Consumption of leaf litter by stream invertebrates: Evaluating leaf size

Consumption of terrestrial leaf litter by stream invertebrates is an important component of leaf breakdown.  The size of leaves comprising a leaf pack can affect breakdown rates, but underlying mechanisms are unclear.  We hypothesize that large leaves (based on width) are preferred by shredders because of high nitrogen content (low C:N).  To test this hypothesis, we investigated consumption rates of autumnal senescent red maple (Acer rubrum) leaves by Gammarus pseudolimnaeus and Pycnopsyche lepida.  Using a factorial design, leaf discs (diameter= 2.5 cm) from small (<7 cm), medium (7-9.5 cm), or large (>9.5 cm) leaves were placed in in-stream containers with shredders (2 G. pseudolimnaeus or 1 P. lepida) for 48-hr feeding trials.  We also examined quality (C:N) of leaves from 5 randomly selected trees that were incubated in the stream for 14 days.  On a biomass basis, G. pseudolimnaeus consumed 4× more leaf material than P. lepida (P<0.001), but consumption rates did not differ with leaf size (P=0.202) or the size-shredder interaction (P=0.831).  However, large leaves had lower C:N than medium or small leaves (P=0.0178), and C:N differed among trees (P=0.008).  Although leaf quality was a function of size, shredders did not show a feeding preference based on leaf size.