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

Dissolved nutrients and temperature tend to increase in a downstream direct ion, whereas shredder density tends to decrease. As a result, the relative importance of microbes (bacteria and fungi) and invertebrates in leaf litte r processing may gradually shift along a stream gradient. Therefore, we hyp othesized 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). T o test this hypothesis, we investigated the processing of alder (Alnus glut inosa) litter in 12 sites ranging from 2nd to 6(th) order in central Portug al during autumn/winter and spring/summer seasons. Mass loss rates (measure d as % ash-free dry mass [AFDM]) were higher in spring/summer than in autum n/winter and higher in coarse- than in fine-mesh bags. No clear relationshi p was observed between river order and litter processing (% AFDM loss). In spring/summer, the difference in remaining mass between fine- and coarse-me sh bags was higher in low-order than in high-order streams and decreased in a downstream direction, supporting our hypothesis. Other evidence for shif ting in processing vectors includes the observations that 1) the biomass an d % of shredders were generally higher in low-order than in high-order stre ams and tended to decrease downstream, 2) high microbial biomass was reache d earlier in high-order than in low-order rivers, and 3) the density of fun gal conidia tended to increase with increasing stream order. No pattern of shifting in processing vectors was observed in autumn/winter, possibly beca use food was nonlimiting.