Tuesday, June 5, 2007
485

Reach-scale geomorphology affects organic matter and consumer δ13C in a forested Piedmont stream (South Carolina, USA)

David M. Walters, US EPA, Office of Research and Development, National Exposure Research Laboratory, 26 West MLK Blvd., MS 640, Cincinnati, OH 45268, Ken M. Fritz, U.S. Environmental Protection Agency, Office of Research and Development, National Exposure Research Laboratory, Cincinnati, OH 45268, and Don Phillips, US Environmental Protection Agency; National Health & Environmental Effects Research Laboratory, 200 SW 35th St., Corvallis, OR 97333.

We investigated seasonal (spring, autumn) and spatial variation of stream organic matter and consumer δ13C in a Piedmont stream. Sites were sampled along a continuum (basin area 30-300 km2) and fit into 2 geomorphic categories: high-gradient, rock-bed (“rock”) or low-gradient, sand-bed (“sand”) sites.  δ13C was generally enriched in spring, but this response varied among foodweb components.  CPOM, seston, and 5 of  9 trophic guilds (e.g., grazers, insectivorous fish) showed spring enrichment, whereas FBOM and one guild were enriched in autumn.  Biofilm, seston, FBOM, and 8 guilds were enriched at rock sites.  δ13C of biofilm and 4 guilds were also positively correlated with drainage area, but the magnitude of enrichment was less than between bed-types.  Consumer δ13C enrichment at rock sites suggests greater reliance on algal carbon than at sand sites; however, biofilm δ13C was also enriched at rock sites.   Thus, geomorphic differences in consumer δ13C could be related to increased utilization of biofilm at rock sites, consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites, or both mechanisms.  Numerous studies have found downstream enrichment of δ13C in stream foodwebs, but longitudinal (and seasonal) effects were minor compared with site-scale geomorphic effects in this study. 

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