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ORGANIC CARBON SUPPLY AND METABOLISM IN A SHALLOW GROUNDWATER ECOSYSTEM
Ecology
... These hotspots consist of high concentrations of sediment-associated organic matter in reduced zones that maintain microbial activity and contain elevated concentrations of highly reduced chemical species (Qafoku et al., 2009;Campbell et al., 2012). Together both surface derived and buried organic matter play a significant role influencing microbial activity and biogeochemistry, controlling metal/radionuclide mobility across the upper Colorado River basin (Baker et al., 2000;Janot et al., 2015). One such aquifer is the Rifle alluvial aquifer, a former U.S. Department of Energy uranium ore-processing site near the city of Rifle, CO. ...
... While we did not test lability of the DOC, correlations with cell (Spearman's ρ = 0.46, p < 0.05) and virus abundance (Spearman's ρ = 0.54, p < 0.05) strongly suggest that DOC was sufficiently bioavailable to stimulate microbial activity. Microbial activity in groundwater is often stimulated by inputs of DOC (Baker et al., 2000;Sobczak and Findlay, 2002;Findlay et al., 2003;Foulquier et al., 2011;Li et al., 2012). The presence of bioavailable DOC and available electron acceptors may thus provide sufficient energy for stimulation of microbial respiration. ...
... Interactions between organic carbon, cells, and viruses may be important in riparian aquifers across the upper Colorado River basin where buried organic matter plays an important role in mediating biogeochemical cycles and metal/radionuclide sequestration. Organic carbon availability can promote microbial activity (Baker et al., 2000;Sobczak and Findlay, 2002;Findlay et al., 2003;Foulquier et al., 2011;Li et al., 2012) and, in turn, virus production, which is reflected by elevated virus abundances. Thus, organic rich sediments may potentially represent regions of increased viral activity as a response to higher microbial metabolic activity. ...
Viruses are the most abundant biological entity on Earth and their interactions with microbial communities are recognized to influence microbial ecology and impact biogeochemical cycling in various ecosystems. While the factors that control the distribution of viruses in surface aquatic environments are well-characterized, the abundance and distribution of continental subsurface viruses with respect to microbial abundance and biogeochemical parameters have not yet been established. In order to begin to understand the factors governing virus distribution in subsurface environments, we assessed microbial cell and virus abundance in groundwater concurrent with groundwater chemistry in a uranium impacted alluvial aquifer adjoining the Colorado River near Rifle, CO. Virus abundance ranged from 8.0 × 10⁴ to 1.0 × 10⁶ mL⁻¹ and exceeded cell abundance in all samples (cell abundance ranged from 5.8 × 10⁴ to 6.1 × 10⁵ mL⁻¹). The virus to microbial cell ratio ranged from 1.1 to 8.1 and averaged 3.0 ± 1.6 with virus abundance most strongly correlated to cell abundance (Spearman's ρ = 0.73, p < 0.001). Both viruses and cells were positively correlated to dissolved organic carbon (DOC) with cells having a slightly stronger correlation (Spearman's ρ = 0.46, p < 0.05 and ρ = 0.54, p < 0.05; respectively). Groundwater uranium was also strongly correlated with DOC and virus and cell abundance (Spearman's ρ = 0.62, p < 0.05; ρ = 0.46, p < 0.05; and ρ = 0.50, p < 0.05; respectively). Together the data indicate that microbial cell and virus abundance are correlated to the geochemical conditions in the aquifer. As such local geochemical conditions likely control microbial host cell abundance which in turn controls viral abundance. Given the potential impacts of viral-mediated cell lysis such as liberation of labile organic matter from lysed cells and changes in microbial community structure, viral interactions with the microbiota should be considered in an effort to understand subsurface biogeochemical cycling and contaminant mobility.
