Natural abundances of stable isotopes trace anthropogenic N and C in an urban stream

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 NO ₃^- flux, which explained 54% of the variation in seston δ¹⁵N. δ¹⁵N was not correlated with NH ₄⁺ 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.