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1 June 2005 Natural abundances of stable isotopes trace anthropogenic N and C in an urban stream
Amber J. Ulseth, Anne E. Hershey
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Important ecological services of low-order streams are greatly affected by urbanization. North Buffalo Creek, in the headwaters of the Cape Fear River basin in Greensboro, North Carolina, receives point- and nonpoint-source pollutants. Natural abundances of the stable isotopes of C (13C) and N (15N) were used to determine the influence of anthropogenic nutrients on seston δ15N, nutrient concentrations, C/N ratios, and patterns of δ13C and δ15N in foodweb components in North Buffalo Creek during different hydrological conditions. Baseflow seston δ15N varied significantly among sampling sites. Baseflow seston δ15N ranged from 3.7‰ to 4.6‰ at forested sites and was slightly enriched at open sites, and probably reflected nonpoint sources of N in North Buffalo Creek. Seston δ15N also reflected point sources of N in North Buffalo Creek. The most enriched seston δ15N values (8.4‰) were found directly downstream of the Waste Water Treatment Plant (WWTP). Seston δ15N values at the Rankin Mill Road (Rankin) site, several km downstream of the WWTP, also were strongly influenced by effluent from the WWTP. The Summit Avenue site (Summit) received textile effluent until June 2001. Before June 2001, seston 15N at Summit was depleted compared to seston 15N at sites upstream of the plant, probably because the textile effluent was depleted in 15N. During storms, seston δ15N was negatively correlated with nutrient concentrations upstream of the WWTP. However, at Rankin Mill Road, seston δ15N was positively correlated with NO3 flux, which explained 54% of the variation in seston δ15N. δ15N was not correlated with NH4 and PO4−3 fluxes at Rankin Mill Road. During storms, seston δ15N was influenced partially by nonpoint sources of N, a pattern consistent with observed C/N ratios. δ13C values for most foodweb components and δ15N values for all foodweb components varied significantly between sites, suggesting that sources of C and N differed among sites. Foodweb components at Rankin were enriched in 15N from incorporation of sewage-derived N from the WWTP effluent, whereas, prior to June 2001, foodweb components at Summit appeared to be depleted in 15N from incorporation of textile effluent. Thus, specific influences of point sources of N could be distinguished in foodweb components. Nonpoint sources and stormwater influenced seston δ15N during storm events, but these sources could not be distinguished in consumers by using natural abundances of stable isotopes.

Amber J. Ulseth and Anne E. Hershey "Natural abundances of stable isotopes trace anthropogenic N and C in an urban stream," Journal of the North American Benthological Society 24(2), 270-289, (1 June 2005).
Received: 6 August 2003; Accepted: 1 February 2005; Published: 1 June 2005

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