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

Radon samples were collected and analyzed in two ways: as discrete grab samples and continuously from the ship’s underway system. This dataset contains the underway (continuous) measurements from July 2023. Discrete grab samples (including February and June/July collections) are provided in the related dataset.

For discrete radon-222 samples, six-liter seawater samples were collected into an 8 L Nalgene high density polyethylene jerrican (hereafter referred to as jerrican) with a three-port cap fitted with tubing, as described by Stringer and Burnett (2004). This approach is similar to the commercially available Big Bottle System (Big Bottle System Manual, 2018) but allows for a sufficient headspace volume of gas, to increase the total sample volume and thus a lower detection limit. Samples were immediately sealed with screw-compressor clamps on the tubing in two locations to prevent radon-222 loss from the headspace. All discrete radon-222 samples were analyzed within 48 hours of collection. Jerricans were connected either to a RAD8 or RAD7, both commercially available radon-in-air detectors (Durridge, Inc.). Detectors were initially purged with Drierite to reduce internal humidity to below 10% and background counts were recorded. Following Stringer and Burnett (2004), jerrican samples were degassed with the built-in air pump for 60 minutes in a closed-air loop. Samples were counted for a minimum of three hours (“Sniff” mode) to reach statistically significant counts. Following, sample volume was precisely measured using a graduated cylinder.

For underway radon-222 analyses, water was pumped into a showerhead gas equilibrator (RAD-AQUA, Durridge, Inc.) at a flow rate > 2 liters min^-1 with a fifteen-minute counting interval, following Burnett et al. (2001).

For both discrete and underway measurements, data files were downloaded using CAPTURE software (Durridge, Inc.). Radon-in-air was calculated by dividing the total counts-per-minute (cpm; background corrected) in window-A by the RAD7/8 detector efficiency. For discrete samples, additional corrections were made as the ratio of total air volume (volume of tubing/drying column connector, jerrican, and chamber) to chamber air volume. For both discrete and underway measurements, in-situ seawater salinity was taken from the ship underway system or CTD (bottom waters), and temperature of the radon-222 sample from the internal air temperature of the RAD7/8 system. Radon-in-air to radon-in-water solubility corrections were made following Schubert et al. (2012), and samples were decay-corrected to the time of collection. Errors are propagated from counting statistics and detector efficiencies. Specific activities are reported as total radon-222 (supported from the in-situ decay of its parent Ra-226 plus excess radon-222).

- Imported "BCO-DMO_STING-II_Radon_Underway_Dataset.csv" into the BCO-DMO system
- Converted "DateTime" into ISO 8601 format (YYYY-MM-DDTHH:MM) and renamed field to "ISO_DateTime_Local"
- Converted "ISO_DateTime_Local" to create a new field "ISO_DateTime_UTC" in UTC timezone
- Created a new field "Rn222_flag" and populated Rn222_flag = ‘BDL’ where Rn222 was below detection
- Removed "BDL" string from the "Rn222" and "Rn222_err" fields so that the fields can be typed as numbers
- Exported file as "988549_v1_sting_radon_underway.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.