Figure - available from: Frontiers in Marine Science
This content is subject to copyright.
Study area. Sampling sites are identified by red dots. The location of the wells and boreholes used in this study are represented with green triangles, while the location of the Miñor river sampling site is shown with a yellow triangle. These data are obtained from Ibánhez et al. (2019). Orthophoto (taken in 2018) and the mapping of local fluvial courses were obtained from the Instituto Geográfico Nacional (www.ign.es). BH denotes borehole.
Source publication
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 ecosy...
Similar publications
Greenhouse gas (GHG) emissions from freshwater streams are poorly quantified in sub-tropical climates, especially in the southern hemisphere where land use is rapidly changing. Here, we examined the distribution, potential drivers, and emissions of carbon dioxide (CO 2 ), nitrous oxide (N 2 O) and methane (CH 4 ) from eleven Australian freshwater s...
Citations
... The seawater infiltration zone marks the only boundary through which larger quantities of fresh OM can enter the reactor, thus regulating the biogeochemical turnover. In the shallow beach subsurface, results from porewater O 2 and nutrient concentration measurements led to the conclusion that aerobic respiration predominates the remineralization of OM (Charbonnier et al., 2013) and that this O 2 consumption is driven by seasonal variabilities in porewater T and the quality and quantity of supplied OM Calvo-Martin et al., 2021). Additionally, sediment-entrapped particulate OM (POM) has been suggested to play a key role in the distribution of reaction patterns (Charbonnier et al., 2022;Kim et al., 2019Kim et al., , 2020. ...
... A previous study from the Delaware Beach aquifer (Kim et al., 2019) also found no correlations between depth, organic matter content, grain size ( Figure S2 in Supporting Information S1), and respiration rate. Other studies reported only subtle changes of TOC content over the season or with depth (Calvo-Martin et al., 2021;Ibánhez & Rocha, 2016). ...
Seawater infiltration into the permeable sands of beach aquifers creates a high input of biogeochemical reactants driven by tides and waves. The upper sand layer acts as a filter, retaining particulate organic matter (POM), which is degraded by bacteria under predominantly oxic conditions. The seasonal variation of seawater POM and oxygen (O2) entering the infiltration zone, combined with the POM filtration efficiency of the highly morphodynamic upper layer, determines the organic matter turnover and subsequent redox gradients along porewater flowpaths. We investigated these effects by quantifying the seasonal O2 consumption rates directly from the incubations of sediments taken along a transect in the seawater infiltration zone at Spiekeroog Beach, Germany. We carried out a two‐monthly year‐long sampling campaign with high spatial resolution measurements down to 1 m depth. In summer, O2 consumption rates of up to 106 μM hr⁻¹ were found in the first decimeters with a significant decline over depth, indicating efficient retention of reactive POM in the surface layer. Seasonal variation in organic carbon of the sand's suspendable particulates indicates rapid turnover and little storage. In winter, rates decreased significantly to below 11 μM hr⁻¹. Integrated over the investigated oxic layer, the estimated carbon mineralization varies between 15 (winter) and 143 (summer) mmol C m⁻² d⁻¹ with a yearly average of 73 mmol C m⁻² d⁻¹. The yearly CO2 production of 35 kg per meter shoreline characterizes the beach as a high‐throughout system with rapid OM remineralization in the retention layer, especially in summer, but with little OM storage.
... Two subterranean estuaries were identified at the tidal, wave-dominated sandy beaches of Panx on and Ladeira in the Ria de Vigo (NW Iberian Peninsula;Calvo-Martin et al. 2021;Ib anhez et al. 2021). There, we used push-pull piezometers to obtain the seasonal, 2D distribution of target solutes to characterize their biogeochemistry, porewater circulation, and microbial communities (Calvo-Martin et al. 2021, 2022. ...
... Two subterranean estuaries were identified at the tidal, wave-dominated sandy beaches of Panx on and Ladeira in the Ria de Vigo (NW Iberian Peninsula;Calvo-Martin et al. 2021;Ib anhez et al. 2021). There, we used push-pull piezometers to obtain the seasonal, 2D distribution of target solutes to characterize their biogeochemistry, porewater circulation, and microbial communities (Calvo-Martin et al. 2021, 2022. Despite their proximate location, Panx on and Ladeira play opposite roles in groundwater-borne N processing due to their contrasting redox conditions. ...
