Denitrification Hotspots or Nutrient Highways? Modeling the Fate of Nutrients in Coastal River Deltas

More than half a billion people worldwide live in coastal river deltas, which provide critical ecosystem services. However, excess nitrate exported from these systems has led to significant environmental challenges, including hypoxic zones like the Gulf of Mexico's dead zone. Islands near the outlet of river deltas can be important last-ditch effort sites for nitrate processing prior to entering the ocean. Over the past decade, research has begun to numerically quantify nutrient transport through delta systems. These studies have traditionally utilized Eulerian models that are spatially-lumped, and nutrient fluxes are largely determined by catchment land use. Additionally, the potential for nutrients to be removed from channels within the islands or secondary channels in delta systems is typically ignored. This research proposes a distributed, Lagrangian modeling framework that follows individual particles through time and space to better understand the fate of nutrients in deltas (and the potential for removal). We accomplish this goal by adding a nutrient transport component to the open-source Python Package dorado, a Lagrangian model for passive particle transport that requires coupling with hydrodynamic outputs. We use dorado to quantify the hydraulic residence time of simulated nitrate particles in Wax Lake Delta of coastal Louisiana. Instead of only measuring nitrate transport through major distributary channels, we model channel-island connectivity, and the consequential differences in residence time distributions as particles “leak” from the channel into island networks. Deltaic islands have the ideal characteristics for increased nitrate processing capacity (slower water velocities, increased vegetation, etc), so these pathways are important to quantify denitrification potential. We couple modeled hydraulic residence time distributions with a first-order nitrate decay model to simulate the removal pathways of nitrate throughout the delta. Results identify the conditions and/or seasons with higher denitrification potential, offering insights into the role of deltas as sinks for excess nutrients. This work demonstrates the importance of deltaic islands in nutrient cycling and highlights how Lagrangian modeling can improve predictions of coastal nutrient dynamics.