Absence or masking of metabolic fractionations of C-13 in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of C-13 due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs. 2. The delta(13)C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (delta(15)N) or lipid content (C/N ratios). 3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their delta(13)C and delta(15)N values, thereby masking any possibility of observing C-13 trophic enrichment. 4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in delta(13)C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.