... Despite their proximate location, Panx on and Ladeira play opposite roles in groundwater-borne N processing due to their contrasting redox conditions. While Ladeira Beach is mostly anoxic, Panx on Beach contains a gravel layer at $ 1 m depth that promotes oxygenation through the beach interior (Calvo-Martin et al. 2021). Panx on and Ladeira beaches further host a highly diverse microbial community that includes methanogens, methanotrophs, and a wide variety of taxa associated with N processing (Calvo-Martin et al. 2022). ...
Continental groundwaters are commonly enriched in nitrous oxide (N2O) and methane (CH4), which can discharge into the coast. The contribution of this diffuse source to coastal N2O and CH4 emissions largely depends on the biogeochemical processes of coastal aquifer exit zones, known as subterranean estuaries. Here, we study the role of subterranean estuaries in modulating N2O and CH4 exports toward the coast. The two studied subterranean estuaries are located at Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula) and had opposite oxygenation at their interior. Groundwater‐borne N2O was detected in the oxygenated subterranean estuary of Panxón Beach, although denitrification attenuated N2O before porewater was discharged into the coast. An N2O hotspot was detected at about 50 cm depth in this subterranean estuary, characterized by the presence of nitrate under suboxic conditions. This N2O also seems to be consumed along the flow path before discharging into the coast. The anoxic subterranean estuary of Ladeira Beach completely removed groundwater‐borne N2O. Yet, nitrification within its suboxic upper saline plume produced and exported N2O toward the coast, hiding the role of this subterranean estuary as an N2O sink. The anoxic subterranean estuary exported CH4 toward the coast. This CH4 was not sourced by continental groundwater, hence it was produced in situ fuelled by the accumulation of organic matter within the beach. The suboxic upper saline plume in Ladeira Beach and the oxygenated subterranean estuary of Panxón Beach acted as CH4 sinks.
... However, the reaction capability of STEs on solutes is highly variable, primarily due to the co-occurrence of different biogeochemical reactions with conflicting effects on target solutes, such as nitrification and denitrification/ biological assimilation (Rocha et al., 2022;Jiang et al., 2023). To achieve a precise quantification of chemical element loading into the coast from SGD, several research projects funded by governments have focused on investigating the spatial and temporal variations of solute concentrations in STEs and aiming to explore the dynamic linkage between environmental variables, such as temperature, salinity, organic carbon supply, and solute concentration changes (Reckhardt et al., 2015;Linkhorst et al., 2017;Calvo-Martin et al., 2021;Jiang et al., 2023). ...
... The similarity in biodiversity observed in our site is likely due to the homogeneity of environmental factors in the sampled seepage face, which is shallow in depth. As the outer regime of STE, the seepage face is highly influenced by tidal setup and wave pumping effects (Fig. 1), which deliver substantial pelagic materials to the highly permeable sediments and improve the mixing between these elements and sandy particles (Calvo-Martin et al., 2021). Consequently, the microbiota attached to the particle surface tends to be evenly distributed along the vertical profile. ...
Subterranean estuaries, i.e., the mixing zone between terrestrial groundwater and recirculated seawater, host a wide range of microbiota. Here, field campaigns were conducted at the mouth of the subterranean estuary at the Sanggou Bay (Shandong Province, China) over four consecutive seasons at a seepage face (0−20 cm depth). The diversity of benthic microbiome was characterized via 16S rRNA gene sequencing and metagenomics, combined with physic-chemical parameters, e.g., organic carbon, total nitrogen and sulfate contents in sediments. During spring, the dominant species were assigned to the phylum Proteobacteria. Important opportunistic species was assigned to Acidobacteria, Actinobacteria and Bacteroidetes. The key components were identified to be species of the genera Pseudoalteromonas, Colwellia and Sphingobium, indicating the involvement of sediment microbiota in the degradation of sedimentary organic carbon, particularly that of pelagic origin, e.g., phytoplankton detritus and bivalve pseudo-feces. During spring, the microbial community was statistically similar along the depth profiles and among the three sampled stations. Similar spatial distributions were obtained in the remaining seasons. By contrast, the dominant species assemblages varied significantly among seasons, with key genera being Thioprofundum and Nitrosopumilus during summer and autumn and Thioprofundum and Ilumatobacter during winter. Network analysis revealed a seasonal shift in benthic nitrogen and sulfur metabolism associated with these variations in microbial community composition. Overall, our findings suggested that macro elements derived from pelagic inputs, particularly detrital phytoplankton, shaped the microbial community compositions at the seepage face, resulting in significant seasonal variations, while the influence of terrestrial materials transported by groundwater on the sediment microbiota at the seepage face found to be minor.
