Nitrogen and carbon cycling and relationships to radium behavior in porewater and surface water: Insight from a dry year sampling in a hypersaline estuary

Abstract

Biogeochemical transformations within highly saline subterranean estuaries (STE) dramatically affect solute cycling, resulting in submarine groundwater discharge (SGD) with distinct chemical signatures. The study hypothesizes that biogeochemical processes within hypersaline bay porewaters (PW) simultaneously affect nitrogen, carbon, and radium cycling. We measured radium isotopes (²²⁶Ra, ²²⁴Ra, and ²²³Ra), nutrients (dissolved inorganic nitrogen [DIN: NH₄⁺ + NO₂⁻ + NO₃⁻], HPO₄²⁻ [DIP], HSiO₃⁻ [DSi], dissolved organic carbon [DOC]), total alkalinity (TA), dissolved inorganic carbon (DIC), stable isotopes, and major cations in PW and surface water (SW) of Baffin Bay, a well-mixed, semi-enclosed estuary along the semiarid northwestern Gulf of Mexico coast, over three seasons in a characteristically dry year. This study's findings show a concurrent increase in NH₄⁺, DIP, DSi, and TA/DIC with reduced metal species (e.g., Mn and Fe) and Ra during the hot and dry seasons, particularly in PW, under increasingly reducing conditions. Principal component analyses (PCA) suggest these increases are primarily driven by dissimilatory nitrate/nitrite reduction to ammonium (DNRA) and dissolution of lithogenic particles and biogenic CaCO₃, modulated by organic matter degradation or remineralization. While more significant terrestrial groundwater inputs may contribute to solutes and Ra supply in the STE, the biogeochemically induced variability in solute concentrations in PW primarily drives larger SGD-derived fluxes, particularly notable in hot months. During a typically dry year, these fluxes, estimated as the average of ²²⁶Ra and ²²³Ra mass balance models (e.g., July/November fluxes in Mmol∙d⁻¹: 0.093/0.092 of NO₃⁻; 0.2/0.02 of NO₂⁻; 72/16 of NH₄⁺; 72.2/18 of DIN; 1.5/0.2 of HPO₄²⁻; 20/9 of HSiO₃⁻; 42/37 of DOC; 503/399 of TA; 582/431 of DIC) are orders of magnitude (∼4 for DIN and DIC, ∼3 for DIP, DSi, and DOC, and ∼2 for TA) greater than surface runoff inputs. These substantial SGD inputs likely sustain phytoplankton growth and potentially fuel harmful algal blooms while countering estuarine acidification.