扇状地の用排兼用幹線水路における炭素・窒素安定同位体比測定による食物網の推定

抄録

  We examined the food webs in the main irrigation and drainage canals on the alluvial fan of the Tedori River (Ishikawa Prefecture, Japan) by measuring the carbon and nitrogen stable isotope ratios in food sources, aquatic insects, other benthic animals, and freshwater fishes in the upper, middle, and lower reaches. The results showed that the food webs in the canals mainly comprised aquatic insects, freshwater shellfish, and fish that originated from the canal bed sediment, and periphyton on the concrete-lined bed and side walls, and cobbles and boulders on the channel bed. The results also showed that the carbon and nitrogen stable isotope ratios of the periphyton increased in the lower reaches due to a decrease in flow velocity and an increase in the anthropogenic nitrogen load; the increases in the stable isotope ratios of the primary producers triggered increases in the stable isotope ratios of the entire food chain. The nitrogen of high stable isotope ratio conveyed by diadromous fish from a sea also might triggered the increase in the nitrogen stable isotope ratio of the entire food chain. Upstream migration of fishes and downstream migration of aquatic insects left gaps in the stable isotope ratios between predators and prey. Sustainable management of the cobbles and boulders will contribute to the conservation and rehabilitation of aquatic fauna and will promote water purification in the canals by increasing the number of aquatic insects that feed on periphyton and the number of fishes that feed on aquatic insects and periphyton.