Organic matter decomposition in a forest mountain stream with a sediment control dam in Japan

Sediment control dams are installed in some mountain regions to prevent streambed erosion and excessive sediment runoff, yet their effects on stream function are not well understood. In this study, we examined the effects of a sediment control dam on organic matter decomposition in a mountain stream by comparing upstream and downstream reaches in benthic and hyporheic zones over 5 seasons using the cotton-strip assay. Total decomposition rates, incorporating both macroinvertebrate-mediated and microbial-mediated decomposition, were lower downstream in the benthic zone in the fall and in the hyporheic zone in the summer and fall. Thus, sediment control dams appeared to alter organic matter decomposition rates downstream in some seasons. Environmental factors measured, such as temperature, NO₃-N concentrations, and contribution of macroinvertebrates, influenced decomposition rates, but no single factor was sufficient to explain the effects of the dam and its seasonality in this study. Sediment control dams are smaller in size compared to water storage dams but widely distributed in mountain regions of Japan, and their cumulative effects on stream ecosystem function should be evaluated further for the management of mountain streams.