We investigated the effects of UV radiation on abiotic decomposition and dissolution of leaf litter from the aquatic macrophyte Phragmites australis. Dead leaves were autoclaved and incubated in quartz tubes with autoclaved Milli-Q water, in darkness, under UVA or under UVA+UVB radiation, using fluorescent tubes with intensities similar to solar radiation. More DOM was produced in irradiated samples compared to dark ones. After exposure, the water was inoculated with a natural assemblage of bacteria and incubated in the dark. Free bacteria reached higher densities in cultures with detritus pre-treated in darkness than in cultures with irradiated substrates. No significant differences in numbers of attached bacteria were found. We suggest that UV-irradiated detritus from P. australis has a negative effect on the growth of free living bacteria, possibly due to the leaching of inhibitory substances from the leaf litter into the water and/or a decrease in DOM bioavailability. Sirnilar experiments, using natural solar radiation, indicated that visible Light has only a minor effect on leaching of inhibitory substances.

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Summary 1. The effects of solar radiation on bacterial and fungal growth on aquatic macrophyte detritus were studied in a microcosm experiment. Senescent leaves of Phragmites australis were incubated for 63 days in shallow water in the shade under photosynthetically active radiation (PAR) together with ultraviolet radiation, or under filters removing either ultraviolet B (UVB) or both UVB and ultraviolet A (UVA). 2. Bacterial abundance and bacterial 3H‐leucine incorporation in the water were measured, together with α‐ and β‐D‐glucosidase activity. In addition, bacterial abundance and fungal biomass associated with the litter were measured. 3. The results indicate that both PAR and UVA affect the micro‐organisms involved in the decomposition of leaf litter. The α/β‐D‐glucosidase activity ratio was less than one in irradiated and more than one in shaded microcosms, suggesting a change in the substrate dissolved organic matter composition towards more β‐ than α‐glycosidic linkages as a result of solar radiation. 4. Microcosms receiving UVB displayed a significantly higher β‐D‐glucosidase activity than shaded microcosms, and those exposed to PAR or PAR + UVA, demonstrating the potential importance of UVB radiation. 5. The free‐living bacteria tended to be dominated by filamentous forms in microcosms subject to solar radiation, especially PAR, and attached microbial communities showed a greater tendency to be dominated by bacteria in irradiated microcosms than in shaded microcosms.

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