Assessment of Submarine Groundwater Discharge and its Role as a Nutrient Source to the West Florida Shelf Using Radium Isotopes
College
College of Sciences
Department
Ocean & Earth Sciences
Graduate Level
Master’s
Presentation Type
Oral Presentation
Abstract
Submarine groundwater discharges (SGD) can impart strong chemical signatures upon coastal waters and provide elevated levels and/or altered forms of macronutrients and trace metals. Over the oligotrophic West Florida Shelf (WFS), SGD may be an important boundary source for dissolved nitrogen (TDN) and phosphorus (TDP). Nitrogen is a limiting nutrient on the WFS and its distribution has a considerable effect on phytoplankton communities, including the toxic red tide-forming dinoflagellate Karenia brevis and nitrogen-fixing diazotrophs such as Trichodesmium species that also influence N distribution. However, the fluxes and variability of these crucial nutrients to the WFS are currently poorly constrained, with much of the required N and P for bloom formation and maintenance as of yet unaccounted for. The first step to assessing the SGD potential input to the WFS is to quantify and characterize the SGD discharging onto the shelf. Here, we used radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) as tracers to estimate the rate of SGD into the West Florida Shelf. A mass balance of 226Ra (t1/2 = 1600 y) was constructed from two cross-shelf cruises in February and July of 2023 as part of GEOTRACES Process Study GApr18. During both seasons, highest 226Ra activities were found nearest to the shore. In February the highest activities were concentrated around the mouth of Tampa Bay, and in July they were found about 60 km further south. Preliminary results indicate an area-normalized discharge of 0.5-1.9 cm d-1 in February and 0.6-2.1 cm d-1 in July to the WFS, across an area bounded by the 50 m isobath and representing 160 km of shoreline. This is similar in magnitude to other studies of the shelf and its estuaries, but notably recirculated SGD is 2-3 orders of magnitude higher in flux compared to discharge from four major rivers into Tampa Bay. Latitudinally, three major coastal and submarine aquifer systems are expressed in the region; spatial and seasonal variations in Ra activities of groundwater from three transects of offshore wells will help assess the aquifers’ contribution to shelf chemistry. Using these estimates of SGD rates and the measured submarine groundwater composition, the magnitude of SGD as a source of nutrients and metals, especially nitrogen and phosphorus, will be considered.
Keywords
groundwater, submarine groundwater discharge, SGD, nutrients, nitrogen, phosphorus, radium, isotope, radioisotope, mass balance, aquifer, Florida, Tampa Bay, West Florida Shelf, Gulf of Mexico
Assessment of Submarine Groundwater Discharge and its Role as a Nutrient Source to the West Florida Shelf Using Radium Isotopes
Submarine groundwater discharges (SGD) can impart strong chemical signatures upon coastal waters and provide elevated levels and/or altered forms of macronutrients and trace metals. Over the oligotrophic West Florida Shelf (WFS), SGD may be an important boundary source for dissolved nitrogen (TDN) and phosphorus (TDP). Nitrogen is a limiting nutrient on the WFS and its distribution has a considerable effect on phytoplankton communities, including the toxic red tide-forming dinoflagellate Karenia brevis and nitrogen-fixing diazotrophs such as Trichodesmium species that also influence N distribution. However, the fluxes and variability of these crucial nutrients to the WFS are currently poorly constrained, with much of the required N and P for bloom formation and maintenance as of yet unaccounted for. The first step to assessing the SGD potential input to the WFS is to quantify and characterize the SGD discharging onto the shelf. Here, we used radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) as tracers to estimate the rate of SGD into the West Florida Shelf. A mass balance of 226Ra (t1/2 = 1600 y) was constructed from two cross-shelf cruises in February and July of 2023 as part of GEOTRACES Process Study GApr18. During both seasons, highest 226Ra activities were found nearest to the shore. In February the highest activities were concentrated around the mouth of Tampa Bay, and in July they were found about 60 km further south. Preliminary results indicate an area-normalized discharge of 0.5-1.9 cm d-1 in February and 0.6-2.1 cm d-1 in July to the WFS, across an area bounded by the 50 m isobath and representing 160 km of shoreline. This is similar in magnitude to other studies of the shelf and its estuaries, but notably recirculated SGD is 2-3 orders of magnitude higher in flux compared to discharge from four major rivers into Tampa Bay. Latitudinally, three major coastal and submarine aquifer systems are expressed in the region; spatial and seasonal variations in Ra activities of groundwater from three transects of offshore wells will help assess the aquifers’ contribution to shelf chemistry. Using these estimates of SGD rates and the measured submarine groundwater composition, the magnitude of SGD as a source of nutrients and metals, especially nitrogen and phosphorus, will be considered.