Litter processing along a stream gradient: the role of invertebrates and decomposers

Dissolved nutrients and temperature tend to increase in a downstream direction, whereas shredder density tends to decrease. As a result, the relative importance of microbes (bacteria and fungi) and invertebrates in leaf litter processing may gradually shift along a stream gradient. Therefore, we hypothesized that differences in litter decay between fine-mesh (invertebrates excluded) and coarse-mesh (accessible to invertebrates) bags will be high in low-order streams (i.e., 4). To test this hypothesis, we investigated the processing of alder (Alnus glutinosa) litter in 12 sites ranging from 2nd to 6th order in central Portugal during autumn/winter and spring/summer seasons. Mass loss rates (measured as % ash-free dry mass AFDM) were higher in spring/summer than in autumn/winter and higher in coarse- than in fine-mesh bags. No clear relationship was observed between river order and litter processing (% AFDM loss). In spring/summer, the difference in remaining mass between fine- and coarse-mesh bags was higher in low-order than in high-order streams and decreased in a downstream direction, supporting our hypothesis. Other evidence for shifting in processing vectors includes the observations that 1) the biomass and % of shredders were generally higher in low-order than in high-order streams and tended to decrease downstream, 2) high microbial biomass was reached earlier in high-order than in low-order rivers, and 3) the density of fungal conidia tended to increase with increasing stream order. No pattern of shifting in processing vectors was observed in autumn/winter, possibly because food was nonlimiting.