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Open AccessGroundwater–Surface-Water Interactions

Groundwater ecosystem services: a review

Abstract

Our daily life depends on many services delivered by the planet’s ecosystems. Groundwater ecosystems deliver services that are of immense societal and economic value, such as: 1) purification of water and its storage in good quality for decades and centuries, 2) active biodegradation of anthropogenic contaminants and inactivation and elimination of pathogens, 3) nutrient recycling, and 4) mitigation of floods and droughts. Many of these services are directly connected to the presence and activity of specific organisms, microorganisms, or metazoa. Sustainable protection and management of important groundwater ecosystem services will require quantitative understanding of processes at different spatial and temporal scales and assessment of their resistance and resilience with regard to common anthropogenic impacts. Our review compiles known groundwater ecosystem services and, where appropriate, highlights important research gaps.

Ecosystem services are the benefits that people obtain from ecosystems for free and are used by humankind to guarantee and increase well-being (Daily 1997, MA 2005). The term ecosystem services emerged in the early 1980s (e.g., Ehrlich and Mooney 1983), but currently >2400 papers related to this topic are listed by the Institute for Scientific Information (ISI) Web of Science (Thomson Reuters, New York), and they have been cited >30,000 times (Costanza and Kubiszewski 2012). Moreover, both numbers are increasing exponentially.

The ecosystem-services framework relates ecosystem functions and environmental health to human health, security, and the material goods necessary for well-being (Brauman et al. 2007). As a result, ecosystem services is now an inter- and transdisciplinary field of research (Costanza and Kubiszewski 2012). During the last 3 decades, the framework has been evolving from purely conceptual toward solution-oriented and ready to support decision-making. Its broad acceptance by economists and politicians is related to the awareness that individual services have an enormous monetary value. A recent meta-analysis of 10 important biomes by de Groot et al. (2012) revealed that each year, the ecosystem services potentially provided by an average hectare of coral reefs can be worth up to 350,000 int$. (The international dollar [int$], a widely used currency in economics, is a hypothetical unit of currency that has the same purchasing power parity as the US dollar had in the USA at a given, fixed point in time.) Rivers and lakes ranked at ∼4000 and inland wetlands at ∼25,000 int$ ha–1y–1. Such large-scale estimates of ecosystem service monetary value are not yet available for groundwater ecosystems, and these ecosystems did not find consideration in the study by de Groot et al. (2012). Given that ground water is one of the most essential resources for the maintenance of human life, this assessment gap calls for attention. Ground water is a major source of drinking water worldwide, serves as a solvent and cooling agent for industrial use, and provides water for irrigation in agriculture. On a global scale, 20% of the irrigation water and 40% of the water used in industry are derived from ground water (MA 2005). Growing industrialization, waste deposition, and the exponentially increasing production and use of synthetic chemicals, which are often released into the environment, put groundwater resources under growing pressure. Today, groundwater quality is poor in many areas of the world (Danielopol et al. 2003). Moreover, groundwater resources are facing quantitative problems. Abstraction of ground water from many large aquifers worldwide significantly exceeds the natural renewal rate, as recently estimated by the groundwater footprint approach (Gleeson et al. 2012). This deficit poses a threat to the health of aquifers and their organisms and to many other groundwater-dependent ecosystems (GDEs), such as rivers and wetlands. During the recent past, the perception of ground water by stakeholders and authorities has been changing slowly from an exclusive focus on economic aspects to one that includes social and ecological aspects (Quevauviller 2005). Moreover, consensus exists that subsurface ecosystems deliver both important water resources and additional ecosystem services and goods that are essential to humankind and GDEs (Danielopol et al. 2004, EU-GWD 2006). These services and goods range from provision of a natural bioreactor for the clean-up of water until it reaches drinking-water quality to maintenance of hydraulic conductivity, drought attenuation and flood mitigation, and recreational and cultural services provided by hot springs (see Fig. 1 and specific sections below).

Figure 1. 
Figure 1. 

Groundwater ecosystem services. GDEs = groundwater dependent ecosystems.

