Earth-Science Reviews 221 (2021) 103742 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev The social implications of Submarine Groundwater Discharge from an Ecosystem Services perspective: A systematic review Aaron Alorda-Kleinglass a, *, Isabel Ruiz-Mallén b, *, Marc Diego-Feliu a, Valentí Rodellas a, Joan Manuel Bruach-Menchén c, Jordi Garcia-Orellana a, c a Institut de Ciència i Tecnologia Ambientals (ICTA), Universitat Autònoma de Barcelona, E-08193 Bellaterra, Catalonia, Spain b Internet Interdisciplinary Institute (IN3), Universitat Oberta de Catalunya, E-08860 Castelldefels, Catalonia, Spain c Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Catalonia, Spain A R T I C L E I N F O A B S T R A C T Keywords: Submarine Groundwater Discharge (SGD) is recognized as a fundamental hydrological process that supports Submarine groundwater discharge many coastal biogeochemical cycles and social-ecological systems. However, very little has been investigated Coastal Ocean about how SGD affects society and, specifically, human well-being. This study systematically examines the Ecosystem Services published scientific literature on the social implications of SGD by using an Ecosystem Service (ES) perspective. Social implications, SGD-ES Coastal services provided by ecosystems dependent on SGD are analyzed and clustered in the four main cate­ gories of Ecosystem Services (i.e., Provisioning, Supporting, Regulating and Cultural), which are in turn divided into subcategories defined as outcomes. This allows identifying and discussing both benefits and threats to coastal societies resulting from SGD outcomes. From the 1532 articles initially reviewed, the most frequently mentioned category was the supporting services (835) due to the mainstream trend in scientific literature to focus on the role of SGD as a process influencing coastal biogeochemical cycles. Conversely, cultural ES were mentioned in only 49 cases, which should not necessarily be interpreted as a lack of research or interest in this topic, but that this type of references are often not found in the scientific literature but in the grey literature. A detailed publication review was additionally conducted, identifying 114 case-studies from 96 different locations worldwide that reported cases in which SGD had social implications on the well-being. Our review also shows how the different types of Ecosystem Services can have multiple synergies and trade-offs between them, resulting in unequal impacts among stakeholder groups. Overall, this study identifies research gaps related to Ecosystem Services provided by SGD as well as opportunities for further studies, while developing an analytical framework that relies on the Ecosystem Services approach to guide future research on the social implications of SGD. 1. Introduction Johannes, 1980; Kohout et al., 1979), when it was recognized to play a relevant role in coastal hydrological and biogeochemical cycles (Knee Humans have historically used and managed coastal ecosystems and and Paytan, 2011; Luijendijk et al., 2020; Ma and Zhang, 2020; Santos their resources as services towards their own benefit. Currently, coastal et al., 2021; Slomp and Van Cappellen, 2004). SGD is composed of environments support a wide range of economic sectors (e.g., tourism, groundwater with different compositions (e.g., fresh, brackish or saline fisheries, and mining) and provide important socio-environmental groundwater), origins (e.g., terrestrial, marine) and driven by a wide benefits (e.g., temperature regulation, shoreline protection against range of physical processes (e.g., terrestrial hydraulic gradient, waves, storms and floods, and water quality maintenance) (Alder et al., 2006). tides, etc.) (Garcia-Orellana et al., 2021; Moore, 2010; Santos et al., The ecological richness of a coastal habitat, as well as its complexity, is 2012; Taniguchi et al., 2019). SGD is characterized by its often diffusive in part dependent upon land-ocean interactions. The supply of fresh­ inflows along large areas and below the water surface, resulting in an water and solutes from terrestrial sources, such as rivers, streams, gla­ “invisible” process in comparison to surficial and point-sourced riverine ciers, and groundwater, support coastal ecosystems (Alder et al., 2006). discharge. SGD is also characterized by a unique biogeochemical Among these sources, Submarine Groundwater Discharge (SGD) has signature (i.e., characteristic physiochemical parameters and solute received increasing attention since the 1980s (Bokuniewicz, 1980; concentrations) compared to other sources, owing to the * Corresponding authors. E-mail addresses: aaron.alorda@uab.cat (A. Alorda-Kleinglass), iruiz_mallen@uoc.edu (I. Ruiz-Mallén). https://doi.org/10.1016/j.earscirev.2021.103742 Received 11 January 2021; Received in revised form 11 July 2021; Accepted 14 July 2021 Available online 17 July 2021 0012-8252/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 biogeochemical transformations occurring in the coastal aquifer, which studied, but there is a lack of studies examining SGD from an ES are often mediated by bacteria (Ruiz-González et al., 2021; Santos et al., perspective or evaluating the synergies and trade-offs derived from SGD- 2021). related ES. Only two studies have preliminary explored this topic; Ero­ SGD is an important source of fresh groundwater representing be­ state et al. (2020), who discussed the policies and management of ES tween 0.6% and 1% of the total global freshwater inputs into the oceans linked to groundwater-dependent coastal ecosystems in Mediterranean (Luijendijk et al., 2020; Zhou et al., 2019). Although it represents a small regions; and Moosdorf and Oehler (2017), who reviewed social uses of fraction of freshwater discharge into the ocean on a global scale, the SGD-derived freshwater (e.g., drinking, hygiene, agriculture, fishing, high spatial variability of SGD fluxes results on locally important fluxes tourism or culture). in specific areas (Luijendijk et al., 2020). Fresh SGD might thus ensure In this study, we conduct a systematic review of the scientific liter­ water resources for people's livelihoods, particularly in arid or semi-arid ature to gather the available and existing knowledge on the social im­ coastal regions were freshwater resources are limited (Erostate et al., plications of SGD worldwide. Specifically, we review the peer-reviewed 2020; Moosdorf and Oehler, 2017). In addition, SGD supplies solutes to scientific academic literature published in English to analyze coastal ES the coastal ocean (e.g., Cho et al., 2018; Johannes, 1980; Moore, 2008; derived from SGD, understood as both positive and negative effects on Rodellas et al., 2015), which directly impact the productivity of coastal human well-being and quality of life, and classify the information ob­ ecosystems (e.g., Erostate et al., 2020; Johannes, 1980; Johannes and tained using the Millennium Ecosystem Assessment (MEA, 2005) as a Hearn, 1985; Lecher and Mackey, 2018; Taniguchi et al., 2019; Valiela conceptual framework baseline. Furthermore, we review the direct so­ et al., 1990). SGD can also supply dissolved contaminants to the coastal cial impacts of SGD, in terms of synergies and trade-offs towards well- ocean derived from anthropogenic sources (e.g., agriculture, industrial, being, and the research gaps and opportunities for further studies. The mining activities, domestic wastewaters) (e.g., Alorda-Kleinglass et al., new insights derived from this review will allow the development of an 2019; Pavlidou et al., 2014; Rodellas et al., 2014; Sternal et al., 2017; ES-based analytical framework that will guide future research on the Szymczycha et al., 2020; Trezzi et al., 2016), which can endanger the social implications of SGD. This review also highlights the importance of coastal ecosystems and the well-being of local population living around SGD from a social perspective, closing the gap between physical and them. In this regard, societies living around SGD-influenced zones may social disciplines that have often worked independently. benefit or be harmed by the services and goods provided by the eco­ systems influenced by SGD. 2. Analytical framework The benefits that are obtained from ecosystems that support, directly or indirectly, the survival and quality of life can be