Abstract
Subterranean estuaries (STEs), where continental groundwaters and saltwaters meet, are zones of intense biogeochemical reactivity. As such, STEs significantly modify groundwater-borne nutrient fluxes to the coastal zone. Thus, evaluating their reactive role is crucial to anticipate impacts of submarine groundwater discharge (SGD) over coastal ecosystems. Here, we studied the nitrogen biogeochemistry of two STEs with contrasting wave-exposure and redox conditions in Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula). Seasonal surveys were performed at the permanently saturated zone of both beaches during low tide in February, May, July, and October 2019. Sediment was sampled and porewater samples collected using push-pull piezometers. Salinity, 222Rn and 226Ra activities were used to trace water circulation inside each beach. Porewater nitrate, ammonium, nitrite and dissolved oxygen were used to evaluate the role of these STEs as reactive sinks or sources of inorganic nitrogen. Our results showed a marked seasonal variability of water circulation inside both beaches, with strong salinity gradients in February and May and weakened circulation in July and October. The presence of a gravel layer in Panxón beach completely altered the typical structure of STEs by increasing porewater transport and mixing through the beach interior. As a result, Panxón beach profiles were highly enriched in nitrate and oxygen. Conversely, suboxic, and anoxic conditions were prevalent in Ladeira beach during the study period, with ammonium being the prevailing inorganic nitrogen form. High nitrate concentrations occurred associated to the tidal circulation cell during February and May, being the only effective mechanism of sediment oxygenation in Ladeira beach. Although nitrate reduction and production were observed in both STEs, comparison with averaged conservative mixing porewater profiles showed that Ladeira beach acted as a net nitrogen sink whereas Panxón beach acted as a net nitrogen source. The presence of a gravel layer oxygenates the interior of Panxón beach, thus limiting nitrate reduction and promoting the amplification of groundwater-borne nitrogen fluxes to the coast.
Highlights
Submarine groundwater discharge (SGD) is defined as any and all water flow on continental margins from the seabed to the coastal ocean, including both the discharge of meteoric groundwater from coastal aquifers and the recirculation of seawater through the sea floor (Burnett et al, 2003)
Fractured granites and shales dominate the geology of the NW Iberian Peninsula (Llerena et al, 2013) and are part of the Monteferro-O Rosal geological complex (Pliego Dones et al, 1978) that surrounds Baiona Bay
These geological substrates are naturally enriched in isotopes from the 228U decay chain (Tollefsen et al, 2016), such as 226Ra and 222Rn
Summary
Submarine groundwater discharge (SGD) is defined as any and all water flow on continental margins from the seabed to the coastal ocean, including both the discharge of meteoric groundwater from coastal aquifers (fresh SGD) and the recirculation of seawater through the sea floor (Burnett et al, 2003). Solute flux estimations associated to SGD need to take into account the high biogeochemical reactivity commonly found at the exit sites that can significantly modify the composition of the circulating waters prior to discharge (Slomp and Van Cappellen, 2004; Spiteri et al, 2008; Santos et al, 2008, 2009a,b; Rocha et al, 2009; Ibánhez et al, 2011, 2013; Ibánhez and Rocha, 2014, 2016, 2017; Jiang et al, 2018, 2020) This zone is called subterranean estuary (STE), by analogy with surface estuaries (Moore, 1999; Burnett et al, 2003)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
