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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) and low in high-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.