defined as Ecosystem SGD is defined as “the flow of water through continental and insular Services (ES) (Costanza et al., 2017, 1997; Harrington et al., 2010). ES in margins from the seabed to the coastal ocean, regardless of fluid composition coastal areas have been the foundations for many coastal civilizations or driving force” (Garcia-Orellana et al., 2021; Burnett et al., 2003; (Costanza et al., 1997). For instance, coastal environments have pro­ Taniguchi et al., 2019). SGD includes thus both fresh groundwater vided water and food for human consumption for millennia and main­ discharge (fresh or terrestrial SGD) and seawater circulating through the tained coral reefs or mangroves, which have buffered wave storms and coastal aquifer (saline or marine SGD). Fresh and marine SGD can be protected the coastline erosion (Hassan et al., 2005). Those ES have also supplied via 5 different pathways: 1) Terrestrial groundwater discharge inspired myths, tales or even gods, and provided habitats for the fish­ (usually fresh groundwater); 2) Density-driven seawater circulation; 3) eries that coastal societies rely on (Alder et al., 2006). Conversely, the Seasonal exchange of seawater; 4) Shoreface seawater circulation; and effects of global change (e.g., increase of demographic pressure, climate 5) cm-scale porewater exchange (PEX) (Garcia-Orellana et al., 2021). change, groundwater squeeze, increased industrial pollution) are Whilst pathways 1 and 2 may represent a net input of freshwater to the threatening coastal environments (Islam and Tanaka, 2004; Michael coastal ocean, pathways 3, 4, and 5 only involve the recirculation of et al., 2017; Paoli et al., 2017; Williams, 1996). ES provided by rivers or seawater through permeable sediments. In this study, we will only refer streams (Yeakley et al., 2016), lakes (Schallenberg et al., 2013), estu­ to the fresh or saline fraction of SGD, regardless of the pathway driving aries (Barbier et al., 2011) or mangroves (Queiroz et al., 2017) are well the discharge of groundwater (Fig. 1). Previous research has highlighted Fig. 1. Conceptual diagram of a coastal society influenced by Submarine Groundwater Discharge and its four categories of Ecosystem Services: a) Supporting; b) Provisioning; c) Regulating; d) Cultural. The different components of SGD are also shown: 1) Fresh component of SGD; 2) Saline component of SGD. Figure based on Garcia-Orellana et al. (2021). 2 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 that coastal ecosystems provide diverse and valuable goods and services indirect since they do not directly affect human well-being but as a result of fresh SGD (Erostate et al., 2020; Moosdorf and Oehler, are fundamental to the existence of the other categories identified 2017). However, coastal ecosystems can also be supported by saline in the MEA (2005). In this sense, we consider any SGD-driven SGD, which constitutes a relevant source of dissolved solutes to the input of water and solutes, including chemical dissolved com­ coastal ocean (Cho et al., 2018; Moore et al., 2008; Rodellas et al., pound (e.g., nutrients, bacteria, trace metals, oxygen, or rare- 2015). Following Erostate et al. (2020) and Richardson et al. (2011), we earth elements) or water that modifies the physicochemical define coastal groundwater dependent ecosystem (coastal GDE) as the characteristics (e.g., salinity, temperature or pH) of the coastal marine coastal ecosystems that require permanent or intermittent access environment, and the biogeochemical implications of these in­ to groundwater (including fresh and saline SGD) for maintaining their puts. For example, coastal human societies do not use nutrients as biological communities, their ecological processes, and the associated they are delivered to the environment, but they are essential to ecosystem services. Furthermore, we define the ecosystem services support the photic zone, where primary production will be pro­ inherent to coastal GDE, that are directly provided by submarine duced, and therefore will be able to support the production of groundwater discharge as SGD-ES. food (e.g., algae or fish) for human and animal consumption, or to To develop a common and interdisciplinary framework to assess the maintain the habitat. As Outcomes derived from this category, we social implications of SGD, we used the ES approach based on the Mil­ consider: Water Cycle, Nutrient Cycling, Primary Production, and lennium Ecosystem Assessment (MEA, 2005). The MEA (2005) was used Habitat. as baseline due to its sound relevance as the first robust framework to a. Water Cycle as the role of SGD within the hydrological cycle. classify and quantify the social benefits and losses that humans obtain b. Nutrient cycling as the transfer of nutrients delivered by SGD from the functioning of ecosystems. An academic debate exists about from the coastal aquifer into the coastal GDE, from the inor­ what to call “negative” effects provided by ecosystems. Some authors ganic compounds to the assimilation of super depredators at define harmful effects as Ecosystem Dis-services, being “the ecological the top of the chain. processes that affect human well-being in negative ways, causing harm c. Primary Production as the transformation of energy and inor­ or costs” (Barnaud et al., 2018; Zhang et al., 2007). On the other hand, ganic compounds delivered by SGD into organic compounds other researchers argue that such dichotomy is a matter of perception as by those organisms living in the coastal GDE. some people can be damaged while others can benefit from them, d. Habitat as the coastal GDE that promotes life due to the highlighting that such an approach does not reflect reality (Saunders and physicochemical and biological conditions which are sus­ Luck, 2016). Following this second line of thinking, in this study we tained by SGD. consider both positive and negative coastal processes impact on human well-being as ES. This facilitates detecting and discussing potential (ii) Provisioning ES are defined as products that SGD provides to so­ trade-offs and synergies between organisms, ecosystems and human ciety. As Outcomes we consider Freshwater and Food. activities that cannot be related to a single ES (Norberg, 1999; Saunders a. Freshwater as the fresh component of SGD that is directly used and Luck, 2016). as a water resource for human consumption, agriculture or We divided our conceptual framework into the four broad ES cate­ other industrial purposes. gories identified in the MEA (Fig. 2): (i) Supporting; (ii) Provisioning; b. Food as organisms that have their habitat in coastal GDE or (iii) Regulating; and (iv) Cultural, which are in turn subdivided in that their survival depends on SGD and are consumed by Outcomes, reflecting the different services of each category. A wide society. range of outcomes derived from each ecosystem service category were already established by MEA (2005) framework, but only those relevant (iii) Regulating ES are defined as services that control crucial processes for SGD-influenced ecosystems have been selected in the framework. for habitats and coastal ecosystems influenced by SGD. As Out­ comes we consider Biological Control and Human Disease (as (i) Supporting ES are defined as services provided by SGD that sustain Disease Regulation in the MEA (2005)). the existence of other ecosystem services. These services are Fig. 2. SGD-ES conceptual framework. SGD derives into the four ES categories and their outcomes (represented by squared boxes). Those outcomes depend on each other by creating synergies or trade-offs shown with blue and red arrows, respectively, as an example. Finally, those interactions influence the coastal society well- being, based on the MEA (2005). