Decomposition of allochthonous organic matter is an essential process in headwater streams. Damming of streams alters decomposition rates in the benthic zone downstream, but little is known about the effects on hyporheic decomposition. We examined the effects of dams on hyporheic and benthic organic matter decomposition, using the cotton-strip assay over five seasons, in two forest mountain streams in western Japan. The decomposition rates in the hyporheic zone were lower downstream of the dams than at the unregulated reach in spring, rainy season and fall, but they did not differ in winter and summer. Hyporheic decomposition rates were comparable to the benthic rates in one river and were lower in the other river. Decomposition rates did not differ between coarse- and fine-mesh bags in many seasons at all sites, and the densities of macroinvertebrates were low, suggesting that the contribution of macroinvertebrate to decomposition in the hyporheic zone was small. These results showed that the hyporheic zone is an important zone for decomposition and that presence of dams altered the hyporheic decomposition in some seasons. Thus, it is crucial to examine both the benthic and hyporheic zones when addressing the effects of dams and reservoirs on stream ecosystem processes.
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