Within the Millennium Ecosystem Assessment classification scheme (MA 2005), groundwater ecosystem services and goods partition into all 4 categories: 1) supporting services, 2) provisioning services, 3) regulating services, and 4) cultural services. The most frequently discussed services and goods related to these categories are shown in Fig. 2, with a special focus on those affiliated to groundwater ecosystems. For example, the presence of ground water itself is a supporting service because most terrestrial and surface aquatic ecosystems depend on its availability in good quality and sufficient quantity. Ground water is an important global source for drinking water and, thus, is a provisioning service. Regulating services include purification of water and, particularly, in situ biodegradation of contaminants and elimination of pathogenic microorganisms and viruses, which in turn, contributes to disease control. Last, cultural services include large water bodies in caves that constitute tourist attractions and hot springs that are used for recreation. In many places, ground water feeds springs that are of high spiritual importance, such as the sacred spring in the Grotto of Massabielle in Lourdes, France; the Chalice Well at Glastonbury, UK; sacred hot springs in Hierapolis, Turkey; the Ban Ban Springs, an Aboriginal Cultural Heritage site in Queensland; and similar sites in Australia (e.g., McDonald et al. 2005).

Figure 2. 
Figure 2. 

Selected examples of ecosystem services and goods sorted into the 4 categories of services as defined in the Millennium Ecosystem Assessment report (MA 2005). The goods and services that are directly related to groundwater ecosystems are highlighted in bold print. GW = groundwater, GDEs = groundwater dependent ecosystems.

The framework of ecosystem services is a powerful tool to raise awareness in human society of the various benefits we receive and use from ecosystems each day—an important prerequisite for their appreciation. However, the consequent next step of this awareness, i.e., appropriate protection and sustainable management of ecosystems, including their organismic repertoire and diverse set of functions, awaits efficient implementation. Moreover, the ambitious goal of routine implementation of the ecosystem services concept in water-regulation practice is not yet achieved (Carpenter et al. 2007). This situation also applies to groundwater ecosystem management and exploitation, particularly because the concept has become part of the scientific interests in this field only in the last few years, and its wide-spread use in current studies is just starting to develop. The objectives of our article are to review the various recognized services and goods provided by groundwater ecosystems and to identify specific, current knowledge gaps.

Ground water—an open system within the hydrological cycle

Groundwater systems are an important compartment of the hydrological cycle. About 30% of all freshwater is terrestrial ground water, whereas the world’s lotic (streams and rivers) and lentic (lakes) systems contribute only 0.3% (Danielopol et al. 2003). With respect to the available (liquid) fresh water, terrestrial ground water contributes 94%. Hydrological recharge of aquifers is geographically very variable and strongly dependent on climate, geology, soil type, vegetation, and land use, among other factors (Scanlon et al. 2002). Groundwater recharge via precipitation is complemented by naturally infiltrating surface water or by artificial recharge. In the opposite direction, ground water leaves the subsurface via springs and wetlands (sustaining groundwater-dependent ecosystems = supporting service, drought attenuation = regulating service), enters surface waters (provision of water and nutrient cycling = supporting services), or is being exploited for different types of usage, the latter directly contributing to the category of provisioning ecosystem services and goods. Moreover, individual functions may serve different service categories and different ecosystems at the same time.

A key feature of groundwater ecosystems is that they are open systems that are closely connected to other terrestrial and aquatic ecosystems, many of which depend on ground water in high quality and quantity—the so-called groundwater-dependent ecosystems (GDEs; Boulton 2005, Humphreys 2006, Murray et al. 2006). The boundaries and transition zones (ecotones) of aquifers are particularly important (Gibert et al. 1990) because they play a significant role in regulating the flux of matter and energy across systems. Ecotones are characterized by steep physicochemical and biological gradients, high activities, and high biological diversity (Naiman and Décamps 1997, Ward and Wiens 2001). From the perspective of ecosystem health, transition zones are significant biobarriers and filters for external impacts. Important ecotones are the hyporheic zone of streams and rivers (Gibert et al. 1990, Brunke and Gonser 1997, Boulton et al. 2010), the transition between the unsaturated and saturated zone (capillary fringe and groundwater table) (Madsen and Ghiorse 1993, Lahvis et al. 1999), and transitions between geological strata (e.g., unconsolidated sediments and rock, highly conductive sand layers and clay lenses; McMahon et al. 1992, Ulrich et al. 1998, Krumholz 2000).