... The local aquifers are configured in a two-layered system based on the dominant weathering zonation: the upper aquifer is contained within the highly weathered regolith that presents variable thickness typically from 5 to 20 m depth ( Fig. 2; Raposo et al. 2013). These upper parts commonly show large horizontal flow transfer following the local topography (e.g., Roques et al. 2014;Calvo-Martin et al. 2021) and provide the majority of the groundwater storage Fig. 1 Location of the Ría de Vigo basin (NW Iberian Peninsula, a, within the autonomous region of Galicia (Spain; b population density, c geological composition, d and hydrographic network are shown, together with the location of the sampled rivers, wells, springs, and boreholes. Note that different symbols denote the different sampling strategies followed by the study. ...
The Ría de Vigo catchment is situated in the largest radon-prone area of the Iberian Peninsula. High local indoor radon (²²²Rn) levels are the preeminent source of radiation exposure, with negative effects on health. Nevertheless, information on radon levels of natural waters and the potential human exposure risks associated with their domestic use is very sparse. To elucidate the environmental factors increasing human exposure risk to radon during domestic water use, we undertook a survey of local water sources, including springs, rivers, wells, and boreholes, over different temporal scales. Continental waters were highly enriched in ²²²Rn: activities ranged from 1.2 to 20.2 Bq L⁻¹ in rivers and levels one to two orders of magnitude higher were found in groundwaters (from 8.0 to 2737 Bq L⁻¹; median 121.1 Bq L⁻¹). The geology and hydrogeology of local crystalline aquifers support one order of magnitude higher ²²²Rn activities in groundwater stored in deeper fractured rock compared to that contained within the highly weathered regolith at the surface. During the mean dry season, ²²²Rn activities nearly doubled in most sampled waters in comparison to the wet period (from 94.9 during the dry season to 187.3 Bq L⁻¹ during wet period; n = 37). Seasonal water use and recharge cycles and thermal convection are postulated to explain this variation in radon activities. The high ²²²Rn activities cause the total effective dose of radiation received from domestic use of untreated groundwaters to exceed the recommended 0.1 mSv y⁻¹. Since more than 70% of this dose comes from indoor water degassing and subsequent ²²²Rn inhalation, preventative health policy in the form of ²²²Rn remediation and mitigation measures should be implemented prior to pumping untreated groundwater into dwellings, particularly during the dry period.
... The reported capability of STEs to modulate land-ocean NO 3 À transfer varies significantly. This shift between acting as a net source and sink, on both temporal and spatial scales, depends on a wide range of environmental factors, e.g., NO 3 À concentration, organic matter standing stocks and supply, temperature and advection rates (Ibánhez and Rocha, 2017;Calvo-Martin et al., 2021;Jiang et al., 2022). Previous research work has assessed insitu NO 3 À biogeochemistry in more than 20 sites at a global scale (Cho et al., 2018), with particular focus on reaction pathway identification and rate quantification, while the environmental drivers of reactivity received less attention. ...
... The studied sediment column shifted rapidly from acting as a net NO 3 À sink or source, especially at the 20 cm depth layer. Such variability in NO 3 À accumulation and removal with depth, or along the main porewater transport flow lines, was also found in the outer boundary of the STE in the Ria Formosa Lagoon (Rocha et al., 2009), Rarotonga, Cook Islands and Ría de Vigo (Calvo-Martin et al., 2021). During spring, net NO 3 À depletion occurred below 12 cm depth. ...
