• bankfull width;
  • carbon/nitrogen ratio;
  • nitrogen cycling;
  • riparian forest management;
  • river continuum;
  • spatial scaling;
  • stable isotopes;
  • stream–terrestrial coupling;
  • threshold effect


  1. Spatial scale of processes is necessary information to design appropriate ecosystem research and management. In-stream ecological processes are tightly linked with their adjacent terrestrial environment and impacted by modifications of riparian environments, particularly in small, forested streams. Quantitative understanding of how the dynamic coupling of these adjacent ecosystems varies along stream continua is necessary to enhance stream research and management.
  2. In temperate, forest-dominated watersheds of British Columbia, Canada, we examined the stream–forest linkage along a stream-size gradient based on biogeochemical analysis of fine particulate organic matter retained in streambed sediment (FPOMS). We analysed FPOMS at 48 stations in 14 cobble-bed streams from 2·2 to 39 m bankfull widths during summer low-flow conditions and tested their relationships with local, forest-related, environmental factors (light penetration, benthic coarse POM biomass, riparian-soil properties) along the stream-size gradient.
  3. Carbon/nitrogen ratio of FPOMS (FPOMS-C/N) was distinctly lower in stream bankfull widths larger than an estimated threshold around 7·6 m and corresponded with decrease in N/chlorophyll a of FPOMS and inorganic nitrogen concentration. We found significant linkages of FPOMS-C/N with local streamside environments in streams narrower than the threshold (but not in widths <~3 m). For wider streams, reduced canopy cover was likely to enhance in-stream nitrogen cycling by algae and consistently lowered FPOMS-C/N regardless of streamside environment. These indicate a nonlinear attenuation of the strength of linkage between in-stream biogeochemical processes and forest-related environments along the stream-size gradient.
  4. Synthesis and applications. The relationship of biogeochemical proxies of in-stream fine particulate organic matter with local environments can be a practical tool for measuring the strength of stream-forest linkage. Furthermore, regional- and stream-specific threshold stream widths dividing in-stream biogeochemical regimes can be an important landmark for ecologists and managers to refine designs for stream research and management at appropriate spatial scales. For stream widths narrower than a threshold, reach-scale linkages between stream and terrestrial environments can be a major focus, and riparian conditions need to be carefully managed to protect stream habitats. For stream widths wider than a threshold, catchment impacts on streams at larger spatial scales would be a more important focus.