Photochemical and microbial decomposition of chromophoric dissolved organic matter during long (months–years) exposures

Photochemical and microbial decomposition of chromophoric dissolved organic matter (CDOM) was studied at time scales in which terrestrially derived CDOM is transported from many fresh water watersheds to coastal waters. During 70-day exposure, solar radiation decomposed 96% of freshwater CDOM but only 41% of DOC. Apparent quantum yields for photochemical decomposition decreased in proportion to the dose of absorbed photons by CDOM. When apparent quantum yield for photochemical decomposition of CDOM was applied to an optical model, it revealed that solar UV radiation was responsible for 75% and 44% of the decomposition of CDOM at the surface and in the whole water column, respectively. The modeled rates of photochemical decomposition were in good agreement with the measurements in a 2-m deep photolytic stratum. The photochemical decomposition of CDOM attenuated in the water column similarly to violet-blue irradiance. During 500-day long incubations in darkness, plankton decomposed CDOM as much as was decomposed at the surface by solar radiation in a week. Thus, solar radiation was much more important than plankton in the decomposition of CDOM at the surface.