1. The magnitude and spatial scales of human effects on stream habitats need to be correctly measured to achieve sound ecosystem management. We investigated whether the biogeochemical properties of fine particulate organic matter (FPOM) might be indicative of reach-scale vs. catchment-scale effects on forested stream ecosystems.
2. Along each of three forested streams, we established 4–5 sampling stations at 0·6–2·0-km intervals, which represent a range of local riparian forest conditions (e.g. vegetation types and forestry practices). At each station, rock biofilm (considered as a representative of stream-origin POM), FPOM in sediment (FPOMS) and suspended in water (FPOMW) and three species of benthic macroinvertebrates were collected during a summer low-flow period.
3. Measures of δ13C, C:N and chlorophyll a:C for FPOMW, FPOMS and biofilm were longitudinally heterogeneous, reflecting the reach-scale, local environment. δ13C, C:N and chlorophyll a:C of FPOMS were significantly related to irradiance and streambed coarse particulate organic matter (CPOM) abundance, suggesting that the relative contribution of in-stream primary production and CPOM breakdown are dominant controls of FPOM properties.
4. In redundancy analysis, variations in FPOM properties were principally explained by local environmental factors correlated with stream size and/or longitudinal position of the sampling stations (irradiance, streambed CPOM, stream gradient, discharge, riparian vegetation). No significant effect of riparian forestry activities (logged, riparian reserve or no harvest in the past 75 years) was found on FPOM properties.
5. Resource use by primary consumers was species dependent. δ13C of larval Lepidostoma roafi depended on the relative abundance of stream-origin POM in pools of stored FPOM. Selective intake/assimilation of stream-origin POM by this species was probably enhanced when stream-origin POM was abundant. δ13C of larval Despaxia augusta or larval Paraleptophlebia spp. were not related to FPOMSδ13C, suggesting no linkage with FPOM composition.
6. Synthesis and applications. The tight linkages between biogeochemical properties of FPOM, resource use by primary consumers, and reach-scale environment indicate that local effects were greater than signals transmitted from upstream in this study. The use of in-stream FPOM properties can help managers to measure reach-scale effects of environmental changes on forested stream habitats.