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 a. Biological control as those changes or conditions induced by communities, due to the subjectivity of the observer. As Out­ SGD that affect the prevalence of certain species, ecosystems, comes we consider Recreational Activities or Ecotourism, Sense or limit the entrance of other species into coastal GDE. of Place, Religion, Cultural Heritage and Aesthetic or Inspira­ b. Human Disease refers to the transport or restriction of patho­ tional values. gens or pollutant compounds delivered by SGD that can a. Recreational Activities or Ecotourism as any leisure activity compromise human health. (economically exploited or not) developed on an environment influenced by SGD. (iv) Cultural ES are defined as the non-material benefits provided by b. Sense of place as the feeling of belonging to a certain site or SGD that contribute to human values and influence behavior. The toponyms that have been given to certain places after the perception of those ES can vary across stakeholders or occurrence of SGD, as well as buildings' names. c. Religion as those stories, tales, myths, or religious ceremonies that are based on SGD. Table 1 d. Cultural heritage as those ways of doing, traditions, knowledge, Data selection criteria. objects, or values related to SGD that the present society Stage Criteria Screened Selected continues from older generations. studies studies e. Aesthetic or Inspirational values as the subjective sensory- Preliminary Key words 30 2 emotional values provided by SGD, such as inspiration, “Submarine Groundwater intrigue to explore nature, or beauty. Discharge” “Soci*” Each of those Outcomes can be related with each other by means of First Key words 1532 503 “Submarine Groundwater synergies or can be prioritized by coastal societies by trade-offs in order Discharge” to achieve their well-being. Following the MEA (2005) we define: “Submarine Spring” Second Title analyses 503 92 - Well-being as “the capacity of an ES to provide the conditions for SGD impacts physical, social, psychological, and spiritual fulfillment”. Third Abstract analysis 92 32 Social implications - Synergies as the relation between two or more outcomes that benefit Fourth Detailed review 32 32 (114 cases) mutually due to their existence. Social implications - Trade-off as the choice taken by society that involves prioritizing one outcome in exchange of another one or more. Fig. 3. Worldwide distribution of locations where Ecosystem Services provided by SGD were identified and reported in scientific literature (A). Pink, turquoise, dark blue and yellow quadrant of the circle corresponds to sites where Provisioning, Supporting, Regulating and Cultural Ecosystem Services were identified, respectively. Zooms into the areas where most of the studies are located are also provided: B) North America; C) Japan and the Korean peninsula; D) Hawaiian archipelago; E) Mediterranean Sea; F) Indonesia and North Australia. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 3. Data collection and analysis The scientific literature review process has been carried out in five stages (Table 1). In order to provide an overview on the state of the art of the SGD-ES, according to well-established guidelines for systematic re­ views by Petticrew and Roberts (2006), a systematic literature review was conducted by using the search engine Web of Science (hereinafter, WoS). As a preliminary review, we used “submarine groundwater discharge” and “soci*” as keywords in WoS. Results from this pre­ liminary review search provided 30 studies of which only two (i.e., Duarte et al., 2010; Moosdorf and Oehler, 2017) explicitly reported direct social implications. Considering the limited number of studies obtained from the pre­ liminary review, a broader search using the terms “Submarine Groundwater Discharge”, or “Submarine Springs” was conducted as first Fig. 4. Number of reported cases in the first stage for each SGD-ES for the stage of the systematic review. A total of 1532 studies were registered systematical search key words using WoS. with these search terms from 1900 to April 2020. Considering the MEA (2005) as a baseline, a re-search within this sample of articles was done by establishing a set of keywords for each ES category and outcome 8%, and 3%, respectively, of the examined literature, have been signi­ derived from SGD-ES (see Supplementary material). ficatively overlooked in research pertaining to SGD (Fig. 4). Notice that The content of the articles (reported in Supplementary material) according to the MEA (2005), supporting ES are services that create indicated that most studies could be categorized as supporting ES, sug­ synergies between categories, or their effects are so long term that they gesting that these articles did not explicitly examine the social impli­ cannot be perceived by the coastal societies (Fig. 2). Therefore, ac­ cations of SGD. To readdress the review and focus on the social cording to this definition, any provisioning, regulating or cultural ES are implications of SGD, the title of all the publications identified at the first related to the Supporting category and there is an overlap of some SGD- stage (n = 1532) was reviewed to only include those studies which ES. In this direction, the same study can be accounted for the supporting focused on SGD impacts as the second stage of the systematic review. category and any other. For instance, Duarte et al. (2010) showed that With the remaining (n = 503), a detailed publication review was made an aquiculture macroalgae farm that is provided by the nutrients of SGD to double check for suitable publications that established direct relations is directly related to the provision of food by the algae farm (provi­ between SGD processes, ES, and social implications. As third stage the sioning ES) and, indirectly, but not less important, the support of habitat abstracts of the manuscripts were screened to select only publications for those organisms (supporting ES). In this direction, any other example that referred to social implications of SGD. The fourth and final stage described in the following sections is always related to supporting ES. consisted on the full text analysis of the selected manuscripts (n = 92), Focusing on the studies explicitly referring to SGD social implica­ which was carefully reviewed to identify the social implications of SGD. tions (fourth stage), the identified 114 cases from the 32 publications in As a result, 32 publications were finally included in the analysis of social the systematic review showed the following results in terms of the re­ implications, in which a total of 114 cases from different locations ported ES: 100% Supporting, 80% Provisioning, 25% Regulating, and worldwide were identified and analyzed by using the MEA-based 41% Cultural ES (Fig. 5). The higher incidence of Provisioning services framework described in Section 2 as a baseline (Fig. 3). seems to be the result of the major scientific effort done over the last 40 years to link SGD as a fresh water resource (Moore, 2010; Taniguchi et al., 2019). Accordingly with results obtained in the first stage, the 4. Results and discussion smaller relevance of Regulating and Cultural ES suggests that these topics have only recently received scientific attention. In the following sections we describe how SGD is directly related to those ES and how those interact with each other, what dependences they have and what consequences affect the coastal societies. As it will be explained, supporting, provisioning, regulating and cultural ES can have multiple synergies between them (see Section 4.1). In any case, those ES do not interact spontaneously but the different stakeholders from any coastal society interact with the SGD process. Either to take advantage or remove the threat to guarantee their well-being, there are trade-offs between ES that will take place in each society (see Section 4.2). Therefore, those situations will make difficult to achieve a win-win scenario, which can result into social conflicts. 