Experimental shading alters leaf litter breakdown in streams of contrasting riparian canopy cover

Abstract

1. Headwater stream ecosystems are primarily heterotrophic, with allochthonous organic matter being the dominant energy. However, sunlight indirectly influences ecosystem structure and functioning, affecting microbial and invertebrate consumers and, ultimately, leaf litter breakdown. We tested the effects of artificial shading on litter breakdown rates in an open-canopy stream (high ambient light) and a closed-canopy stream (low ambient light). We further examined the responses of invertebrate shredders and aquatic hyphomycetes to shading to disentangle the underlying effects of light availability on litter breakdown. 2. Litter breakdown was substantially slower for both fast-decomposing (alder, Alnus glutinosa) and slow-decomposing (beech, Fagus sylvatica) leaf litters in artificially shaded stream reaches relative to control (no artificial shading) reaches, regardless of stream type (open or closed canopy). 3. Shredder densities were higher on A. glutinosa than on F. sylvatica litter, and shading had a greater effect on reducing shredder densities associated with A. glutinosa than those associated with F. sylvatica litter in both stream types. Fungal biomass was also negatively affected by shading. Results suggest that the effects of light availability on litter breakdown rates are mediated by resource quality and consumer density. 4. Results from feeding experiments, where A. glutinosa litter incubated under ambient light or artificial shade was offered to the shredder Gammarus fossarum, suggest that experimental shading and riparian canopy openness influenced litter palatability interactively. Rates of litter consumption by G. fossarum were decreased by experimental shading in the open-canopy stream only. 5. The results suggest that even small variations in light availability in streams can mediate substantial within-stream heterogeneity in litter breakdown. This study provides further evidence that changes in riparian vegetation, and thus light availability, influence organic matter processing in heterotrophic stream ecosystems through multiple trophic levels.

Periphytic algae decouple fungal activity from leaf litter decomposition via negative priming

“…Increased light availability enhances C lability through photolytic (ultraviolet‐induced) degradation of recalcitrant C compounds (e.g., humic acids) into fatty acids and carbohydrate monomers (King, Brandt, & Adair, ; Wetzel, Hatcher, & Bianchi, ), but considerably less emphasis has been placed on the potential for light‐mediated effects via algal growth and C exudation and its subsequent stimulation of heterotrophic decomposers (Danger et al, ; Kuehn et al, ). On leaf litter, active periphytic algae can double bacterial and fungal growth rates (Kuehn et al, ), and enhance C‐ and nitrogen (N)‐acquiring enzyme activities (Rier, Kuehn, & Francoeur, ), and speed decomposition by 20%–126% (Danger et al, ; Halvorson et al, ; Lagrue et al, ). Algae can also increase overall microbial biomass in the litter‐periphyton complex, and because algae are N‐ and phosphorus (P)‐rich relative to litter, this increases nutrient uptake and reduces C:N and C:P ratios (Danger et al, ; Halvorson et al, ).…”

Benthic algae stimulate leaf litter decomposition in detritus‐based headwater streams: a case of aquatic priming effect?

“…(B) Under low-nutrient conditions, competition for inorganic nutrient is high, resulting in reduced algal growth, since decomposers are superior to plants in competition for nutrients (Currie and Kalff 1984). Recent studies have established links between light, aquatic hyphomycetes, invertebrate detritivores, and leaf litter decomposition, which have all been interpreted as indirect evidence of a response of litter consumers to litter-associated algae (Franken et al 2005, Rier et al 2007, Albarin˜o et al 2008, Lagrue et al 2011. The use of this labile C by decomposers should, in turn, increase the mineralization of recalcitrant organic matter through a ''priming effect'' (Guenet et al 2010).…”

Is smaller necessarily better? Effects of small-scale forest harvesting on stream ecosystems

“…4a and 4b). The genuine importance of riparian canopy cover in determining key ecosystem properties needs to be confirmed by further empirical research based on streams spanning a broad gradient of canopy openness and experimental studies examining the interplay between autotrophic and heterotrophic pathways in streams (e.g., Lagrue et al, 2011).…”