Submarine Groundwater Discharge Along the West Florida Shelf: Is Groundwater an Important Nutrient Source for Florida's Red Tides?
Harmful algal blooms have been observed along the west Florida shelf and adjacent water bodies for more than 150 years (some suggest as long ago as 1570), with the first historically documented bloom dating back to 1854. Modern harmful algal blooms, commonly referred to as "red tides," are dominated by the brevetoxin-producing dinoflagellate Karenia brevis. Brevetoxins are neurotoxins that pose a threat to marine and human health. The greatest densities of K. brevis blooms generally occur along the west Florida shelf between Pinellas and Lee Counties, Florida.
One of the main questions about harmful algal blooms on the west Florida shelf is, how do these blooms initiate and maintain themselves in an area that appears to have limited available nitrogen, one of the nutrients that algae require? Several hypotheses have been proposed to explain the sustained and increased growth of large K. brevis blooms on the nitrogen-limited west Florida shelf, including transport of nitrogen-enriched Mississippi River water to the area during summer months, utilization of nitrogen fixed by Trichodesmium spp. (a blue-green alga that fixes atmospheric nitrogen into ammonia, which can be used by other organisms as well as by Trichodesmium), and a supply of nitrogen internal to the bloom (excreted by zooplankton, for example, or generated by the decay of fish killed by the bloom).
Another possible source of nitrogen for fueling K. brevis blooms is the movement of nitrogen from the land to the ocean through submarine groundwater discharge—the flow of water from underground aquifers into the ocean, either through discrete submarine springs or by more diffuse flow from sediment into overlying seawater. To date, very few studies of benthic flux—the exchange of water between aquifers and the ocean—have been conducted along the west Florida shelf, leaving a great deal of uncertainty as to the role of submarine groundwater discharge in delivering nutrients to the coastal ocean.
Currently, U.S. Geological Survey (USGS) scientist Christopher Smith is addressing this uncertainty by measuring nutrient fluxes related to submarine groundwater discharge along the west Florida shelf and evaluating whether this source is significant relative to the nutrient demands of recurrent harmful algal blooms. Smith is a Mendenhall Postdoctoral Research Fellow at the USGS Florida Integrated Science Center office in St. Petersburg. His current research builds on his earlier Ph.D. work (Louisiana State University), which focused on using uranium-series radionuclides to quantify submarine groundwater discharge to Indian River Lagoon, Florida, and to understand how mixing between fresh and saline groundwater within the "subterranean estuary" affects the use of these radionuclides as groundwater tracers.
To explain the extensive harmful algal blooms observed throughout the year 2005, Peter Swarzenski of the USGS and Chuanmin Hu and Frank Muller-Karger of the University of South Florida (USF) have hypothesized that the precipitation associated with Hurricanes Charley, Frances, Ivan, and Jeanne had a twofold effect on the movement of nutrients from the land to the west Florida shelf: (1) an immediate effect by enhancing surface runoff and (2) a delayed effect by recharging the coastal aquifer and increasing submarine groundwater discharge along the coast. Most of their coastal-groundwater data linking submarine groundwater discharge to nutrients were derived from studies conducted in Tampa Bay. To test for a link between submarine groundwater discharge and nutrients on the west Florida shelf, Smith is measuring radon (222Rn) and radium (223,224,226,228Ra) isotopes (common submarine-groundwater-discharge tracers), as well as dissolved-inorganic-nutrient and metallic-ion concentrations in surface, ground, and pore waters. One area that he is investigating is the coastal water off the densely populated Pinellas County peninsula, which lies between Tampa Bay and the west Florida shelf.
In descriptions of Florida's groundwater system, the most commonly discussed features are freshwater springs, sinkholes, and the highly transmissive Floridan aquifer. Not much attention is paid to the shallow deposits that overlie the Floridan aquifer, especially in coastal areas; with respect to human demands, many of these deposits simply do not produce enough water to be viable primary water sources. In coastal areas, however, it is these shallow aquifers that are directly recharged by precipitation and are the most sensitive to human activity—such as application of pesticides, herbicides, and fertilizers and installation of underground storage tanks. One such aquifer in northwestern Pinellas County consists of a relatively thick (20 to 30 m) section of mixed siliciclastic and carbonate sedimentary deposits that form a prominent ridge near the coast (hereinafter referred to as the "Pinellas Ridge"). This local high is a recharge zone for the surficial aquifer and, in this area, the underlying Floridan aquifer. The elevation of water within the aquifers in this ridge causes groundwater to flow down toward Tampa Bay and the Gulf of Mexico.