... C dynamics within the hyporheic zone vary with space and time because of the extent of mixing of different source waters (Schindler and Krabbenhoft 1998, Battin 1999, Baker et al. 2000 and biogeochemical processes that occur in the flood plain and the hyporheic zone (Findlay et al. 1993, Jones et al. 1995a, Shibata et al. 2001. Mixing of different water sources is generally controlled by interactions between geomorphic and hydrologic settings of streams (Cardenas and Wilson 2006, Poole et al. 2006, Ward et al. 2012, Wondzell and Gooseff 2013, Hester et al. 2017. ...
... Some studies have shown that DOC can accumulate along anaerobic flowpaths, including both CH 4 and other compounds produced from buried organic matter Krabbenhoft 1998, Helton et al. 2015). The C fluxes from riparian and hillslope flow paths to streams are likely to vary seasonally (e.g., during snow melt in alpine streams ;Battin 1999), and C flushed from the shallow subsurface can stimulate hyporheic metabolism (Baker et al. 2000). Similarly, soil CO 2 produced from root respiration and oxidation of organic matter in the vadose zone can contribute DIC (Tsypin and Macpherson et al. 2012). ...
... The groundwater contains copious DOC [7][8][9], which varies the redox values, disturbing the nature of groundwater in provincial aquifers [10,11]. Studies [12,13] have inferred that the significant contribution of DOC to the local ecosystem is mainly governed by the transport of DOC from shallow groundwater. Further, DOC plays a key role in the geochemical variations in shallow/perched and deeper/regional groundwater in the aquifers [14,15] Bacterial activity in an ecosystem is fundamentally triggered by the availability of DOC from the degradation of plant matter [16,17]. ...
... Hydrogen ions, manganese and iron all respond to rising DOC-DO linear/hyperbolic performance [67]. Groundwater [13,68] with high concentrations of DOC and DO may originate in irrigated areas, which are a major source of bioavailable [69] (bioactive or biologically active) carbon. In contrast to the low bacterial activity elsewhere, the high DO in the research area indicates that oxygen is being used by microbes. ...
The quality of groundwater has been severely impacted by urbanization around coasts. The change in climate and land use patterns has deteriorated the quality and availability of groundwater. One of the main issues in contemporary groundwater quality research is dissolved organic carbon (DOC) in the water. The influence of DO, water level and water temperature on DOC in groundwater was identified in the current study by sampling 68 groundwater samples. The analytical results revealed that ~18% of total samples have DOC > 5 mg/L. The groundwater samples represented in the urban regions show high DOC. The samples with higher DOC correlated positively with dissolved inorganic ions, such as Ca, K, NO3, Fe and DO. Domestic wastewater, agricultural runoff and local geology all have an impact on the DOC of groundwater. Groundwater chemistry is shown to be controlled by both aerobic and anaerobic conditions based on the DOC’s interactions with other ions. The study interrelates various sources, such as land use, geology, water level and temperature, to the DOC in groundwater and infers that the levels are higher in shallow groundwater, predominantly around the built-up region followed by the agricultural region. The temperature changes enhance the DOC in groundwater due to the variation in microbial activity. The shallow water level with a lower temperature shows the maximum DOC. Apart from the sediment organic matter and microbes, the study also attributes land use pattern to the source of DOC in groundwater.
... We know little about FPOM transport in groundwater ecosystems, but FPOM is smaller than CPOM and is thus more likely to be transported throughout groundwater ecosystems. A substantial fraction (15%e34%) of DOM can be derived from the processing of FPOM and CPOM along transport pathways (Shen et al., 2015), whereas a significant proportion of DOM is also retained due to microbial consumption, adsorption to soils, and flocculation (Baker et al., 2000;Pabich et al., 2001;Simon et al., 2010;Mermillod-Blondin et al., 2015;Hofmann and Griebler, 2018;Hofmann et al., 2020). For example, deeper groundwater systems will generally receive less DOM than shallower systems due to differences in water transport times (Pabich et al., 2001;Simon et al., 2010). ...
