Submarine groundwater discharge in a river-dominated Florida estuary

Eutrophication in the coastal zone has largely been driven by changing land use practices that lead to nutrient-enhanced runoff. While in most studies the overland component of this nutrient vector has been well documented, the role of groundwater in coastal nutrient mass balances is often poorly constrained. Here, we used radium isotopes to quantify SGD and associated nutrient fluxes to the Caloosahatchee River estuary (Florida, USA) during the wet and dry seasons of 2009–2010. Like many estuaries worldwide, the nutrient balance and ecology of the Caloosahatchee has been negatively impacted by excessive nutrient-laden runoff from fertilizer use and other anthropogenic sources. A four endmember mixing model was used to quantify the magnitude of SGD and the relative importance of terrestrial and marine groundwater sources. Terrestrial groundwater comprised 44% of the total SGD in April 2009, but 98–100% of the total groundwater flux during all other time periods. SGD rates were highly seasonal ranging from a low of 8.5 × 104 m3 d− 1 in April 2010 to a high of 1.3 × 106 m3 d− 1 in October 2010 (average = 4.8 ± 5.5 × 105 m3 d− 1). For the four time periods, these fluxes ranged from 2 to 140% (average = 43%) of the river discharge through Franklin Lock, a water control structure at the head of the estuary and the only previously quantified source of nutrients to the system. The groundwater total dissolved nitrogen (TDN) flux to the estuary averaged 450 ± 490 kg d− 1 for the four time periods, while dissolved inorganic nitrogen (DIN) and soluble reactive phosphorous (SRP) averaged 241 ± 267 kg d− 1 and 93 ± 111 kg d− 1, respectively. On average, the surface water freshwater fluxes for TDN exceeded the SGD fluxes by a factor of 6. However, the SGD fluxes of DIN and SRP, highly bioavailable forms of N and P, were only 3 and 1.5 times lower than the river flux, respectively. The major form of nitrogen carried by groundwater to the estuary was ammonium; this highly labile form of nitrogen is likely rapidly consumed within the estuary by primary producers (both macro- and microalgae). Our results suggest that during extended dry periods when water releases from Franklin Lock are at a minimum, SGD will remain a substantial source of nutrients to the system.