Mechanical abrasion and organic matter processing in an Iowa stream

Rates of organic matter processing are key parameters for studies of stream ecosystem function and stream community ecology. Most studies of organic m atter processing in streams use leaves in leafpacks or litterbags, which ar e immobilized and partly shielded from contact with stones in transport and in the stream bottom. As a result, these studies may underestimate the con tribution of mechanical abrasion to overall processing rate (of coarse orga nic detritus to fine particles). We compared leaf processing rates in litte r bags with and without stones (5 cm crushed limestone ballast) in Big Mill Creek, eastern Iowa. In two of three experiments, processing was significa ntly more advanced in bags with stones than in bags without stones: the fra ction of leaf mass reduced to small fragments (1.4-9.5 mm) was 45% and 93% higher in bags with stones. In a fourth experiment, we compared the effects of stones and shredders (Gammarus pseudolimnaeus, at near-natural densitie s) on fragmentation of leaves in litterbags. This experiment indicated that mechanical and biological agents of processing are roughly equally importa nt in Big Mill Creek. Our results indicate that mechanical abrasion can be an important contributor to organic matter processing in streams. If so, it may be an important source of the finer particles used by collectors. Litt erbag and leafpack experiments may underestimate total processing rates and overestimate the relative importance of processing by microbes and inverte brates.