Export of invertebrates and detritus from fishless headwater streams in southeastern Alaska: implications for downstream salmonid production

Abstract

1. We examined the export of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters to aquatic habitats downstream in the coastal mountains of southeast Alaska, U.S.A. Fifty‐two small streams (mean discharge range: 1.2–3.6 L s−1), representing a geographic range throughout southeast Alaska, were sampled with 250‐μm nets either seasonally (April, July, September) or every 2 weeks throughout the year. Samples were used to assess the potential subsidy of energy from fishless headwaters to downstream systems containing fish.2. Invertebrates of aquatic and terrestrial origin were both captured, with aquatic taxa making up 65–92% of the total. Baetidae, Chironomidae and Ostracoda were most numerous of the aquatic taxa (34, 16 and 8%, respectively), although Coleoptera (mostly Amphizoidae) contributed the greatest biomass (30%). Mites (Acarina) were the most numerous terrestrial taxon, while terrestrial Coleoptera accounted for most of the terrestrial invertebrate biomass.3. Invertebrates and detritus were exported from headwaters throughout the year, averaging 163 mg invertebrate dry mass stream−1 day−1 and 10.4 g detritus stream−1 day−1, respectively. The amount of export was highly variable among streams and seasons (5–6000 individuals stream−1 day−1 and <1–22 individuals m−3 water; <1–286 g detritus stream−1 day−1 and <0.1–1.7 g detritus m−3 water). Delivery of invertebrates from headwaters to habitats with fish was estimated at 0.44 g dry mass m−2 year−1. We estimate that every kilometre of salmonid‐bearing stream could receive enough energy (prey and detritus) from fishless headwaters to support 100–2000 young‐of‐the‐year (YOY) 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.