Groundwater impacts on stream biodiversity and communities: a review

Abstract Groundwater discharge into streams influences the biodiversity and health of groundwater-dependent stream ecosystems. These localized upwelling zones may act as biodiversity hotspots, or areas with a heightened amount of endemic species richness and abundance when compared to the surrounding locality. This input water creates environments with unique chemical compositions and water temperatures that serve as ideal habitat for various species within the ecosystem. Although difficult to identify and sample, these underwater groundwater–surface water interaction zones are important for fish spawning, benthic macroinvertebrate biodiversity, microbial communities, and aquatic and riparian vegetation. In this review, we highlight the groundwater characteristics that influence stream biodiversity and community structure. We argue for the importance of increased research on biodiversity indicators of groundwater upwelling zones as well as more public involvement through citizen science practices on the indirect and direct relationships between groundwater and dependent stream ecosystems. KEY POLICY HIGHLIGHTS The unique characteristics that differentiate groundwater from surface water in a stream environment can be ideal conditions for fish, benthic macroinvertebrates, vegetation, and bacteria. Groundwater upwelling zones act as biodiversity hotspots that may have a key role in the overall health of a stream community. Research that revolves around groundwater-stream interactions is limited and difficult to accomplish, with no standardized hydrologic or ecological methods of measurement. It is crucial to not only make measurement collection practices and data interpretation commonplace but increase awareness of the relationship between groundwater-stream interactions and biodiversity hotspots.


Introduction
Streams are groundwater-dependent ecosystems (Hare et al. 2021).Groundwater discharges to these surface water features through upwelling zones, which are composed of diffuse seeps and preferential discharge, often making up a considerable proportion of the stream water.Fish, benthic macroinvertebrate, microbial, and vegetation species depend on these groundwater-stream interaction zones, yet there is often a disconnect between research on the ecological and the hydrological processes that occurs in these localized stream habitats.An understanding of both the physical and biological factors controlling these biodiversity hotspots is crucial for evaluating the health of groundwater-dependent ecosystems.
The biodiversity of an ecosystem correlates to the area's health and population stability in the face of drastic changes like climate change, invasive species, or habitat fragmentation (Wilson 1988).Biodiversity hotspots are areas with a heightened endemic species richness and abundance when compared to the surrounding locality.(Myers et al. 2000;Araujo 2002).These pockets are often a driving focus for conservation efforts due to their ecological significance and current lack of conservation resources (Jepson and Canney 2001).Hotspots can be defined in numerous ways, including increased species richness (Reid 1998;Myers et al. 2000), community structure (McNeely et al. 1990), or richness of rare (Dobson et al. 1997) or taxonomically unusual species (Flather et al. 1998).The boundary of a biodiversity hotspot can be determined by biological commonalities (Myers et al. 2000) or physical characteristics that determine the limiting factors of the ecological island.We argue that the presence of high concentrations of groundwater seepage in groundwater-dependent streams may mark ecologically significant stream hotspots.
Extensive hydrologic research has been conducted to attempt to characterize the many ways in which groundwater interacts with streams (e.g.Constantz 1998;Malard et al. 2002;Boulton and Hancock 2006;Boulton et al. 2010;Marmonier et al. 2010;Briggs et al. 2014;Lewandowski et al. 2019;Hare et al. 2021;DelVecchia et al. 2022;Feng et al. 2022;Stewart et al. 2022), but this exchange can be difficult to identify and sample.Much of the literature focuses on the hyporheic zone, or the region of sediment and porous space beneath and alongside a stream bed, where groundwater and surface water mix (Mugnai et al. 2015).Because hyporheic processes interact across a range of spatiotemporal scales, researchers still know little about the complex interactions and feedbacks (Ward 2016; Magliozzi et al. 2018).Here, we argue that groundwater upwelling zones from long groundwater flow paths that exchange groundwater from beyond the hyporheic zone are equally as complex and potentially even more understudied.In this review, we (1) give context to how groundwater is different from and impacts surface water systems, (2) draw attention to literature on groundwater-stream interactions that extend beyond the hyporheic zone, (3) synthesize physical, chemical, and biological processes that make these groundwater-stream interaction zones important for stream ecology, and (4) argue for more interdisciplinary research on these biodiversity hotspots.

