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Abstract

We compared the relative importance of abiotic oxygen consumption due to photochemical oxidation with microbial respiration and primary production in samples from lakes with different humic content. In light-exposed samples, the abiotic oxygen consumption rate increased with increasing DOC content, while dark-exposed samples showed none, or slight, abiotic oxygen consumption. There was no relation between primary production and DOC content or electron-transport-system (ETS) activity, but ETS activity was correlated with DOC, suggesting that heterotrophic activity increased as DOC content increased. The ratio of primary production to the sum of photochemical oxygen consumption and ETS activity showed a negative trend as DOC increased, indicating the increasing importance of oxygen consuming processes in DOC rich waters. ETS activity increased after incubation of samples, but this increase was higher for light incubated samples than for samples kept in the dark. The increase in ETS activity was more pronounced in water rich in DOC, indicating that bacterial substrate may be produced from humic substances by photochemical processes.