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Abstract Subterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p < 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were <8 mmoles N m−2 d−1. The total groundwater discharge rate was 38 ± 11 L m−2 d−1; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m−2 d−1for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to >100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for <10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics.
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This content will become publicly available on February 1, 2026
Submarine groundwater discharge as a major nutrient source in river‐fed vs. tidally dominated estuaries
Abstract The tidal tributaries of the lower Chesapeake Bay experience seasonally recurring harmful algal blooms and the significance of submarine groundwater discharge (SGD) as a nutrient vector is largely unknown. Here, we determined seasonal SGD nutrient loads in two tributaries with contrasting hydrodynamic conditions, river‐fed (York River) vs. tidally dominated (Lafayette River). Radon surveys were performed in each river to quantify SGD at the embayment‐scale during spring and fall 2021. Total SGD was determined from a222Rn mass balance and Monte Carlo simulations. Submarine groundwater discharge rates differed by a factor of two during spring (Lafayette = 11 ± 17 cm d−1; York = 6 ± 10 cm d−1) and a factor of six during fall (Lafayette = 19 ± 27 cm d−1; York = 3 ± 7 cm d−1). Groundwater N concentrations and fluxes varied seasonally in the York (4–7 mmol N m−2d−1). In the Lafayette River, seasonal N fluxes (22–37 mmol N m−2d−1) were driven by seasonal water exchange rates, likely due to recurrent saltwater intrusion. Submarine groundwater discharge–derived nutrient fluxes were orders of magnitude greater than riverine inputs and runoff in each system. Additionally, sediment N removal by denitrification and anaerobic ammonium oxidation would only remove ~ 1–11% of dissolved inorganic nitrogen supplied through SGD. The continued recurrence of harmful algal blooms in the Bay's tidal tributaries may be indicative of an under‐accounting of submarine groundwater‐borne nutrient sources. This study highlights the importance of including SGD in water quality models used to advise restoration efforts in the Chesapeake Bay region and beyond.
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- Award ID(s):
- 1737258
- PAR ID:
- 10579806
- Publisher / Repository:
- Limnology and Oceanography
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 70
- Issue:
- 2
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- 426 to 442
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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