Abstract
We investigated the distribution of Nuphar oguraensis Miki var. akiensis Shimoda (Nymphaeaceae), an endangered floating-leaved macrophyte, with particular focus on water quality and river course modification history in the Koda River, Kochi City. The Koda River is geomorphologically divided into upper and lower reaches, and water quality clearly changed across the boundary of the two reaches. In particular, values of pH and electrical conductivity in the lower reaches were lower than those in the upper reaches throughout the year. Based on the relationship between RpH and pH, downstream water likely contained seepage ground water that was oversaturated with dissolved CO_2. Because dissolved CO_2 stimulates photosynthesis and can stimulate Nuphar growth, this oversaturation of CO_2 may be one reason why Nuphar populations were restricted to lower reaches of the river. Two locations where Nuphar populations were observed corresponded to reaches of the original course that remained after river modification. The presence of these original reaches was confirmed by comparing actual courses to those found on old maps and aerial photographs. Because the recruitment of populations has not been successful in recent years, facilitating seed production and population establishment is necessary for the conservation of Nuphar.
