Stable carbon, nitrogen, and sulfur isotope ratios in riparian food webs on rivers receiving sewage and pulp-mill effluents

We examined spatial trends in ratios of stable isotopes of nitrogen (delta N-15), carbon (delta C-13), and sulfur (delta S-34) in riparian food webs leading to a riparian, insectivorous bird species, the tree swallow (Tachycineta bicolor), on rivers receiving pulp-mill effluent and municipal sewage and compared them with ratios of these isotopes in components of the effluents themselves. The study was conducted on two rivers in western Canada at sites upstream and downstream from sewage and pulp mill effluent sources. Municipal sewage contained materials with delta N-15 or delta C-13 values that were distinct from background conditions in the receiving environment. Pulp-mill effluent contained materials with delta S-34 values that were distinct from those at upstream sites on the rivers. At both locations, effluent-derived nitrogen and sulfur were observed in algae and suspended sediments at sites downstream from the effluent sources. This was also observed in adult aquatic insects, the one exception bring that uptake of effluent-derived nitrogen was detected isotopically at only one location. Tree swallows' use of sewage-derived nitrogen was evident only at one location, while use of pulp-mill-derived sulfur was evident only at the other location. Our study demonstrates the usefulness and limitations of measuring stable isotopes for tracing the movement of nutrients derived from sewage and pulp-mill effluent in freshwater ecosystems and, further, indicates that such tracing may be extended to riparian species feeding on aquatic prey.