Longitudinal and seasonal changes in the origin and quality of transported particulate organic matter along a gravel-bed river

Particulate organic matter (POM) plays an important role in nutrient dynamics in river ecosystems, but little is known about changes in the origin and quality of POM in relation to physical and seasonal changes along rivers. Using stable isotope and stoichiometric analyses, we investigated the changes in origin and quality of POM of three different size fractions (fine [FPOM], 1.2–100 μm; medium [MPOM], 100–250 μm; and coarse [CPOM], 250–1,000 μm) at 14 sites along a gravel-bed river over four seasons. FPOM and MPOM accounted for 90% of all POM at all study sites. At each site, the δ 13 C level was lower for FPOM (range: −29.0 to −21.1‰) than for MPOM (−26.9 to −17.2‰) and CPOM (−27.5 to −16.3‰). The C:N ratio was lower for FPOM (6.9–15.6) than for MPOM (6.3–17.4) and CPOM (5.7–27.1). The contribution of periphyton to POM of all size fractions had a tendency to increase downstream, though the trend was less clear and varied seasonally for MPOM and CPOM between sites in middle and downstream reaches. Contrastively, the C:N ratio in all size fractions of POM consistently decreased downstream. The downstream decrease in the C:N ratio of POM can be partly explained by the increase in the contribution of periphyton, which seems to be associated with increased discharge and enhanced periphyton dislodgement, especially in winter. In addition, an increase in bacterial biomass associated with the greater nutrient availability in pool areas is another possible reason for the decrease in the C:N ratio of POM downstream.