Subterranean estuaries (STEs) form in the subsurface where fresh groundwater and seawater meet and mix. Subterranean estuaries support a variety of biogeochemical processes including those transforming nitrogen (N). Groundwater is often enriched with dissolved inorganic nitrogen (DIN), and transformations in the STE determine the fate of that DIN, which may be discharged to coastal waters. Nitrification oxidizes ammonium (NH4+) to nitrate, making DIN available for N removal via denitrification. We measured nitrification at an STE, in Virginia, USA using in situ and ex situ methods including conservative mixing models informed by in situ geochemical profiles, an in situ experiment with15NH4+tracer injection, and ex situ sediment slurry incubations with15NH4+tracer addition. All methods indicated nitrification in the STE, but the ex situ sediment slurries revealed higher rates than both the in situ tracr experiment and mixing model estimations. Nitrification rates ranged 55.0–183.16 μmol N m−2 d−1based on mixing models, 94.2–225 μmol N m−2 d−1in the in situ tracer experiment, and 36.6–109 μmol N m−2 d−1slurry incubations. The in situ tracer experiment revealed higher rates and spatial variation not captured by the other methods. The geochemical complexity of the STE makes it difficult to replicate in situ conditions with incubations and calculations based on chemical profiles integrate over longer timescales, therefore, in situ approaches may best quantify transformation rates. Our data suggest that STE nitrification produces NO3−, altering the DIN pool discharged to overlying water via submarine groundwater discharge.
Within fluvial networks, lakes can be sinks or sources of dissolved organic carbon (DOC) and nutrients, yet the controls over sink‐source behavior remain unclear. We investigated the influence that an in‐network lake exerted on DOC and nutrient export. Our investigation consisted of: (1) injecting a conservative tracer to determine lake travel times and flow paths; (2) sampling lake inflow, outflow, and surrounding groundwater to determine water and nutrient budgets; and, (3) sampling internal lake profiles to ascertain in‐lake physico‐chemical patterns through time. Conservative tracer data indicated considerable in‐lake retention and combined with inflow‐outflow discharge measurements revealed a decoupling of kinematic and solute pulses. Nitrate (NO3) was the dominant form of dissolved inorganic nitrogen (DIN) at lake inflow whereas ammonium (NH4) became the dominant component at lake outflow. The lake was a sink for NO3‐N and PO4, but a source for NH4‐N, DON, TDN, and DOC. We observed hydrologic controls on DOC concentrations and export patterns, but redox controls on DIN dynamics. Our results indicate that lakes within fluvial networks can be sources of dissolved organic material and reduced nitrogen (NH4) while simultaneously being sinks for NO3‐N and PO4‐P. Determining controls on sink‐source behavior and the cumulative effect of lakes on DOC and nutrient budgets is a necessary first step toward improved understanding of the role of lakes in network‐ to regional‐scale dynamics.
more » « less- NSF-PAR ID:
- 10444072
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 52
- Issue:
- 11
- ISSN:
- 0043-1397
- Page Range / eLocation ID:
- p. 8668-8684
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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