Influence of shredders in mediating breakdown rates of beech leaves in circumneutral and acidic forest streams

Abstract

From December 1997 to July 1998, the breakdown of beech leaves (Fagus sylvatica) and associated macroinvertebrate assemblages was investigated in seven headwater forested streams that differed in mean pH (4.64 to 7.20) due to acidification. Comparisons were made between leaves held in 0.3 mm, and 5 mm mesh bags in order to assess the contribution of macroinvertebrate shredders in litter breakdown. In addition, shredding efficiencies of five acid-tolerant and acid-sensitive invertebrate taxa (Protonemura sp., Leuctra sp., Sericostoma personatum, Potamophylax cingulatus, Gammarus fossarum) were investigated under laboratory conditions. Results showed that breakdown of leaves enclosed in fine-mesh bags was very low in all the streams, indicating that direct microbial contribution to the mass loss of beech leaves was small. On the contrary, breakdown of leaves enclosed in coarse-mesh bags was drastically reduced under acidic conditions (mass loss = 15 to 23 % after 270 days) when compared to circumneutral conditions (mass loss = 91.7 to 97.7 % after 270 days). Shredder assemblages associated with bags in acidic streams were dominated by Nemouroidea stonefly larvae, which showed low shredding efficiencies in laboratory experiments, particularly the early-instar larvae. Beech leaf breakdown rates rapidly increased at a mean stream pH>5.5, apparently due to the increasing abundance of the acid-sensitive shredders, S. personatum and G. fossarum, which shredded beech leaves more efficiently in laboratory feeding trials (even the first instars in the case of G. fossarum). The results of this study suggest that invertebrate shredders play an important role in controlling rates of beech leaf breakdown in forested Vosgian streams that differ in levels of anthropogenic acidification.