What is groundwater?
The physical characteristics of groundwater depend on the organization of subsurface geology and the chemical interaction between water and subterranean rocks (van der Kamp 1995;de Vries and Simmers 2002).The flow, temperature, and chemical composition of groundwater are determined by the hydrogeologic processes during recharge and flow below the land surface (Freeze and Cherry 1979).Discharge of groundwater to streams can therefore influence the physical and chemical conditions of surface water and influence the stream ecosystem function.
The groundwater flow system begins when meteoric water infiltrates permeable localized or regional recharge zones.Flow from these various sources converges and mixes to form the composite groundwater found within the transmissive layers of the subsurface, that is, aquifers.The shape of the water table (top of the groundwater) often is a subdued replica of the land surface-highest in the hills and sloping toward depressions and valleys.
Along groundwater flow paths, the chemical composition of the water is influenced by hydrogeologic conditions and physical processes.Groundwater recharge can influence the water's chemical signature through airborne (Schroeder et al. 2021) or surface contamination (Pitt et al. 1999;Li et al. 2021) or contact with plant material (Ploum et al. 2020).Evaporation and transpiration of soil water increase the concentration of dissolved solids (Deverel and Fujii 1988).Enrichment of carbon (Mazariegos et al. 2017) and oxidation of minerals (Lakshmanan and Kannan 2007) occur as water percolates from the ground surface to the water table, or the top of the saturated zone.Depending on the reactiveness of the materials along the flow path, the water chemistry may further change as the water is transmitted through aquifers.Ion exchange (Priyadarshanee et al. 2022), redox reactions (Mcmahon et al. 2011), and dissolution of soluble minerals, like the dissolution of carbonates in karst landscapes (Díaz-Puga et al. 2016), also leave behind chemical signatures.These chemical characteristics, as well as thermal stability, distinguish groundwater from surface water counterparts.
Depending on the length of flow paths, groundwater may reach equilibrium with the subsurface temperature.Groundwater gains or loses heat by conduction, chemical reactions, and friction as it moves along the groundwater flow path (Burns et al. 2016).Although the shallow subsurface can warm or cool in a muted pattern controlled primarily by air temperature, the deep subsurface (greater than 10 m below ground level) remains relatively constant.Groundwater that discharges to the surface, therefore, is often closer to the stable ground temperature than the surface air temperature.This thermal difference is a major draw for stream species seeking thermal refuge and maximizing growth rate, spawning, and overall safety during extreme temperature conditions above or below a species' thermal tolerances (Ojo et al. 2012).

How does groundwater interact with streams?
Water exchange with the surface occurs when porous aquifers outcrop with the land surface.This exchange between surface water and groundwater occurs within the hyporheic zone (Feng et al. 2022).When the water pressure in the aquifer exceeds the surface pressure, groundwater discharges to the surface.Therefore, rivers commonly receive groundwater flow that originates from further upland due to the pressure gradient.The discharge can be diffuse seepage to springs, streams, and lakes; or the groundwater discharge may flow through preferential pathways, such as through highly conductive sand or gravel substrate or through fractured rock.This discharge of groundwater into streams through hyporheic exchange is also largely influenced by preexisting geomorphic structures within the body of water (Feng et al. 2022).This groundwater then serves as the stream baseflow, or the portion of the streamflow that is sustained between precipitation events.This constant feed of water is the dominant control on the amount of water, chemical composition, and temperature of rivers, especially in humid climates.
Groundwater contributes to most streams, even if inflow is limited to specific seasons.The amount of water that groundwater contributes to streams can be estimated from stream hydrographs.The groundwater component of stream flow can range from 10 to 90% of the stream discharge (Haith and Shoemaker 1987) based on landscape and underlying rock type (Foks et al. 2019).Although the quality and quantity of groundwater can fluctuate, many groundwater discharge zones can remain relatively constant in comparison to the short-term variation in the surface system.Recession analysis is the classical method for determining groundwater contributions to streamflow, yet the effect of regional factors (e.g.geomorphology, climate, and soil hydraulics), local factors (e.g.soil moisture and evapotranspiration), and seasonal factors (e.g.seasonal storage and time-lagged evapotranspiration) make calculations uncertain (Haith and Shoemaker 1987;Famiglietti 2014;McNutt 2014;Miller et al. 2016).With changes in climate and human use of water, baseflow may become an even more important sustaining source of flow in drought conditions, pressuring water managers to see groundwater and surface water as a single resource (Famiglietti 2014;McNutt 2014;Miller et al. 2016).

