Quality and quantity of organic matters discharged into the river ecosystems are changed depending on the flow regimes, and riparian land use and land cover. These organic matters sustain riverine food webs by being incorporated through microbes. Recent studies have shown that among microbes, fungi are especially important decomposers and consumers. However, little is known on how fungal community changes along river gradients.

Generally, kinds and amount of organic matters increase toward the downstream because of increase in flow rate and diversifying land use and land cover. Therefore, we hypothesized that species diversity of fungi changes along the river gradients. To examine it, we analyzed species diversity of fungi grown on cobble surface at various sites of the Natori river system in fall, 2011. The fungal species diversity was quantified using a DGGE method assuming that the gel electrophoresis bands indicated species. Then, three diversity indices was estimated at each site: average number of species (α-diversity), total number of species (γ-diversity) and spatial variation among cobbles (β-diversity = γ/α).

The analyses showed that toward downstreams the α- and γ-diversity increased while the β-diversity decreased. These results suggest that local fungal community in river systems reflects quantity and quality of organic matters.