Factors characterizing phosphate oxygen isotope ratios in river water: an inter-watershed comparison approach

Abstract

We compared the oxygen isotope ratio of dissolved phosphate $\left( {\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }} } \right)$ in two rivers with different land-cover and geological features (Ado River and Yasu River) within Lake Biwa basin, central Japan, to explore what factor primarily characterizes the $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$. Mean values of $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ in river water were 19.0 ± 2.4‰ (n = 7) in Ado River and 13.1 ± 2.3‰ (n = 15) in Yasu River, which were significantly different. Comparisons of $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ between river water and potential sources of phosphate revealed that in the Ado River, the $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ was similar to that in rocks from the accretionary complex and decreased with increasing sedimentary rock coverage. In the Yasu River, the $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ was low in the upper forested areas, but increased with paddy field coverage. These results demonstrate that river $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ strongly reflects inputs from geological substances, but is also impacted by land-use activities and varies with anthropogenic land coverage in the watershed. Thus, river $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ relates to land or bedrock coverage differentially in each river. Regression analysis showed that residuals of the $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ tended to converge to zero with increasing drainage area, suggesting that river $\delta^{{{18}}} {\text{O}}_{{{\text{PO}}_{{4}} }}$ more explicitly reflects land-cover and geological features on a larger watershed scale.