How does groundwater chemically influence streams?
In addition to flow, the discharge of groundwater into a stream affects surface-water quality and temperature.Stream water is a mixture of source waters from (1) distinct flow paths at different subsurface depths and (2) meteoric water.As explained above, the shallow and deep, as well as long and short, flow paths carry distinct water chemistry.Deep groundwater is often rich in base cations (Shand et al. 2005), while groundwater from shallow soils is typically rich in organic materials (Neal et al. 2012;Stewart et al. 2022).These chemical differences create localized physical conditions that can be either hazardous or beneficial for aquatic life, impacting fish and benthic macroinvertebrate feeding and digestion patterns and overall survival throughout all life stages (Camargo et al. 2005).Chemical compounds easily mobilized by groundwater flow, such as nitrogen-and phosphorus-rich nutrients, are often sourced from groundwater upwelling zones (Camargo et al. 2005).Although necessary to support the growth of aquatic plants and algae, nitrogen and phosphorus can lead to eutrophication if discharged in excessive amounts (Holman et al. 2008).These impacts can lead to population declines of several aquatic species that typically live in stream environments (Camargo et al. 2005;Conceição et al. 2012).
Stream temperature is a primary water-quality parameter because it directly influences the nutrient cycling, productivity, and metabolic rates and physiology of aquatic species.Stream temperature is controlled by a combination of natural variables, such as solar radiation, air temperature, precipitation, surface water inflow, ground temperature, and groundwater inflow (Sinokrot and Stefan 1993).Diurnal and annual stream temperature fluctuations are expected but elevated water temperature from human-caused land-use practices is reason for concern.Temperature changes in groundwater are minimal in most regions, and therefore groundwater discharge to streams can reduce variation in stream temperature.Locations of groundwater inflow often act as thermal refugia, protecting aquatic species from extreme thermal disturbances (Torgersen et al. 1999;Dole-Olivier 2011).

Fish biodiversity hotspots
Groundwater seepage influences fish behavior, migration, and species distribution (Figure 1).The presence of groundwater in streams is essential for habitual fish migration and reductions to groundwater influx within a stream can be detrimental to fish activity and function (Power et al. 1999;Stelzer et al. 2022).For example, in the arctic, streams can freeze completely or be reduced to scattered groundwater-fed reaches or deep pools suitable for overwintering fish.To survive in such places, Arctic Grayling (Thymallus arcticus), may make long migrations to overwintering habitats that are almost always associated with groundwater inflows (West et al. 1992).This fish migration is precarious and very dependent on the height of the water table, and therefore groundwater discharge to the stream.Low flows in dry autumns can lead to high mortality of the fish population.Furthermore, experimental studies have explored the impacts that groundwater reduction in a stream can have on the surrounding ecosystems (Stelzer et al. 2022).Groundwater reduction was the ultimate cause for the decrease in the density and biomass of Brown Trout (Salmo trutta) and total fish in experiments.A change in groundwater input to a stream can also impact food availability for Brown Trout (Salmo trutta), as more stable thermal conditions provided by groundwater allow for an increase in abundance of various benthic macroinvertebrate species (French et al. 2014).Decreased flow and water velocity lead to increased deposition of fine sediment (Buendia et al. 2013).The decreased water depth and the lack of available coarse substrate in the streambed made the stream habitat unsuitable for a variety of fish taxa (Stelzer et al. 2022), inducing species distribution shifts.With decreased groundwater inflow, the stream's low flow can also expose in-stream barriers (e.g.large woody material) that further obstruct fish migration (Walker 1985).
As ectotherms, the physiology of fish is greatly affected by temperature.The thermal stability of groundwater makes springs or upwelling groundwater zones desirable habitats for fish.Research conducted in Shenandoah National Park concluded that thermal stability proved critical to Brook Trout (Salvelinus fontinalis) populations in headwater streams (Snyder et al. 2015).Similarly, Atlantic Salmon (Salmo salar) and Brown Trout (Salmo trutta) also use groundwater to their advantage during the cooler seasons by burrowing in the substrate during the day to keep warm due to the temperature stability of groundwater (Power et al. 1999).Warmwater stream fish, such as those in the Missouri Ozarks, may also concentrate in groundwater discharge zones during coldwater periods (Peterson and Rabeni 1996).The stable thermal conditions of groundwater are optimal for many fish species as they provide a controlled temperature environment.Fish commonly gravitate towards these points of discharge when surface water temperatures exceed various species' thermal tolerances-whether water temperatures are too high in warm seasons or too low in cold seasons (Torgersen et al. 1999).
Many studies have discovered a relationship between spawning patterns and the outputs of groundwater into streams (Garrett et al. 1998;Brabrand et al. 2002;Stelzer et al. 2022).Groundwater characteristics are hypothesized to be advantageous for embryo development due to the stable and favorable temperature conditions as well as increased water exchange rate past the embryos.Extensive research on the preferential use of spring or groundwater upwelling sites for spawning has been reported for Bull Trout (Salvelinus confluentus) (Baxter and McPhail 1999), Sockeye Salmon (Oncorhynchus nerka) (Gendaszek and Sheibley 2021), Pink Salmon (Oncorhynchus gorbuscha) (Weber Scannell 1992), Rainbow Trout (Oncorhynchus mykiss) (Garrett et al. 1998), Brook Trout (Salvelinus fontinalis) (Curry et al. 1994).In many of these studies, fish populations show increased hatch or survival of fish embryos in stable flows provided by groundwater (Brewer 2013).

