The importance of submarine groundwater discharge to the nearshore nutrient supply in the Gulf of Aqaba (Israel)

We used two short-lived radium isotopes (223Ra, 224Ra) and a mass balance approach applied to the radium activities to determine the nutrient contribution of saline submarine groundwater discharge to the coastal waters of the northern Gulf of Aqaba (Israel). Radium isotope activities were measured along transects during two seasons at a site that lacked any obvious surficial water input. An onshore well and an offshore end member were also sampled. For all samples, nutrients and salinity data were collected. Radium isotope activities generally decreased with distance offshore and exhibited significant tidal variability, which is consistent with a shorederived tidally influenced source. Submarine groundwater contributes only 1–2% of the water along this coast, but this groundwater provides 8–46% of the nutrients. This saline groundwater is derived predominately from tidally pumped seawater percolating through the unconfined coastal aquifer and leaching radium and nutrients. This process represents a significant source of nutrients to the oligotrophic nearshore reef. The importance of atmospheric and groundwater inputs as sources of new nutrients to coastal systems has been suggested by many studies in the last three decades (Valiela et al. 1978; Dollar and Atkinson 1992; Paerl 1997). Since groundwater is typically elevated in nutrients compared to coastal waters, even a small amount of groundwater discharge can significantly elevate the nearshore nutrient supply (Li et al. 1999). Coastal coral reef ecosystems, residing in seas of low nutrient concentrations, may be particularly dependent on and influenced by nutrient input from groundwater discharge (Hamner and Wolanski 1988; Hatcher 1997). However, non–point source groundwater discharge into the coast is difficult to identify or quantify (Burnett et al. 2002). Moore and collaborators over the last 30 yr have pioneered the use of the naturally occurring radium isotope quartet as tracers for riverine and saline groundwater input to coastal systems (e.g., Elsinger and Moore 1980; Moore 1997; Krest and Harvey 2003). The divalent cation radium is bound to soil particles in freshwater but readily desorbs via ion exchange in the presence of higher ionic strength solutions (Elsinger and Moore 1980; Yang et al. 2002). This results in radium enriched saline groundwater, and, thus, radium can be used as a tracer of such water, particularly when mixed into seawater with low radium concentrations (Moore 2000). Indeed, the four isotopes, which vary in half-life (223Ra, 11.4 d; 224Ra, 3.7 d; 226Ra, 1,600 yr; and 228Ra, 5.7 yr), have been used to study mixing processes on a wide range of timescales and to quantify submarine groundwater fluxes to the coast (Moore 2003). Radium isotopes have been used to calculate groundwater fluxes in relatively wet regions of the world, such as the Carolina coast, Florida, and New England (Moore 1996; Charette et al. 2001; Burnett et al. 2002). However, few published studies have applied the use of radium isotopes to explore the influence of groundwater in arid coastal regions (Boehm et al. 2004) or to explore groundwater influence on the nutrient supply to coral reefs (Paytan et al. 2006). It has been assumed that groundwater discharge and associated nutrient supply in dry areas are negligible because of the low precipitation and groundwater recharge rates. In the present study, we use the two short-lived radium isotopes (223Ra and 224Ra) and a mass balance approach to determine the nutrient contribution of saline submarine 1 Present address: U.S. Geological Survey, California Water Science Center, Placer Hall, 6000 J Street, Sacramento, California 95819-6129.