GApr18

Overview

The Submarine Groundwater Discharge, Trichodesmium, Iron, and Nitrogen on the Gulf Shelf (STING) project is a U.S. GEOTRACES process study (GApr18) that investigates how submarine groundwater delivers important nutrients to the coastal ecosystems of the West Florida Shelf (WFS). Preliminary studies indicate that groundwater may supply both dissolved organic nitrogen (DON) and iron to this region. In coastal ecosystems like the WFS that have very low nitrate and ammonium concentrations, DON is the main form of nitrogen available to phytoplankton. This study investigates the sources and composition of DON and iron, and their influence on the coastal ecosystem.

The STING project (GApr18) is a U.S. GEOTRACES process study funded by the U.S. National Science Foundation. The work quantifies the fluxes and isotopic signatures of submarine groundwater discharge (SGD)-derived DON and dissolved iron to the WFS, and evaluates whether SGD stimulates nitrogen fixation by Trichodesmium. The project tests two central hypotheses: (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, increases nitrogen fixation rates, and modifies DON and iron composition.

The lead PI and Chief Scientist was Angela Knapp (Texas A&M University), Co-PIs included Rene Boiteau (University of Minnesota), Kristen Buck (Oregon State University), Dreux Chappell (University of South Florida), Tim Conway (University of South Florida), Chris Smith (USGS -St. Petersburg, FL), and Joe Tamborski (Old Dominion University). The first cruise was conducted on the R/V Atlantic Explorer out of St. Petersburg, FL from February 20-March 6, 2023, sampling 44 stations across the WFS. The second cruise was on the R/V Endeavor out of St. Petersburg, FL from July 1–13, 2023, and sampled 43 stations. 

Major Findings, last updated May 2026

Initial findings reveal that submarine groundwater discharge (SGD) accounts for a significant component of total dissolved nitrogen and total dissolved phosphorus — primarily in organic form — and more than 95% of the silicate and dissolved iron delivered to the inner shelf. Radium isotopes were used to quantify the seasonal flux of nutrient-rich groundwater entering and leaving the West Florida Shelf, confirming SGD as a major nutrient source to these coastal waters. The study also found extremely high nitrogen fixation rates on the inner shelf, driven by Trichodesmium erythraeum, UCYN-A, and the Richelia/Hemiaulus diatom-diazotroph assemblage, while Trichodesmium thiebautii dominated offshore diazotroph populations.