Modeling absorption by CDOM in the Baltic Sea from season, salinity and chlorophyll

In the Baltic Sea the underwater light regime is to a large extent governed by absorption by colored dissolved organic matter (CDOM), where the largest source is riverine input. We have applied a conservative mixing model of CDOMs optical properties to a dataset collected in the Baltic Sea during 11 years of continuous observations in 1993–2004. The majority of observations agreed well with the model indicating that conservative mixing is important in the area, as terrestrial organic matter is diluted with open sea water. Deviations from the conservative mixing pattern mainly occurred at salinities over 6.8, and chlorophyll a concentrations over 1.5 mg m− 3, and were located in open sea waters, coastal zone and Pomeranian Bay. The seasonal dependence between the light absorption coefficient by CDOM aCDOM(375) and salinity and chlorophyll a concentrations was explored. In March, April and November, months of intensive mixing and high riverine discharge, most of the variability in aCDOM(375) values could be explained by dilution of terrestrially derived CDOM alone. In February, May, and September, months of thermal stratification, reduced riverine discharge and enhanced biological activity, inclusion of chlorophyll a concentration resulted in significantly better models. Autochthonous production of CDOM was found to be a significant source of CDOM in the Southern Baltic Sea in these months. A series of algorithms for the prediction of CDOMs optical properties is presented.