4.1. Submarine groundwater dependent ecosystem services Most of the SGD studies are focused on understanding the role of SGD Fig. 5. Number of reported cases for each SGD-ES categories and derived in the water cycle and its biogeochemical impacts on coastal and marine outcomes from the fourth stage. WC: water cycle; NC: nutrient cycling; PP: ecosystems. Indeed, the results derived from the first stage of the sys­ primary production; H: habitat; FW: freshwater; F: food; BC: biological control; tematical review reinforce this fact; 55% of the literature has been HD: human disease; LA: leisure activities or ecotourism; SP: sense of place; R: focused on evaluating supporting ES of SGD. This evidences that direct religion or myth; CH: cultural heritage; A: aesthetical values. ES such as providing, regulating, and cultural, which only comprise 7%, 5 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 The analyzed 114 cases where at least one SGD-ES can be identified habitats. For instance, SGD from karst springs may reduce the net with a direct social implication correspond to 96 different locations calcification capacity of corals, and therefore the extension of habitat worldwide (Fig. 3). Nevertheless, there were vast areas of the planet that they can provide to other species by provisioning high amounts of where few SGD studies were performed and little scientific information CO2 stabilizing low pH (Álvarez-Góngora and Herrera-Silveira, 2006; is available on ES provided by SGD. These areas include the coast of Crook et al., 2012; Troccoli-Ghinaglia et al., 2010). Also the low salin­ Africa, South America, the Arabian Peninsula, Antarctica or the Indian ities provided by fresh SGD were reported to reduce the diversity and subcontinent, which altogether represent less than 10% of the cases richness in coral reefs (Lirman et al., 2003) and meiofaunal communities found in the review (Fig. 3). This distribution of case studies is mainly (Kotwicki et al., 2014; Migné et al., 2011). Contrarily, diatoms (Welti explained by the inherent bias in this search because most of the et al., 2015), cyanobacteria from benthic communities (Lee and Kim, research conducting SGD studies is concentrated in specific areas (e.g., 2007) and juvenile snails (Lobatus gigas) showed to find stable habitats USA, Australia, Europe) and the selection of scientific publications due to freshwater conditions supported by SGD (Stieglitz and Dujon, written only in English. 2017). Moreover, SGD inputs from karstic springs showed to represent the principal source of freshwater to some coastal lagoons, contributing 4.1.1. Supporting ecosystem services to maintain them under non-hypersaline conditions for most of the year As one process involving the transference of water across the land- and thus playing a relevant role for coastal lagoon ecosystem func­ ocean interface, SGD has a role on the global Water Cycle (Church, tioning (Rodellas et al., 2018; Stieglitz et al., 2013). 1996; Zhou et al., 2019), which is understood as a SGD-ES. During the SGD can also contribute to stabilize temperatures of coastal ecosys­ 1960s, several studies were conducted to evaluate the fresh groundwater tems, which might have a key role on the maintenance or weakening of driven by SGD as part of the local water budget (Burdon and Papakis, coastal habitats. On the one hand, SGD-driven inputs of nutrients 1961; Isbister, 1966; Muir, 1968; Newport and Haddor, 1963) and as accompanied with stable temperatures promote primary production and part of the global budget that was estimated to represent 5% of surface support more complex organisms across the trophic chain and richer runoff (Nace, 1967). Since then, other attempts were performed to es­ habitats. Such effects are known to create biological hotspots, where timate the fresh SGD contribution to the water cycle (Church, 1996; biomass, richness, diversity, net community production and ecosystem Taniguchi, 2002; Zektser and Loaiciga, 1993), including the recent in­ complexity are enhanced (Encarnação et al., 2015; Foley, 2018; Garcés vestigations that have estimated that the fresh component of SGD rep­ et al., 2011; Utsunomiya et al., 2017). For example, in Salses - Leucate resents ~1.3% of river discharge (Zhou et al., 2019) or ~0.6% of the coastal lagoon (France) the higher temperature and nutrients avail­ total freshwater into the global ocean (Luijendijk et al., 2020). However, ability related to SGD were correlated to the higher growth of Medi­ when the saline or brackish component of SGD is integrated, SGD has a terranean mussels (M. galloprovincialis) (Andrisoa et al., 2019a). On the broader influence into the world oceans representing between 80% and other hand, the same constant supply of nutrients and stable tempera­ 160% of the amount of freshwater entering the Atlantic Ocean from ture can enhance the dominance of opportunistic species, which can rivers (Moore et al., 2008), or up to 4 times taking also into account the displace others or affect them by cascading effect (Lecher and Mackey, Indo-Pacific Oceans (Kwon et al., 2014). 2018). The dominance of opportunistic species inevitably reduces In addition to its relevance for the hydrological cycle, SGD also richness and diversity in some coastal ecosystem. These consequences supplies nutrients from natural or anthropogenic sources into the coastal were studied in coral reefs of Japan (Blanco et al., 2008) and benthic ocean (Basterretxea et al., 2010; Johannes, 1980; Johannes and Hearn, communities in Delaware (USA) (Miller and Ullman, 2004), where zones 1985; Lecher and Mackey, 2018; McClelland et al., 1997; Santos et al., directly influenced by SGD had communities with less ecological rich­ 2021; Slomp and Van Cappellen, 2004; Valiela et al., 1990). Nutrients ness than others which were not influenced. supplied to the coastal sea by SGD, which form part of the nutrient In addition, such processes can support Harmful Algal Blooms cycling, continue their cycle interacting with the biota. Indeed, micro­ (HAB's) such as red tides, brown tides, cyanobacteria blooms or mac­ biota (bacterioplankton or phytoplankton) are the first organisms to roalgal green tides (e.g., Gobler and Sañudo-Wilhelmy, 2001; Hu et al., transform those nutrients and make them available for other secondary 2006; Hwang et al., 2005; Kwon et al., 2017; Lapointe, 1997; Lapointe producers (Lecher and Mackey, 2018). One of the most important pro­ et al., 2005; Laroche et al., 1997; Lee et al., 2009; Su et al., 2012) cesses in this nutrient cycling is to support the primary production. Many enhancing primary production but at the same time destroying the publications have related the role of SGD-driven nutrients to support habitat of other species and ecosystems. The presence of HAB's sup­ higher productivity of phytoplankton blooms in coastal areas (Garcés ported by SGD-driven anthropogenic nutrients may have cascading ef­ et al., 2011; Machado and Imberger, 2014; Troccoli-Ghinaglia et al., fects to entire ecosystems, as the massive kills observed in the USA (Hu 2010), cyanobacteria blooms (Blanco et al., 2011, 2008; Umezawa et al., et al., 2006) and South Korea (Lee et al., 2010). The presence of dense 2002), macroalgae blooms (Amato et al., 2016; Derse et al., 2007; Ouisse phytoplankton blooms was reported to reduce light availability for et al., 2011; Yoshioka et al., 2016) or enhance macrophytes spatial benthic communities, in seagrass meadows (Short and Burdick, 1996) coverage, leaf growth and meadow productivity (Carruthers et al., 2005; and coral reefs (Laroche et al., 1997; Richardson et al., 2017). Similar Dadhich et al., 2017; Darnell and Dunton, 2017; Kamermans et al., effects were produced by macroalgal blooms in coral reefs and turf algae 2002; Peterson et al., 2012). Primary producers, from both the benthos in Hawaii, where the massive presence of those organisms covering the and the water column, are able to incorporate inorganic nutrients into benthos has prompted the habitat to change (Amato et al., 2016). the trophic chain upholding nutrient cycling and food for more complex organisms (Lecher and Mackey, 2018). Isotopic analysis of ∂15N and 4.1.2. Provisioning ecosystem services ∂13C and N:P ratio have allowed identifying the direct uptake of inor­ Freshwater resources in coastal areas, particularly in arid or semi-arid ganic nutrients by phytoplankton (e.g., diatoms and dinoflagellates) and zones, are of vital importance to any type of human settlement. Several macrophytes (Amato et al., 2016; Andrisoa et al., 2019b; Hata et al., studies documented the use of the freshwater component of SGD since 2016; Lecher and Mackey, 2018; McClelland et al., 1997). The utilized the Phoenician times along the Mediterranean coasts (Kohout, 1966) nutrients can then be transferred to zooplankton and to higher trophic and the ancient Rapa Nui civilization in Easter Island (Brosnan et al., levels in the food web (Lecher and Mackey, 2018). 