... Though the lack of information on N cycling in deep zones of the STE (e.g., Erler et al., 2014;Calvo-Martin et al., 2021), the d 15 N-NO 3 À in terrestrial groundwater endmember is assumed to be in the range of 0 (estimated from Fig. 3F) and 7.5‰ (bottom porewater), and d 18 O-NO 3 À falls in the range of À3‰ to 6.5‰, suggesting NO 3 À originated from chemical fertilizers used on land . The d 2 H-H 2 O and d 18 O-H 2 O in porewaters remained almost unchanged during summer and winter (cf. the linear curve in Fig. 3G), suggesting that the terrestrial groundwater endmember was the same throughout the year (i.e., from the same coastal aquifer; e.g., Rocha et al., 2016), but the estimated end-member groundwater NO 3 À concentration from Fig. 3D was higher in autumn and winter compared to spring-summer. ...
Subterranean estuaries are highly dynamic in processing dissolved inorganic nitrogen (DIN). Here we investigate DIN turnover in surface sediments (0–20 cm depth) at the higher, medium and lower intertidal of a seepage face, i.e., the outer “mouth” of the subterranean estuary, during four consecutive seasons in Sanggou Bay, China. Throughout the studied period, ammonium (NH4+) and nitrite (NO2−) concentrations in the sampled porewaters did not vary significantly with depth or season. In contrast, peaks in porewater nitrate (NO3−) concentration and decreases in δ15N-NO3− and δ18O-NO3− were observed in the 15–20 cm depth (bottom) sediment, particularly during summer and autumn. Coupled with NO3− production, the sediment total nitrogen was also markedly peaking in the bottom layer of the studied seepage face. Together with abundant heterotrophic microbes in the sediment, this NO3− accumulation was linked to a reaction chain including organic matter decomposition, ammonification and nitrification. During winter, porewater enrichment in total nitrogen occurred closer to the surface of the seepage face but triggered also active NO3− production. This pattern reinforced the importance of pelagic organic matter supply on NO3− production. In the shallower depths of the seepage face (
... The contribution of sewage to inorganic nitrogen inputs has reduced to less than 4% (unpublished data) by a recently commissioned wastewater treatment plant. Groundwater discharge was recently found to be a significant source of freshwater to the Ría de Vigo, particularly important at the inner San Sim on and Baiona Bays ( Figure 1A; Ib anhez et al., 2021), and the solute transport associated with this previously overlooked source of continental waters is starting to emerge (Calvo-Martin et al., 2021). ...
... Unimodal fine sands dominate there, with a mean grain size of 0.234 mm and high carbonate content (60%-65%; Alvarez-V azquez et al., 2003;Queralt et al., 2002). Over the period covered by this study (February to October 2019), both beaches showed similar porosity (0.47 AE 0.02 in Panx on and 0.46 AE 0.06 in Ladeira), but slightly different particulate organic matter content (1.33 AE 0.10% dry weight in Panx on and 0.93 AE 0.40% dry weight in Ladeira beach) and sediment organic C:N molar ratio (10.7 AE 3.5 in Ladeira and 3.2 AE 0.8 in Panx on; Calvo-Martin et al., 2021). ...
... continental groundwater influence). Source: Data used in the schemes correspond to those obtained during may in Ladeira beach and October in Panx on beach (taken from the study by Calvo-Martin et al., 2021). Sampled depths are marked with circles. ...
Subterranean estuaries (STEs) modulate the chemical composition of continental groundwater before it reaches the coast but their microbial community is poorly known. Here, we explored the microbial ecology of two neighbouring, yet contrasting STEs (Panxón and Ladeira STEs; Ría de Vigo, NW Iberian Peninsula). We investigated microbial composition (16S rRNA gene sequencing), abundance, heterotrophic production and their geochemical drivers. 10,150 OTUs and 59 phyla were retrieved from porewater sampled during four surveys covering each STE seepage face. In both STEs, we find a very diverse microbial community composed by abundant cosmopolitans and locally restricted rare taxa. Porewater oxygen and dissolved organic matter are the main environmental predictors of microbial community composition. More importantly, the high variety of benthic microbiota links to biogeochemical processes of different elements in STEs. The oxygen‐rich Panxón beach showed strong associations of the ammonium oxidizing archaea Nitrosopumilales with the heterotrophic community, thus acting as a net source of nitrogen to the coast. On the other hand, the prevailing anoxic conditions of Ladeira beach promoted the dominance of anaerobic heterotrophs related to the degradation of complex and aromatic compounds, such as Dehalococcoidia and Desulfatiglans, and the co‐occurrence of methane oxidizers and methanogens. This article is protected by copyright. All rights reserved.
