Landscape-scale effects of urban nitrogen on a chain of freshwater lakes in central

We measured nitrogen (N) transport to and storage in nine lakes linked by the Qu’Appelle River, Saskatchewan, Canada, to quantify the unique effects of N on the eutrophication of phosphorus (P)-replete lakes. 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 that receive wastewaters from the City of Regina and that drain into Pasqua Lake. Urban effluent (d15N ,16 6 2%) enriched dissolved N isotope ratios of river water by up to 15% but was not stored in lotic sediments. Instead, urban N increased d15N signatures of lotic periphyton and POM by 10–15% and was transported to Pasqua Lake, where sedimentary d15N 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 5 0.84, p , 0.0001) and to a 300% increase in the production of Pasqua Lake (as fossil pigments) since ca. 1880 (r2 5 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. Eutrophication of surface waters by point and diffuse sources of nutrients remains a potent cause of aquatic ecosystem degradation despite decades of intensive study (Carpenter et al. 1998; Smith 2003). In general, phosphorus (P) supply is thought to regulate algal production in freshwater lakes (Schindler 1977), whereas nitrogen (N) supply is the predominant control in many estuaries and shallow marine environments (Vitousek et al. 1997; Rabalais 2002). However, to date, few studies have investigated the unique effects of N loading on freshwater ecosystems during eutrophication (Kilinc and Moss 2002; James et al. 2003), despite recognition that many phytoplankton communities can exhibit N limitation (Elser et al. 1990), particularly if lakes lie in catchments with P-rich soils (e.g., Hall et al. 1999b). Further, it is increasingly evident that long-term agricultural practices can saturate soils with P (Bennett et al. 1999; Foy et al. 2002), thereby increasing P export to surface waters (Bennett et al. 2001) and creating conditions in which diffuse and point sources of N degrade water quality (Bunting et al. 2005). Unfortunately, investigators have rarely quantified the degree to which such P-sufficient lakes are degraded by N, nor the ability of these lakes to sequester or eliminate N (sedimentation, ammonia volatilization, denitrification), thereby reducing environmental effects of N on downstream lakes or estuaries. Eutrophication arising from N influx may be particularly severe in lakes of the northern Great Plains, sites often characterized by high nutrient content, low N : P ratios, surface blooms of N2-fixing cyanobacteria, and periodic fish kills (Hammer 1971; Haertel 1976). In many cases, supply of P from glacial tills and soils is great (Allan and Kenney 1978; Klassen 1989), leading to poor predictive relationships between P fluxes and algal abundance (Allan 1980; Campbell and Prepas 1986). Instead, recent paleoecological evidence suggests that changes in N influx from urban and agricultural sources are correlated with increased lake production and biological change during the 20th century (Hall et al. 1999b; Dixit et al. 2000; Quinlan et al. 2002). However, because agricultural and urban development has often been simultaneous, such retrospective analyses have been incapable of distinguishing among the effects of diffuse and point sources of N. Fortunately, analysis of stable isotopes can provide a means of differentiating among urban and agricultural sources of N, provided that their isotopic signatures can be distinguished (e.g., Wayland and Hobson 2001; deBruyn et al. 2003; Anderson and Cabana 2005). Further, when combined with paleolimnological analyses of past lake production, it is possible to quantify how ecosystem production and community composition have varied in response to changes in N loading (e.g., Schindler et al. 2005). 1 Corresponding author (Peter.Leavitt@uregina.ca).