Nutrient transformations between rainfall and stormwater runoff in an urbanized coastal environment: Sarasota Bay, Florida

Abstract

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 δ15N 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 δ15N values were -11.6‰ to -0.3‰, and nitrate δ15N values were -5.1 to +3.8 over the 2-yr period. Decreases on the order of 50% in ammonium concentration and δ15N enrichments as great as +24‰ relative to rainwater ammonium were typically observed in the evolution of rainwater into stormwater. Stormwater δ15NH4 values were +7 to +18 ‰. Nitrate (NO3) concentrations were typically elevated in stormwater relative to rainwater, although this trend was not statistically significant, and δ15N values were generally slightly enriched in stormwater. Rainfall phosphate (PO4) concentrations were always low (<2.1 μmol L-1), whereas stormwater consistently had elevated PO 4 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.