... Hynes (1975) stated that ". in every respect the valley rules the stream", meaning that the terrestrial environment has a strong influence on surface streams, and groundwater ecosystems are no different. Land-cover and precipitation patterns play roles in shaping groundwater communities (Baker et al., 2000;Boulton et al., 2008;Español et al., 2017). Rainfall can increase dissolved oxygen levels and transport organic matter to aquifers, which can trigger changes in community composition (Datry et al., 2005;Dole-Olivier et al., 2009;Brankovits et al., 2017;Hofmann et al., 2020;Saccò et al., 2020a,b). ...
... The vertical fluid flow connecting surface habitats to the aquifers allows the transport of dissolved as well as particulate plant derived organic carbon into the groundwater [8][9][10][11]. Hence, insoluble plant polymers as well as partially degraded and soluble polymeric material can represent relevant carbon sources for heterotrophic microorganisms, thereby shaping the aquifer microbiomes [12][13][14]. The relative importance of surface inputs compared to lithoautotrophy and rock-derived organic carbon, and the impact of subsurface heterotrophy on the global carbon cycle, however, are largely unknown. ...
... In DNA-SIP, a separation of 13 C labelled (heavy) and unlabelled (light) DNA is possible by density gradient ultracentrifugation [22]. Traditionally, in DNA-SIP experiments the presence or absence of OTUs in the 13 C heavy fraction and the 12 C heavy fraction is evaluated to identify labelled microbial taxa [22], e.g. microbial taxa are deemed to be labelled when their DNA is present in the heavy fraction of 13 C samples but absent from the heavy fraction of 12 C samples. ...
Light driven primary production by plants is the main source of biomass in terrestrial ecosystems. But also in subsurface habitats like aquifers, life is fueled largely by this plant-derived biomass. Here, we investigate the degradation of plant-derived polysaccharides in a groundwater microbiome to identify the microbial key players involved, and compare them to those from soil of the groundwater recharge area. We quantified the activities of enzymes degrading the abundant plant polymers starch, cellulose and hemicellulose in oligotrophic groundwater samples, despite the low cell numbers present. Normalized to 16S rRNA gene copy numbers, these activities were only one order of magnitude lower than in soil. Stimulation of the groundwater microbiome with either starch or cellulose and hemicellulose led to changes of the enzymatic activity ratios, indicating autochthonous production of enzymes in response to the plant polymers. Furthermore, DNA stable isotope probing with ¹³C labelled plant polymers allowed us to identify microbes involved in the degradation of these compounds. In (hemi)cellulose microcosms, Bacteroidia and Candidatus Parcubacteria were active, while the active community in starch microcosms mostly comprised Candidatus Saccharibacteria, Cytophagia, and Actinobacteria. Not a single one of the active OTUs was also found to be labelled in soil microcosms. This indicates that the degradation of plant-derived polysaccharides in groundwater is driven by organisms completely distinct from those active in soil. The involvement of members of the candidate phyla Cand. Parcubacteria and Cand. Saccharibacteria, organisms known to be abundant in groundwater, in plant-derived organic matter degradation might strongly impact subsurface carbon cycling.
... Judd et al. 2006;Freixa et al. 2017), availability of inorganic nutrients and terminal electron acceptors (e.g. Baker et al. 2000;Ito et al. 2002), and temperature (e.g. Ylla et al. 2012Ylla et al. , 2014Mori et al. 2019). ...
Purpose
Hyporheic biofilms are the central site for biogeochemical cycling in streams and rivers. In view of global warming and increasing human pressures, this study aimed to compare the response of hyporheic biofilm biomass and activities from an unpolluted reference stream reach surrounded by forest with those from a stream reach exposed to agricultural and urban land use using a mesocosm experiment in which the water temperature and dissolved organic carbon (DOC) contents were manipulated.
Methods
Hyporheic sediments collected in the field from the two study reaches (i.e. reference and impacted) were incubated in the laboratory at two different temperatures (10 °C, 14 °C) and wetted with three types of synthetic water (control [C] – 0 mg L⁻¹; low DOC – 5 mg L⁻¹; high DOC – 30 mg L⁻¹) for four weeks. The responses of the hyporheic biofilms were measured weekly using structural (total protein content [TPC] as a proxy for biofilm biomass) and functional measures (electron transport system activity [ETSA] and community-level physiological profiling [CLPP]).
