O. Dangles et F. Guerold, Influence of shredders in mediating breakdown rates of beech leaves in circumneutral and acidic forest streams, ARCH HYDROB, 151(4), 2001, pp. 649-666
From December 1997 to July 1998, the breakdown of beech leaves (Fagus sylva
tica) and associated macroinvertebrate assemblages was investigated in seve
n 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 shred
ders 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, indicati
ng that direct microbial contribution to the mass loss of beech leaves was
small. On the contrary, breakdown of leaves enclosed in coarse-mesh bags wa
s drastically reduced under acidic conditions (mass loss = 15 to 23 % after
270 days) when compared to circumneutral conditions (mass loss = 91.7 to 9
7.7 % after 270 days). Shredder assemblages associated with bags in acidic
streams were dominated by Nemouroidea stonefly larvae, which showed low shr
edding efficiencies in laboratory experiments, particularly the early-insta
r larvae. Beech leaf breakdown rates rapidly increased at a mean stream pH
> 5.5, apparently due to the increasing abundance of the acid-sensitive shr
edders, S. personatum and G. fossarum, which shredded beech leaves more eff
iciently in laboratory feeding trials (even the first instars in the case o
f G. fossarum). The results of this study suggest that invertebrate shredde
rs play an important role in controlling rates of beech leaf breakdown in f
orested Vosgian streams that differ in levels of anthropogenic acidificatio
n.