... DSi was the nutrient with the highest values in the sampled porewaters, followed by DIN. The dominant form of DIN was NH 3 + NH 4 + , indicative of reducing conditions within the sediment (1.5 m depth), that can be originated from fertilizer and wastewater leakage, NO 3 − reduction, OM remineralization, and the lack of oxygen would limit the nitrification process, thus accumulating NH 3 + NH 4 + in the porewaters (Slomp and Van Cappellen, 2004;Ibánhez and Rocha, 2017;Calvo-Martín et al., 2021). Furthermore, under reducing conditions, phosphate is generally mobilized (Slomp and Van Cappellen, 2004;Griffioen, 2006). ...
Continental runoff is an important source of solutes and nutrients to coastal coral reefs. Nevertheless, they can also act as local stressors due to high coastal urbanization and associated release of untreated effluents. Here we aimed to evaluate qualitatively the effects of seasonality over reefs from the Costa dos Corais Marine Protected Area (Brazil) and to identify the influence of rivers and submarine groundwater discharge (SGD) over the reefs' environmental status. Surface waters from the reef lagoon, local rivers and shoreline porewaters were sampled monthly during a year and abiotic parameters were determined. The results showed that the reef lagoon is oligotrophic throughout most of the year, with conditions that generally favor biological activities, such as calcification. Chlorophyll-a, with an average of 0.78 ± 0.43 mg m−3, was significantly correlated with salinity and radon in excess. This indicates the influence of rivers and SGD over the reefs, especially during the rainy season. These are important sources of nutrients, regulating the primary production. However, signs of anthropogenic pressure were found in the local rivers. In addition, porewaters had a N:P ratio (27.68 ± 24.45) which could potentially impact surface waters through SGD by increasing the local trophic status. In summary, the studied reefs still present good water quality conditions, although continental runoff is a significant transport vector of anthropogenic solutes to the area.
... Low C stocks and the refractory nature of both particulate and dissolved organic matter in the sub-surface landocean continuum limit heterotrophic NO 3 − reduction processes and can explain the large NO 3 − concentrations reaching STEs (Hartog et al., 2004;Rivett et al., 2008). On the other hand, fresh marine organic matter brought into the STE at the seepage face fuels a spatially heterogeneous cascade of biogeochemical processes that can determine final N loads to the coast (Ibánhez et al., 2013;Pain et al., 2019;Calvo-Martin et al., 2021). Due to the high mobility of N compounds in the environment and the role N plays in microbial and primary producer metabolism, anthropogenic N enrichment of STEs could also promote alterations on the cycling of other elements such as C. Hence, mobilization of organic C from high residence time reservoirs like peat bogs (Bragazza et al., 2006), the Artic tundra (Mack et al., 2004), or alpine steppes (Chen et al., 2020) is increasingly linked to the enhancement of microbial and enzymatic activity caused by anthropogenic N enrichment. ...
Due to the widespread pollution of coastal groundwaters with fertilizers, submarine groundwater discharge (SGD) is often thought to be a large dissolved inorganic nitrogen (DIN) source to the ocean. Whether this N is autochthonous or allochthonous to the subterranean estuary (STE), the availability of large quantities of DIN can nevertheless interact with the cycling of other elements, such as carbon (C). In previous studies, we documented the discharge of large quantities of freshwater and NO3– from the mouth of an STE into the Ria Formosa lagoon (SW Iberian Peninsula). For the period covered in this study (2009–2011), the same STE site was dominated by recirculating seawater due to a prolonged fall in piezometric head in the coupled coastal aquifers. Total SGD rates remained similarly high, peaking at 144 cm day–1 at the lower intertidal during fall. We observed a progressive increase of NO3– availability within the STE associated with the recovery of piezometric head inland. Interestingly, during this period, the highest SGD-derived dissolved organic C and DIN fluxes (112 ± 53 and 10 ± 3 mmol m–2 day–1, respectively) originated in the lower intertidal. NO3– enrichment in the STE influences the benthic reactivity of fluorescent dissolved organic matter (FDOM): when seawater recirculation drives STE dynamics, only small changes in the benthic distribution of recalcitrant humic-like FDOM are observed (from −2.57 ± 1.14 to 1.24 ± 0.19 10–3 R.U. “bulk” sediment h–1) in the absence of DIN. However, when DIN is available, these recalcitrant fractions of FDOM are actively generated (from 1.32 ± 0.15 to 11.56 ± 3.39 10–3 R.U. “bulk” sediment h–1), accompanied by the production of labile protein-like FDOM. The results agree with previous studies conducted with flow-through reactor experiments at the same site and suggest that DIN enrichment in the STE enhances the metabolic turnover of sedimentary organic matter up to the point of discharge to surface waters. DIN pollution of coastal aquifers may therefore promote a contraction of the residence time of particulate organic C within the STE, driving carbon from continental storage into the sea.
