The nitrogen isotopic composition of biotic components in Northern Biscayne Bay and Associated Waterways, South Florida USA: implications for sources of nitrogen

Over an approximate twelve-month period, between June 2021 and June 2022, the δ¹⁵N values of aquatic vegetation and sediments and the δ¹⁸O and δ¹⁵N values of the NO₃⁻ in water samples were analyzed in order to understand the sources and processes affecting nitrogen entering the northern portion of Biscayne Bay, an urban area located to the west of Miami, in the southern United States. Samples were collected from 22 sites, primarily located in waterways feeding into the Bay (Miami River, Little River, Biscayne Canal, and Snake Creek). Additional samples were also collected in the central region of Biscayne Bay, around Virginia Key, Key Biscayne, Coconut Grove, and Matheson Hammock. The highest δ¹⁵N values were found in algae and plants collected from the Biscayne Canal, Little River, and Miami River, inland sites heavily influenced by land-based source of pollution such as storm water, septic tanks, and industrial runoff, while lower values were measured at the central Biscayne Bay sites. The Little River sites showed a strong seasonality with δ¹⁵N values of the vegetation ranging from + 8‰ in the wet season to over + 13‰ at the start of the dry season, while the Miami River and Biscayne Canal sites exhibited δ¹⁵N values of between + 9 to + 13‰ throughout the year. The elevated δ¹⁵N values are associated with the input of anthropogenic waste, presumed to originate from septic tanks along the canals. Elevation of δ¹⁵N values in excess of typical values found in humans is driven by fractionation of nitrogen during assimilation, nitrification, denitrification, volatilization of NH₄⁺ and anaerobic ammonium oxidation in septic tanks with clogged drain fields. We propose that dissolved inorganic nitrogen, derived from septic tanks with elevated δ¹⁵N values, mixes with nitrogen derived from the natural environment as well as that sourced from artificial fertilizers (both with lower δ¹⁵N values) to produce pools of nitrogen with intermediate δ¹⁵N values. These are then further enriched in ¹⁵N during the process of assimilation and nitrification which fractionates the δ¹⁵N value of the residual NO₃⁻. At all locations there is close to a 1:1 relationship between the δ¹⁵N and δ¹⁸Ο values of the nitrate, expected as a result of fractionation during assimilation. Mixing between natural sources of nitrogen, or nitrogen arising from anthropogenic sources or other animal waste and nitrogen arising from contamination by fertilizer produces a positive association between NO₃⁻ concentration and δ¹⁵NNO3- values at some locations.