Fluorescence quantum yield of CDOM in coastal zones of the Arctic seas

ABSTRACT

Along with traditional optical indices, calculated from absorption and fluorescence spectra to describe chromophoric dissolved organic matter (CDOM) naturally occurring in water, the fluorescence quantum yield (FQY) becomes significant. Knowledge of CDOM optical properties is important for satellite remote sensing as well as for lidar ground-true measurements. The FQY as a function of excitation wavelength within 240–500 nm range for a variety of the Arctic shelf waters was determined for the first time in order to identify the characteristic chromophores peculiar to different regions of the Arctic basin affected by freshwater runoff. The surface water samples were collected during several cruises in 2015–2017 in the following sites: the mouth areas of the Khatanga and Lena Rivers (the Laptev Sea), the delta area of the Northern Dvina River (the White Sea), desalinated waters of the Kara Sea (influenced by freshwater of the Ob and Yenisei Rivers) and the East Siberian Sea (influenced by freshwaters of the Indigirka and Kolyma Rivers), as well the shelf areas of those seas not affected by terrigenous runoff. To characterize DOM, conventional optical indices SR, HIX, and BIX were calculated. In most cases, important humic character of DOM was established, while the contribution of autochthonous organic matter varied from low to intermediate level. For the samples with terrestrial impact, the FQY decreased from excitation at 240 nm to 270–280 nm and then increased, demonstrating two peaks at 340 and 380 nm, with constant decrease towards longer excitation wavelengths; at λ_ex = 380 nm FQY varied from 1.4% to 3.1%. In some cases, additional maximum at 270 nm of FQY-excitation dependency was observed as an indicator of autochthonous nature of biological material. Minimal FQY was measured for the White Sea surface waters, the maximal for the Laptev and East Siberian seas.

Acknowledgments

The researches in the Siberian Arctic seas were performed in the framework of the state assignment of FASO Russia (theme No. 0149-2018-0005). The study of the Laptev and Kara seas optical properties was funded by RFBR, according to the research project No. 16-35-60032 mol_a_dk. The calculations of fluorescence quantum yield were funded by RFBR research project No. 18-016-00078. The authors thank A.I. Kochenkova, N.M. Mahnovich, and A.S. Lohov and the crew of the RV Akademik Mstislav Keldysh for their assistance in the fieldworks.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the Federal Agency for Scientific Organizations [0149-2018-0005]; Russian Foundation for Basic Research [16-35-60032 mol_a_dk, 18-016-00078];