Supply of bio-labile dissolved organic matter (DOM) has been assumed to be a key factor for the intensity of nitrate (NO3–) removal in permeable coastal sediments. In the present study, a series of flow through reactor experiments were conducted using glucose as a N-free bio-labile DOM source to permeable sediments from a sandy beach seepage face to identify its effect on benthic NO3– removal. The results revealed a shift from the dominance of NO3– production to removal processes when NO3– input concentration increased from 10 to 80 μM under oxic conditions. Sediment microbiota information suggests that nitrification (e.g., Nitrosomonas and Nitrososphaera) and denitrification (e.g., Marinobacter and Bacillus) were dominant pathways for benthic NO3– production and removal in the studied sediment. Compared with the active response of sediment microbiota to NO3– additions, the supply of glucose (approximately 300 μM final concentration added) did not significantly change the NO3– removal efficiency under aerobic conditions (dissolved oxygen saturation approximately 100%). Similarly, an insignificant increase of NO3– removal rate after glucose amendment of the circulating water was obtained when dissolved oxygen (DO) saturation decreased to approximately 70% in the input solution. When DO at the input solution was decreased to 30% saturation (sub-oxic conditions), the removal rate of NO3– in the group amended with glucose increased, suggesting that glucose stimulated denitrifiers. These results revealed that NO3– removal relied mainly on the anaerobic environment at particle surfaces, with a dependence on the sedimentary organic matter as an electron supplier under bulk aerobic conditions, while the bio-labile DOM was consumed mainly by aerobic respiration instead of stimulating NO3– reduction. However, the respiration triggered by the over-supply of bio-labile DOM reduced the DO in the porewater, likely depressing the activity of aerobic reactions in the permeable sediment. At this point, the benthic microbiota, especially potential denitrifiers, shifted to anaerobic reactions as the key to support nitrogen metabolism. The glucose amendment benefited NO3– reduction at this point, under sub-oxic conditions.
Submarine groundwater discharge is recognized as a major source of chemicals to the global ocean, exerting large control over coastal water composition. Radon and 226 Ra are used to evaluate, for the first time, the occurrence and magnitude of submarine groundwater discharge in the Ría de Vigo, a large, highly productive embay-ment affected by seasonal, wind-driven upwelling. The system is naturally enriched in 222 Rn due to the regional granitic basement geology: high 222 Rn activities (up to 10 6 Bq m −3) are detected in wells and boreholes in the drainage basin of the embayment. High 222 Rn activities (>400 Bq m −3) are also measured in certain areas of the embayment. Comparatively lower 226 Ra activities (<4 10 3 Bq m −3) were measured in the freshwater sources to the bay. Mass balances obtained with a box model are used to perform a volumetric estimate of fresh and saline submarine groundwater discharge in the Ría de Vigo under contrasting circulation patterns. Fresh groundwater is shown to be a relevant hydrological component of the Ria de Vigo water balance, equivalent to 9% AE 4% and 23% AE 9% of the volume discharged by tributary rivers during winter and summer, respectively. On the other hand, recirculation of seawater through permeable sediments is capable of filtering the entire upper volume of the Ria de Vigo through its seafloor in <100 days and might thus be a previously overlooked major source of regenerated solutes to the system.
