Ambient solar radiation effects on Phragmites australis leaf and culm litter were investigated in a shallow and eutrophic south Swedish lake littoral. Leaf and culm litter was exposed to natural sunlight in the lake at a water depth of a few centimeters. For the leaf litter, an additional subset of the experiment was exposed to solar radiation in the air, to evaluate effects of the solar radiation on the leaf litter in a dry state. Radiation treatments (shaded, photosynthetic active radiation [PAR], PAR + ultraviolet-A [UVA] and PAR + UVA + ultraviolet-B [UVB]) were achieved by screening with Plexiglas and Mylar film. Decomposition was measured as dry weight loss, and fungal and bacterial abundance on the detritus was estimated as ergosterol and bacterial numbers, respectively. We found no differences in either weight loss or bacterial abundance among radiation treatments. The fungal biomass in the dry leaf litter was unaffected by the radiation. In the wet leaf litter, however, the ergosterol content in PAR, PAR + UVA and PAR + UVA + UVB treated samples was about one third of the amounts found in the initial material and in the samples kept darkened. Similarly, the fungal biomass associated with the culm litter was negatively affected by solar PAR + UVA + UVB radiation, but in culms exposed only to PAR or to PAR + UVA it was not significantly different from the fungal biomass in darkened samples. These results suggest that the net effects of radiation differ between fungi and bacteria, with the fungi being more susceptible to suppression by solar radiation than the bacteria. Our experiments mimic more closely than previously published studies the conditions that can be expected in natural environments. Hence, we propose that previous reports of strong radiation effects on aquatic liner degradation should be applied very carefully to natural conditions.