Correlated dissolved organic nitrogen and dissolved iron concentrations on the West Florida Shelf: signatures of submarine groundwater discharge and Trichodesmium thiebautii

Phoebe Dreux Chappell, Old Dominion University, Ocean, Earth and Atmospheric Sciences, Norfolk, United States, Kristen N Buck, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, Corday Selden, Cuernavaca, Mexico, Salvatore Caprara, University of South Florida, College of Marine Science, St Petersburg, FL, United States, Brent A Summers, University of South Florida, College of Marine Science, St Petersburg, United States, Travis Mellett, University of Washington Seattle Campus, School of Oceanography, Seattle, United States, Kristina Confesor, Heidelberg University, Tiffin, OH, United States, Kimberly Powell, Old Dominion University, Ocean, Earth & Atmospheric Sciences, Norfolk, VA, United States, Laura Donahue, Haverford College, Haverford, United States, Rene Boiteau, Pacific Northwest National Laboratory, Richland, WA, United States, Tim M Conway, University of South Florida, College of Marine Science, St. Petersburg, United States, Matthew A Charette, Woods Hole Oceanographic Institution, Marine Chemistry & Geochemistry, Woods Hole, MA, United States, Joseph Tamborski, Old Dominion University, Ocean and Earth Sciences, Norfolk, United States, Amy M McKenna, Florida State University, National High Magnetic Field Laboratory, Tallahassee, FL, United States and Angela N Knapp, Florida State University, Earth, Ocean, and Atmospheric Sciences, Tallahassee, FL, United States
Sampling along the West Florida Shelf (WFS) in June 2015, March 2018, and April 2019, led to novel observations of significantly positively correlated surface ocean dissolved organic nitrogen (DON) and dissolved iron (dFe) concentrations. In addition to dFe and DON, samples were collected for chlorophyll, isotopic analysis of DON, additional dissolved metal concentrations, and DNA and RNA analyses. Samples for analysis of the isotopic composition of dFe were collected starting in March 2018 and samples for radium isotopes were added in April 2019. We consider potential explanations for the correlation between dFe and DON across the WFS, including that dFe availability supports new production, a fraction of which is released as DON. One likely candidate for this production-associated DON release is the cyanobacterial diazotroph Trichodesmium spp. that was observed along the transect. Clade-specific quantitative polymerase chain reaction methods were used to quantify the gene abundance of the two clades of Trichodesmium (Trichodesmium thiebautii and Trichodesmium erythraeum). In higher salinity waters near the 50 m isobath, DON concentration positively correlated with T. thiebautii abundance. Additionally, T. thiebautii gene abundance was negatively correlated with the d15N of DON, which was as low as -8 per mil in samples where the highest abundance of T. thiebautii was found. These observations suggest that this clade may be releasing low d15N DON under certain conditions on the shelf. However, in samples collected closer to shore, the concentration of DON exceeded 17 μM, which N2 fixation could not realistically supply. Instead, significant positive correlations between the concentrations of DON, dFe, salinity, barium, and silicate suggest a potential submarine groundwater source of DON and dFe to nearshore shelf waters. Taken together, our results suggest multiple, dynamic sources of DON on the WFS.