Benthic macroinvertebrate biodiversity hotspot
Benthic (meaning 'bottom-dwelling') macroinvertebrates are small aquatic animals and the aquatic larval stages of insects that are commonly used as indicators of the biological condition of water bodies.Certain taxa of benthic macroinvertebrates are more vulnerable to high levels of pollution, while others are incredibly tolerant to environmental stressors (Doughty 1994;Klemm et al. 2003).Since they spend all or most of their lives in water, are easy to collect, and differ in their tolerance to pollution, benthic macroinvertebrates are commonly used as reliable indicators of stream health and water chemistry (Slooff 1983).This common collection technique is often used in citizen science opportunities for volunteers.However common, it is important to note that this method of benthic macroinvertebrate collection is subject to scientific limitations due to increased errors in accurate identification and a lack of consistency in collection practices (Davies 2001).Although there is a lack of research on the relationship between benthic macroinvertebrates and groundwater, it is expected that zones of groundwater discharge establish niche habitats for benthic macroinvertebrates due to the stability of water temperature, water quality, and spatiotemporal refugia (Stanford and Ward 1993).
Temporal and spatial refugia can be key to maintaining species diversity and structuring biotic responses to extreme disturbances in frequently disturbed ecosystems (Winemiller et al. 2010).In streams, surface-flow permanence can be the primary requirement for benthic macroinvertebrate refugia (Erman and Erman 1995;Datry et al. 2007;Chester and Robson 2011;Walters and Post 2011).During droughts in prairie streams, for example, groundwater discharge from spring and hyporheic exchange remain saturated and were refugia to Chironomidae, Ceratopogonidae, Tipulidae, Oligochaeta, and a variety of microcrustaceans (Burk 2012;Burk and Kennedy 2013).Microcrustaceans, Amphipoda, and Isopoda often inhabitat upwelling groundwater sites (Marmonier et al. 2010), elevating population densities and richness (Datry et al. 2007).In another study, species such as water mites (Hydrachnidia), and small crustaceans were found living exclusively in groundwater discharge habitats (Różkowski and Dumnicka 2010).Other fauna, sometimes called hyporheos, must live a portion of or their entire life cycle in the hyporheic zone, such as meiofauna, groundwater specialists, and early instar insect larvae (Beracko and Revajová 2019).The unique aquatic biodiversity hotspots would be lost without the groundwater upwelling zones.
The thermal stability of springs and groundwater discharge can also provide refugia in temperate regions with alternating warm and cold seasons (Skalicky et al. 2017).Although benthic macroinvertebrates typically have a univoltine life cycle (i.e. one brood of offspring per year), the constant thermal regime of groundwater may allow for multi-voltinism (i.e.multiple broods per year) (de Moor 1989; Haidekker and Hering 2008;Heggenes et al. 2017;Skalicky et al. 2017).Water temperature is an important control on biological processes, such as embryonic development, larval growth, emergence, and metabolism (de Moor 1989; Heggenes et al. 2017;Beracko and Revajová 2019).Some studies have shown that zones of groundwater discharge may actually allow benthic macroinvertebrates to mature more rapidly (Timm and Martin 2015;Beracko and Revajová 2019).
Although it is much more commonplace to use benthic macroinvertebrates as water quality and pollution indicators, microbial populations can also rely on high-quality stream waters.A research study where both aquatic insects and microorganisms were collected in various stream conditions reflected a similar correlation between pollution and decreased abundance and biodiversity in these communities (Lear et al. 2009).The analysis of the abundance and biodiversity of both benthic macroinvertebrates and microorganisms as indicators of polluted stream waters and groundwater influx would greatly benefit the accuracy behind this method of research collection.

