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

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^-.