When snowmelt and heavy precipitation cause floods, aquifers and wetlands act as sponges that receive and retard significant amounts of surface water (Postel and Carpenter 1997). Water also is purified (see water clean-up below) and is later available for irrigation purposes (provisioning service). Conversely, base flow or dry-weather flow in rivers and streams may be fed exclusively by ground water entering the stream channel (Allan 1995). Without this quantitatively significant contribution, many rivers and streams would be intermittent or ephemeral (supporting and regulating services). Last, the open corridor between surface waters and aquifers enables individual surface species to find temporary refuge in times of harsh conditions (e.g., floods and droughts, temperature elevations caused by anthropogenic impacts and climate warming; Hose et al. 2005, DiStefano et al. 2009).

Natural production and storage of drinking water

In Europe, 75% of the drinking water is produced from ground water, and on a global scale, ⅓ of the population uses ground water as their main source of drinking water (Sampat 2000), a provisioning service that frequently is taken for granted. Covered by active soil and sediment layers, ground water is often much better protected than surface water from the negative impacts of human activities. In some areas, a substantial portion of the drinking water is produced from bank filtration of surface waters that carry a diverse load of chemicals and pathogens (Tufenkji et al. 2002), thus making use of the regulating service of water purification.

Water from the tap, when stagnant in in-house distribution pipes or kept open to the atmosphere at room temperature, changes in quality within hours or days (Lautenschlager et al. 2010). In contrast, a healthy aquifer, like an active biofilter, keeps and further improves water quality and provides safe water storage for centuries. Drinking-water production uses this service and benefits directly from the integrity of groundwater ecosystems (Tufenkji et al. 2002). Moreover, sustainable management of the catchment area can be translated directly into economic values. For example, the 4 European cities Berlin, Munich (both Germany), Vienna (Austria), and Oslo (Norway), each supply their citizens (500,000–3.5 million people) with drinking water derived from ground water. The ground water originates from areas with sustainable management in which protective measures range from strict protection (Vienna) to partial conversion of areas from conventional to organic agriculture (Munich). The drinking water in Vienna and Munich is untreated ground water. In Oslo and Berlin, where ground water needs to be treated, the water works benefit from the very low contamination levels resulting from catchment protection. The annual economic benefit of natural water purification and provision ranges from 17 to 108 million €, which translates into an average reduction in the water bill by 45 € per capita or 200 € per household and year (ten Brink et al. 2011).

Sink and source of C and nutrients

Aquifers are open systems, so they may be a sink and source for C and nutrients. Water infiltrating into the subsurface is continuously depleted in C during the passage through soil and sediment (Tufenkji et al. 2002), and as a result, ground water is typically poor in dissolved organic C (DOC). Nevertheless, in sum, aquifers are fueled continuously by considerable amounts of organic C. In pristine aquifers, mineralization of organic C, in terms of respiration (O2 consumption) and biomass production (growth rates), is extremely low and at the lower range of sensitivity of the methods routinely used in freshwater microbiology (Kieft and Phelps 1997). However, facilitated by the huge volume of aquifers and the comparably long residence times of organic matter in the subsurface, groundwater ecosystems contribute significantly to the turnover of C and, hence, to the purification of water. At present, which organismic guilds are primarily responsible for the C turnover in aquifers is poorly understood. Nevertheless, evidence is increasing that stable, attached microbial communities play a much greater role than dynamic, low-activity suspended communities (Alfreider et al. 1997, Griebler et al. 2002, Wilhartitz et al. 2009, Flynn et al. 2013). Furthermore, seasonal hydrological dynamics are sometimes strong and may control bacterial activity (heterotrophic production), diversity (Zhou et al. 2012), and the distribution of microorganisms between the sediment and water phases (Griebler et al. 2002) in pristine groundwater ecosystems.

