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Comparison of organic matter decomposition rates between benthic and hyporheic zones in a headwater stream

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Abstract 渓流の水生生物を支えている陸生有機物は,河床だけでなく,河川間隙水域にも保持されている。本研究では有機物の分解に着目し,分解速度を河床と河川間隙水域で比較することで,分解の場としての河川間隙水域の相対的重要性を5つの季節(冬・春・梅雨・夏・秋)で評価すること,また分解を制御する主要な要因である大型無脊椎動物の貢献度を評価することを目的とした。有機物分解速度の指標として,綿ストリップの引張強度の減少...率を用いた。大型無脊椎動物の貢献度は,目開き5mm 0.1mm のメッシュに入った綿ストリップの引張強度減少率の差から評価した。河床と河川間隙水域ともに,有機物分解速度は冬に最も低く,夏に最も高い傾向を示し,水温が分解速度の季節性に大きく影響していることがわかった。河床と河川間隙水域の分解速度を比較すると,春から夏にかけては河床の方が有意に高いが,秋から冬にかけては有意な差はなく,河川間隙水域の分解の場としての相対的重要度は季節によって異なることがわかった。大型無脊椎動物の貢献度は,河床では冬を除いてすべての季節で,河川間隙水域では夏と秋のみ有意であった。大型無脊椎動物が,河床での有機物分解だけでなく,季節的にではあるが河川間隙水域の有機物分解においても重要であることが示唆された。
Allochthonous organic matter that supports aquatic organisms is retained in the benthic and hyporheic zones in headwater mountain streams. In this study, we examined relative importance of the hyporheic zone as the zone of decomposition over 5 seasons (winter, spring, rainy season, summer and fall) and contribution of macroinvertebrate to organic matter decomposition. Cotton-strip assay and tensile strength loss were used to measure organic matter decomposition rates, and relative contribution of macroinvertebrate was estimated using the differences in tensile strength loss of cotton strips in 0.1 mm and 5 mm mesh-opening bags. Both benthic and hyporheic zone showed seasonality, where lowest decomposition rate observed in winter and highest in summer. The relationship between organic matter decomposition rate and water temperature suggested that water temperature was a primary factor resulting in seasonal variation. The decomposition rates in the hyporheic zone were similar to the benthic zone in fall and winter, but from spring to summer, the rates were significantly lower. Thus, as the zone of decomposition, the relative importance of the hyporheic zone differ depending on seasons. Contribution of macroinvertebrate to decomposition was significant in all seasons but winter in benthic zone and was significant only in summer and fall in the hyporheic zone. Thus, macroinvertebrate plays important role even in the hyporheic zone in some seasons.
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Created Date 2022.04.13
Modified Date 2023.11.20

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