Marc Diego-Feliu

Subterranean estuaries (STEs) are key bioreactors regulating the quantity and chemical composition of groundwater‐derived nitrogen (N) reaching coastal ecosystems. Yet, the microbial controls on N‐cycling within these groundwater–seawater mixing zones remain poorly understood. We investigated the spatio‐temporal variations in microbial communities and their N‐cycling potential within an alluvial Mediterranean STE with high N concentration. We explored changes in microbial abundance, heterotrophic activity, taxonomic composition, and the abundance of N‐cycling genes across groundwater samples collected at several depths and distances from the shoreline in winter and summer. Microbial abundance, activity, and diversity varied strongly across hydrochemical zones according to physicochemistry and aquifer depth but showed limited seasonality. Functional predictions suggested a complex, spatially structured suite of N pathways encoded by diverse taxa occupying different STE zones, and quantitative‐PCR revealed niche partitioning between ammonia‐oxidizing archaea, prevalent in fresh‐groundwater, and bacterial denitrifiers enriched in deep‐saline layers. Multiple linear model predictions showed a stronger fit for NO ₂ ⁻ and NH ₄ ⁺ concentrations when using microbial properties than when using environmental variables, highlighting their importance for understanding N cycling in STEs. Our results suggest that the functional potential of the STE microbiome is complex and spatially structured across hydrochemical zones, explaining spatial variations in STE N‐cycling.

Submarine groundwater discharge (SGD) is a widely recognized pathway for nutrient transport to coastal systems. Prior studies overlook the unique biogeochemical signatures of different SGD pathways, neglecting their differences in biogeochemical transformations and spatiotemporal factors. Here, we present an integrated assessment of nutrient delivery through three SGD pathways (fresh SGD, long‐scale recirculated SGD, and short‐scale porewater exchange), alongside surface water inputs, to evaluate their seasonality and impact on nutrient dynamics in a coastal lagoon. We conducted five field surveys in March, July, and November 2021, July 2024, and March 2025 in the Mar Menor lagoon, an ecosystem facing severe ecological degradation, to analyze nutrient concentrations in stream water, groundwater, and lagoon water. Concentrations in discharging waters were shaped by watershed‐scale land use and local redox‐driven biogeochemical processes. Recirculated SGD (long‐scale and short‐scale), often overlooked, emerged as the dominant source of ammonium (; 100–160 kmol d ⁻¹ ), dissolved organic nitrogen (DON; 270–900 kmol d ⁻¹ ), dissolved inorganic phosphorus (DIP; 4–30 kmol d ⁻¹ ), and dissolved silica (DSi; 200–270 kmol d ⁻¹ ) across seasons, while continental discharge (streams and fresh SGD) was the main source of nitrate and nitrite (NO _x ; 30–210 kmol d ⁻¹ ). Short‐scale recirculation flows controlled seasonal nutrient limitation by delivering additional DIP inputs (24 kmol d ⁻¹ ) in summer, increasing eutrophication risk. Identifying preferential transport pathways for each nutrient into coastal systems is crucial for predicting and managing ecosystem vulnerability.

Submarine groundwater discharge (SGD) is a complex hydrological process influenced by multiple mechanisms with various spatiotemporal scales. Quantifying SGD is complex because it depends on various driving forces (e.g., waves, tides, hydraulic gradient) and on the underlying geological context that modulate the occurrence and magnitude of SGD. This poses challenges to the accurate quantification of SGD, especially in heterogeneous nearshore environments. However, there is still limited understanding on SGD spatiotemporal patters hindering the accurate assessment of SGD implications for the coastal ocean.The aim of this study is to use amphibious electrical resistivity tomography (AERT), which combines terrestrial and marine instruments, to assess spatiotemporal patterns of FSGD (Fresh Submarine Groundwater Discharge) in the land-sea interface. This cost-effective method enables prospecting SGD dynamics at the land-sea transition zone based on the resistivity variations in the subsurface induced by salinity changes. The resistivity study was conducted regularly to assess hourly spatiotemporal variations in two aquifers near Barcelona, Spain, with different geological contexts, including detrital and karst formations. The resistivity data in both study sites have been validated with in situ pore water sampling for physicochemical parameters, with a good agreement between resistivity and subsurface salinity. The time-lapse results indicate that even in micro-tidal environments such as the Mediterranean Sea, SGD patterns are highly dynamic, with the karst environment exhibiting a more significant proportion of freshwater in the marine sediments and faster changes than the alluvial context. This methodology proves effective for the spatial and temporal assessment of FSGD. These findings offer a valuable approach for monitoring subsurface salinity changes in coastal aquifers and enhance our understanding of small-scale and short-term SGD variations, which is fundamental to deriving reliable SGD and nutrient flux estimates.

