Landscape-scale effects of urban nitrogen on a chain of freshwater lakes in central North America
Limnol. Oceanogr., 51(5), 2006, 2262-2277 | DOI: 10.4319/lo.2006.51.5.2262
ABSTRACT: We measured nitrogen (N) transport to and storage in nine lakes linked by the QuAppelle River, Saskatchewan, Canada, to quantify the unique effects of N on the eutrophication of phosphorus (P)-replete lakes. Stable isotope content (δ15N) was measured for dissolved N, periphyton, particulate organic matter (POM), and sediment samples collected at 10 stations along Wascana Creek and the QuAppelle River, lotic ecosystems that receive wastewaters from the City of Regina and that drain into Pasqua Lake. Urban effluent (δ15N ~16 ± 2%) enriched dissolved N isotope ratios of river water by up to 15% but was not stored in lotic sediments. Instead, urban N increased δ15N signatures of lotic periphyton and POM by 10-15% and was transported to Pasqua Lake, where sedimentary δ15N values increased from ~6.5% during the 19th century to 14.0% by the 1990s. This increase was linearly correlated both to the mass of dissolved N released from Regina (r2 = 0.84, p < 0.0001) and to a 300% increase in the production of Pasqua Lake (as fossil pigments) since ca. 1880 (r2 = 0.69, p < 0.0001). Similar isotopic enrichment was recorded in five downstream lakes, but not three reference ecosystems, although the degree of downstream enrichment declined rapidly, mainly as a result of sequestration of urban N in lake sediments. Together, these patterns demonstrate that urban N can directly degrade surface waters of P-sufficient lakes, but that these ecosystems can eliminate urban effects through permanent storage of wastewater N in their sediments.