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.

中文翻译：

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水坝和水库对日本西部森林山区溪流中有机物分解的影响

溪流和河流的筑坝会改变下游的生态系统过程，因此了解其影响对于管理森林山区河流至关重要。这项研究考察了五个水库中的水坝对两个相邻山区河流中有机物分解及其季节变化的影响。棉条试验和抗张强度损失的测量被用于评估分解速率。此外，还测量了环境因素，例如水位，水温，无机氮浓度和大型无脊椎动物的贡献。与上游和不受管制的地点相比，有储层的大坝在所有季节都降低了下游的分解率。所有地点的分解率也因季节而异，夏季分解率较高，冬季分解率最低，季节性变化大于两条河流水库的影响。分解速率的季节性变化与水温变化相吻合，表明水温的影响很大。但是，温度差异不能解释水库的影响，因为下游站点的水温始终较高。下游位置的硝酸盐浓度较低，且无脊椎动物的贡献较大，这些因素可能比水温影响更大。因此，由于营养物和生物效应的改变，溪流的筑坝可能降低下游位置的分解速率。但是，水温的季节性变化可能会产生更明显的影响，从而导致季节性变化比站点之间的差异更大。