Small forested streams typically are dominated by heterotrophic processes and rely on retained allochthonous material, including coarse particulate organic matter (CPOM), to support stream food webs. Within the Ottawa National Forest (ONF), Michigan, 3 small streams of similar size (discharge <130 L/s) were used to measure the retention (e.g., particle travel distance) of various allochthonous particles. Allochthonous CPOM consisted of natural (Ginkgo biloba leaves, white pine needles, and sugar maple sticks) and analog (wood dowels of 3 lengths, paper triangles, and paper rectangles) particles to test if particle type affected travel distance and if analog particles would be a good surrogate for natural particles. Travel distances of the particles were measured monthly from June to November 2003. Physical parameters, such as velocity, depth, wood density and volume, and substrate size, were measured in each reach to determine if instream characteristics affected coarse particle travel distances. Mean travel distance was >7 m for all particle types. Travel distance was not significantly different between particle analogs and natural particles or among natural particles. Significant differences in particle travel distance (p < 0.001) occurred among wood dowel lengths and between paper rectangles (mean ± standard error, 4.9 ± 0.3 m) and paper triangles (5.9 ± 0.5 m). For all particles, travel distance was positively related to current velocity, discharge, and stream depth and negatively related to instream wood volume and density (p < 0.05), but travel distances were not significantly different among months. Visual observation also indicated that particle flexibility and surface area reduced particle travel distance. Our results suggest that: 1) low-gradient streams in mixed coniferous–deciduous forests of Upper Michigan are highly retentive of a range of CPOM types and are more retentive than are other low-gradient streams; 2) the analog particles were reasonable mimics of natural particle transport, and the different lengths of wood dowels affected travel distance; and 3) instream hydraulic and physical parameters, especially large woody debris, were major factors in CPOM retention. In forested headwater streams, the ability of the channel to efficiently retain multiple types of allochthonous material might be crucial to overall stream productivity.
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Vol. 27 • No. 3