Poster Session 1, Monday,  October  3, 15:40–18:00

Poster 53

Thermo-haline stratification controls the vertical profiles of optical properties and Fluorescent Dissolved Organic Matter composition in the Baltic Sea

Vertical profiles of Fluorescent Dissolved Organic Matter (FDOM) collected in the Baltic Sea from 2014 to 2020 with the use of a three-channel WET-Star in situ fluorometer have been analyzed. Sampling sites in the Bornholm and Gdansk Basins with depths exceeding 70m were selected to include mesohaline deoxygenated deep water layers below the permanent halocline. Spectral characteristics of FDOM fluorometer enabled to observe profiles of marine and terrestrial humic-like, I CH1 , I CH2, and protein-like, I CH3 DOM fractions, and their seasonal evolutions. Winter conditions were set as reference when convective mixing forced a homogenous distribution of IOP and considered FDOM fractions down to a permanent halocline. Surface layer warming throughout spring and the development of thermal stratification impacted FDOM vertical profiles. I CH3 followed the pattern of phytoplankton biomass indicated by phytoplankton pigment absorption line height, a LH (676). At the subsurface maximum, I CH3 was ca. 20% higher relative to observations below the thermocline. Vertical profiles of I CH1 and I CH2 were characterized by decreased values in the mixed layer compared to fluorescence intensities of respective fractions measured below the thermocline. At the end of summer, when thermal stratification was strongest, there were observed elevated values of I CH3 and significantly depleted values of I CH1 and I CH2 . FDOM intensities below halocline were controlled by periodic inflow of mesohaline water from Danish Straits and DOM diffusion from bottom sediments. The non-linear processes impacting respective FDOM fractions in distinct vertically stratified water masses are discussed.

Piotr Kowalczuk, Institute of Oceanology Polish Academy of Sciences, [email protected], 0000-0001-6016-0610 

Monika Zabłocka, Institute of Oceanology Polish Academy of Sciences, [email protected], 0000-0002-1803-7042

Karolina Borzycka, Institute of Oceanology Polish Academy of Sciences, [email protected], 0000-0002-4160-2548

Alexnadra N. Loginova, Institute of Oceanology Polish Academy of Sciences, [email protected], 0000-0003-2805-7386

Elena Terzić, Institute of Oceanology Polish Academy of Sciences, [email protected], 0000-0002-9335-6174

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