Factors influencing retention of coarse particulate organic matter in streams

Input of terrestrial coarse particulate organic matter (CPOM: leaves and small wood, >1 mm) adds food resources and habitat to streams. Restoration projects often aim to increase CPOM through riparian planting, but this relies on inputs being retained within the restored reach, and quantitative knowledge of factors influencing retention processes is limited. We investigated retention characteristics under base‐flow conditions in 65 stream reaches in pasture, native forests and pine plantations (with and without recent logging) in five contrasting New Zealand regions, using three CPOM analogues, namely 30 cm dowels, soaked ginkgo leaves and waterproof paper triangles. These differed significantly in geometric mean retention distance (Sp), with greatest retention (smallest Sp) of dowels > ginkgos > triangles. Retention efficiency was greater in riffles and glides than in pools. Objects trapping CPOM differed between land uses and regions in predictable ways. For example, in relatively unshaded pasture streams, instream and encroaching vegetation riparian was more important, and wood less important, than in forest. Catchment‐area‐ and width‐specific Sp values were more informative for comparisons amongst sites than deposition velocities (vdep). Catchment‐area‐specific Sp values varied 10‐fold amongst regions and threefold amongst land uses. Stepwise multiple regression models explained 71–74% of variance in Sp for the different CPOM analogues and showed that flow (+ influence; i.e., Sp increases with flow) was the most important factor, followed by submerged wood (−), with periphyton biomass (−) also significant in all models. A comparison with results from Spain showed similar relationships between Sp of plastic CPOM analogues and discharge under base‐flow conditions. Restoring riparian vegetation is expected to ultimately enhance instream CPOM by increasing input and retention by wood. Our findings indicate that this should prioritize small streams if the aim is to optimize the restoration of CPOM influence on instream processes. Copyright © 2007 John Wiley & Sons, Ltd.