Results
The response of hyporheic biofilms to temperature changes and DOC enrichment was site-specific for all studied measures (TPC, ETSA and CLPP, including measured average well colour development [AWCD]). The addition of DOC to biofilms from the pristine stream reach significantly heightened the responses at 10 °C, a temperature within the normal environmental temperature ranges of the reference location, but not at 14 °C, which was here, a temperature outside normal environmental range. On the other hand, biofilms from the impacted stream reach exhibited increased responses following DOC enrichment under both temperature regimes, with a particularly pronounced response at 14 ºC, in this case, both experimental temperatures were within the normal environmental temperature ranges of the study locations.
Conclusion
Hyporheic biofilms were shown to be, like benthic biofilms, sensitive to temperature changes and organic enrichment, but their response to temperature changes and enrichment caused by climate change and/or other anthropogenic pressures (i.e. point and non-point pollution, removal of the riparian zone, hydromorphological modifications, etc.) was not simply linear but site-specific. The intensity of the response, characterized by increased activity and biomass production, appears to be constrained within the temperature ranges prevalent in the environment from which the biofilms originate. These findings emphasize the importance of site-specific considerations in predicting the impacts of climate change and anthropogenic pressures on these critical components of river and stream ecosystems.
... We have long been concerned about the occurrence of non-stygobiotic species in subterranean environments due to the impact these species can have on the availability of food resources and predator-prey dynamics [67]. The contribution of non-stygobiotic organisms to the carbon resource in groundwater has not been extensively studied until now [68]. While some non-stygobiotic copepod species can be predators [53], exerting relevant pressure on subterranean copepods, other species are herbivorous, feeding on algae, and may encounter death due to starvation in subterranean habitats. ...
Understanding trait selection factors is vital for decoding the processes shaping species’ assemblages. However, trait-based studies in freshwater crustacean copepod assemblages are scarce, especially in groundwater environments. We explored how environmental filtering influences functional traits in copepod assemblages across four freshwater habitats (an alluvial aquifer, a hyporheic zone, a stream benthic zone and a lake littoral) along a depth gradient. Each habitat had distinct environmental templates based on light, temperature and dissolved oxygen. We analysed 4898 individuals from 43 copepod species and examined 12 morphological and life history traits. The results revealed significant differences in copepod traits among habitats, notably in ovigerous female biomass, egg biomass and ovigerous female percentages. Furthermore, despite some statistical uncertainty, notable differences were also observed in the number of juveniles, male-to-female abundance ratios and overall biomass. No significant differences were observed in juvenile biomass, egg characteristics, body size dimorphism or juvenile-to-adult ratios among habitats. The trait variations offer insights into copepod-mediated ecosystem services, particularly carbon recycling. To gain a deeper understanding of copepod adaptations to environmental features and anthropogenic changes, future research should consider additional functional traits, such as locomotion and feeding habits.
... The potability of groundwater is directly linked to DOC concentrations, which influence water chemistry and microbial levels [47][48][49][50][51]. As a result, there have been several studies on the concentrations, sources, and diffusion of natural DOC in aquifers [52][53][54]; however, to date, few have focused on human health. DOC consists of the carbon part of dissolved organic matter (DOM), typically representing more than 90% of the whole organic carbon content in natural groundwater reserves [55]. ...
Groundwater pollution is a current issue that may result in considerable negative effects on human health and the ecological balance. In the present study, the authors evaluated pollutants in groundwater in Maricá Municipality, located on the east side of Rio de Janeiro state in Brazil. The evaluated parameters were temperature, pH, electrical conductivity, Eh, dissolved oxygen, chlorides, nitrates, dissolved organic carbon, total inorganic carbon, phosphates, and total and thermotolerant coliforms. Due to the large number of evaluated points, they were divided into zones according to the respective hydrographic basin. The local accelerated urbanization accompanying income from oil production has led to uncontrolled population growth and associated groundwater pollution. The results of the present study suggest that sewage pollution of Maricá groundwater is already a significant issue. The lack of investment in basic sanitation has led to an imbalance in the local groundwater reservoir. In certain locations of the municipality, dissolved organic carbon (DOC), nutrient, and bacteria concentrations increase and spread in the aquifers because of domestic waste disposal. As aquifers are the main source of freshwater for the residents, contamination of them represents a potential threat to local public health.
