Quantifying the effects of in-stream habitat structure and discharge on leaf retention: implications for stream restoration

Input, trapping, and retention of coarse particulate organic matter are among the key determinants of resource availability for consumers in forested headwater streams. Retention efficiency is regulated by the interaction between bed roughness and hydrological characteristics. We examined short-term retention of leaf analogs in artificial flumes, where we manipulated streambed structure and discharge using a factorial design. The substrate structures used are widely applied in stream restoration and represent a gradient of increasing bed complexity: channelized streams, channelized streams + moss, boulder additions, boulders + moss, addition of wood (low-wood and high-wood). We assessed whether, and by how much, the retentive efficiency of a stream is enhanced by various restoration structures and whether the effect of discharge on retention efficiency is similar in channelized and differently restored streams. All restoration structures enhanced leaf retention considerably, and even channelized streams containing mosses were highly retentive. At low discharge, channelized streams lacking mosses retained leaf litter less effectively than did the restored streams, whereas retention differed little among restoration treatments. At high discharge, most restoration structures were much less retentive, and differences in retentiveness between restored and channelized streams became insubstantial, except that streams with abundant wood remained highly retentive at all discharges. Our results suggest that all types of structures added to improve stream retentiveness may enhance leaf retention, and thereby contribute to organic matter dynamics of a stream, but the addition of wood of variable sizes is the best long-term solution to forested stream restoration.