ISOTOPIC INSIGHTS INTO INORGANIC AND ORGANIC NITROGEN DYNAMICS: A PATHWAY TO RESTORATION IN A SEMI-ARID ESTUARY (Invited Presentation)

Coastal nutrient enrichment studies focus on dissolved inorganic nitrogen (DIN) and often overlook dissolved organic nitrogen (DON) as a vital component in estuarine nitrogen cycles. Here we take a comprehensive isotopic approach to understanding nitrogen dynamics in a DON dominated low-inflow, inverse estuary (Baffin Bay, TX, USA). We conducted high-resolution spatial sampling of ground, pore and surface waters throughout the watershed over an 18-month period. Isotopic compositions of both DIN (δ¹⁵N-NH₄⁺, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-) and DON (δ¹⁵N-DON) were measured and used to investigate N sources and processing. To determine the relative source apportionment of nitrate and DON in well and surface waters, we employed Bayesian-type isotope mixing models that incorporated fractionation factors. Our results indicate that the observed high nitrate concentrations (averaging 1064 µM) in wells can be attributed to competing sources, primarily wastewater and fertilizer inputs. Allochthonous sources (i.e., manure, wastewater, wet deposition) were found to contribute substantially to DON concentrations in surface waters but autochthonous sources (i.e., particulate organic matter) can also play a significant role. The isotope data from the various sample matrices provided evidence of multiple nitrogen processes, such as nitrification, denitrification, dissimilatory nitrate reduction to ammonium, and remineralization, shedding light on the transformation pathways of each nitrogen species before reaching the bay via groundwater discharge. The isotopic evidence and observed low inflow coupled with collaborative groundwater discharge studies, suggest groundwater as the primary delivery mechanism of nitrogen to the Bay.

This investigation identified potential sources and mechanisms behind the declining health of Baffin Bay which has been facing challenges such as intermittent harmful algae blooms, fish kills, and general symptoms of eutrophication. Our findings have played a crucial role in informing watershed management plans and guiding a new multi-decade "Bringing Baffin Back" restoration initiative. Continuous monitoring of changes in nitrogen concentrations and isotopic composition remains a central focus of future work, enabling us to evaluate the efficacy of restoration efforts over time.