The role of sewage in a large river food web Adrian M.H. deBruyn, David J. Marcogliese, and Joseph B. Rasmussen Abstract: We evaluated the role of sewage as a resource for the littoral food web of the fluvial St. Lawrence River near Montreal, Quebec. Stable isotope analysis indicated that macroinvertebrate primary consumers were feeding on local epiphytic production at sites outside the sewage plume, but shifts in δ 15 N of primary and secondary consumers revealed a substantial uptake of sewage-derived resources within the plume, up to 10 km from the outfall. Daily sec- ondary production of macroinvertebrates was 1.8- to 4.1-fold higher at sewage-enriched sites, and the fraction of this production attributable to larval Chironomidae increased from 46% (outside the plume) to 85% (at sewage-enriched sites). Sewage enrichment also stimulated increases in daily fish production based on algivory–detritivory (1.3- to 4.4- fold), invertivory (1.7- to 10-fold), and piscivory (11- to 73-fold). We estimate a daily flux of 13 tonnes of sewage- derived particulate matter, 184 kg of total nitrogen, and 13 kg of total phosphorus into the food web over 1.2 km 2 of the littoral zone within 10 km of the outfall. These values represent no more than a few percent of the total daily dis- charge of sewage-derived resources but were sufficient to support an overall fivefold increase in secondary production relative to sites outside the plume. Résumé : Nous avons évalué le rôle des égouts comme ressource dans le réseau alimentaire littoral du tronçon fluvial du Saint-Laurent, près de Montréal, Québec. Une analyse des isotopes stables révèle que les consommateurs primaires parmi les macroinvertébrés se nourrissent de la production locale des épiphytes aux sites en dehors du panache des égouts; toutefois, dans le panache, des changements de δ 15 N chez les consommateurs primaires et secondaires indiquent qu’il y a une ingestion importante de ressources provenant des égouts, jusqu’à 10 km en aval du déchargement. La production secondaire journalière des macroinvertébrés est de 1,8 à 4,1 fois plus élevée aux sites enrichis par les égouts et la proportion de la production attribuable aux larves de chironomidés augmente de 46 % (hors du panache) à 85 % (aux sites enrichis par les égouts). L’enrichissement par les égouts entraîne aussi une augmentation de la produc- tion journalière de poissons en conséquence de la consommation d’algues et de détritus (augmentation de 1,3 à 4,4 fois) d’invertébrés (1,7 à 10 fois) et de poissons (11 à 73 fois). Nous estimons à 13 tonnes le flux quotidien de matière particulaire provenant des égouts qui entre dans le réseau alimentaire dans les 1,2 km 2 de la zone littorale qui s’étend sur les 10 km en aval de la décharge; l’azote total représente 184 kg et le phosphore total 13 kg. Ces valeurs ne repré- sentent qu’un faible pourcentage du déversement quotidien total de ressources provenant des égouts, mais elles sont suffisamment élevées pour expliquer un accroissement général de l’ordre de 500 % de la production secondaire, par comparaison aux sites situés hors du panache de déversement. [Traduit par la Rédaction] deBruyn et al. 1344 Introduction River ecologists have developed a small but powerful body of general theory, central to which is the overriding impor- tance of longitudinal (upstream–downstream) and lateral (watershed–river) connections. These connections interact with physical forces that change along the course of a river, producing a continuum of morphological and hydrological features. The result is a shift in the relative importance of basal resources, from allochthonous litterfall in headwater streams to autochthonous producers in midsized rivers (Vannote et al. 1980). The metabolism of larger rivers is dominated by fine particulates transported in the main chan- nel, although autochthonous production in the littoral zone and allochthonous inputs from riparian vegetation and flood- plains are energetically more important to higher trophic levels (Thorp and Delong 1994, 2002). Most of the world’s large rivers are influenced to some extent by human activities and this often includes substantial pollution with sewage (Meybeck and Helmer 1989), but the role of sewage as a resource in river food webs is frequently overlooked. Previous work on sewage in rivers has tended to focus on nuisance blooms of algae or on systems so heavily enriched that hypoxia is a dominant environmental factor. In Can. J. Fish. Aquat. Sci. 60: 1332–1344 (2003) doi: 10.1139/F03-114 © 2003 NRC Canada 1332 Received 17 January 2003. Accepted 30 July 2003. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 21 November 2003. J17294 A.M.H. deBruyn 1,2 and J.B. Rasmussen. Department of Biology, McGill University, 1205 Docteur Penfield Ave., Montreal, QC H3A 1B1, Canada. D.J. Marcogliese. St. Lawrence Centre, Environment Canada, 105 McGill St., Montreal, QC H2Y 2E7, Canada. 1 Corresponding author (e-mail: adebruyn@sfu.ca). 2 Present address: School of Resource and Environmental Management, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada.