Monster Seminar JAM - Degradation and Destabilization of Freshwater Ecosystems by Diffuse and Urban Nitrogen Sources
Dr. Peter R. Leavitt, University of Regina
Few studies have quantified the unique impacts of nitrogen (N) on lake eutrophication, despite increasing recognition that many lakes are replete with phosphorus (P) from natural or anthropogenic sources. Further, little is known of the ability of such P-sufficient lakes to retain N and thereby mediate environmental degradation of downstream lakes or estuaries. We addressed these issues by measuring N transport to and storage in nine lakes linked by the Qu'Appelle River, Saskatchewan, Canada. Stable isotope content (d15N) was measured for dissolved N, periphyton, particulate organic matter (POM) and sediment samples collected at 10 stations along Wascana Creek and the Qu'Appelle River, lotic ecosystems which receive wastewaters from the City of Regina and which drain into Pasqua Lake. Urban effluent (d15N ~16+20) enriched dissolved N isotope ratios of river water by up to 150 but was not stored in lotic sediments. Instead, urban N increased d15N signatures of lotic periphyton and POM by 10-150 and was transported to Pasqua Lake, where sedimentary d15N values increased from ~6.50 during the 19th century to 14.00 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 ~1880 CE (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 due to 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 impacts through permanent storage of wastewater N in their sediments. Results of this study are compared with loss of ecosystem integrity arising from N inputs to Lough Neagh, Northern Ireland, and Lake Washington, United States.
Date and Time:
March 9, 2006,
11:00 am - 12:30 pm