Abstract
A variety of human activities were reported to affect stream ecosystems as multiple stressors. However, few studies have simultaneously evaluated the impacts of anthropogenic factors on stream invertebrates in mountain streams. This study aimed to elucidate the impacts of catchment land use and riparian forest logging on invertebrate scrapers, via altering nutrient concentrations and light condition, in Japanese mountain streams. Field survey was conducted in stream reaches with and without riparian forests in two mountain streams in Ehime Prefecture, of which catchments have different intensities of human land use.
Inflow of domestic and agricultural wastewater was found to increase nutrient concentrations (NO3-N and PO4-P) in the stream with human land use. Canopy opening was higher in stream reaches without riparian forest than with, owing to the loss of the shading by forest logging. Total density, biomass and taxon richness of scrapers were higher in the stream reaches without riparian forest than with. Decreased canopy shading and associating high productivity of periphyton was suggested to cause these increase. In contrast, the effect of catchment land use via nutrient concentration did not prevail in this study. These results supported the hypothesis that the light condition is a primary limiting factor of in-stream primary productivity in heavily shaded mountain streams. There were not significant difference in periphyton biomass between streams and present/absent of riparian forests, probably caused by strong grazing pressure by abundant scrapers in the stream reaches without riparian forest. We suggest that our multi-factor approach can contribute to gain insight into the impacts of various human activities on stream ecosystems.
