Shifts in the trophic base of intermittent stream food webs |
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Authors: | Matthew P Dekar Daniel D Magoulick and Gary R Huxel |
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Institution: | (1) Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA;(2) U.S.G.S., Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA;; |
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Abstract: | Understanding spatial and temporal variation in the trophic base of stream food webs is critical for predicting population
and community stability, and ecosystem function. We used stable isotope ratios (13C/12C, and 15N/14N) to characterize the trophic base of two streams in the Ozark Mountains of northwest Arkansas, U.S.A. We predicted that
autochthonous resources would be more important during the spring and summer and allochthonous resources would be more important
in the winter due to increased detritus inputs from the riparian zone during autumn leaf drop. We predicted that stream communities
would demonstrate increased reliance on autochthonous resources at sites with larger watersheds and greater canopy openness.
The study was conducted at three low-order sites in the Mulberry River Drainage (watershed area range: 81–232 km2) seasonally in 2006 and 2007. We used circular statistics to examine community-wide shifts in isotope space among fish and
invertebrate consumers in relation to basal resources, including detritus and periphyton. Mixing models were used to quantify
the relative contribution of autochthonous and allochthonous energy sources to individual invertebrate consumers. Significant
isotopic shifts occurred but results varied by season and site indicating substantial variation in the trophic base of stream
food webs. In terms of temporal variation, consumers shifted toward periphyton in the summer during periods of low discharge,
but results varied during the interval between summer and winter. Our results did not demonstrate increased reliance on periphyton
with increasing watershed area or canopy openness, and detritus was important at all the sites. In our study, riffle–pool
geomorphology likely disrupted the expected spatial pattern and stream drying likely impacted the availability and distribution
of basal resources. |
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