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

Abstract

SUMMARY1. 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-lm 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 . 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.

Disproportionate Relative Importance of a Terrestrial Beetle Family (Coleoptera: Scarabaeidae) as a Prey Source for Central Appalachian Brook Trout

“…However, recent examinations of salmonid diets in headwater watersheds reveal that terrestrial organisms may outweigh aquatic organisms in energy provided to fish (Kawaguchi and Nakano 2001;Sweka and Hartman, in press;Utz and Hartman 2007). When taxonomic detail of terrestrial invertebrates is considered relative to nutrient input to streams, ecosystems, and salmonid diet, results suggest that certain organisms are of particular importance while others are of negligible importance (Allan 1981;Nielsen 1992;Wipfli and Gregovich 2002;Utz and Hartman 2007). Despite the notion that a variety of terrestrial organisms may be of critical importance to lotic salmonids, a large number of salmonid diet studies classify all terrestrial organisms into one broad category (Cada et al 1987;Forrester et al 1994;Bridcut and Giller 1995;Mookerji et al 2004).…”

Red Alder-Conifer Stands in Alaska: An Example of Mixed Species Management to Enhance Structural and Biological Complexity

“…For instance, sediment produced in headwater systems moves through channel networks and alters channel morphology [123], creating various channel environments and modifying patterns of riparian structure and hyporheic exchange in downstream reaches [125,126]. Movement of detritus and invertebrates from headwaters to downstream food webs may alter productivity, population density, and community structure of stream biota in downstream reaches [127].…”

“…Also noteworthy was the trophic influence that upland red alder had on more distant habitats. Fish habitats downstream of red alder-dominated reaches were supplied with invertebrates (food for fish) and organic detritus (food for invertebrates) from upstream headwaters [119,127]. Also, aquatic food webs downstream of red alder received more prey and detritus than those below conifer dominated reaches [115].…”

Effects of clearcut logging on organic matter and invertebrate drift exported from headwater streams in the interior of British Columbia.

“…The result o f these characteristics is that headwater streams export unique resources that subsidize downstream reaches Wipfli and Gregovich 2002).…”

“…However, fish such as salmonids are seasonally food limited and may receive an important subsidy o f organic matter from headwater reaches, especially when considering their cumulative export (Wipfli and Gregovich 2002). It is important to consider cumulative impact because small streams exist in the highest numbers among the continuum of freshwaters and have the closest link between terrestrial and aquatic ecosystems .…”