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.
Highlights
Riparian leaves fuel aquatic food webs with organic matter (OM) that serves as a food source for a diverse array of microorganisms and macroinvertebrates
For U. dioica, a difference in leaf disk mass loss was found between treatments, in which leaf disks exposed to sunlight degraded faster than leaf disks in shaded conditions (ANOVA: F = 6.139, p = 0.029; figure 1), but there was no significant impact of microorganisms or an interaction effect between sunlight exposure and presence of microorganisms
The control experiment without invertebrates to assess the effect of illumination on biofilm formation on the DECOTABs revealed a reduced (Wilcoxon signed rank test: V = 0, p = 0.002) number of substrates used by the microbial community in the illuminated area compared to the shaded area
Summary
Riparian leaves fuel aquatic food webs with organic matter (OM) that serves as a food source for a diverse array of microorganisms and macroinvertebrates. The relative importance of key factors that govern the degradation of OM remains a central issue. OM-degradation is typically considered to be primarily governed by biotic factors such as microbial decomposition and invertebrate consumption (Odum and de la Cruz 1963), in which key abiotic factors such as water current (physical abrasion) and temperature (promoting detritivore activity) are considered to contribute to the overall process (Throop and Archer 2009). Despite the importance of detritivores for OM degradation, many studies have difficulties reconciling OM degradation rates with the composition of aquatic invertebrate communities (e.g. Gonçalves et al 2006, Lagrue et al 2011, Hunting et al 2016), suggesting other (abiotic) variables contribute to OM-degradation in freshwater ecosystems. Provided that sunlight is able to penetrate to the sediment in shallow freshwater bodies and fringe areas of lakes, where substantial amounts of OM accumulate (Morris et al 1995), it is conceivable that sunlight could potentially affect OM degradation in aquatic systems
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