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Presented at the NABS Annual meeting, Athens, Georgia, 2003
in Food Webs
Spatial variability of stable isotopes in stream macroinvertebrates: patterns along a river continuum.
M.I. Hornberger, A.R. Stewart, and S.N. Luoma. U.S. Geological Survey, 345 Middlefield Rd., MS 465, Menlo Park, CA 94025
Stable isotope (SI) analysis is commonly used in aquatic systems when reconstructing food webs. However, identifying trophic linkages across large spatial gradients, such as in rivers, requires an understanding of isotopic variability at the lowest trophic level (isotopic baseline). Baseline values of δ13C and δ15N were determined in aquatic insects collected along a 400 Km segment of the Clark Fork River in 2000-2001. Insects were sorted to species and represented three functional feeding groups: filter/collectors, predators, and shredders. Results indicate that functional groups within a station were not significantly different in isotopic signatures. However, insect δ13C and δ15N varied spatially in both years. Values of δ13C in taxa from the upper 20 Km were enriched (δ13C –22‰) relative to δ13C values in the same taxa from stations further downstream (δ13C –31‰). Insect values of δ15N were also highest upstream and ranged from 12–15‰ in the upper reach to 6‰ downstream. These data suggest that while stream insects can be used to establish isotopic baseline values in lotic systems, there can be considerable variability along a river continuum. Therefore, identifying the spatial pattern of isotopic baseline is essential when reconstructing site-specific trophic relationships in lotic systems.
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