Damming of streams and rivers alters downstream ecosystem processes, and understanding its effects is essential in managing forested mountain streams. This study examined the effects of dams with a reservoir on organic matter decomposition and its seasonality in two neighbouring mountain streams over five seasons. The cotton-strip assay and measurement of tensile strength loss were used to evaluate decomposition rates. In addition, the environmental factors, such as water level, water temperature, inorganic nitrogen concentrations and contribution of macro invertebrates were measured. The dams with a reservoir lowered decomposition rates downstream compared to upstream and unregulated sites, in all seasons. The decomposition rates also varied by season at all sites, with higher rates in summer and lowest in winter, and the seasonal variation was larger than the effects by the reservoirs in both streams. Seasonal variation in decomposition rates coincided with water temperature variation, suggesting strong influence of water temperature. However, the temperature differences did not explain the effects of reservoirs because water temperature was always higher at downstream sites. The downstream sites had lower nitrate concentrations and contribution of macroinvertebrates, and these factors may have greater effects than water temperature. Thus, damming of streams may lower the decomposition rates at downstream sites due to altered nutrient and biological effects. However, seasonal variation in water temperature may have more pronounced effects, resulting in greater seasonal variation than the difference among sites.
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