EGU24-12823, updated on 09 Mar 2024
EGU General Assembly 2024
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Nitrogen Isotopic Studies in an Urban Estuary: Not Just a story of Anthropogenic Influences

Peter Swart1, Amel Saied1, Maribeth Gidley2, Aliza Karim3, Elzabeth Kelly3, Rachel Silverstein3, Chistopher Sinigalliano2, Tiffany Troxler4, and Sean Ahearn5
Peter Swart et al.
  • 1University of Miami, RSMAES, MGS, Miami, United States of America (
  • 2NOAA/AOML, 4301 Rickenbacker Causeway, Miami Fl, 33149
  • 3Miami Water Keepers, P.O. Box 141596, Coral Gables, FL 33114-1596
  • 4Florida International University, SE Environmental Research Ctr. & Department of Biological Sciences, 11200 SW 8th Street, Miami, FL 33199
  • 5Beta Analytic,4985 SW 74th Court Miami, Florida 33155 USA

Over a twelve-month period, between June 2021 and June 2022, a study was carried out that investigated possible causes of water quality decline in northern Biscayne Bay (South Florida, USA).  During this investigation a large number of different water quality parameters were monitored at 22 sites and waterways feeding into the Bay (Miami River, Little River, Biscayne Canal, and Snake Creek) in this area. Additional samples were also collected in the central region of Biscayne Bay around Virginia Key and Key Biscayne, Coconut Grove, and Matheson Hammock. At each site water and vegetation samples were collected.  Water samples were analyzed for standard nutrient parameters (TN, TP, NO3-, NH4, chlorophyll, turbidity, salinity, and dissolved oxygen content) and vegetation samples analyzed for their δ15N and δ13C values, and CN ratios. A smaller number of additional water samples were collected for the analysis of the δ18O and δ15N values of the NO3- and sediment samples were analyzed for their dδ15N values at each site. This presentation concentrates principally on the nitrogen isotopic portion of the study. The highest δ15N values were found in algae and plants collected from the Biscayne Canal, Little River, and Miami River sites, while lower values were measured at the central Biscayne Bay sites.  The Little River sites showed a strong seasonality with δ15N values ranging from +8 ‰ in the wet season to over +13‰ at the start of the dry season, while Miami River and Biscayne Canal sites remained at values of between +9 to +13‰ throughout the year.   While elevated δ15N  values are associated with the input of anthropogenic waste, leaking from septic tanks along the canals, elevation of the d15N values in excess of typical human δ15N values is driven by fractionation during nitrification and incorporation by assimilation. We propose that nitrogen derived from septic tanks ,with elevated δ15N  values, mixes with nitrogen derived from the natural environment as well as from artificial fertilizers (both with lower δ15N values) to produce a pool of nitrogen with intermediate δ15N values.  This reservoir is then further enriched in 15N during the process of assimilation and nitrification which fractionates the δ15N  value of the residual NO3-

How to cite: Swart, P., Saied, A., Gidley, M., Karim, A., Kelly, E., Silverstein, R., Sinigalliano, C., Troxler, T., and Ahearn, S.: Nitrogen Isotopic Studies in an Urban Estuary: Not Just a story of Anthropogenic Influences, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12823,, 2024.