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[MIS22-08] Trends of seasonal stream water chemistry in a forested ecosystem in the Shimanto River Basin
Keywords:season, trend, rock weathering, water chemistry, nitrogen
Stream water chemistry in forested watersheds is very important for providing drinking water in the area. In the 1970 stream water chemistry was affected by acidic deposition in some ecosystems whereas in recent years, it would be affected by ongoing climate change. The Shimanto River Basin in Shikoku island is located at a remote area and it is considered that the effects of acidic deposition were modest. However, the results of long-term monitoring for 20 years suggested that the stream water chemistry was affected by acidic deposition (Inagaki et al 2025 Ecol Res). The annual average concentration was analyzed in the study and trend of each season is not known. In the present study, we aimed to investigate the trends of stream water chemistry in each season.
The study area of this study is in a fir-dominated natural forest (18.7ha) in Takatori area in the Shimanto River Basin. Stream water was collected from 1995 to 2019. The runoff rate was recorded at the sample collection. Water chemistry was predicted by runoff rate, sunlight hours, year and season in the multiple regression analysis. For season, samples collected from March to May, June to August, September to November, and December to February were categorized as spring, summer, autumn, and winter, respectively. To analyze the tends of water chemistry in each season, we performed the multiple regression analysis, separately.
Mean values of stream water chemistry in each season showed the different patterns among the elements. Potassium, sodium, calcium and magnesium had higher concentration in autumn and winter. Nitrate concentration was highest in winter then decreased to the lowest in the spring, and it increased gradually in the summer and autumn. Sulfate and chloride concentrations were higher in winter and spring then it decreased in summer and autumn. Bicarbonate concentration was higher in autumn and winter and it showed similar pattern with potassium concentration. During the 24-year monitoring, concentrations of cation in each season did not show significant trend. Sulfate and chloride concentration decreased in all season. Bicarbonate concentration increased in spring and summer. Nitrate concentration decreased in spring and autumn but it did not change in summer and winter. From these results, it is suggested that decrease of anthropogenic sulfate and chloride is offset by the increase of bicarbonate derived from higher soil respiration during the spring and summer. Moreover, nitrate concentration decreased in spring and autumn due to higher plant uptake of nitrate whereas in summer the plant nitrate uptake is inhibited by the high temperature in summer. These findings suggest that trend analysis of each season can provide valuable insights about the mechanism about the changes of water chemistry in forested ecosystems.
The study area of this study is in a fir-dominated natural forest (18.7ha) in Takatori area in the Shimanto River Basin. Stream water was collected from 1995 to 2019. The runoff rate was recorded at the sample collection. Water chemistry was predicted by runoff rate, sunlight hours, year and season in the multiple regression analysis. For season, samples collected from March to May, June to August, September to November, and December to February were categorized as spring, summer, autumn, and winter, respectively. To analyze the tends of water chemistry in each season, we performed the multiple regression analysis, separately.
Mean values of stream water chemistry in each season showed the different patterns among the elements. Potassium, sodium, calcium and magnesium had higher concentration in autumn and winter. Nitrate concentration was highest in winter then decreased to the lowest in the spring, and it increased gradually in the summer and autumn. Sulfate and chloride concentrations were higher in winter and spring then it decreased in summer and autumn. Bicarbonate concentration was higher in autumn and winter and it showed similar pattern with potassium concentration. During the 24-year monitoring, concentrations of cation in each season did not show significant trend. Sulfate and chloride concentration decreased in all season. Bicarbonate concentration increased in spring and summer. Nitrate concentration decreased in spring and autumn but it did not change in summer and winter. From these results, it is suggested that decrease of anthropogenic sulfate and chloride is offset by the increase of bicarbonate derived from higher soil respiration during the spring and summer. Moreover, nitrate concentration decreased in spring and autumn due to higher plant uptake of nitrate whereas in summer the plant nitrate uptake is inhibited by the high temperature in summer. These findings suggest that trend analysis of each season can provide valuable insights about the mechanism about the changes of water chemistry in forested ecosystems.