Spatial variability of stream chemistry measured at forested watersheds in the Oki-Dogo Island, western Japan

Abstract

We investigated stream water chemistry at 81 forested watersheds in the Oki-Dogo Island in relation to topography and dominant vegetation in each watershed. Concentrations of Cl^-, Na⁺, K⁺, and Mg²⁺ in Oki-Dogo forest streams were much higher than those in Japanese average values, suggesting strong influence of sea salt. Positive correlations among HCO₃^-, Mg²⁺ and Ca²⁺ indicates that HCO₃^- is important counter anion of Mg²⁺ and Ca²⁺. Relatively low pH and concentrations of HCO₃^-, Mg²⁺ and Ca²⁺ were observed in the stream waters in higher elevation. Therefore, acid neutralizing capacity in the forested watersheds at higher elevation may be small compared to watersheds at the lower elevation forests. Mean NO₃^- concentration was 23.4 μmol L^-1, whereas one-fourth of the sampling sites exhibited higher NO₃^- concentration than expected value at the stream in nitrogen-saturated forest under N deposition level in Oki Islands. NO₃^- concentration was positively correlated to the highest elevation and range of slope steepness within the watersheds, suggesting that nitrogen loss tends to occur frequently in watersheds with hilly topography.