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Submarine groundwater discharge

Groundwater is any water present or moving through the interstitial spaces of soils, sediments and rocks. The salinity of groundwater can vary greatly depending on its source but when it flows directly into the coastal zone is it is referred to as Submarine Groundwater Discharge (SGD). These flows are often difficult to identify and monitor, but in some cases SGD has been known to rival surface water inputs in terms of nutrient delivery and can have major implications for coastal ecosystems. Seagrasses are foundational species in shallow coastal waters, and as rooted submerged plants they are directly impacted by SGD, which percolates through the sediments where seagrass beds are found. Because groundwater differs markedly from the overlying water column, often in terms of nutrient content, temperature, salinity, pH and dissolved oxygen, the interaction between SGD and seagrasses can be complex, and important questions remain about how groundwater discharge influences seagrass productivity, community structure and resilience. 

To address this knowledge gap, researchers from FWRI, Georgia Southern University and Florida International University have teamed up to explore SGD dynamics and their relationship to seagrass communities in western Florida Bay. As part of this three-year EPA-funded study, benthic ecologists and geologists will map SGD in three basins – Rabbit Key, Rankin Lake, and Whipray – during the dry (May/June) and wet (October/November) seasons. 

Two maps side by side showing Florida Bay basins where groundwater was found and extracted for analyzing.

To identify SGD signatures, we will tow an electrical resistivity tomography (ERT) cable and use a radon-222 detector while “mowing the lawn” along gridded transects in each basin. The ERT cable serves as a proxy for salinity measurements and the radon-222 detector “sniffs” out cold- and hotspots of groundwater. At each cold- and hotspot, we will use a piezometer and water pump to extract groundwater that will be processed for radon-222, nutrients and a suite of water quality parameters.

3 photos showing net floats on top of the water, an apparatus with tubes sitting on the side of a boat, and a device with tubes and electrical devices connected to a battery sitting inside a blue plastic storage tub.

Within these same locations, we will classify seagrass and macroalgae using the Braun-Blanquet method to characterize the benthic macrophyte community, as well as harvesting seagrass shoots for measurements of leaf length/width and tissue C:N:P. To map seagrass distribution and extent, we will use satellite imagery groundtruthed using Fisheries Habitat Assessment Program data. Seasonal (wet and dry) measurements will allow us to map patterns in SGD across space and time and to examine the relationship between groundwater and seagrass community composition and resilience in an iconic seagrass-dominated ecosystem.

3 photos showing a person in the water holding onto a pole with a tube, a person sitting inside a boat next to the side with tubing going into the water, and a brown bottle on a boat deck.