Limnol. Oceanogr., 44(5), 1999, 1198–1203

We used variation in algal d13C between river habitats to study the spatial scale of energy flow through river food webs. We found a strong negative relationship between herbivore d13C (which reflects algal d13C) and water velocity in three productive Northern California rivers but not in unproductive streams. The contrast among habitats suggests that water velocity affects algal d13C most strongly when CO2 availability is low relative to photosynthetic rates. Our results help explain the wide variation in published river biota d13C and show that past studies using carbon isotope analyses may have significantly underestimated the importance of algal-derived carbon to river food webs. While flow-related variation in d13C complicates this common application of carbon isotope analysis, we show that it provides a natural tracer of the flux of algal production derived from different habitats within rivers to higher trophic levels. Measurements of consumer d13C showed that most invertebrate and vertebrate consumers relied on local production, except for filter-feeding insects and steelhead trout, which relied on production derived from multiple sources. Stable carbon isotopes may thus be used to spatially delineate the habitats that support river food webs, providing previously unavailable information for understanding and managing river ecosystems. River food webs are based on both local microalgal production and carbon transported from terrestrial or upstream aquatic ecosystems. Determining the sources of production for river food webs is a major challenge to river ecologists because of the complexity of controls over carbon sources and the mobility of consumers (Schlosser 1991; Cummins et al. 1995). Stable carbon isotopes (13C/12C or d13C) hold much promise as a tool for determining the energy base of river and stream food webs because terrestrial and aquatic plants often have different ratios of stable carbon isotopes (Rounick and Winterbourn 1986), and there is relatively small isotopic fractionation associated with trophic transfer of organic carbon (DeNiro and Epstein 1978; France 1996a). Use of carbon isotopes in streams and rivers, however, has been limited by unexplained variability in autotrophic d13C (France 1995, 1996b, but see Doucett et al. 1996a). Some of the variability in algal d13C has been attributed to the different isotopic signatures of the dissolved inorganic carbon (DIC) available to aquatic plants (i.e., atmospheric CO2, biogenic CO2, and weathered bicarbonate) (Rounick and James 1984), but this mechanism cannot account for the large magnitude of variation often observed within rivers. In ocean or lake ecosystems, algal d13C values are often determined by the relative supply and demand for CO2 by autotrophs (Hecky and Hesslein 1995; Fry 1996; Schindler et al. 1997). However, stable carbon isotope studies of river and 1 Present address: Département de Chimie-Biologie, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada G9A 5H7.