgae Chaetoceros spp. A summary of the stations is provided in Table 1 [35,38,40,41]. Samples were taken with a standard Juday plankton net (mouth area 0.1 m 2 , mesh size 180 µm) by vertical trawling at a speed of 0.6-0.8 m sec −1 in the upper 45-m water layer.…”

“…However, no obvious source of such material can be named. The sampling was carried out at the stations located as far as possible from the areas strongly affected by the freshwater runoff bringing high content of suspended particulate matter (Figure 1) [35,39]. The ingress of clay particles as a result of the turbidity of bottom sediments may also be neglected, since no samples were obtained in the near-bottom layer.…”