間隙動物の生息場所に適した砂州内環境の実験的研究

Abstract

モンカゲロウ(Ephemera strigata)などのカゲロウ類では砂州上流端に位置する淵尻の瀬頭（淵から瀬への移行部）に集中的に産卵する行動が知られている。このような産卵場所選択性は、砂州内に発達する河床間隙水域の透水性や溶存酸素濃度などの物理化学的特性と関係していると考えられる。本研究は、平水時の砂州内環境のうち、とくに間隙動物にとって重要な溶存酸素の分布様式や時系列変化に着目し、それらに影響を及ぼす要因について、間隙水の動態と関連付けて検討した。また、モンカゲロウ（Ephemera strigata）を対象に、河床間隙水域の物理化学的特性と卵の孵化率・死亡率、孵化した幼虫の生存率との関係を野外実験によって明らかにし、河床間隙水域の生態学的意義を考察した。その結果、卵の孵化率や幼虫の生存率は、砂州の内陸側よりも瀬水際で高くなった。また、卵の孵化率や幼虫の生存率は間隙水の溶存酸素濃度と強い正の相関が見られた。一方、間隙水の溶存酸素濃度の高い場所は、多くの産卵行動が確認された瀬水際近傍の間隙水流速が大きな場所に限られており、砂州内の大半で低かった。

It has been reported that the ovipositon sites of some aquatic insects such as a mayfly Ephemera strigata are restricted to the shore of the pool tail and riffle head. Ecological implications of the preference of oviposition sites have been attempted in relation to physicochemical environmental conditions of the hyporheic zone. However, few empirical studies have proved the cousal relationship between such habitat selection of stream inhabitants and the ecological functions of the hyporheic zone. The present study focused on the horizontal distribution and seasonal changes in DO concentration in the hyporheic zone of a gravel bar in the Kamo River, in Kyoto city, Japan. Field experiments for rearing eggs of the mayfly E. strigata at various sites of the bar were carried out to examine relations of survival ratio of the eggs and nymphs to the physicochemical conditions in the hyporheic zone. Distribution patterns of the water quality parameters showed that the reduction in DO concentration was strongly related to the hyporheic water temperature as well as the liquidity of hyporheic flow. The egg rearing experiment resulted in higher hutching rate of eggs and higher survivability of nymphs along the riffle shore than inland area of the bar. The egg hutching rate and nymphal survivability showed high correlation with DO concentration of hyporheic waters. The hyporheic flow velocity was also high near the water's edge in the riffle, where E. strigata selected as ovipositon sites, suggesting that their habitat selection funtions to raise survival ratio of their offspring in the egg and early nymphal stages.