Microorganism biodiversity hotspot
Microorganisms are heavily involved in many of the biogeochemical processes occurring at zones of groundwater-surface water interaction in streams.Biogeochemical transformation of the chemical composition of the site of the groundwater influx is mostly caused by a change in redox conditions and the presence of microbial biofilms, comprised of viruses, bacteria, protozoa, and fungi, in streambed sediments (Knapp et al. 2018;Zhu et al. 2020;Jekatierynczuk-Rudczyk et al. 2021).These microbial transformations have implications for carbon processing, nutrient cycling, and contaminant mobility and therefore influence both macroinvertebrate and algal assemblages and may play a role in the productivity of riparian vegetation (Baerlocher and Murdoch 1989;Jones et al. 1995;Pusch et al. 1998).The resident microbial community structure, dependent on groundwater discharge, is a significant factor in the abundance and biodiversity of higher trophic levels.In addition, the structure of microbial communities may be a useful indicator of the effects of anthropogenic groundwater contamination (Feris et al. 2003).
The microbial activity at zones of groundwater discharge reintroduces the dissolved carbon into the food web (Krause et al. 2011).The upwelling of groundwater in streams often delivers substantial amounts of dissolved organic carbon, which can be immobilized by microorganisms attached to streambed sediments (Fiebig 1997).These organisms filter the organic material from the incoming water, increase their own biomass, and then serve as a food source for predatory fish and invertebrates (Boulton et al. 2010;Krause et al. 2011).
Despite their considerable influence, microbial activity in sediments of large rivers is poorly understood.Research demonstrates that the microbial community present at the groundwater--surface water interaction zone can be particularly complex, containing mixtures of microbial components found in both the river water and deeper groundwater, making the study of individual biogeochemical processes difficult (Zhu et al. 2020).

Aquatic vegetation biodiversity hotspots
Aquatic vegetation in stream systems is commonly referred to as macrophytes, with species like American Pondweed (Elodea canadensis), Water Crowfoot (Ranunculus aquatilis), and invasive Water Millfoil (Myriophyllum spicatum) being observed in these aquatic communities (Visser et al. 2013).Aquatic plants in stream systems play an important functional role in the surrounding environment by regulating steam flow and providing food and habitat (Medina 1996).Aquatic vegetation, just like terrestrial producers, is also a key component within the stream ecosystem food web (Milligan and Humphries 2010).Additionally, aquatic plants serve as pollutant filters in stream systems as they can accumulate high concentrations of contaminants within their root systems and stem structures (Wang et al. 2020).These essential aquatic plants are dependent upon groundwater seepage into streams.Areas of a stream where there is relatively low groundwater discharge have been observed to have lower species richness when compared to areas of the stream with significant amounts of local groundwater seepage (Bornette et al. 1998).Subterranean aquatic plants rely on consistent, clean groundwater seepage, and without that, there are fewer of these natural pollutant filters, and less ecosystem stability due to a lack of safe habitat and food resources for various dependent aquatic species (Milligan and Humphries 2010).These beneficial interconnected relationships between groundwater resources and aquatic vegetation are vital to maintaining the health of stream ecosystems.

