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

We compared the oxygen isotope ratio of dissolved phosphate 18O PO 4 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 18 O PO 4. Mean values of 18O 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 18 O PO 4 between river water and potential sources of phosphate revealed that in the Ado River, the 18O PO 4 was similar to that in rocks from the accretionary complex and decreased with increasing sedimentary rock coverage. In the Yasu River, the 18 O PO 4 was low in the upper forested areas, but increased with paddy field coverage. These results demonstrate that river 18 O 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 18 O PO 4relates to land or bedrock coverage differentially in each river. Regression analysis showed that residuals of the 18 O PO 4tended to converge to zero with increasing drainage area, suggesting that river 18 O PO 4 more explicitly reflects land-cover and geological features on a larger watershed scale.