Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
  • Problem Description
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Project Information
• Funding

Surface water radium isotope samples were collected at all river and estuary stations via a submersible pump lowered ~1.0 m below the water’s surface. River samples were collected at upstream (60 L) and downstream (20 L) stations, and physicochemical parameters were recorded with a calibrated handheld water quality sonde (In Situ Aqua Troll 600). For estuary samples, the water column was characterized by a CTD cast (Seabird 19plus v2) and 20 L of water was collected for radium isotopes. For all sample types, dissolved radium samples were filtered at <1L/min onto a MnO₂-coated acrylic fiber to adsorb Ra. Samples were washed with Ra-free deionized water to remove salts and remaining particles, then dried to a 1:1 fiber:water mass ratio (Moore, 2008).

Short-lived ²²³Ra and ²²⁴Ra were measured using a delayed coincidence counter (RaDeCC) at Old Dominion University (Moore and Arnold 1996), following recommended best-practices (Diego-Feliu et al. 2020). Additional measurements were performed one month (²²⁸Th) and two months (²²⁷Ac) post-collection to assess parent activities supporting ²²³Ra and ²²⁴Ra. ²³²Th standards in equilibrium with ²²⁸Th were measured to assess detector efficiencies for ²²⁴Ra and ²²³Ra (Moore & Cai, 2013). Errors are propagated from counting statistics, chance-coincidence corrections, and detector efficiencies (Diego-Feliu et al. 2020). Following, the Mn-coated acrylic fibers were combusted at 850°C for 16 hours, homogenized, sealed with epoxy resin, and incubated for a minimum of three weeks until ²²²Rn was in secular equilibrium with its parent ²²⁶Ra. Long-lived Ra isotopes were measured via gamma spectroscopy in a Ge well-type detector for ²²⁶Ra (352 keV photopeak) and ²²⁸Ra (average of 338 and 911 keV photopeaks), with efficiencies calibrated using standards prepared in the same fashion as the estuarine samples (Charette et al., 2001). Select estuary samples were analyzed for ²²⁸Ra via ingrowth of ²²⁸Th using the RaDeCC system, following Moore (2008).

- Loaded sheet 1 from "BCO-DMO_STING_Rivers_Estuaries_Radium.xlsx" using the filename as the resource name; missing values flagged as "", "nd", and "NaN"
- Corrected Longitude value at row 57 to -81.97959 (negative sign had been omitted)
- Converted DateTime field from "%m/%d/%y %H:%M" format to ISO 8601 datetime output
- Renamed DateTime to DateTime_local to preserve the local time version
- Created new DateTime field by converting DateTime_local from America/New_York to UTC, formatted as "%Y-%m-%dT%H:%MZ"
- Exported file as "988621_v1_sting_rivers_estuaries_ra.csv"

NSF Award Abstract:
This project will investigate how groundwater discharge delivers important nutrients to the coastal ecosystems of the West Florida Shelf. Preliminary studies indicate that groundwater may supply both dissolved organic nitrogen (DON) and iron in this region. In coastal ecosystems like the West Florida Shelf that have very low nitrate and ammonium concentrations, DON is the main form of nitrogen available to organisms. Nitrogen cycling is strongly affected by iron availability because iron is essential for both photosynthesis and for nitrogen fixation. This study will investigate the sources and composition of DON and iron, and their influence on the coastal ecosystem. The team will sample offshore groundwater wells, river and estuarine waters, and conduct two expeditions across the West Florida Shelf in winter and summer. Investigators will participate in K-12 and outreach activities to increase awareness of the project and related science. The project will fund the work of six graduate and eight undergraduate students across five institutions, furthering NSF’s goals of education and training.

Motivated by preliminary observations of unexplained, tightly-correlated DON and dissolved iron concentrations across the West Florida Shelf (WFS), the proposed work will quantify the flux and isotopic signatures of submarine groundwater discharge (SGD)-derived DON and iron to the WFS, and evaluate the bioavailability of this temporally-variable source using four seasonal near-shore campaigns sampling offshore groundwater wells, estuarine, and riverine endmembers and two cross-shelf cruises. The work will evaluate whether SGD stimulates nitrogen fixation on the WFS, and the potential for the stimulated nitrogen fixation to further modify the chemistry of DON and dissolved iron in the region. The cross-shelf cruises will investigate hypothesized periods of maximum SGD and Trichodesmium abundance (June), and reduced river discharge and SGD (February), thus comparing two distinct biogeochemical regimes. The concentrations and isotopic compositions of DON and dissolved iron, molecular composition of DON, and the concentration and composition of iron-binding ligands will be characterized. Nitrogen fixation rates and Trichodesmium spp. abundance and expression of iron stress genes will be measured. Fluxes of DON and iron from SGD and rivers will be quantified with radium isotope mass balances. The impacts of SGD on nitrogen fixation and DON/ligand production will be constrained with incubations of natural phytoplankton communities with submarine groundwater amendments. Two hypotheses will be tested: 1) SGD is the dominant source of bioavailable DON and dissolved iron on the WFS, and 2) SGD-alleviation of iron stress changes the dominant Trichodesmium species on the WFS, increases nitrogen fixation rates and modifies DON and iron composition. Overall, the work will establish connections between marine nitrogen and iron cycling and evaluate the potential for coastal inputs to modify water along the WFS before export to the Atlantic Ocean. This study will thus provide a framework to consider these boundary fluxes in oligotrophic coastal systems and the relative importance of rivers and SGD as sources of nitrogen and iron in other analogous locations, such as coastal systems in Australia, India, and Africa, where nitrogen fixation and SGD have also been documented.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.