Dissolved organic matter (DOM) source, composition, photochemical alteration and availability affect freshwater ecosystems, their carbon fluxes and thus the global carbon cycle. The aim of this study was to gain an understanding of how these components influence the optical properties and microbial processing of chromophoric DOM (CDOM). The experiments included monitoring of photochemical effects of irradiation on allochthonous and autochthonous dissolved organic carbon (DOC), as well as

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... ent microbial degradation processes. Excitation-emission spectroscopy and absorption spectra were used to describe and quantify the change in optical and fluorescent properties of CDOM from a calcareous headwater stream and from various adjacent water bodies. Microbial respiration was monitored to identify changes in the bioavailability of irradiated DOC samples. Exposure to a spectrum similar to natural sunlight significantly affected the optical properties of CDOM and consequently augmented the bioavailability of DOC to the microbial metabolism; autochthonous DOC from periphytic algae which served as a potential endmember of a DOC continuum in the streamwater did not follow this pattern. The results suggest a large proportion of soil-derived carbon in the stream, which is traditionally thought to be refractory. However, my experimental results suggest that if this DOC is photooxidized, it becomes partially photo-bleached and converted into low-molecular-weight compounds, which enhance microbial metabolism. The photochemical alteration of CDOM provides the opportunity for microbial processing of the photooxidized substrates, eventually increasing CO2 emissions and reducing CDOM concentrations in stream ecosystems.