Subsurface metazoa also are involved in groundwater C cycling. However, their quantitative role is unclear. Only a few investigators have examined the indirect participation of groundwater invertebrates in natural attenuation through grazing on microbial biofilms (see natural attenuation below). Stimulation of C turnover via grazing by meio- and macrofauna on microorganisms, ingestion of microbially colonized sand, and bioturbation has been demonstrated repeatedly in surface aquatic systems and in sediment-column experiments (Mermillod-Blondin 2011, Griebler et al. 2014a).

Groundwater exfiltrating into surface waters and at springs generally is supersaturated with CO2 because of metabolic processes in the subsurface waters. However, in some cases, groundwater ecosystems can act as a source of C. Little attention has been paid to the concept of CO2 fixation and chemolithoautotrophic primary production in ground water. CO2 fixation (linked to photosynthesis) is one of the most important processes on the Earth’s surface (Berg 2011), but our current understanding of its occurrence and importance in groundwater ecosystems (i.e., without the participation of light) is poor (Kinkle and Kane 2000). Recently, a huge diversity and distribution of autotrophic bacteria and functional genes involved in CO2 fixation have been reported from pristine and contaminated aquifers (Alfreider et al. 2003, 2009, Kellermann et al. 2012). Microbial CO2-fixation potential may be of interest with respect to C sequestration and climate change, a putative regulating service.

Depending on the redox conditions and the availability of organic C, nutrients (such as N and P) may be effectively converted, retarded, or immobilized in ground water (e.g., Billen et al. 1991, Lewandowski and Nützmann 2010, Bouwman et al. 2013). This regulating service is used in artificial groundwater-recharge (Schmidt et al. 2012) or stormwater wetlands and infiltration ponds (Datry et al. 2004, Moore and Hunt 2012). However, ground water sometimes can be little more than a transport medium for nutrients, e.g., NO2, which is diluted but not transformed in oxic conditions and in the absence of organic matter. Ground water also can act as a source of P and Si for the surface. Surface vegetation in contact with ground water may directly benefit from the provision of nutrients, but an excessive supply to surface waters and wetlands can cause undesirable eutrophication with massive development of primary producers, e.g., in the form of algal blooms (Freeze and Cherry 1979, Hurley et al. 1985).

Natural attenuation of contaminants and pathogen elimination

Groundwater ecosystems provide biologically mediated transformation and degradation of contaminants (Tufenkji et al. 2002). To date, >75 million chemicals are registered (CAS Registry 2013), and several hundred thousands of these are traded daily on the world market. Accordingly, ground water is severely contaminated at millions of sites (EEA 2007). The list of priority contaminants includes chlorinated solvents, petroleum hydrocarbons, heavy metals, pesticides, and is complemented by chemicals of emerging concern, such as human and veterinary pharmaceuticals, personal care products, and food ingredients (e.g., artificial sweeteners) (Richardson and Ternes 2011). Fortunately, groundwater ecosystems have immense potential to naturally attenuate and aerobically, anaerobically, and syntrophically degrade a huge diversity of pollutants released into the underground (Díaz et al. 2013, Jeon and Madsen 2013, Gieg et al. 2014). Furthermore, microbes can develop new metabolic pathways and, thus, the potential to degrade emerging contaminants (van der Meer 2006, Kolvenbach et al. 2014). In many cases, natural attenuation processes in the subsurface are sufficient to prevent continuous distribution of contaminants over large areas (an important regulating service). Therefore, a steadily growing number of contaminated sites are being managed via monitored and enhanced natural attenuation, a direct economic use of this ecosystem service (Herman et al. 2001).

Pathogenic microorganisms and viruses compose another important class of contaminants. Pathogens regularly enter soils and aquifers via infiltration of surface waters that have received waste water or via seepage of precipitation water from areas where manure has been applied (e.g. Tufenkji et al. 2002, Lucena et al. 2006, Krauss and Griebler 2011, Sinreich et al. 2011). As is the case with organic contaminants, the subsurface systems bear a great potential for effective retardation, inactivation, and elimination of pathogens. Global change is likely to increase the pressure on terrestrial and aquatic environments, and therefore, water carrying pathogenic viruses (e.g., from sewage treatment discharge) probably will be spread into regions used for drinking-water production more often, e.g., by an increasing frequency of large floods (Alley 2001, Green et al. 2011). Therefore, the ecosystem service of pathogen elimination can be expected to gain importance in the future. However, our understanding of the mechanisms behind natural pathogen control and of its limits is still incomplete. Recurring epidemics caused by contaminated surface, ground, and drinking water (Krauss and Griebler 2011) underline the importance of shedding light on the ecology and fate of pathogens in aquatic ecosystems.

