Effects of rising temperature on a functional process: consumption and digestion of leaf litter by a freshwater shredder
Abstract
Climate change will likely affect riparian litter decomposition in freshwater streams in future decades by modifications in both microorganism and invertebrate activities. On the one hand, the expected increase in C:N ratio will decrease the palatability of leaf litter, and consequently slow down decomposition rates of leaves. On the other hand, rising temperatures may stimulate litter decomposition.
We conducted a feeding study that associated the key shredder species Gammarus pulex (Crustacea, Amphipoda) with common riparian leaf litter (from the tree Alnus glutinosa) from European freshwater streams. We assessed the effects of temperature on i) changes in leaf palatability during conditioning (microbial colonisation), ii) leaf consumption rates, and iii) leaf digestion by shredders. Moreover, we compared the responses to rising temperatures between two populations of G. pulex, living in contrasting thermal environments (humid continental climate vs. temperate Mediterranean climate).
We found that the decreased mechanical resistance of leaf tissue, which was induced by the conditioning, was stimulated by an increase in temperature. However, we found no increase in fungal biomass even at the highest temperatures (21 and 24 degrees C). The leaf consumption rate by amphipods increased slightly but not linearly with rising temperatures. Moreover, digestive enzyme activities in amphipod guts were clearly favoured by an increase in water temperature. Catalytic activities of digestive enzymes were generally higher for amphipods living under continental climate conditions.
Our study supported the idea that the effect of rising temperatures on the leaf litter decomposition would not be unidirectional. Higher temperatures would lead to contrasting impacts on both physiology and ecology of organisms involved in this functional process.
