Nutrient transformations between rainfall and stormwater runoff in an urbanized coastal environment: Sarasota Bay, Florida
Dillon, Kevin S., and Jeffrey P. Chanton
Limnol. Oceanogr., 50(1), 2005, 62–69

To determine the relative importance of atmospheric deposition and stormwater runoff as nitrogen sources to Sarasota Bay, Florida, we examined dissolved inorganic nitrogen (DIN) concentrations and d15N compositions of rainwater and stormwater runoff. Rainwater collected in Sarasota, Florida, had ammonium concentrations of 2.1– 29.0 µmol L^-1 in the summers of 1999 and 2000. Corresponding rainwater nitrate concentrations were 3.7–56.0 µmol L^-1. Rainwater ammonium d¹⁵N values were -11.6‰ to -0.3‰, and nitrate d¹⁵N values were -5.1 to +3.8 over the 2-yr period. Decreases on the order of 50% in ammonium concentration and d¹⁵N enrichments as great as 124‰ relative to rainwater ammonium were typically observed in the evolution of rainwater into stormwater. Stormwater d¹⁵NH₄ values were +7 to +18‰. Nitrate (NO₃) concentrations were typically elevated in stormwater relative to rainwater, although this trend was not statistically significant, and d¹⁵N values were generally slightly enriched in stormwater. Rainfall phosphate (PO₄) concentrations were always low (<2.1 µmol L^-1), whereas stormwater consistently had elevated PO₄ concentrations (up to 13.8 µmol L^-1). ¹⁵N-enriched N in the environment has generally been interpreted as an anthropogenic signal (wastewater and agricultural runoff). Our results will require the broadening of that interpretation to include stormwater DIN.