Nitrogen Stable Isotopes of Macrophytes Assess Stormwater Nitrogen Inputs to an Urbanized Estuary

The ¹⁵N composition of seagrass and benthic macroalgae from shallow waters of Sarasota Bay was measured to determine if stable N isotopes can be used to trace stormwater N into macrophyte production within an urbanized estuary. Results show isotopically enriched macroalgae at the landward stations near creeks and bayous in the central Bay and in the southern portion of the Bay. A known sewage outfall at Whitaker Bayou resulted in δ ¹⁵NO₃ values from 0 to +9‰. Isotopically enriched NH₄ values in Phillippi Creek (+10 to +17‰) were similar to the stormwater ¹⁵NH₄ values from the watershed (+7 to +18‰). Enriched N sources supported a significant portion of macroalgae N demands in the southern reaches of the Bay while isotopically depleted N sources (i.e., atmospheric deposition and/or fertilizers) appear to be more important for macroalgae in the northern portion of the Bay. Macroalgae were typically more enriched than seagrass and appear to be better indicators of anthropogenic loadings near creeks and bayous that receive large volumes of stormwater and other anthropogenic N sources. Historically, studies have used enriched ¹⁵N in macrophytes to infer wastewater influences. This study shows that stormwater N inputs need to be considered in nitrogen budgets for aquatic systems that show anthropogenic ¹⁵N enrichment.