Riparian vegetation biodiversity hotspots
Groundwater processes are an important influence on the health and abundance of riparian vegetation (Kuglerová et al. 2014;Ploum et al. 2021).Riparian zone vegetation is important because it acts as the middle ground between aquatic and terrestrial ecosystems (Riis et al. 2020).By comparing plant species richness between areas with groundwater discharge and areas without, a study found that riparian zones with groundwater discharge had 36-209% more species density than stream areas with no groundwater interaction (Jansson et al. 2007).Furthermore, areas near groundwater seepage points have been shown to nurture the growth of more pollution-resistant plant species, indicating greater levels of ecosystem stability (Jansson et al. 2007).Research also suggests a strong positive relationship between groundwater and the abundance of plant species in stream ecosystems.Thus, when groundwater is in decline, it is likely that reduced vegetation cover along stream banks will follow suit (Cey et al. 1998).
Riparian vegetation along a stream acts as a corridor between aquatic and terrestrial ecosystems.Due to this unique assemblage of various flora and fauna, these riparian corridors play a key role in the conservation and restoration of stream ecosystems (Naiman et al. 1993).The relationship between groundwater and surface-water is the main determinant in the conditions of riparian vegetation (Tabacchi et al. 1998).This corridor between aquatic and terrestrial environments also acts as both a carbon source and sink through the structure and natural function of streambank flora (Tabacchi et al. 1998).So, the protection of the groundwater sources that directly influence the riparian buffer can promote biodiversity in the surrounding environment.
Just as vegetation is influenced by groundwater, the quality of groundwater is also impacted by riparian vegetation.Riparian plants help filter out pollutants and chemicals in the water (Riis et al. 2020).The presence and abundance of riparian vegetation also help with sediment retention, sediment stability, and erosion control.Additionally, plants' ability to regulate stream flow and runoff benefits the quality of groundwater discharge and overall stream health (Abdul and Gillham 1989).One study showed that trees use groundwater of depths greater than 10 m below ground surface, and this dependent relationship of tree health relies heavily on the perenniality of the stream (Galeotti and McCullough 2008).The rate and quantity of groundwater seepage into a stream's surface waters can determine the stability of the stream (Hayashi and Rosenberry 2002).These are vital interconnected relationships within an ecosystem that are important to consider and be cognizant of when analyzing groundwater and surface water interactions in stream environments.

