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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 lm; medium [MPOM], 100-250 lm; and coarse [CPOM], 250-1,000 lm) 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 d 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.

1. To understand longitudinal changes in the trophic base of benthic macroinvertebrates from mountain to lowland river sections, we investigated carbon stable isotopic compositions (d 13 C) of macroinvertebrates and their food resources in riffles for four seasons at 14 sites along the main stem of the Toyo River, Japan. 2. At each site, d 13 C was usually highest or nearly highest for periphyton (epilithic biofilm) and was lowest for transported leaf materials. Among macroinvertebrate groups, grazers usually had higher d 13 C values than filterers or predators. 3. During all seasons, d 13 C of periphyton and all macroinvertebrate groups increased downstream from mountain to upland sections, but decreased downstream from upland to lowland sections. In addition, the difference between grazer d 13 C and filterer d 13 C decreased from mountain to upland sections, but increased from upland to lowland sections. 4. The observed changes in d 13 C of periphyton and macroinvertebrates from mountain to upland sections agree with previous reports: the d 13 C of periphyton and consumers increased with stream size and productivity. The decrease in d 13 C of periphyton and macroinvertebrates from upland to lowland sections has not been reported previously, and this may have resulted from an increased importance of terrestrial detritus relative to periphyton production in the lowland section, where riffles were infrequent and pools dominated the reach. 5. A simple mixing model of d 13 C showed that grazers rely mostly on periphyton at all sites, whereas the importance of periphyton for filterers changed longitudinally increasing from mountain to upland sections and decreasing from upland to lowland sections. This longitudinal trend for filterers is possibly associated with the changes in the availability or quality of terrestrial detritus in transported particulate organic matter. 6. Longitudinal changes in the relative importance of autochthonous production and allochthonous detritus appear to be reflected in d 13 C of riffle benthic communities. The longitudinal changes were not monotonic, and specific reach characteristics may be responsible for the greater importance of allochthonous detritus in mountain and lowland sections.

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