Efficient decay of pathogenic viruses takes place in the form of physical, chemical, and biological retardation and through inactivation and destruction. Environmental factors, such as sorption, irradiation (not relevant for groundwater systems), temperature, and ionic strength are currently regarded as the main factors affecting the survival and migration of pathogens in water (Krauss and Griebler 2011). Groundwater microorganisms also can contribute to pathogen elimination. Hirsch and Rades-Rohkohl (1983) showed that >20% of 217 pure bacterial strains isolated from ground water inhibited the growth of Escherichia coli strain K 12. Individual pathogenic bacteria, i.e., E. coli and Vibrio cholerae, which can proliferate in natural waters in the presence of sufficient DOC, did not grow well or were suppressed in the presence of ambient microbial communities (Vital et al. 2007, 2008). Bacteria can inactivate and destroy viruses via exoenzymes (proteases, nucleases) and make use of them as growth substrates after lysis (Cliver and Herrmann 1972, Gerba 1983, Lipson and Stotzky 1985, Nasser et al. 2002). Strong evidence exists that grazing by nanoflagellates may contribute to viral decay (Manage et al. 2002, Bettarel et al. 2005, Deng et al. 2013).

Studies of the potential role of groundwater metazoa in contaminant and pathogen removal are limited. Sinton (1984) demonstrated an essential contribution of invertebrates in C turnover in a sewage-polluted aquifer. The presence of E. coli and coliform bacteria in the guts of isopods provided strong evidence that the isopods had consumed organic material that originated directly or indirectly from the sewage discharge and that the isopods had eliminated a considerable number of pathogens via their feeding activity. Based on the invertebrate standing crop, Sinton (1984) suggested that isopods assimilated 20% of the discharged C (given as calorific equivalents), and based on invertebrate density, total ingestion by invertebrates was estimated as 100 to 200 tonnes of C/y for the contaminated aquifer. This C was converted to animal biomass and CO2 (Fenwick 1998). Thus, in some cases, groundwater invertebrates may play an important role in removing organic contaminants from aquifers. Unfortunately, similar data sets from other sites are missing. In contrast, a high organic load to the subsurface is typically accompanied by a fast depletion of O2 and a switch to anoxic conditions, which would prevent active contribution and survival of invertebrates (Malard and Hervant 1999).

Maintenance of hydraulic conductivity in aquifers

Another ecosystem service attributed to groundwater metazoa, especially invertebrates, is maintenance of hydraulic conductivity in porous sediments through their feeding on microbial biofilms and bioturbation (Eder 1982, Danielopol 1989, Nogaro et al. 2006, Song et al. 2007, Mermillod-Blondin 2011). In the authors’ personal opinion, this service is restricted to energetically favorable and densely colonized areas, such as the hyporheic zone of streams and rivers, shores of lakes, and bottom sediments in artificial groundwater-recharge ponds. Here, the influx of organic matter in dissolved and particulate form sustains the formation of pronounced biofilms and large populations of invertebrates. Burrowing fauna positively affect sediment permeability in sediment-column experiments (Eder 1982, Nogaro et al. 2006) and in gravel and sand filters used in artificial groundwater recharge (Rumm 1993, see also Mermillod-Blondin 2011). Even low invertebrate densities may significantly enhance the permeability of fine sand by creating burrows that constitute preferential flow paths (C. Stumpp [Helmholtz Zentrum München] and G. C. Hose [Macquarie University], personal communication). Ward et al. (1998) used the term ecosystem engineers (Jones et al. 1994) when describing the ecological function of invertebrates in aquifers. However, given the generally low densities of invertebrates in most pristine groundwater systems, this role awaits further evaluation.

