Quantifying Submarine Groundwater Discharge on the West Florida Shelf Using Radium Isotopes

Submarine groundwater discharge (SGD) can have a considerable effect on biogeochemical cycling in coastal environments. One region in which SGD may play a major role in delivering biologically-available iron and nitrogen is the West Florida Shelf, an oligotrophic region. In this study, we quantified spatial and temporal variations in the transport of submarine groundwater into the West Florida continental shelf region of the Gulf of Mexico. Measurements of radium (Ra) activities in seawater were collected across two cross-shelf cruises during February and July 2023. Additional Ra activity measurements were carried out for rivers, estuaries, and offshore groundwater wells in the region to classify local water sources. Cross-shelf spatial patterns were consistent between cruises, with short-lived Ra most highly concentrated near the coast and to the south of Tampa Bay. In February, the average surface activity of excess 223Ra within the 50 m isobath was 2.2 ± 1.1 dpm 100 L-1, while the average surface activity of excess 224Ra was 4.9 ± 2.6 dpm 100 L-1. In July, the average surface activity of excess 223Ra within the 50 m isobath was 1.4 ± 0.9 dpm 100 L-1, while the average surface activity of excess 224Ra was 2.8 ± 1.7 dpm 100 L-1. River measurements displayed higher short-lived Ra activities in May than in February or November. Ra isotopes will be used in quantitative estimates of cross-shelf transport rates, groundwater discharge rates and apparent age of water masses. Resolving the input of SGD into the shelf environment will offer meaningful insight into the distribution and sourcing of crucial biogeochemical constituents in the Gulf of Mexico.