Tracing water mass mixing in the Baltic–North Sea transition zone using the optical properties of coloured dissolved organic matter

The distribution and characteristics of coloured dissolved organic matter (CDOM) in the Baltic – North Sea transition zone were studied. The aim was to assess the validity of predicting CDOM absorption in the region on the basis of water mass mixing alone and demonstrate the utility of CDOM as an indicator of water mass mixing in coastal seas. A three-end-member mixing model representing the three major allochthonous CDOM sources was sufficient to describe the patterns in CDOM absorption distribution observed. The three-end-member water masses were the: Baltic outflow, German Bight and the central North Sea. Previously, it was thought that water from the German Bight transported northwards in the Jutland coastal current only sporadically influenced mixing between the Baltic and North Sea. The results from this study show that water from the German Bight is detectable at salinities down to 12 in the Kattegat and Belt Sea. On average, 23% of the CDOM in bottom waters of the Kattegat, Great Belt, Belt Sea, Arkona Sea and the Sound originated from the German Bight. Using this conservative mixing model approach, local CDOM inputs were detectable but found to be limited, representing only 0.25% of CDOM in the surface waters of the Kattegat and Belt Sea. The conservative mixing of CDOM makes it possible to predict its distribution and characteristics and offers a powerful tool for tracing water mass mixing in the region. The results also emphasize the need to include the Jutland Coastal current in hydrodynamic models for the region.