They are what We eat : Tracking incorporation of sewage particulates into the food web of a large river ecosystem

English Abstract:

Anthropogenic inputs of inorganic N having tell-tale high N-isotope ratios now dominate N budgets of rivers. This contrasts with punctual sewage inputs of particulate organic N from the human food chain which have low d¹⁵N and may have only local effects, as shown by studies of sessile organisms or fish with little mobility sampled dowstream of municipal effluents. How spatially important is this contribution to the riverine food web and can it be detected in large (>1 kg) fish potentially roaming up and down the river and feeding at different trophic levels? To answer these questions, we investigated the isotopic discontinuity observed in 15 species of fish along a 200-Km transect of the St-Lawrence River above and below the city of Montreal, one of the largest sewage treatment plant in North America. Baseline d¹⁵N organisms (mollusks and crustaceans) showed a clear precipitous decline in N- signature below the effluent, an abrupt decrease not observed elsewhere along this transect. Depleted (by 6-10 permil) d¹⁵N values were observed near the effluent and gradually increased back to normal levels 40 km dowstream in large species (e.g. sturgeon, walleye, and redhorse). Thus riverine food web N-incorporation is dominated by these anthropogenic inputs in highly mobile and long-lived organisms over spatial scales of 10’s of kilometers. This pattern should grealty help in tracing urban ecotoxicological impacts on fish in river ecosystems.