2018). In this regard, SGD has been intensively studied in several The discharge of groundwater to the coastal sea can also modify or countries (e.g., southern coasts of France, Lebanon, Libya and Greece) to stabilize temperature, pH, salinity or water transparency conditions of be exploited as a freshwater resource (Bakalowicz, 2018; Fleury et al., the receiving water bodies, which might be essential to maintain or 2007; Mijatović, 2006; UNESCO, 2004). Fresh SGD is also used for other support many coastal habitats and ecosystems (e.g., coral reefs or sea­ purposes, such as agriculture or livestock (Moosdorf and Oehler, 2017; grass meadows). However, the effects of SGD also may harm those Pereira et al., 1996). Still today, the fresh SGD is used for drinking, 6 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 laundering or hygiene in several islands of Indonesia (e.g., Java, Lom­ (Stieglitz, 2005), or create aquaculture in Japan (Hosono et al., 2012; bok, Bali) (Moosdorf and Oehler, 2017) or built-in tap water facilities in Utsunomiya et al., 2017). the Mediterranean (e.g., Trieste bay, Italy; Port-Miou, France; Chekka, Human diseases can also be introduced into the coastal waters by Lebanon; or Benghazi, Libya) (Mijatović, 2006). This SGD-ES can be bacteria or viruses supplied by SGD. Polluting microorganisms can be crucial in semi-arid regions, which are strongly dependent on ground­ driven by groundwater (Abaya et al., 2018; De Sieyes et al., 2016; water resources, especially under climate change forecasts that consider Paytan et al., 2004; Yau et al., 2014), delivering bacterial foreign these regions as hot spots due to their sensitivity to climatic disturbances communities (Knee et al., 2008) or viruses (Futch et al., 2010) into the (IPCC, 2014). Forecasting models predict changes in rainfall seasonality coastal environments. Either from leaks or spills from septic tanks or patterns, an increase of evapotranspiration and a declining of ground­ injection wells from water treatment plants, sewage can infiltrate in water reserves (IPCC, 2007). Aside from its importance as a direct good, coastal aquifers polluting groundwater with high concentrations of fecal fresh SGD has numerous synergies with supporting, regulating and bacteria and viruses, which can be transported by SGD to the coastal cultural ES provided by SGD that are explored in the following sections seawater. In different study sites in California (De Sieyes et al., 2016; (see 4.1.1, 4.1.3 and 4.1.4). Paytan et al., 2004; Yau et al., 2014), Florida (Futch et al., 2010) and The provisioning of food through fishing or aquaculture activities is Hawaii (Knee et al., 2008) it was demonstrated that zones influenced by another ES provided by SGD (UNEP-MAP, 2015). SGD can play a key SGD from coastal aquifers contaminated with wastewater, had elevated role on the support of productivity and functioning of coastal ecosystems levels of Fecal Indicator Bacteria (FIB). According to Yau et al. (2014), (see 4.1.1), resulting in favorable habitat conditions to provision food to the discharge of fecal bacteria into Avalon beach (California, USA) could human societies. This creates a synergy between the supporting ES and be directly related to certain diseases among swimmers who entered in this provisioning ES. Due to the enhancement on high productivity contact with the SGD influenced zones. derived from the SGD-driven nutrients, secondary producers can grow, The provisioning of freshwater is also used to reduce human diseases habitats and ecosystems can develop, and ultimately, human consumed and improve hygiene. Moosdorf and Oehler (2017) reported the use of species are present in those areas. Sites where SGD contributes to pro­ groundwater to make laundry, bathing or to heal wounds in Indonesia, visioning of food include algae aquaculture in Hawaii (Duarte et al., Fiji and Mozambique. The use of SGD in these societies contributes to 2010; Pongkijvorasin et al., 2010), crustacea in Portugal (Silva et al., the citizens health via two ways: directly, by using SGD for hygiene 2012), fish in Japan (Burnett et al., 2018, 2015; Fujita et al., 2019; purposes, and indirectly, by allowing saving their cleaner freshwater Utsunomiya et al., 2017), mussels in south France (Andrisoa et al., resources only for drinking purposes. SGD can also represent a pathway 2019a), or oysters in China and USA (Chen et al., 2018; Spalt et al., delivering pollutants into the coastal ocean from anthropogenic activ­ 2020). SGD can also endanger the provision of food through the supply ities and settlements (e.g., cities, harbors or mines), which could of pollutants by deteriorating the ecosystems that support aquaculture represent a great threat towards people's health that have not yet been and fisheries and endangering local communities' health (Erostate et al., evaluated. These pollutants supplied by SGD include nutrients, which 2020). SGD can also introduce large amounts of metals from mining can trigger HABs that may liberate toxins and can pollute commercial activities (Alorda-Kleinglass et al., 2019; Trezzi et al., 2016), which can species of shellfish (Anderson et al., 2000; Laroche et al., 1997), and act as pollutants instead of micronutrients, affecting primary producers finally endanger the health of the consumers; metals, which can accu­ and subsequently higher trophic levels used for human consumption. In mulate on commercial mussel species (e.g., M. edulis grown in SGD- addition, SGD-driven HAB's may not directly affect humans but their influenced sites had twice the concertation of Hg on soft tissue toxins can be accumulated in food that then will be consumed by compared to non-SGD influenced mussels (Laurier et al., 2007); as well humans (Lee et al., 2010). as other contaminants such as radionuclides from nuclear power plant accidents (Fukushima, Japan) (Sanial et al., 2017) or from high natu­ 4.1.3. Regulating ecosystem services rally occurring radioactive areas, which can bioaccumulate in biota Biological control has been observed where fresh SGD influences the (Garcia-Orellana et al., 2013; 2016). Other studies have reported that salinity levels of coastal areas. This process regulates the presence of SGD can also supply pharmaceutical and caffeine residues (CEC's) (Knee species depending on their tolerance to low salinities as in Okinawa et al., 2010; Szymczycha et al., 2020), pesticides and persistent organic (Japan) (Blanco et al., 2008), Florida (USA) (Lirman et al., 2003) and pollutants (POP's) (Dzierzbicka-Głowacka et al., 2019; Pavlidou et al., Roscoff Aber Bay (France) (Migné et al., 2011). This process, often 2014) to the coastal ocean. referred to as zonation (Kohout and Kolipinski, 1967), can also be pro­ duced by the reduction of the pH due to the influence of SGD and, 4.1.4. Cultural ecosystem services therefore, the difficulties of some organisms with external carbonate or Recreational activities or Ecotourism resulting from the existence of silicate structures to live (e.g., coral reefs or foraminifera) (Crook et al., SGD are mainly linked to the presence of submarine springs (Burnett 2013, 2012; Martinez et al., 2018; Prouty et al., 2017). For example, it et al., 2015; Lougheed, 2006; Moosdorf and Oehler, 2017). Sites- was recognized that juvenile teleost fish had higher growth rates in influenced by submarine springs receive the attention of divers and coastal GDE (Lilkendey et al., 2019) or that the higher temperatures in swimmers due to the existence of a biodiversity hotspot supported and winter supported by SGD provided shelter for other species (Miller and regulated by SGD. A clear example of the cultural relevance that these Ullman, 2004). SGD may thus concurrently provide habitat (supporting springs have is the case of La Source (“The Spring”, in French) in Tahiti, ES) and biological control (regulating ES). Excessive nutrient or con­ where leisure companies have established guided routes to visit SGD- taminants loadings supplied by SGD can enable the presence of those influenced sites as a result of their enhanced biodiversity (Moosdorf species that are adapted to these live conditions (e.g., Ulva spp.) (Hwang and Oehler, 2017). Other recreational activities linked to the occurrence et al., 2005; Kwon et al., 2017; Yoshioka et al., 2016) or opportunistic of SGD can also be found through grey literature sources. On the coasts species of diatoms, dinoflagellates or cyanobacteria (Blanco et al., of the Balearic Islands (western Mediterranean Sea), several caves in 2011). SGD investigations showed that eelgrass