Nitrate Transport Across the Northern Gulf of Mexico Shelf During Historically High Mississippi River Discharge

Abstract

This study characterizes the spatial and seasonal dynamics of radium isotopes ( ²²³ Ra, ²²⁴ Ra, and ²²⁸ Ra) and nitrate plus nitrite (NO ₃ ⁻  + NO ₂ ⁻ ) transport on the northern Gulf of Mexico shelf during 2019, when the Mississippi River remained at flood stage for 226 days. Sampling focused on two contrasting shelf settings: the Atchafalaya‐influenced Wax Lake Delta region and the Mississippi‐influenced Barataria Basin (BB) region. High river discharge in June produced elevated radium activities and stronger low‐salinity nitrate enrichment relative to September. Radium activity ratios yielded apparent surface‐water ages of approximately 5–10 days during peak discharge, indicating rapid transport of river‐associated constituents across the shelf. We further compared radium‐derived apparent effective mixing estimates with ADCP‐based advective nitrate fluxes measured at a fixed shelf location. These two approaches emphasize different, but not fully separable, components of transport in a dynamic river plume system. The radium‐derived estimates are model‐dependent and may include effects of unresolved plume advection, whereas ADCP‐derived fluxes represent velocity‐driven transport of nitrate fields that have already been shaped by mixing and dispersion. A first‐order mass balance suggests that the shallow Louisiana shelf retained or transformed only 39% of riverine nitrate during the high‐discharge period, with the rest being transported rapidly to deeper water beyond 50 m isobath.