The role of sewage in a large river food web

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 δ15N of primary and secondary consumers revealed a substantial uptake of sewage-derived resources within the plume, up to 10 km from the outfall. Daily secondary 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 km2 of the littoral zone within 10 km of the outfall. These values represent no more than a few percent of the total daily discharge of sewage-derived resources but were sufficient to support an overall fivefold increase in secondary production relative to sites outside the plume.