Trophic structure and energy flow in backwater lakes of two large floodplain rivers assessed using stable isotopes Brian R. Herwig, Daniel A. Soluk, John M. Dettmers, and David H. Wahl Abstract: Carbon and nitrogen stable isotopes ( δ 13 C and δ 15 N, respectively) were measured for locally abundant pri- mary producers, detritus, aquatic invertebrates, and fish in backwater lakes of two large floodplain rivers in an effort to understand patterns of energy dependence and trophic structure within these habitats. We observed trophic enrichment values for 15 N that were within the ranges observed for other aquatic systems but were often considerably lower than the 3.4‰ typically assumed in stable isotope studies. Production based on benthic and terrestrial material, combined with planktonic production for larval fish, appears to be an important energy source supporting consumers in backwater lakes of large floodplain rivers. Our isotopic data challenge the conventional wisdom that macrophytes are centrally in- volved in supplying carbon to aquatic consumers in shallow lakes and that food webs in most lakes are sustained al- most exclusively by carbon fixed by phytoplankton. Résumé : Nous avons mesuré les isotopes stables de carbone et d’hydrogène (* 13 C et * 15 N) chez les producteurs pri- maires, le détritus, les invertébrés aquatiques et les poissons communs des lacs situées sur la plaine de débordement de deux grandes rivières dans le but de comprendre les patterns de dépendance énergétique et la structure trophique de ces habitats. Nous avons observé des valeurs d’enrichissement trophique de 15 N comprises dans la même gamme que celles des autres systèmes aquatiques, mais ces valeurs étaient souvent considérablement plus faibles que le 3,4 ‰ qu’on assume communément dans les études sur les isotopes stables. Une production basée sur du matériel benthique et ter- restre et une production planctonique utilisée par les larves de poissons semblent être les sources d’énergie qui suppor- tent les consommateurs dans ces lacs situés sur les plaines de débordement de grandes rivières. Nos données sur les isotopes mettent en doute la croyance commune que les macrophytes jouent un rôle central dans l’apport de carbone aux consommateurs dans les lacs peu profonds et que les réseaux trophiques de la plupart des lacs sont alimentés par le carbone fixé par le phytoplancton. [Traduit par la Rédaction] Herwig et al. 22 Introduction The use of measurements of the natural abundance of car- bon and nitrogen stable isotopes ( δ 13 C and δ 15 N, respec- tively) has become common place in assessing energy sources and trophic structure in freshwater food webs (Peterson and Fry 1987; Vander Zanden and Rasmussen 1999). Tradi- tionally, food web structure has been characterized using gut content analysis in combination with field and laboratory ob- servations of feeding. These methods provide information about recent ingestion but not about feeding history and as- similation (Rounick and Winterbourn 1986). Stable isotope analysis has an advantage over traditional techniques in that the δ 13 C and δ 15 N ratios of a consumer provide a time- integrated measure of assimilated prey, rather than a list of prey items merely encountered in the diet. Specifically, the δ 13 C ratio of a consumer reflects (within 1‰) the δ 13 C signa- ture of its diet (DeNiro and Epstein 1978; Peterson and Fry 1987), and the δ 15 N ratio of a consumer reflects its trophic position because there is an average enrichment of 3.4‰ per trophic level (Minagawa and Wada 1984). Carbon sources often considered important in lakes in- clude phytoplankton, benthic and epiphytic algae, and aquatic vascular plants (Bunn and Boon 1993; Hecky and Hesslein 1995; Keough et al. 1996). Limnologists often con- sider feeding pathways derived from phytoplankton to be most important for secondary consumers and fish growth in lakes (Gu et al. 1994; Yoshioka et al. 1994; Gu et al. 1996). Recently, there has been a growing appreciation that benthic primary producers and allochthonous organic matter from ri- Can. J. Fish. Aquat. Sci. 61: 12–22 (2004) doi: 10.1139/F03-139 © 2004 NRC Canada 12 Received 9 January 2003. Accepted 19 October 2003. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 30 January 2004. J17270 B.R. Herwig 1,2 and D.A. Soluk. Illinois Natural History Survey, Center for Aquatic Ecology, 607 East Peabody Drive, Champaign, IL 61820, U.S.A. J.M. Dettmers. Illinois Natural History Survey, Lake Michigan Biological Station, 400 17th Street, Zion, IL 60099, U.S.A. D.H. Wahl. Illinois Natural History Survey, Kaskaskia Biological Station, R.R. 1, Box 157, Sullivan, IL 61951, U.S.A. 1 Corresponding author (email: brian.herwig@dnr.state.mn.us). 2 Present address: Minnesota Department of Natural Resources, Section of Fisheries, 2114 Bemidji Avenue, Bemidji, MN 56601, U.S.A.