Table of Contents

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

Sample Collection

SGD Wells

Submarine groundwater samples were collected by U.S. Geological survey (USGS) small boat from three offshore well transects, each with three well location. Samples were taken from well head (GWd) and from surface (SB) and bottom waters (BW). Divers, using acid-cleaned Teflon tubing attached to a trace metal clean diaphragm pump. First, bottom waters were collected, then the well uncapped, adequately purged, and sampled. All samples were filtered through 0.2 um Acropak Supor Cartridges (Pall, USA) into acid-cleaned LDPE bottles. NC5, NC10, NC15 = Nature Coast Transect, IRB25, IRB30, IRB35 = Indian Rocks Transect, VH45, VH50, VH55 = Venice Headlands Transect. More details are provided in Hunt (2025).

Rivers and Estuaries

River Sample sites were located on land and samples were collected from five rivers (Hillsborough, Alafia, Manatee, Peace, and Caloosahatchee), using trace metal clean protocols. Estuary samples were collected in collaboration with Mote Marine Laboratory via small boat (RV Eugenie Clark), also using trace metal clean protocols. All water samples for dissolved Fe and d56Fe were syringe filtered using acid-cleaned 50 mL Henke Jet HDPE syringes and 0.2 um PVDF membrane Titan3 syringe filters into acid-cleaned LDPE bottles. AR1-2 = Alafia River, HB1-2 = Hillsborough, MR1-2 = Manatee, PR1 = Peace River, CR1 = Caloosahatchee, CS and E= Estuary sample locations. More details are provided in Hunt (2025).

Sample Processing

Samples were acidified back on shore at the University of South Florida (USF) using 2.4 mL 10 M Teflon-distilled HCl per liter of sample and stored for at least 6 months before processing. Samples were then processed for dissolved Fe isotope and concentration analysis at the University of South Florida following Sieber et al. (2019), modified from Conway et al (2013). Briefly, an Fe double-spike was added prior to batch extraction using Nobias PA-1 chelating resin, followed by purification by anion-exchange chromatography using AG-MP1 resin. Concentration and isotope analyses were performed on a Thermo Neptune Plus MC-ICPMS (Multicollecter Inductively Coupled Plasma Mass Spectrometer) in the Tampa Bay Plasma Facility at the University of South Florida using the double spike technique via a ~100 uL min^-1 PFA nebulizer and Apex Ω introduction system, Pt Jet Sampler cone and an Al X skimmer cone.

Sample Analysis

Dissolved Fe stable isotope ratios are expressed in delta notation (δ⁵⁶Fe) relative to the IRMM-014 standard. A secondary Fe standard, NIST-3126, was analyzed over 56 sessions to provide an estimate of long-term instrumental precision. We obtain a value of +0.36 ± 0.06‰ (2SD, n = 772; runs = 56), in agreement with consensus values (Hunt 2025; Conway et al., 2013). Therefore, we consider a 2SD uncertainty of 0.06‰ as an estimate of analytical precision, and have applied it to all samples, except for low concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Dissolved Fe concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ⁵⁷Fe/⁵⁶Fe ratios measured simultaneously with isotope analysis. We express uncertainty (1SD) on Fe concentrations as 2%, based on Conway et al. (2013).

Data was processed in Microsoft Excel using an in-house double spike reduction scheme from Sieber et al (2021), based on Siebert et al (2001).

- Loaded sheet 1 from "BCO-DMO_STING_Rivers_Estuaries_Wells_Conway V1.0.xlsx" (xlsx format) using filename as resource name; missing values treated as empty string, "nd", and "NaN"; duplicate headers deduplicated
- Converted DateTime field from "%m/%d/%y %H:%M" to "%Y-%m-%dT%H:%M" datetime format, stored back to DateTime field
- Edited row 57: set Longitude to "-81.97959" from "81.97959"
- Renamed deduplicated fields: "n (1)" to "n_dFe", "n (2)" to "n_d56Fe", "DateTime" to "DateTime_Local", "2SD" to "SD_2", to comply with BCO-DMO naming conventions (parameter should not start with number)
- Converted DateTime_Local from "%Y-%m-%dT%H:%M" (America/New_York) to "%Y-%m-%dT%H:%MZ" (UTC), stored in new field DateTime_UTC
- Exported file to "995970_v1_sting_fe_and_fe_isotopes.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.