Significance of sunlight for organic matter degradation in aquatic systems

Abstract

Degradation of organic matter (OM) is generally considered to be primarily governed by biotic factors in aquatic environments. However, a number of abiotic processes also play key roles in mediating OM-degradation. Sunlight can act as a principal abiotic driver of the degradation of terrestrial organic matter, but its importance for freshwater ecosystems and possible interactions with biotic drivers remains poorly understood. We carried out two microcosm experiments which focused on the role of sunlight on microbial and invertebrate-mediated OM degradation using two species of plant leaves and the aquatic invertebrate Asellus aquaticus. Results indicated that sunlight was the primary driver of leaf mass loss during the early stages of decomposition, whereas microbial communities had a negligible effect. Sunlight was observed to strongly affect invertebrate behavior as invertebrates avoided direct illumination. This alteration of behavior resulted in a reduction in the consumption of a leaf surrogate (DECOTAB) by A. aquaticus. Together, these results indicate that sunlight has the potential to strongly influence structural and functional attributes of shallow freshwater systems, and hence serve as an appraisal to consider sunlight as a significant direct and indirect physical driver governing OM degradation in shallow aquatic systems.