Photodegradation of aquatic humic substances: An important factor for the Baltic carbon cycle?

Although humic substances (HS) are often considered as chemically stable and non-biodegradable in a marine environment, it is well known that exposure to light will lead to a decomposition of these substances. Laboratory experiments of photodegradation by UV-irradiation of aquatic humic substances and natural water resulted in a rapid and quantitative degradation. Seasonal variations in the humic fraction of the total organic carbon (TOC) was indicated in field studies in humic-rich water in Sweden. The HS/TOC-ratio was considerably higher during the dark part of the year than during the light season. Both the laboratory results and the field studies indicate the potential importance of photodegradation of HS and formation of CO2 for the global carbon cycle, though other mechanisms like microbial degradation are possible. Furthermore, the formation of biodegradable low-molecular weight organic acids from terrestrial humic substances may be essential for partly heterotrophic systems like the Bothnian Bay.