A study was conducted on the seasonal change in the decomposition process in a woodland stream in Hokkaido, Japan, throughout a year to examine the effect of allochthonous inputs with remarkable temporal variability. Two peaks were observed in the litter retention and the pattern was similar to that of litter input in this stream. In addition to a large peak in the autumn, another small peak of the retention after snowmelt was observed when a small amount litter was brown in from channel sides. Litter input and litter retention was significantly correlated both in the two types of microhabitat, i.e. leaf-pack and log, and there was no significant difference in the composition of detrital items between them. These results implied that the seasonal change of litter retention in the stream was strongly regulated by the litter input and the effect was similar between the different kinds of microhabitats.

Seasonal change of the detritivore biomass was similar to the litter retention. Macroinvertebrate biomass was often dominated by shredder, one of the functional feeding groups (FFG) which feed on fallen leaves directly, in the two types of microhabitats. However, the biomasses of the predominant detritivores were almost constant throughout the year and the correlation between litter retention and each of the biomass was not significant for all FFGs. Autumnal increase of shredder biomass appeared to be the results of their aggregation from the riffle.

Macroinvertebrate community was different between the microhabitats. There were significant differences in the FFG composition and shredder biomass. Significant correlation in biomass between shredder and other dominant detritivores biomass was found only in the leaf-pack microhabitat. On the other hand, correlation between predator and prey biomass, which is the sum of the biomass of FFGs other than predator, was significant only in the log microhabitat. These results implied the difference of structure in the food web between the type of micro-microhabitat. Indirect interaction between detritivores through organic matter conversion from litter to fine particulate organic matter (FPOM) might be dominant only in leaf-pack microhabitat. However, in log microhabitat, bottom-up regulation might be dominant for predator and its prey. This difference might have been caused by the difference in physical environment such as water velocity.
A distinct seasonality in standing stock of FPOM was observed and it showed inverse of the seasonal pattern of litter retention. The rate of fecal production was measured in situ for two predominant shredder species (Amphipoda and Capniidae) and the contribution of feces to FPOM in a riffle on annual basis was estimated to be 0.29, which was higher than the values reported so far.

I also examined bacterial biomass on FPOM surface for one year, to determine their effect on the seasonality in the decomposition process. The CN ratio of FPOM varied seasonally; low just after the autumnal litter input when fecal production was high. Bacterial biomass varied considerably throughout the year without clear seasonality. The bacterial biomass was negatively correlated with CN ratio of FPOM. There was no significant difference in bacterial biomass per unit FPOM surface area among particle size fractions. However significant differences were observed for bacterial biomasses per FPOM weight among different particle sizes. The dynamics of bacterial biomass were controlled mainly by the quality of substrates, not by the seasonal change of DOC concentration.