An assessment of karstic submarine groundwater and associated nutrient discharge to a Mediterranean coastal area (Balearic Islands, Spain) using radium isotopes
Résumé
Short and long-lived radium isotopes (Ra-223, Ra-224, Ra-226, Ra-228) were used to quantify submarine groundwater discharge (SGD) and its associated input of inorganic nitrogen (NO3 (-)), phosphorus (PO4 (3-)) and silica (SiO4 (4-)) into the karstic Alcalfar Cove, a coastal region of Minorca Island (Western Mediterranean Sea). Cove water, seawater and groundwater (wells and karstic springs) samples were collected in May 2005 and February 2006 for radium isotopes and in November 2007 for dissolved inorganic nutrients. Salinity profiles in cove waters suggested that SGD is derived from shallow brackish springs that formed a buoyant surface fresh layer of only 0.3 m depth. A binary mixing model that considers the distribution of radium activities was used to determine the cove water composition. Results showed that cove waters contained 20% brackish groundwater; of which 6% was recirculated seawater and 14% corresponded to freshwater discharge. Using a radium-derived residence time of 2.4 days, a total SGD flux of 150,000 m(3) year(-1) was calculated, consisting of 45,000 m(3) year(-1) recirculated seawater and 105,000 m(3) year(-1) fresh groundwater. Fresh SGD fluxes of NO3 (-), SiO4 (4-) and PO4 (3-) were estimated to be on the order of 18,000, 1,140 and 4 mu mol m(-2) day(-1), respectively, and presumably sustain the high phytoplankton biomass observed in the cove during summer. The total amount of NO3 (-) and SiO4 (4-) supplied by SGD was higher than the measured inventories in the cove, while the reverse was true for PO4 (3-). These discrepancies are likely due to non-conservative biogeochemical processes that occur within the subterranean estuary and Alcalfar Cove waters.