Boulton et al. (2008) carried out an initial study in which they emphasized the ecological role of groundwater fauna at well characterized alluvial aquifers in Australia and New Zealand. They categorized important ecosystem service providers (ESPs) among the invertebrates found in riverbed sediments and alluvial aquifers and proposed a conceptual model of the contribution of individual groups of invertebrates to 2 ecosystem services (biogeochemical filtration and particulate organic matter [POM] breakdown). In this conceptual model, Boulton et al. (2008) assigned a significant role in C turnover to stygobitic invertebrates, particularly amphipods and isopods. However, they also pointed out several gaps in understanding of the functional importance of most stygofauna and highlighted the need for further research in this area.

Bioindication and Biomonitoring

Groundwater organisms live in an energy-limited habitat with comparably predictable environmental conditions. Thus, they may be very sensitive to anthropogenic impacts and environmental changes. This sensitivity would make them potential candidates as bioindicators that could provide decision makers and groundwater managers with useful information on ecosystem status (Griebler et al. 2010), an important cultural ecosystem service (Fig. 2). The significant economic and environmental value of aquifers and ground water makes detailed understanding and monitoring of the behavior and status of these ecosystems crucial. However, to date, ground water and aquifers have been perceived mainly from a resource-oriented, economic perspective (Danielopol et al. 2004). In Europe, this attitude started to change with the December 2006 release of the European Groundwater Directive (EU-GWD), which mandates at a political level that ground water is more than just a resource and aquifers are more than just drinking-water reservoirs, and that both are also unique habitats. Ecological assessment of ecosystem status is done routinely for surface waters. Metazoa (invertebrates and fishes), macrophytes, phytoplankton, and diatoms are used frequently as bioindicators. Groundwater ecosystems lack algae and higher plants, but native invertebrate communities harbor sensitive sentinels available for an ecologically oriented assessment (Notenboom et al. 1995, Malard et al. 1996, Mösslacher 1998, 2000, Dumas et al. 2001, Hahn 2006, Schmidt et al. 2007, Bork et al. 2009, Brielmann et al. 2009, Stein et al. 2010) and biomonitoring (Moesslacher et al. 2001, Marmonier et al. 2013). However, invertebrate densities are low and many ground waters are naturally anoxic, so ubiquitous microorganisms and microbially related variables, such as total cell number, ATP concentration, and specific activities, may be promising bioindicators and ecological criteria (Claret 1998, Feris et al. 2009, Pronk et al. 2009, Griebler et al. 2010, Stein et al. 2010). Several attempts are in progress to develop an assessment scheme for groundwater systems comparable to the ones routinely used for surface-water ecosystems (Steube et al. 2009, Korbel and Hose 2011, Griebler et al. 2014b).

Biodiversity

Ground water and aquifers are habitats for diverse microbial communities (Ghiorse and Wilson 1988, Hirsch et al. 1992, Madsen and Ghiorse 1993, Novarino et al. 1997, Goldscheider et al. 2006, Griebler and Lueders 2009) and metazoan fauna (Marmonier et al. 1993, Danielopol and Pospisil 2000, Culver and Pipan 2009, Deharveng et al. 2009). The diversity and activity of groundwater organisms is linked directly to the provision of individual ecosystem services. Therefore, groundwater quality depends on biological activity and ecosystem health. Moreover, the unique organisms found only in ground water (see below) have high existence and bequest value. Bequest value, in economics, is defined as ‘the willingness to pay for the satisfaction derived from endowing future generations with a natural environment’ (Greenley et al. 1981), i.e., the value people may place on knowing that a resource exists even if they never use it directly and the value derived from preserving the option to use a service in future (that may not be used at present) by others or by future generations (MA 2005).

Groundwater metazoa (stygofauna) have been investigated for >100 y. Groundwater ecosystems harbor a vast diversity of living fossils and endemic species (Marmonier et al. 1993, Danielopol and Pospisil 2000, Ferreira et al. 2007, Humphreys 2008, Gibert and Culver 2009, Malard et al. 2009). Moreover, as indicated by the cumulative richness curves of stygobitic species that have been reported in various studies (e.g. Deharveng et al. 2009), a major part of groundwater metazoan diversity still awaits discovery.