living in coastal GDE karstified limestone massifs influenced by SGD are visited by divers, can have fewer herbivory organisms (Peterson et al., 2012). Turtle grass swimmers, and recreational underwater fisherman due to the specific (Thalassia testudinum) can change their biological strategy, under high low light biodiversity that lives in them (Rützler, 1996). During the cold nutrient levels, by not growing flowers and developing bigger leaves winters of Canada, it was also reported that a polynya was maintained (Darnell and Dunton, 2017), while the Australian dhufish (Glaucosoma due to the discharge of groundwater with higher temperatures, allowing hebraicum) can use low salinities to remove parasites (Pironet and Jones, Arctic shipping (Sadler and Serson, 1980). SGD can also affect recrea­ 2000). Humans take direct advantage of such sites, where the abun­ tional activities, particularly when SGD-driven HAB's are occurring or dance of certain species are a source of food to catch fish in Australia when swimming beaches are closed for sanitary reasons due to the 7 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 presence of FIB derived from SGD inputs (Yau et al., 2014). Although passed across fishers' generations. Examples of SGD-linked fishing hot­ less frequent, it was observed that navigation of recreational small boats spots that can be considered cultural heritage include the “wonky holes” was compromised along Croatian shorelines as result of point-sourced of the Great Coral Reef in Australia (Stieglitz, 2005), the “Mud Hole” in large-flow groundwater discharge (Alfirevic, 1966; Keller, 1963). Florida (USA) (Kohout et al., 1979) and in the Yucatan Peninsula Several toponyms along world shorelines were named after the (Mexico) (Stieglitz and Dujon, 2017). occurrence of SGD, especially in areas where groundwater inputs rep­ Additionally, Zektser et al. (1973) mention submarine springs as “the resented a freshwater resource or lead to biological hotspots, giving most spectacular manifestation of groundwater discharge to the seas”, people a sense of place. Examples of such particularity are: “Olhos de indicating the aesthetic value of SGD. Submarine springs or SGD continue Água” in Portugal (“eyes of water” in Portuguese) (Carvalho et al., 2013; to inspire and motivate new generations of authors and researchers. The Encarnação et al., 2015; Foley, 2018), “Punalu'u” in Hawaii (USA) inspiration has gone from one of the first documents that explained SGD, (“diving spring” in Hawaiian language) (Moosdorf and Oehler, 2017), where Aristotle on his treatise “Meteorology” (ca. 350 BCE), explains “Es Dolç” or “Sa Font de Sa Cala” in the Balearic Islands (“the fresh­ how karstic streams sink underground and travel short distances to water” and “the spring of the cove” in Catalan) (Tovar-Sánchez et al., discharge into sea (Clendenon, 2009), to today's latest publications on 2014). Symbolic constructions and places were also built in areas linked SGD. This aesthetical value of SGD has reached ancient poets as Lucre­ to SGD. Some human settlements located in coastal areas with a semi- tius or geographers as Pausanias and Strabo, to inspire songs in the arid climate were built near SGD springs because they depended on Hawaiian folklore (Pukui, 1949). Nowadays, people continue to find the the freshwater resource provided by SGD. These sources of freshwater aesthetic value to SGD by visiting submarine springs on a scuba diving were often protected or venerated through the construction of defense, experiences or by discovering new features in new SGD investigations. strategic or mystical buildings. Examples of such are the Cala Figuera in Maó (Menorca, Balearic Islands, western Mediterranean), where the 4.2. SGD trade-offs Romans already exploited a spring with the construction of a nym­ phaeum and ships stopped to load freshwater (Murray-Mas, 2006). In The ES provided by SGD and the synergies between them cannot be Rapa Nui (Easter Island), it is believed that the original civilization that fully understood without a detailed consideration of the interactions of built the famous face statues (moai) around the island were able to the different stakeholders from any coastal society with SGD-influenced survive thanks to the construction of wells and “punas” (dams) taking ES. Therefore, trade-offs (i.e., the prioritization of one service in ex­ advantage of the fresh SGD, which created a sense of belonging to the change of another one) play a key role in understanding the social im­ place (Brosnan et al., 2018). The Yokokujo Castle of Hiji, Ohita Pre­ plications of SGD. For example, if a community perceives SGD as a fecture (Japan), was built at the shores of the beach to allow the freshwater resource and decides to collect and use this water for their catchment of the Marbled flounder (Pseudopreulonectes yokohamae), a own consumption, this action could reduce the flux of water and solutes highly-prized fish species that thrives in a SGD-dependent ecosystem to the coastal ecosystem. In that case, the community will trade-off the (Shoji and Tominaga, 2018). provision of freshwater resource in exchange to reduce the regulating SGD is also related to religion or myths, both through iconic buildings and supporting ES of SGD derived from the supply of water and nutrients and legends. A clear example is the Hindu temple Tanah Lot, Bali to the ecosystem. This action might be in detriment of nearby coastal (Indonesia), which was built to protect a spring that was magically communities that could see their provision of food reduced (Duarte moved from inland to the sea (Lubis and Bakti, 2013; Moosdorf and et al., 2010; Pongkijvorasin et al., 2010), or also see affected their cul­ Oehler, 2017). The ancient Greek civilization also created different tural heritage if those consumed SGD dependent species formed part of myths and legends related to the occurrence of SGD. According to the their culture (McDermid et al., 2019). This example is summarized in ancient Greek geographer Pausanias (2.5.3), an old legendary tale Fig. 6, highlighting the synergies between the four categories of ES (blue explained how the Turkish River Meander went under the Aegean double arrows) and the effects of a human decision that choses to trade- seabed to the surface 390 km away, in the northeast Peloponnese off most of the outcomes by just the provision of the freshwater (red (Clendenon, 2009). This same legend, according to Strabo (6.2.271) was arrows). Further trade-offs related with SGD-ES can become especially originated by the Greek lyric poet Ibycus around the 6th century B.C. complex when economic, cultural or political interests are at stake. (Clendenon, 2009). Pausanias (8:VII) also described how the inhabitants Thus, in order to achieve or maintain the well-being of a coastal com­ of Argos made sacrifices to Poseidon in the location of a SGD spring, munity, policies and management strategies need to be developed currently named Kiveri spring (Leake, 1830; Moosdorf and Oehler, considering the synergies and trade-offs between the different SGD-ES. 2017). One of the best-known and oldest myths related to SGD is the One of the main targets of the coastal policies and management story of the spring nymph Arethusa and the river god Alpheus, origi­ strategies dealing with coastal ecosystem services is to guarantee the nated in the 8th century BCE (Bilić, 2009; Clendenon, 2009). In this supply and shortage of materials or goods necessary for good life and Greek myth, Arethusa transformed into water and traveled underground economic sustainability (Costanza et al., 2017). In this regard, SGD has through the Ionian Sea to escape from Alpheus's amorous advances. been used and managed for centuries as a freshwater resource. Nowa­ Arethusa resurfaced as freshwater spring in Syracuse (Sicily, Italy) days, climate change-induced drought and high anthropogenic pressure together with Alpheus that had followed her and traveled the same (e.g., groundwater withdrawal, irrigation) makes management of fresh submarine journey, remaining always fresh by never mixing with the sea SGD more necessary (Stigter et al., 2014; UNEP/MAP, 2012). This is (Clendenon, 2009). critical for those societies that have scarce supplies of freshwater (e.g., These SGD-linked myths, stories, buildings and