Benthic assemblages in riffles and pools show different resource dependencies. Light condition is also one of the major factors known to affect resource use in benthic assemblages. Although there are studies on resource use comparing different reaches or streams, resource use heterogeneity of benthic assemblages within a single reach are remain under-evaluated. In tropical pristine forests, gap-dynamics make heterogeneous light conditions on streams. Therefore, we have studied the habitat-scale heterogeneity of autochthony among benthic assemblages at 12 sites in a single reach of a pristine montane stream in Borneo. We have selected three open-pools, three open-riffles, three close-pools and three close-riffles. The stable isotopes ratios of Nitrogen and Carbon were used to estimate the autochthony of each benthic taxon and compared their ratios of invertebrates and their sources among the habitats. Further, the autochthony of benthic assemblages were estimated based on their taxon-specific autochthony and their biomass.
A total of twenty-six invertebrate families were collected. While majority of invertebrates were found to depend on fine particulate organic matter (FPOM), some were epilithic algae dependent. The FPOM were mainly derived from terrestrial detritus (~90%) compared to stream algae (~10%). However, the stable isotope ratios of FPOM-feeders showed high dependence on stream-derived algae as their trophic source, indicating selective assimilation. The carbon stable isotope ratio of algae-feeders varied with the ratio of local algae suggesting their habitat-scale local dependence on algae. On the other hand, FPOM-feeders had fixed stable isotope ratio in all sites despite stable isotope ratio of local algae varied widely. It suggests that they depend on well-mixed flowing matters rather than local resources. The assemblage autochthony was highest in open-riffles (~70%) compared to the close-pools (~50%). These results suggest that the habitat-scale light condition and geomorphology can induce change in resources usage of benthic assemblages even within a reach scale.