Keywords:Stable isotope analysis, Source of phosphate, Diffuse pollution, Lake Biwa, Accretionary complex
Excess phosphorus (P) utilization by human activities has resulted in a large amount of P loss from terrestrial to aquatic ecosystems, which in turn can induce eutrophication and subsequently algal bloom in enclosed waters. To control the P loss, the phosphate oxygen isotope ratio (δ18OP) technique is expected to be applied for clarifying P dynamics in terrestrial ecosystems. This is because δ18OP in river water could reflect sources of phosphate within the river watershed. However, very few studies have applied the δ18OP technique to clarifying watershed-scale P dynamics and thus little information is available about what the river δ18OP value indicates specifically within the watershed. To examine this, we compared the δ18OP values in river water and rocks between two watersheds with different land-use and geological compositions. For this, we collected river water and rock samples and analyzed their δ18OP in subwatersheds of the Ado River and the Yasu River watersheds, which were dominated by forests and covered by large areas of agricultural lands, respectively, belonging to the Yodo River system in central Japan. The river δ18OP value was significantly higher in the Ado River watershed than in the Yasu River watershed (u-test, p < 0.05). This could not be explained by the forest area ratio within a subwatershed. The relationship between river δ18OP values and subwatershed areas revealed that the δ18OP value tended to increase and reach a plateau as the subwatershed area increased. Additionally, the result showed that the river δ18OP value was higher in the Ado River watershed than in the Yasu River watershed at a given subwatershed area. These findings are attributable to the fact that high values of δ18OP in river water could derive from those in the accretionary complex. This is because geological compositions in the subwatershed with large area (≥ 10 km2) contain the accretionary complex in the Yasu River watershed and also because geology in the Ado River watershed is composed mostly of the accretionary complex. On the other hand, the river δ18OP value could change not only by the geological factor, but also by biologic uptake of phosphate. It is possible that changes in the river δ18OP value with the subwatershed area resulted partly from an increase in the opportunity for the biologically-mediate oxygen isotope exchange between water and phosphate associated with the increased river length.