Assessing alteration of leaf litter breakdown rate influenced by dam operation in Nakatsugawa River and Arakawa River, Central Japan

The process of leaf litter breakdown is essential as a main energy source for aquatic life in stream ecosystems. This process is complicated due to the interaction among the stream's physical, chemical, and biological characteristics as well as the composition of litter. This study investigated the variation of litter breakdown rates with different water qualities and microbial activities influenced by dam operation. Litter bags were installed along Nakatsugawa River, which is regulated by Takizawa Dam, and Arakawa River in Saitama Prefecture, central Japan, during autumn and winter. Three dominant leaf species were studied, namely Quercus serrata, Carpinus cordata, and Acer mono. The factors possibly affecting the litter breakdown process were: (1) a 4 °C higher water temperature downstream than upstream of the dam in the Nakatsugawa; (2) high bacterial productivity in the Nakatsugawa; (3) high silt deposition on the litters in the Arakawa; and (4) low contents of Dissolved Organic Carbon (DOC) and nutrients in the Arakawa. In the Nakatsugawa, neither a contribution of the dam to a higher breakdown rate nor recovery of the rate to upstream conditions far downstream was found. However, the breakdown rate was lower in the Arakawa than in the Nakatsugawa. Inorganic deposition on litter, DOC, Na⁺, Mg²⁺, Ca²⁺ and NO₃⁻ were believed to play a main role in litter breakdown.