Solar radiation effects on decomposition and decomposers of plant litter in lakes

Abstract: This thesis summarizes the results of a series of experiments, from test tube up to field study scale, performed to increase the understanding of solar radiation effects on decomposition and microbial decomposers of plant litter in stagnant water of littoral zones. In conclusion, I suggest that fungi play a significant role in the decomposition of plant litter submersed in lakes, and that their distribution is dependent on detritus quality and water chemistry, but also tends to be inversely related to the occurrence of bacteria attached to the litter. Solar radiation affected fungi and bacteria differentially. Compared to bacteria, fungi were more seriously inhibited by solar radiation, suggesting a higher relative importance of bacteria in solar exposed habitats. This thesis also shows possible ways in which solar radiation can affect plant litter decomposition in aquatic environments. These effects include dissolution, photo-mineralization, changed spectral properties of dissolved organic matter, modified extracellular enzyme activity, and altered biomass of both bacteria and fungi. Not only UV-radiation but also visible light is an important agent in these processes. These findings are valid for controlled laboratory systems, but the results were only to a minor extent verified in field experiments. Plant litter was confined to sheets or small boxes open to colonization by invertebrates, placed close to the water surface in a lake littoral, and shielded by filters to exclude different fractions of the solar radiation spectrum. The different radiation regimes did not induce any differences in net decomposition after about 2 months. However, the composition of microbial communities was sensitive to radiation. Fungal biomass attached to the litter was negatively affected, resulting in a higher relative abundance of bacteria compared to fungi in solar exposed habitats.