We describe an intermediate scale experimental field site located in a coastal alluvial aquifer at the mouth of the Argentona ephemeral stream on the Maresme coastline (Barcelona, Spain). We have been monitoring Seawater Intrusion (SWI) and Submarine Groundwater Discharge (SGD) for several years using geological (lithological description and core samples analyses), geophysical (downhole and cross-hole measurements), hydraulics (pumping and tidal response tests) and hydrochemical (major and minor elements), and geophysical methods (cross-hole electrical resistivity. We have found that apparently minor silt layers control the distribution of salinity, with SWI and freshwater SGD occurring at multiple layers. This multiplicity of salinity levels promotes unstable mixing, which is very active and leads to a surprising bio-geochemical activity in the mixing zone. In parallel, instability makes it hard to sample SGD and makes it clear that the traditional SWI-SGD paradigm needs to be revised.

Coastal aquifers are characterized by their unique land-sea interaction and the main difficulty in studying these systems is the complexity of their biogeochemical cycles. Important processes in the “mixing zone” of coastal systems are associated with the ecosystem’s diversity, and supply and exchange of chemical compounds. This coastal body known as a “subterranean estuary” (STE), is characterized by a free connection to the sea, creating an interface between freshwater and seawater. The distribution and composition of substances flowing from land to sea change because the submarine groundwater discharges (SGD) considerably dilute the seawater that has invaded the aquifer through the free connection to the sea (SWI, Sea Water Intrusion).These processes were studied in an area 100 m long inland from the coastline and 30 m wide, in the alluvial aquifer of Argentona, Mataró, northeast of Barcelona, Catalonia (Spain). This experimental site was established in 2015 and has been equipped for intensive monitoring of coastal aquifer processes such as SWI and SGD. Currently, it is monitored with 25 piezometers (2 meters screened) consisting of 5 nests with 4 piezometers each (at 10m, 20m, 15m, and 25m intervals) and 4 individual piezometers. In this system described as a “multi-aquifer and reactive system”, we are characterizing the physicochemical and hydrogeochemical conditions associated with biogeochemical processes at different depths and seasonal variations. For this presentation, we will show new results of hydrogeochemistry, nitrogen isotopes, and microbiological parameters associated with different periods of monitoring that characterize the dynamics in the subterranean estuary. Acknowledgments: This work was funded by the Spanish Government (grant no. PID2019-110212RB-C21 and PID2019-110212RB-C22), the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency, and the SENACYT – IFARHU – BID Scholarship.

The Mar Menor coastal lagoon (Spain) is a critical ecological and socioeconomic ecosystem and the first in Europe to be granted rights of personhood. However, pollution from past and present activities such as mining, agriculture, urbanization, and tourism threatens its health and ecological stability. Previous research has shown the importance of metal contamination in the lagoon and its link to nearby mining activities, but little consideration has been given to historical changes in this industry and in other potential metal sources. In this work, metal concentrations have been analyzed in 12 sediment cores dated with 210Pb, allowing the reconstruction of the recent (last ~150 years) metal contamination in the lagoon. The main metal sources have been identified by using multivariate statistical methods. Metal contamination from mining activities (point-source pollution) peaked in the mid-20th century, whereas nonpoint-source metal contamination reached its highest level in more recent decades. Despite the current decrease in metal deposition trends, concentrations in surface sediments still exceed sediment quality and ecotoxicological thresholds in areas close to former mining sites. Therefore, they need to be considered in future management strategies, which should also include the evaluation of sources and processes that are still supplying them to the lagoon.

Submarine Groundwater Discharge (SGD) is recognized as a fundamental hydrological process that supports many coastal biogeochemical cycles and social-ecological systems. However, very little has been investigated about how SGD affects society and human well-being. Coastal services provided by ecosystems dependent on SGD can be analyzed and clustered into the four main categories of Ecosystem Services (i.e., Provisioning, Supporting, Regulating and Cultural), which are divided into subcategories defined as outcomes. This enables identifying and discussing both benefits and threats to coastal societies resulting from SGD outcomes. Due to the lack of academic literature on this matter, here we explore the academic and local knowledge of the social perception toward SGD and its ecosystem services (ES). This research is conducted through two case studies, the island of Mallorca and the Region of Salento, to unravel the similarities and particularities of each Mediterranean society regarding the SGD-ES identified and their historical evolution. Such evolution transitions from the management of the fresh groundwaters for human consumption to the exploitation by the tourism industry of cultural ecosystem services related to the same discharge. Our review also shows how compiling different search possibilities (e.g., local languages, including paper-based documents; grey literature; local knowledge; academic literature) has resulted in a significant increase in the reported ES and its understanding. In this direction, combing traditional and academic knowledge are key to accessing society's perception of most cultural ES. Therefore, SGD-ES studies are extremely locally-dependent, and thus regional or global require an in-depth understanding of all areas comprehended in the study. Overall, the research presented in this study contributes to a better understanding of the complexity of the SGD and its social implications. Therefore, this research presents to the academic community new insights from traditional knowledge and an opportunity to integrate multidisciplinarity into a study subject that has usually only been looked from the prism of natural sciences.