Gulf of Mex DON dFe - Jun

Motivated by three prior cruises to the West Florida Shelf (WFS), the proposed work will quantify the flux and isotopic signatures of submarine groundwater (SGD)-derived dissolved organic nitrogen (DON) and dissolved iron (dFe) to the WFS as well as evaluate the bioavailability of this temporally variable DON and dFe source using four near-shore sampling campaigns and two cross-shelf cruises. The proposed work will evaluate whether SGD stimulates nitrogen fixation, and the potential for that nitrogen fixation to further modify the chemistry of DON and dFe on the WFS. The cross-shelf cruises will sample during hypothesized periods of both maximum SGD and Trichodesmium spp. abundance (June), and reduced river discharge and SGD (Feb.), providing comparison between the two distinct regimes. The four small boat sampling campaigns will sample submarine groundwater wells and the overlying seawater, as well as Tampa Bay and the Hillsborough and Caloosahatchee Rivers. The concentration, isotopic composition of DON and dFe, and in the case of DON, the molecular composition, will be characterized. Fluxes of DON and dFe from SGD and rivers across the WFS will be quantified with radium isotope mass balance budgets. These measurements will test the hypothesis that submarine ground water discharge (SGD) is the dominant source of bioavailable dissolved organic nitrogen (DON) and dissolved iron (dFe) on the WFS. This hypothesis will also be tested with incubations of submarine groundwater with phytoplankton communities collected at the 50 and 20 m isobaths. The proposed work will evaluate the potential for SGD, rivers, and nitrogen fixers to modify the DON and dFe imported into the Gulf of Mexico from the Atlantic Ocean.