In contrast, groundwater microbiology and microbial ecology have much shorter histories. These disciplines have their early roots in the middle of the 20th century. This research was triggered by industrial activities and hygienic aspects of the extraction of oil and water (e.g., Leenheer et al. 1976, Godsy and Ehrlich 1978). From today’s standpoint, it is not surprising that diverse microbial communities were found. Recent studies show that microbes are present even hundreds and thousands of meters below our feet (Ghiorse 1997, Griebler and Lueders 2009).

With respect to ecosystem services, biological diversity can be seen from 2 perspectives. First, it stands for a certain repertoire of functions (provisioning service). Second, it represents the richness of species (supporting service), some of which may be rare and in need of protection (cultural service). Processes in individual habitats or ecosystems are a direct result of the functional-trait diversity within biotic communities (Griebler and Lueders 2009). Therefore, changes in diversity may lead to changes in ecosystem processes (Humbert and Dorigo 2005). Moreover, high biodiversity (although not in every case) is linked to functional stability and flexibility of ecosystems or habitats and, hence, stands for functional resilience (e.g., Girvan et al. 2005, Eisenhauer et al. 2012; reviewed by Loreau 2000, McCann 2000, Schwartz et al. 2000, Cottingham et al. 2001). The occurrence of microbes with a similar functional repertoire but slightly different niches (with respect to temperature or other biophysical and chemical conditions, substrate affinity, uptake and storage of nutrients, C degradation kinetics, growth rates) secures system functioning in the face of environmental dynamics and disturbances (e.g., the “insurance hypothesis”; Botton et al. 2006). Microbial species in aquifers may display functional redundancies, but like in other environments, each species also may possess very specialized catalytic abilities. One prominent example for the direct benefit of microbial diversity in aquifers is the enormous intrinsic potential for degradation of a variety of contaminants (Aamand et al. 1989, Haack and Bekins 2000, Röling and van Verseveld 2002, Griebler and Lueders 2009). Moreover, the subsurface may have in store an almost untapped reservoir of processes and biological compounds (e.g., enzymes, antibiotics) useful for novel biochemical and biotechnological applications (Pedersen 2000, Griebler et al. 2014a).

Geothermal energy usage

Subsurface heat and cold are increasingly important as sources of sustainable energy that has a comparably low impact in terms of CO2 emissions (Lund et al. 2011). Geothermal energy is abiotic and arises from the geophysical properties of the planet rather than as a result of biological activity. Therefore, this service should be considered more a system service than an ecosystem service per se. Nevertheless, the provision of geothermal energy is mentioned here because it depends on the availability of ground water and, as such, on the provisioning service of groundwater ecosystems. Furthermore, use of geothermal energy can directly affect groundwater biodiversity and other ecosystem services.

Depending on the depth below surface, the use of geothermal energy differs. Deep subsurface installations aim exclusively at extraction of heat, which is then converted to electricity in geothermal power plants or is used directly for heating purposes. In contrast, geothermal energy use from the shallow subsurface (<400 m depth) encompasses 3 strategies: 1) open-loop systems, 2) closed-loop systems, and 3) aquifer storage systems (Malin and Wilson 2000). Closed-loop systems influence aquifer ecosystems by seasonally decreasing and increasing average groundwater temperatures by only a few °K (Rybach and Sanner 2000, Sanner et al. 2003), but open-loop systems may produce extensive heat plumes several hundred meters in length, thus resulting in absolute increases in temperature ≥20°C (Brielmann et al. 2009). Furthermore, aquifer heat-storage systems may even exhibit temperatures of 30–90°C (Sanner 2004). These temperature alterations act on an ecosystem that would be (below a depth of 10–20 m) unaffected by seasonal temperature fluctuations under natural conditions and would maintain constant temperature conditions (Matthess 1994), e.g., between 8 and 14°C in Central Europe. Thus, apart from the clear benefits of using geothermal energy sources, altering the local thermal regime of aquifers and introducing seasonal temperature fluctuations could affect aquifer biogeochemical processes and consequently, water quality (Bonte et al. 2011, 2013a, b). Temperature is a key regulator of (micro-) biological activities and often is more important than other limiting factors, such as the availability of substrates or nutrients (Peters et al. 1987). Changes in temperature cause changes in aquifer hydro- and geochemistry and, thus, influence the thermodynamics of biogeochemical processes (Bonte et al. 2013a). Thus, increasing use of the subsurface as storage capacity for heat and cold is a putative danger because it is not clear how groundwater ecosystems are affected structurally and biologically by regularly induced temperature changes (Guimarães et al. 2010, Brielmann et al. 2011).