Groundwater contamination
Just as the presence of high concentrations of groundwater seepage in groundwater-dependent stream may mark ecologically significant stream hotspots, the presence of contaminated groundwater discharge may delineate ecologically threatened zones (Hancock 2002).Urban and agricultural settings contain a myriad of different activities commonly associated with groundwater contamination (Vroblesky et al. 1991;Conant et al. 2004;Roy and Bickerton 2012;Sonne et al. 2018).In regions of groundwater-surface water exchange, these contaminants can create groundwater plumes influencing the health of fish, macroinvertebrates, microbial stream communities.
Fish are susceptible to poisonous chemicals and toxins through polluted groundwater from anthropogenic activities (Thompson et al. 2021).Preservation of the quality of groundwater would directly benefit fish populations.Groundwater discharge zones can represent a unique exposure pathway of contaminants, including naturally produced phytoestrogens and commonly applied herbicides to stream ecosystems (Thompson et al. 2021).Because of the ecological importance of these upwelling habitats, groundwater discharge areas may represent an important exposure pathway for aquatic organisms that utilize these areas.Industrial contaminants, like trichloroethene, can enter streams through groundwater discharge, creating spatially discrete 'hotspots' at specific locations in the streambed with high concentrations and longer residence times (Weatherill et al. 2014).For example, an excess amount of nitrogen, often attributed to high levels of nitrate in groundwater (Spalding and Exner 1993), causes a decline in fish health and impairs their reproductive capabilities (Hrubec et al. 1996).Increased nitrate levels can be toxic to freshwater fish and can adversely impact a fish's metabolism (Camargo et al. 2005;Conceição et al. 2012).Nitrate levels in groundwater can be caused by abundant rainfall, elevated temperatures, heavy agricultural usage, and high contents of organic carbon in soil (Spalding and Exner 1993).
A decline in fish population and health can cause trophic cascades and lead to reduced levels of biodiversity in groundwater-dependent ecosystems (Holmlund and Hammer 1999).Fish aid in the regulation of the stream trophic structure through their consumption patterns (Holmlund and Hammer 1999).A fish's dynamic diet throughout different life stages influences a stream's entire food web.A decline in a stream's fish population can lead to consequences, such as changes in a stream's algal production and organic matter (Whiles et al. 2006).
The impacts of contaminated groundwater flow on macroinvertebrate communities have been documented in a few studies where stream water samples were collected in the proximity of a contaminated groundwater inflow zone (Plotkin and Ram 1984;Dickman and Rygiel 1996;Rasmussen et al. 2016).A study has reported a reduction of benthic macroinvertebrate density by over 50%, compared to upstream, uncontaminated sites, occurring at the polluted groundwater discharge zones (Rasmussen et al. 2016).Anthropogenic activities, such as gold mining, are harmful to benthic macroinvertebrates due to associated mercury exposure possible through groundwater flow.Mercury concentrations are orders of magnitude higher in stream sediments than in river water due to groundwater discharge (Dickman and Rygiel 1996).These highly contaminated habitats support sediment-dwelling benthic macroinvertebrates that develop high concentrations of mercury, beginning the bioaccumulation process through the aquatic trophic levels (Picado et al. 2010).More studies are needed to investigate the structural changes of sediment-dwelling invertebrate communities along streams that intersect with contaminated groundwater inflow.
In addition to nutrient transformation, microbial responses to anthropogenic contamination and pollution are an area of research interest.Microbially enhanced oxidation of dissolved metals can lead to net decreases in heavy metal load in watersheds (Harvey and Fuller 1998).However, heavy metal contamination due to acid mine drainage in groundwater has shown inverse correlations with total microbial biomass (Fuller and Harvey 2000).Flow rates, contaminant concentrations, and chemical reaction rates are important considerations to determine the impact of contamination in hyporheic exchange.It is uncertain how widespread contaminated groundwater-surface water interactions reduce instream biodiversity despite the potential of upwelling zones delineating biodiversity hotspots.

Call for research
Even though our knowledge of the complexity of stream biodiversity has improved over the past century, there are still more open questions than answers.To date, there have been few studies holistically examining locations of groundwater upwelling to streams as biodiversity hotspots (Araujo 2002;Marmonier et al. 2020).This research is extraordinarily challenging because this topic requires intimate knowledge of multiple disciplines, investigation across multiple spatiotemporal scales, and requires a full mechanistic understanding of the system.Nevertheless, there is a need for joint investigations and more thorough experimentation of indirect and direct relationships between groundwater and groundwater-dependent stream ecosystems.Streams with greater groundwater discharge have been associated with enhanced rates of river microbial activity and more complex, and possibly productive food webs, and are likely hotspots for the diversity and productivity of riparian plants and aquatic invertebrates.
Increased implementation of citizen science projects could greatly benefit research efforts on groundwater impacts on stream biodiversity.Citizen science is the engagement of scientific research in the community through public participation.Despite claims that public participation practice is wildly inaccurate and insufficient within the realm of scientific research, citizen science has made many contributions to the world of academia (Kobori et al. 2016;Elliott and Rosenberg 2019), and the significance and impact of citizen science have continued to grow as environmental stressors, like climate change, have become ever more prevalent in modern society (Kobori et al. 2016).Data obtained from groups of citizen scientists is a beneficial method to be used for collecting various hydrologic data (Lowry and Fienen 2013), and this public engagement helps bridge the knowledge and interest gaps between scientific researchers and the public.The more people who are aware and interested in understanding these important biodiversity hotspots, the more public interest will accrue, and resources will be prioritized to further research on groundwater-dependent stream ecosystems (Kobori et al. 2016).Collaborative efforts to study the hydrological, chemical, and biological processes at these biodiversity hotspots is imperative to protect a precious natural resource and the respective dependent ecosystems.

Figure 1 .
Figure 1.conceptual diagram of groundwater upwelling zones on biodiversity of streams.