Table of Contents

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

Collection

Samples were collected on STING I and STING II during cruises AE2305 (R/V Atlantic Explorer, Feb 20th - Mar 5th 2023), EN704 (R/V Endeavor Jul 1-12, 2023), and 23WFS02 (R/V Sallenger, Jun 3-7, 2023). Surface water salinity and temperature measurements were taken from the ship underway system (AE2305 and EN704) and a flow-cell (23WFS02; YSI ProDSS; Chestang et al., 2025), and from the ship CTD for intermediate and bottom waters.

Surface samples were collected at all stations via submersible pump lowered 1.5 m below the water’s surface. Intermediate depth and bottom water samples were collected at stations along three transects that crossed the entire shelf from nearshore to the shelf break via Niskin bottles on the CTD Rosette. Samples ranged from 114 to 170 L in volume on AE2305 and EN704, and ~45 – 60 L in volume on 23WFS02 (Chestang et al., 2025).

Samples were passed through a 1 µm prefilter to remove suspended particles before being 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).

Measurements

Short-lived 223Ra and 224Ra were measured underway using a delayed coincidence counter (RaDeCC) (Moore and Arnold 1996), following recommended best-practices (Diego-Feliu et al. 2020). Additional measurements were performed in the laboratory 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 separately in the lab and underway to assess detector efficiencies for 224Ra and 223Ra (Moore & Cai, 2013); background counts were similarly measured both in the lab and at sea. Errors are propagated from counting statistics, chance-coincidence corrections, and detector efficiencies (Diego-Feliu et al. 2020). Short-lived Ra isotope specific activity from 23WFS02, with accompanying physicochemical parameters, is reported in Chestang et al. (2025). 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 measured via gamma spectroscopy in a Ge well-type detector for 226Ra (352 keV photopeak) and 228Ra (average of 338 and 911 keV photopeak’s), with efficiencies calibrated using standards prepared in the same fashion as the seawater samples (Charette et al., 2001).

- Imported "BCO-DMO_STING-I_Radium_Dataset.xlsx" and "BCO-DMO_STING-II_Radium_Dataset.xlsx" into the BCO-DMO system without any formatting
- Concatenated the data sources together, creating a cruise name column to identify "STING I" and STING II"
- Created flag column for all measurements to indicate BDL
- Converted local datetime into a new UTC datetime field and removed the local timestamp, as requested by submitter
- Exported file as "982223_v1_sting1_sting2_radium.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.