| Summary: | Because riparian canopy controls most energy inputs to stream
ecosystems, it directly affects the structure of aquatic food webs and the
ecological processes that govern interactions among trophic levels. This
study addresses the interdependence among riparian canopy, benthic
community structure, and the carrying capacity of high desert streams for
salmonid fishes. In streams in the lower John Day River Basin in eastern
Oregon, algal, invertebrate, and fish communities were compared in reaches
with varying densities of riparian canopy. Water temperatures varied with
the density and upstream extent of canopy. Densely canopied sites were
cool, while sites with high irradiances had temperatures exceeding the upper
lethal limit for salmonids. Periphyton and grazer biomasses were greater in
well-lighted sites, but 90% of grazer biomass consisted of Dicosmoecus
gilvipes, a large caddisfly inedible by juvenile trout. Warmer water
increased metabolic demands for salmonids, while the overwhelming
dominance of Dicosmoecus in open sites shifted energy flow away from
trout and shrunk their food base. High water temperatures, however,
provided suitable habitat for many warmwater fishes which would otherwise
not enter tributaries of this size. At higher elevation study sites in Camp
Creek, light levels were higher and less variable than at the lower sites.
Periphyton and invertebrate abundances were not correlated with irradiance.
Rather, periphyton was maintained at low levels by grazers, particularly
Dicosmoecus and snails. Manipulations of fish densities in enclosures
showed that trout and dace had no negative impacts on numbers of
invertebrate prey, and that grazers played a larger role in regulating lower
trophic levels than did fish. Dicosmoecus acted as a keystone species in
the benthic food web of Camp Creek by simultaneously influencing the
trophic level both below and above its own. When irradiance was
experimentally reduced under artificial canopies, periphyton standing crops
were not different from those in open control pools after 4 wks. However,
grazers were more abundant in open pools. The cropping of periphyton to
uniform levels in both sunlight and shade indicated that mobile grazers
targeted sites of varying productivities. Comparisons between benthic
communities in Camp Creek and in a densely canopied reference stream
suggested that benthic community structure shifted to accommodate
changes in energy resources that occur when canopy density is altered. === Graduation date: 1998
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