Mixing anomaly in deoxygenated Baltic Sea deeps indicates benthic flux and microbial transformation of chromophoric and fluorescent dissolved organic matter

Optical measurements have indicated an anomalous distribution of Colored Dissolved Organic Matter absorption coefficient – aCDOM(λ), and dissolved organic matter fluorescence – FDOM intensity, in the salinity gradient in deep Baltic Sea waters below the permanent pycnocline. The two steps mixing model of the aCDOM(λ) with salinity have been derived based on 3521 CDOM absorption observations done in Baltic Sea, from 1993 until April 2014 and 1011 CDOM absorption observation done in the Danish Straits from August 2006 until February 2008. Disproportionate decrease and even increase of CDOM absorption coefficient at 350 nm, aCDOM(350), with increased salinity was observed in deep waters. There was a significant increase of the aCDOM(350) mixing model residual values in deep water compared to overlaying waters. The aCDOM(350) mixing model residual values tended to increase significantly with increasing salinity in deep waters. The CDOM and FDOM composition has been assessed by spectral slope coefficient, S300–600, the Humification Index (HIX), calculated from measured DOM fluorescence Excitation–Emission Matrix spectra and from ratio of the humic-like and protein-like DOM fluorescence measured in situ. The S300–600 values decreased significantly in deep waters compared with overlaying waters. There was a strong (R2 = 0.705) inverse linear relationship between S300–600 and aCDOM(350) mixing model residuals. The HIX, and the values of the ratio of the humic-like to protein-like DOM fluorescence increased significantly in the deep waters, what indicated increase of humic-like and decrease of protein-like DOM fraction in the deep waters samples compared to control surface waters samples. An exponential relationship between the humic-like to protein-like FDOM intensity ratio and the apparent oxygen utilization – AOU was observed in the Gulf of Gdansk. Presented results suggested that possible DOM emission from sediments into the adjacent near bottom water in anoxic conditions, could explain the aCDOM(λ) mixing anomaly. Microbial DOM reprocessing in anoxic condition, documented by direct in situ measurements, significantly changed the CDOM and FDOM composition.