Chemical Forms of Nitrogen in Runoff from Diverse Florida Watersheds
Voelker, Jessica Emily
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Watershed use along the Indian River Lagoon (IRL) on the central east coast of Florida has changed drastically during the past century, bringing with it declining water quality, eutrophication, hypoxia and fish kills. The decline of the lagoon has been directly linked to nutrient overloading; however, the sources and chemical forms of nutrients to the IRL are still not well constrained. This study was designed to determine how watershed land use impacts concentrations and fluxes of the various forms of dissolved nitrogen to the IRL. Five tributaries to the IRL were sampled monthly from July 2015 – September 2016. Storm flow was independently sampled 15 times during an eight-day event (May 17 – 24, 2016) at two locations. Concentrations and transport of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN as ammonium, nitrate and nitrite) were determined near U.S. Geological Survey gauging stations. DON was the dominant form of dissolved nitrogen in each watershed making up an average of ~72 ± 7% of the total dissolved nitrogen (TDN). The tributary with greatest percent natural and agricultural land use had the largest mean DON concentration (54 ± 7 µM; 0.76 ± 0.10 mg N/L). Tributaries with greatest urban land use had significantly lower DON concentrations and higher values of nitrate and nitrite. Concentrations of the various forms of dissolved nitrogen during both non-storm and storm conditions did not vary greatly with flow; therefore, mean concentrations during storm and non-storm surveys were used to calculate nitrogen fluxes to the IRL. The annual flux of TDN from Turkey Creek was ~60 tons of N which is ~15 times greater than inputs of dissolved nitrogen (all as ammonium) from muck sediments in Turkey Creek. Ammonium inputs from runoff from Turkey Creek (~4 tons N/year) were essentially equal to the muck flux. The cumulative flux of total dissolved nitrogen from all five tributaries was ~220 tons N/year. Creek fluxes of DON were ~3 times greater than DIN; fluxes of nitrate made up ~70% of the DIN flux. These relationships between land use, flow rates, and creek and sediment fluxes of chemical forms of nitrogen show that inputs of inorganic nitrogen have increased over the past three decades, likely due to increased urbanization.