Smith has been collecting data near the north end of the Pinellas Ridge at an offshore freshwater spring known locally as Crystal Beach Spring (near Crystal Beach, Florida) on the Gulf of Mexico side of the ridge. The spring is approximately 150 m offshore in St. Joseph Sound and approximately 4 km from the ridgetop. Spring-discharge estimates made by the Southwest Florida Water Management District in 2002 range from 0.15 to 0.63 m3/s. Using a surface-water mass balance of 222Rn, Smith estimated the discharge in January 2009 at 0.22 to 0.81 m3/s. The discharge from the spring and the dissipation of the freshwater plume throughout St. Joseph Sound are moderated by tides and by wind-driven variations in water level. During the Southwest Florida Water Management District study, surface water entered or was siphoned into the spring vent during spring high tides; during subsequent spring low tides, a mixture of surface water and groundwater discharged from the spring. Smith and Keith Ludwig (USGS, St. Petersburg), along with Inia Soto (USF doctoral candidate), conducted sampling trips in January and February 2009, when the tidal range was moderate (0.5 m). At that time, siphoning was not observed, and the discharging fluid had a constant salinity (3.4). This vent is one of many known offshore springs along the coastal sections of Pinellas, Pasco, Hernando, and Citrus Counties; however, according to a spring survey conducted by the Southwest Florida Water Management District in 2002-03, Crystal Beach Spring is the only spring discharging large volumes of relatively fresh groundwater; the others appear to have stopped flowing. What keeps this spring flowing while so many others in the area have become extinct?
Smith hypothesizes that recharge along the Pinellas Ridge creates a steep hydraulic gradient between the groundwater and adjacent surface-water bodies, subsequently driving nutrients derived from human activities (such as fertilizer application and septic-system seepage) to the coastal ocean. Accompanying the Crystal Beach Spring effluent are relatively elevated levels of dissolved inorganic nitrogen (141.6 mM), primarily in the form of nitrate. Judging from estimated spring-discharge rates and measured nutrient concentrations, the total dissolved inorganic nitrogen being discharged by the spring is estimated at 103 to 104 mol/d, a similar magnitude as in major rivers feeding Tampa Bay (104.5 mol/d of dissolved inorganic nitrogen). If the spring is fueled by local recharge along the topographic high, then similar nutrient fluxes may be expected along the rest of Pinellas County adjacent to the ridge. So, the next question to address is whether there is evidence of submarine groundwater discharge offshore from the Pinellas Ridge in areas besides St. Joseph Sound.
In late February 2009, Smith accompanied Lisa Robbins (USGS), Paul Knorr (USGS/USF), and Xuewu Liu (USF) during a cruise to map surface-water CO2 concentrations (read about similar research cruises in Sound Waves, April 2009 story, "Research Cruises Collect Measurements on the West Florida Shelf for Modeling Climate Change and Ocean Acidification"). To test for evidence of submarine groundwater discharge along the west Florida shelf, Smith measured surface-water 222Rn, a naturally occurring radioactive gas with a short half-life (3.8 days) that is much more concentrated in groundwater than in surface water. Interestingly, surface-water 222Rn activity was 2 to 3 times higher off the Pinellas County peninsula than in coastal waters with similar water depths, suggesting enhanced submarine groundwater discharge off the peninsula. Similar 222Rn enrichments were observed in the waters off Pinellas County during the mid-1980s, indicating a persistent source of 222Rn. Smith is conducting additional surveys to refine the spatial extent of the discharge area and to determine how this source may influence nutrient budgets on the west Florida shelf.
Smith joined the USGS after completing his Ph.D. in Oceanography and Coastal Sciences at Louisiana State University under the direction of Jaye Cable. Smith received his B.S. and M.S. degrees at East Carolina University, where he worked on a project with the North Carolina Coastal Geology Cooperative that was primarily funded by the USGS Coastal and Marine Geology Program. His M.S. thesis was focused on deciphering the late Holocene stratigraphy of two sections of the Outer Banks of North Carolina, using lithology, benthic-foraminiferal assemblages, ground-penetrating radar, and geochronology.
For more information about Smith's study of submarine groundwater discharge along the west Florida Shelf, contact Christopher G. Smith, U.S. Geological Survey, 600 Fourth Street S., St. Petersburg, FL 33701, telephone (727) 803-8747 (ext. 3035), fax (727) 803-2032, e-mail firstname.lastname@example.org.
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