Role of transported particulate organic matter in the macroinvertebrate colonization of litter bags in streams

1. The exposure of mesh litter bags has been widely used to investigate the role of benthic macroinvertebrates in leaf processing in freshwaters. In t his sense, several studies have related litter bag breakdown rates to the p resence of colonizing invertebrates. A possible confounding factor in such experiments is that the litter bags trap suspended particulate organic matt er that can itself attract invertebrate colonists, irrespective of the inte nded experimental treatment. 2. We attempted to quantify the accumulation of particulate organic matter (POM) within litter bags and to investigate its possible impact on macroinv ertebrate density and richness. In seven headwater forested streams we expo sed mesh bags filled either with beech leaves (Fagus sylvatica) or with pla stic strips of an equal surface area. 3. Principal component analysis (PCA) showed that bag type and stream were the main explanatory variables for invertebrate colonization and POM accumu lation within the bags. In contrast, there was little variation among sampl ing dates (6.4% of the total inertia). 4. The accumulated POM within the bags was substantial (up to 8.83 g ash-fr ee dry mass (AFDM)) but highly variable among sites (mean from 0.32 to 4.58 g AFDM). At each of the seven sites, both richness and abundance of invert ebrates were positively correlated with the mass of accumulated POM in bags . Macroinvertebrate colonization (notably taxon richness) was directly link ed with the quantity of POM accumulated. 5. Our findings provide evidence of a potential pitfall in linking inverteb rates to litter processing in mesh bags, particularly when large amounts of POM, entrained in stream flow, accumulate within the bags. An evaluation o f the POM mass trapped in litter bags could account for the erratic pattern s sometimes observed in their colonization by invertebrates.