Mineral water and hot springs

Deep subsurface ground water delivers mineral water to human society which is then either bottled for daily consumption, applied in medical therapy, or enjoyed in natural hot springs and centers for recreation (spas). The idea that water has magical properties to heal and confer vitality has deep historical roots in sacred springs and wells that were seen as sources of spiritual knowledge and wisdom (Strang 2004). Research of this mainly cultural service is found in the social and economic science disciplines and is not further discussed here.

Discussion and conclusions

Groundwater ecology has become a modern subdiscipline of ecology and limnology, and has undergone several paradigm changes from the ‘living fossils’ tradition to a more holistic ecosystem research field that includes many socioeconomic aspects and that is slowly catching up with general aquatic and terrestrial ecology (Danielopol and Griebler 2008). However, compared to other disciplines, hypothesis-driven research is obviously lacking. Such research is needed to provide mechanistic explanations that may allow the predictions that are necessary for the protection and sustainable management of ecosystem integrity and services (Larned 2012, Griebler et al. 2014a). Much of the scientific work on groundwater systems in the past has been purely descriptive. The concept of ecosystem services supports the recent trend to move beyond describing observed structures, e.g., community composition of communities, to focus on the functions and individual processes that are connected to them. Unraveling the ecological principles will allow prediction of ecosystem functions and services in the face of short- and long-term disturbances, such as contaminant spills and global change, respectively. Such predictive tools are urgently needed by practitioners and policy makers to develop appropriate instruments for risk assessment and resource protection (Griebler et al. 2014a).

As a prerequisite, ecologists must conduct inter- and transdisciplinary work while studying groundwater ecosystems to fill still-open research gaps. Detailed information is lacking for important groundwater ecosystem properties, such as structural heterogeneity; ecosystem size, borders, and connectivity to neighboring systems; inflow and outflow of matter; and the spatial and temporal distribution of substrates (organic C and nutrients) (Larned 2012). Proper evaluation of these patterns will improve our qualitative and quantitative understanding of groundwater ecosystem services. However, filling these gaps will require the intimate involvement of expertise in the fields of geohydrology, geophysics, and geochemistry.

A currently almost untouched topic is groundwater–foodweb interactions, especially between micro- and macroorganisms, and their links to processes (C and nutrient cycling) and services (attenuation of contaminants; reviewed by Marmonier et al. 2012 for the hyporheic zone). Even in low numbers, invertebrates may significantly influence sediment permeability via burrowing activity and, thereby, affect the transport and distribution of matter, which is later transformed by microbes. Stygofauna may be a vector for the distribution of key microbes and functions. Within the microbial food web, the roles of grazers and viruses in shaping of communities and underlying processes, such as the turnover of C, still await evaluation. The general prediction that ongoing loss of biodiversity feeds back on ecosystem responses (Hooper et al. 2012) is likely to apply to groundwater ecosystems. Thus, future researchers will have to address interdependencies among increasing pressures (e.g., pollution, overexploitation, use of ground water for heating) and the loss of biodiversity, ecosystem reactions, and potential loss of ecosystem services. Without doubt, the ecosystem services scheme is a useful tool for raising awareness of the importance of groundwater ecosystems. Monetary values of ecosystem services, where these can be assessed in a meaningful way, can provide convincing arguments in public discourse, politics, and legislation.

The authors thank Colette Whitfield for language editing, Guest Editor Kathleen Rugel, and 2 anonymous referees for their valuable comments and suggestions, as well as editor Pamela Silver for the final editing of the manuscript. Colleagues in the field of groundwater ecology are acknowledged for ongoing critical and fruitful discussions. Financial support to Maria Avramov was granted by the German Federal Environmental Foundation in terms of a PhD scholarship and is also highly appreciated (DBU, grant 20009/005, 2009–2012).

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