Award: OCE-2341997

This project investigated the role of submarine groundwater discharge in delivering key nutrients to the coastal ecosystems of the West Florida Shelf. The West Florida Shelf is somewhat unique in that it receives low levels of nutrients like nitrogen, particularly in forms most readily used by phytoplankton, such as nitrate and ammonium. Despite this, it still supports periodic harmful algal blooms, including those caused by the toxic alga Karenia brevis, which threaten coastal waters and economies. The growth of harmful algal blooms and other phytoplankton depends heavily on nutrients supplied from land. Although major land-based nutrient sources, such as rivers, are well monitored, submarine groundwater discharge across the continental shelf has emerged as a substantial nutrient source whose fate and contribution to these recurring blooms remain poorly understood. Additionally, in coastal low-nutrient regions like the West Florida Shelf, we know very little about the role of nitrogen fixation, a process that allows certain bacteria to convert otherwise unavailable dinitrogen gas into ammonium and organic nitrogen.

Through a coordinated research effort, including two cross-shelf cruises in different seasons and seasonal sampling of offshore groundwater wells and rivers, this study found that submarine groundwater-derived nutrients can supply a major share of the nitrogen and phosphorus that sustain K. brevis blooms and deliver enough iron to fuel the nitrogen-fixing cyanobacteria Trichodesmium spp., demonstrating that models predicting algal blooms must account for submarine groundwater discharge (Figures 1 and 2). Delivery of these nutrients via submarine groundwater discharge to coastal waters is significantly modified by storm surges. This study also found that the shelf environment supported some of the highest measured marine nitrogen fixation rates (Figure 3). Nitrogen fixation rates were highest nearshore in the summer, but high rates were also measured offshore in the summer and nearshore in the winter. The offshore nitrogen-fixing community was dominated by Trichodesmium thiebautii year-round, whereas the nearshore nitrogen-fixing community was more variable and included both clades of Trichodesmium and cyanobacterial nitrogen fixers that are symbiotic with both diatoms and haptophytes. Collaborators on the broader project are still analyzing samples, but initial statistical analyses show that T. thiebautii correlates with offshore properties (especially in winter), while the dominant nitrogen-fixing diatom symbiont (het-2) correlates with nearshore properties, particularly radioisotopes associated with submarine groundwater discharge and submarine groundwater-derived nutrients (Figure 4).  

In addition to the intellectual merit of this proposal's scientific advances, this project also impacted the broader community. This project directly supported multiple undergraduate students, who were introduced to cutting-edge research. It also supported a master’s student and two PhD students. One student-led manuscript has already been published, another is currently under review, and a third is in final review by collaborators before submission. The project also supported a postdoctoral scholar, who is working on a manuscript based on his work. Finally, the project supported multiple outreach efforts targeting elementary and high school students that were held both in person and virtually.

Last Modified: 08/10/2026
Modified by: Phoebe Dreux Chappell