villages have become North African and Middle East coasts). However, management strategies part of the current Cultural Heritage. Different cultures around the globe rarely take trade-offs into account. Research findings have assessed, place high value to keep alive the old uses that SGD had for their an­ conceptually and economically, the impacts of management in­ cestors. In Australia, the Aboriginal community of Kaurna finds an terventions aimed to exploit such resources in these regions. For important part of their identity to the story of the ancient creator Tjil­ example, Ayoub et al. (2002) developed a cost-benefit survey in order to bruke, who wept for his nephew, and from his tears freshwater springs identify the viability of exploiting offshore SGD through a pumping were created on the beach (Amery, 2016; Moosdorf and Oehler, 2017). freshwater project in Cheka Bay (Lebanon). They found that the In the Island of Kona, Hawaii (USA), many of the algae (e.g., limu exploitation of fresh SGD would drastically reduce the marine zonation manauea (Gracilaria coronopifolia)), which are harvested in sites influ­ around the submarine springs, and consequently the availability of enced by SGD, are valued by the indigenous cultures for centuries commercial fish species, affecting the well-being of fishermen commu­ (Duarte et al., 2010; Pongkijvorasin et al., 2010). Fishing spots related to nities. In contrast, local citizens and industries would gain an additional SGD has been part of communities' traditional knowledge, which has volume of freshwater and therefore security for their future good living. 8 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 Fig. 6. Diagram exemplifying how the SGD-ES framework is applied to a hypo­ thetical case described in the text. Filled boxes correspond to outcomes identified and empty boxes to outcomes that have been removed or do not exist. Blue double arrows correspond to synergies between outcomes and red arrows correspond to trade-offs between outcomes that were preferred (filled box) in exchange of those renounced (empty box). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Other concerning trade-offs that exist between freshwater and SGD- threatened by another individual or group that is also exploiting the dependent commercial species have been described for algae in same resource, confrontation can converge into a social conflict. Access Hawaii (USA) (Duarte et al., 2010; Pongkijvorasin et al., 2010) and fish to freshwater is the ES provided by SGD that is more likely to lead to in Obama Bay (Japan) (Burnett et al., 2018, 2015). In these cases, there conflicts, due to its importance for human survival. Due to the political were direct linkages between the use of groundwater as a freshwater need and willingness to explore SGD as an option to face freshwater resource and the exploitation of nearshore biomass productivity fueled shortages, fresh groundwater is widely studied around the Mediterra­ by SGD, where a gain of one goes in detriment of the other. Guaran­ nean (Ayoub et al., 2002; Bakalowicz, 2018; Bakken et al., 2012; Fleury teeing a minimum provision of food stock might require significantly et al., 2007; Ghannam et al., 1998; Mijatović, 2006; Tardieu and Poité, decreasing groundwater extraction (Duarte et al., 2010; Pongkijvorasin 2015). In Hawaii (USA), serious political battles are already raging with et al., 2010). On the contrary, according to another investigation in respect to terrestrial anthropogenic impacts on nearshore environments Japan (Burnett et al., 2018), the economic benefits that provisioning of related to the supply of terrestrial anthropogenic pollutants through groundwater might provide, in terms of freshwater stock, could be SGD (Duarte et al., 2010; Pongkijvorasin et al., 2010). These impacts can greater than the losses to the nearshore fisheries. The management in turn effect the algae aquaculture, feeding social conflicts because the strategy implemented should thus deal with the dichotomy between the cultural ES of the algae aquaculture are deeply rooted into the Hawaiian benefits of provisioning freshwater and the benefits of provisioning food community (McDermid et al., 2019). On the one side, most political stock. parties understand that groundwater extraction reduce SGD and can Anthropogenic actions devoted to improve the management of hy­ impact coastal marine ecosystems. On the opposite side, landowners and drological or biological coastal resources can also indirectly affect other developers insist that the effects of reducing SGD in exchange of the ES (e.g., the construction of subterranean dams, grout curtains, conducts freshwater supply are irrelevant. Such discrepancy between both sides is or pipes, and tapping water plants) (Mijatović, 2006; Tamborski et al., what leads to a conflict towards the future management of this coastal 2020; Tardieu and Poité, 2015). Similarly, in the exploitation of society (Duarte et al., 2010). Recently, for the first time, the US Supreme brackish submarine springs as a freshwater resource, desalinization of Court has ruled in favor to protect the connection between the coastal mined groundwater is required and the brine produced can have haz­ aquifer and the coastal ocean (SGD) (Cornwall, 2020). The case was ardous consequences for the social-ecological system when it is dis­ based on the demonstration that the injection of wastewater effluents charged into the sea (e.g., destruction of the surrounding habitats and into the coastal aquifer directly affected the coastal ecosystem (Glenn potential commercial species, Bakken et al., 2012). et al., 2013) and should be protected according to the Clean Water Act. Coastal management strategies shaping the SGD-ES can lead to That sets a new base on the social conflicts induced by SGD (Santos et al., confrontation between the different stakeholders involved. When one 2021). individual or group perceives that their gains on the SGD-ES are 9 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 5. Future perspectives of the Ecosystem Services linked to SGD (SGD-ES), as well as the syn­ ergies and trade-offs between different SGD-ES. However, the scarcity of This review has also identified that studies on direct ES (provision­ literature relating SGD to ES and the fact that the results obtained pro­ ing, regulating or cultural ES) are vastly outnumbered by publications vide a point-in-time view, means that we only understand a small related to supporting ES. This is a consequence of the large effort made fraction of the current interactions between coastal dwellers and SGD- during the last 40 years on the SGD research field to demonstrate the ES. Importantly, the links between SGD and ES are of a dynamic na­ importance of this process on the hydrological and biogeochemical cy­ ture, implying that their synergies and trade-offs are likely to continu­ cles. The consulted scientific literature has also evidenced lack of ously evolve together with societies and the coastal environment, both attention towards potential impacts and benefits that supporting ES at a local, regional and global scale. This is particularly relevant in the have on coastal societies or other provisioning, regulating or cultural ES. actual context of global change. In this regard, the overpopulation of In this regard, future studies on ES provided by SGD should consider coastal areas is likely to continue increasing, rising the demand of expanding their research to address how their findings could have po­ freshwater and therefore the provisioning SGD-ES. In addition, fresh tential social implications or be related to any of the other SGD-ES resources would be prejudiced by the likely decrease of mean pre­ categories and outcomes. There is thus a need on the SGD research cipitations, due to climate change, and increase of evapotranspiration in field to develop more interdisciplinary studies involving social scien­ many mid-latitude and subtropical regions (IPCC, 2014). Such change tists, hydrologists and oceanographers to work together. would involve a major reduction on the fresh SGD input into the oceans (Kundzewicz and Döll, 2009; Stigter et al., 2014). Sea level rise is also 5.1. Exploration of grey literature expected to reach up to ~0.8 m by 2100 (IPCC, 2014), affecting large coastal areas and reducing