Abstract. Discharge of groundwater and associated chemical compounds into coastal karstic regions, which are abundant in the Mediterranean basin, is envisaged to be significant. In this study, we evaluate the groundwater discharge and its nutrient load to the open karstic site of Badum (Castelló, East Spain). Salinity profiles evidenced that groundwater discharge from coastal brackish springs causes a buoyant fresher layer, as identified with thermal infrared images. Chemical tracers (radium isotopes, dissolved inorganic silicate and seawater major elements) have been used to determine a brackish groundwater proportion in coastal waters of 36% in October 2006 and 44% in June 2007. Based on a radium-derived residence time of 2.7 days in October 2006 and 2.0 days in June 2007, total SGD fluxes have been estimated in 71 500 and 187 000 m3 d−1, respectively, with fresh-SGD contributions representing 71% and 85%. The calculated SGD-associated nutrient fluxes, most likely of natural origin, were 1500 and 8300 μmol m−2 d−1 of DIN and 19 and 40 μmol m−2 d−1 of DIP in October 2006 and June 2007, respectively. These inputs may actually lead to or enhance P limitation, thereby altering the structure of biological communities in the area.

Subterranean estuaries are biogeochemically active coastal sites resulting from the underground mixing of fresh aquifer groundwater and seawater. In these systems, microbial activity can largely transform the chemical elements that may reach the sea through submarine groundwater discharge (SGD), but little is known about the microorganisms thriving in these land-sea transition zones. We present the first spatiallyresolved characterization of the bacterial assemblages along a coastal aquifer in the NW Mediterranean, considering the entire subsurface salinity gradient. Combining

Abstract. Current studies of submarine groundwater discharge (SGD) are commonly conducted under aquifer baseflow conditions, neglecting the influence of episodic events that can significantly increase the supply of nutrients and water. This limits our understanding of the social, biogeochemical, and ecological impacts of SGD. In this study, we evaluated the influence of an extreme precipitation event (EPE) on the magnitude of SGD. To do so, three seawater sampling campaigns were performed at a Mediterranean ephemeral stream-dominated basin after an EPE (∼ 90 mm in few hours) and in baseflow conditions. Results indicate that the groundwater flows after the EPE were 1 order of magnitude higher than those in baseflow conditions. The SGD induced by EPEs, which only take place a few days per year, represented up to one third of the annual discharge of groundwater and associated nutrients at the study site. This work accentuates the need to account for episodic increases in the supply of water and nutrients when aiming to provide reliable annual SGD estimates, particularly in the current context of climate change, since the occurrence of such events is expected to increase worldwide.

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