Linking the chemical and optical properties of dissolved organic matter in the Baltic–North Sea transition zone to differentiate three allochthonous inputs

Optical and chemical properties of dissolved organic matter (DOM) were resolved in the mixing zone between the North Sea and the Baltic Sea. The results from four cruises undertaken from August 2006 to February 2007 are presented. Relationships between the optical (absorption and excitation–emission matrix (EEM) fluorescence), dissolved organic carbon (DOC), and dissolved lignin concentrations were determined and carbon stable isotope values (δ13C) of DOM were also used to evaluate DOM sources. Chromophoric DOM (CDOM), measured as absorption at 300 nm, was a strong predictor for DOC and lignin concentrations. The DOM fluorescence (FDOM) characteristics were modeled by parallel factor analysis (PARAFAC) and a six component model was derived. The intensity of one terrestrial humic-like fluorescent component (Fmax3) was strongly correlated to dissolved lignin concentrations. DOC concentrations were best modeled by the combination of this component with an amino acid-like fluorescent component. A three end-member mixing model developed for the region using CDOM–salinity relationships was used to estimate end-member lignin and DOC concentrations, which produced results consistent with other published results from the region. The Baltic Sea outflow was determined to be the dominant source of dissolved lignin in these waters. Despite the high riverine influence in the Jutland Coastal Current, the DOM originating from these waters contained little lignin and was predominantly autochthonous. Additionally CDOM and carbon stable isotope measurements revealed substantial autochthonous production in the Kattegat. Finally, we used the fluorescence–DOC relationships to estimate the net export of terrestrial C from the Baltic Sea at 0.8 Tg year− 1, which represents 45% of the calculated net DOC flux (1.7 Tg year− 1).