2002 Volume 63 Issue 1 Pages 11-19
In order to evaluate the effects of acid deposition and the function of forests on fresh water environments, mountain stream waters have been monitored on Yakushima Island. The concentrations of SO42-, NO3-, and other anions and cations in the rainwater undergo seasonal change and vary with the level of precipitation. In particular, the pH in rainwater shows a tendency to fall lower in the winter than in the other seasons. The characteristics of water quality in the central area of the island, that is, the high mountainous area, are lower pH and EC, and also lower concentrations of SO42- and NO3-. On the other hand, the water quality of the western part of the island has a distinguishing feature, that is, a low pH and a higher concentration of anions. However, from calculations of the distance index, no difference is shown in the ion constituents of each mountain stream. The watershed of the Kawara streams, which is located in the western part of the island, is covered with one of the most extensive evergreen broad-leaved forest in Japan. The area is very steep with an average incline of 20 degrees from the summit of Mt. Kuniwaridake to the coastline. As to the reasons why higher concentrations of acid substances are found in the stream waters in the western area, the densely growing canopy may play a role in trapping air pollutants, and acid substances deposited on the canopy would be discharged through fall and a stem flow. Therefore, there is a future risk of acidification of the stream waters due to an increase in the acids loaded from the Asian continent on Yakushima Island.
