In this study, we investigate the optical group of Coloured Dissolved Organic Matter (CDOM) in surface water bodies in the central Lena Delta, Arctic Siberia. Within the framework of Russian-German expeditions we sampled surface water from lakes and the Lena River during summers 2013 to 2016. Main study objects were lakes on Samoylov Island, a Holocene river terrace, and on Kurungnakh Island, an erosional remnant of the Late Pleistocene Yedoma formation. Supplementary samples were taken from the Lena River Olenekskaya branch bordering Kurungnakh and Samoylov. We investigated absorption and the type of CDOM. The magnitude of CDOM is given by its absorption, the type and freshness or source of CDOM is revealed by the CDOM Slope (S) value in the ultraviolet and the visible wavelength regions of CDOM absorption. Results show different characteristics for thermokarst lakes on Samoylov and Kurungnakh Island in contrast to oxbow lakes on Samoylov. The type of pristine tundra thermokarst lakes on Samoylov Island shows lowest CDOM by absorption and no fresh terrestrial matter characterization. Some thermokarst lakes on Samoylov and on Kurungnakh show fresher terrestrial CDOM with equivalent low CDOM Slope ratio (Sr) values ≤1. We assume that on the Holocene river terrace of Samoylov, the lakes with CDOM Sr values ≤1 and higher CDOM absorption are influenced by the Lena River spring flood. The Lena River spring flood with high water level brings fresh terrestrial-derived CDOM in through flooding the lakes that are on lower topographic levels. The lake group that seems to be influenced by the Lena spring flood in every year are the floodplain lakes and oxbow lakes showing highest CDOM early in summer. The thermokarst lakes on Kurungnakh on the Yedoma plateau do not seem to be influenced by the Lena River spring flood. They show low to medium CDOM absorption. We could define three different lake class types according to the event scale of terrestrial input: (1) pristine thermokarst tundra lakes on the Holocene terrace, (2) topographically low-lying lakes episodically influenced by the Lena River spring flood, and (3) thermokarst lakes on the Yedoma upland with steep topography and shore erosion processes to bring in terrestrial matter into the lakes. Our study represents the first CDOM data collection and the first assessment of CDOM regimes in the lakes in the Lena River delta. This assessment sets reference conditions facilitating detecting future trends of CDOM over time.
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