the hydrologic gradient and consequently the This review reveals that the published scientific literature on Sub­ quantity of fresh SGD (Robinson et al., 2018). Moreover, important marine Groundwater Discharge has neglected its social dimension, disruptions are expected on the biogeochemical cycles due to contami­ particularly regarding those cultural Ecosystem Services provided by nation of multiple anthropogenic factors (e.g., the agriculture nutrient SGD. Rather than being reported in conventional scientific research, pollution of aquifers, mining and industrial wastes, growing cities waste many of the cultural ES related to SGD have only been published waters) (Lafortezza and Chen, 2016). Such changes on coastal aquifers through grey literature publications. This grey literature can be defined will directly affect the quality of SGD inflowing into the coastal ocean (e. as “all documents except journal articles that appear in widely known, g., fluxes of nutrients, trace metals, contaminants) and thus the role of easily accessible electronic databases will be considered grey literature” SGD on the nutrient cycle, productivity or habitat support. Sea level rise (Rothstein and Hopewell, 2009). These publications are generally will also likely affect nutrient cycling and productivity in coastal areas, written in local languages, and thus we acknowledge that by only mobilize terrestrial anthropogenic pollutants to the sea and critically focusing on English written scientific publications, this review has impact those habitats with low-salinity conditions that are supported by excluded both academic publications in other languages and grey SGD (Danielopol et al., 2003; Michael et al., 2013; Pope et al., 2011). literature. Gathering together all the available information on social Global change will thus definitely impact the supporting Ecosystem implications of SGD is challenging both because local and regional Services supported by SGD, including water cycle, nutrient cycle, pri­ documents are not easily accessible and also because this research re­ mary production or habitats. Once the supporting ES are affected, all the quires involving people proficient with those local languages. As an other dependent ES could be affected by cascading effects. For instance, example to highlight this complexity, there are more than 85 languages the reduction of the habitats, due to the lack of freshwater, could make spoken in the Mediterranean region. Focusing on the 44 submarine disappear the biological control, reduce the amount of food due to a springs around the Mediterranean mentioned by Gilli (2020), the sys­ reduction of productivity or displacement of species. In addition, some tematic review conducted in this study only allowed identifying direct of the cultural ES could disappear or be endangered (e.g., leisure ac­ ES (providing, regulating or cultural) in two sites. If literature available tivities related to the fresh SGD). Therefore, in order to develop new online (both scientific and non-scientific) is screened using English, management programs to preserve SGD-ES for next generations, it is Italian, French, Catalan and Spanish (languages in which the authors are necessary to study the evolution of SGD-ES under the current global proficient) the number of sites with reported ES increases to 14. For change scenario, with special emphasis on new synergies and trade-offs. example, in France, the spring of Estramar is related to leisure activities such as fishing and speleology. The springs of Port-Miou (France) are 5.3. Valuation of submarine groundwater discharge ecosystem services documented as an important freshwater resource for human consump­ tion and also used as a diving spot by different companies. In Italy, the Ecosystem Services where born under the question of “how much are spring of Galeso has become a Tourist attraction to go and see the “citri”, nature's services worth?” (Westman, 1977). Since then there was a terminology used to describe the bubbling pools inside the sea surface pursue towards valuing those services, until Daily (1997) and Costanza that SGD creates under vigorous flow. Widening the search possibilities et al. (1997) came up with their respective publications, boosting the (e.g., more languages, including paper-based documents) would have research on Ecosystem Services during more than two decades (Costanza surely resulted in a significant increase of the reported ES. A compre­ et al., 2017). Since then, the economic value of world ecosystem services hensive understanding of the ES provided by SGD should thus try to was initially estimated to represent trillions of USD (1012) (Costanza incorporate this locally-based knowledge, and this can only be attained et al., 1997; Pimm, 1997) and researchers continue to look for new by conducting local investigations that focus on social and cultural techniques and categorizations to refine these evaluations (Costanza perceptions and local people's experience and knowledge. Further et al., 2017). However, there are ES that can hardly be economically research should thus attempt to incorporate this grey literature, dealing valued, such as the supporting or cultural ES. Therefore, alternative with the challenge of tackling different languages in which most of the frameworks that can integrate the non-monetary values of the SGD-ES information is provided. In this regard, engaging citizens and commu­ (TEEB, 2012) might be more accurate to study these services and are nities (citizen science) to inform about these SGD-social links can deci­ being applied worldwide. sively contribute to produce a comprehensive understanding of social Ecosystem Services have already been valued in seagrass meadows, implications of SGD. salt marshes, mangroves and coastal fisheries (Himes-Cornell et al., 2018; Tuya et al., 2014). In the SGD literature, most studies normally 5.2. A global change perspective evidenced the importance of SGD indirectly relating it to the fisheries or economical activities that surrounded the discharging areas. However, This study provides, for the first time, an overview and classification to our knowledge, SGD has only been valued in two studies, where the 10 A. Alorda-Kleinglass et al. Earth-Science Reviews 221 (2021) 103742 authors estimated the economic value of i) fresh SGD for human con­ fellowship (RYC-2015-17676). V. Rodellas acknowledges financial sumption and SGD-dependent commercial algae species in Hawaii support from the Beatriu de Pinós postdoctoral program of the Gen­ (Duarte et al., 2010; Pongkijvorasin et al., 2010) and ii) fish supported eralitat de Catalunya autonomous government (2019-BP-00241). M. by SGD-driven nutrient inputs in Japan (Burnett et al., 2018, 2015). Diego-Feliu acknowledges the economic support from the FI-2017 fel­ Although there remains great ambiguity on which is the value of SGD-ES lowships of the Generalitat de Catalunya autonomous government (Burnett et al., 2018, 2015; Duarte et al., 2010; Pongkijvorasin et al., (2017FI_B_00365). The authors would also like to acknowledge the help 2010), this study offers an opportunity for natural and social scientists to and financial support provided by the Foundation Iniciatives del Medi­ work together and develop monetary and non-monetary approaches to terrani. Also, the authors deeply appreciate the comments and revisions estimate SGD-ES values which results can support and guide future done by Dr. J. Tamborski and Professor S. Rossi. Finally, the authors management and policies for the evaluation and preservation of thank the proofreading by Ruby and Tania Easton. Ecosystem Services supported by Submarine Groundwater Discharge. Appendix A. Supplementary data 6. Conclusions Supplementary data to this article can be found online at https://doi. This study reviews the existing knowledge on the social implications org/10.1016/j.earscirev.2021.103742. of Submarine Groundwater Discharge (SGD) from an ecosystem services perspective. It also offers for the first time a conceptual and analytical References framework to identify and classify the ecosystem services provided by SGD and their effects on local societies' well-being based on the Mil­ Abaya, L.M., Wiegner, T.N., Beets, J.P., Colbert, S.L., Carlson, M., Kramer, K.L., 2018. 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