Photochemically induced changes in bioavailable carbon and nitrogen pools in a boreal watershed

In several recent studies, a net stimulation of bacterial growth has been d emonstrated after exposing humic surface waters to solar radiation or artif icial ultraviolet radiation. This stimulation has been attributed to a phot ochemical release of bioavailable carbon or nitrogen compounds (ammonium). In a synoptic experiment, we exposed 0.2 mu m filtered water from 12 differ ent habitats in a river system, dominated by allochthonous carbon input, to mild artificial UV radiation. A significant photochemical release of carbo xylic acids of low molecular weight occurred. Furthermore, the exposure inc reased carbon-limited bacterial yield on average by a factor of 1.7. No pho tochemical production of free ammonium could be detected, which was in acco rdance with the lack of effects of radiation on bacterial growth yield unde r nitrogen-limited conditions. We conclude that, in boreal systems dominate d by allochthonous carbon input, photochemical production of bioavailable c arbon rather than nitrogen compounds is likely to positively influence the total substrate pool available for bacterial utilization.