Trophic linkages between headwater forests and downstream fish habitats: implications for forest and fish management

This study examined the fluvial transport of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters in alternatives-to-clearcutting (ATC) harvest units to aquatic habitats downstream in the coastal mountains of southeastern Alaska. Fifty small streams (mean discharge 2.7 L s−1, range 0.1-128.1 L s−1) representing three geographic areas (ATC installations) throughout southeastern Alaska were sampled with 250 μm nets three times per year (April, July, September). Samples were used to assess the subsidy of energy from fishless headwaters to downstream systems containing or potentially containing fish, and to obtain preharvest data on streams for the ATC study. Invertebrates of both aquatic and terrestrial origin were captured, with aquatic taxa averaging roughly 3/4 of the total individuals sampled. Invertebrates and detritus were transported from headwaters during all sampling periods, averaging 163 mg invertebrate dry mass stream−1 day−1 (1.7 mg m−3 water) and 10 g detritus stream−1 day−1 (0.05 g m−3 water), respectively. Based on the frequency of headwater streams in the watersheds studied, and the average amount of food delivered to downstream habitats by these streams, every kilometer of salmonid-bearing stream could receive enough energy from fishless headwaters to support 100-2000 young-of-the-year salmonids. These results illustrate that headwaters are source areas of aquatic and terrestrial invertebrates and detritus, linking upland ecosystems with habitats lower in the catchment. ATC activities are predicted to have variable effects on headwater stream productivity, and subsequent food resources for downstream salmonids. Partial canopy removal is expected to increase solar penetration to the stream and primary and secondary production within these partially harvested sites, and full canopy removal will likely reduce allochthonous inputs of leaf litter into the streams, and reduce stream invertebrate abundance.