... The highest DOC concentrations were in Modern, unconfined groundwater, reflecting organic carbon input with recharge (Bakalowicz, 2003). Decreases in DOC along aquifer flow paths by ecological metabolism is a recognized process (e.g., Baker et al., 2000), although the kinetics of DOC attenuation are relatively rapid (Chapelle et al., 2016). These processes are reflected in the Edwards aquifer, where the DOC distribution is largely bimodal. ...
Karst aquifers are a vital groundwater resource globally, but features such as rapid recharge and conduit flow make them highly vulnerable to land‐surface contamination. We apply environmental age tracers to the south‐central Texas Edwards aquifer, a karst resource in a rapidly urbanizing and drought‐prone region, to assess vulnerability to land‐surface contamination and risks unique to karst aquifers. We show that vulnerability of Edwards aquifer groundwater follows similar spatial and depth patterns common to porous‐media type aquifers, despite complicated karst hydrogeologic features. Shallow and unconfined parts are more vulnerable to land‐surface contamination than the deeper and confined parts, although even the oldest groundwater is mixed with some recent recharge. When modeled age‐tracer results are coupled with other independent geochemical tracers of water‐rock interaction specific to karst settings, they can yield insight into residence time and associated vulnerability.
... Changes to carbon inputs, such as from sewage or stormwater (Foulquier et al., 2011), strongly influence microbial assemblages and ecosystem functions. Aquifer productivity, which can be measured by rates of heterotrophic metabolism and aerobic respiration and determined by CO 2 and CH 4 generation and the depletion of dissolved oxygen (Baker et al., 2000) is a valuable indicator of ecosystem functioning. ...
The concept of ecosystem health is now widely used to communicate the status or condition of a natural environment and is embedded in environmental policies globally. The concept has underpinned ecological assessments for decades but has only recently been applied to groundwater ecosystems. The aim of this chapter was to provide a critical review of current methods for monitoring and assessing the health of groundwater ecosystems, and a discussion of future directions to progress the understanding of ecosystem health, and the provision of ecosystem services in groundwaters.
The assessment of ecosystem health is frequently based on the measurement of a suite of indicators at a site, which should reflect the organization and function of the ecosystem, and the presence of stressors as an early warning indicator. For groundwaters, this should include characterization of biotic (microbial, invertebrate and vertebrate) and abiotic parameters, and specifically include the provision of functions that provide ecosystem services. A number of indicators and approaches have been developed for assessing the health of groundwater ecosystems. These include methods in the fields of community ecology, functional ecology, and ecotoxicology and recently integrated molecular approaches. A holistic approach is needed for managing groundwater ecosystems, and the same should be applied to monitoring ecosystem health. Approaches should be based on sound science, engagement with stakeholders, and consider the interconnected nature of groundwater and surface waters, with the goal to preserve the unique subterranean biodiversity.
... Its response to changing environments is also better understood in recent years (McDonough et al., 2020a, b). Groundwater DOM, consisting of tens of thousands of formulae (McDonough et al., 2020c(McDonough et al., , 2022, generally originates from the direct infiltration of surface DOM, plant litter, and soil (Baker et al., 2000;Kalbitz et al., 2000;Shen et al., 2015) and its environmental or microbial transformation (Donn and Barron, 2013) or water-rock interactions (Walter et al., 2017). Additionally, human activities (e.g., agriculture, urbanization) provide additional organic molecules to the DOM pools preserved in the aquifers, resulting in remarkable alterations in the composition, transformation, and fate of groundwater DOM (Wang et al., 2013). ...
Groundwater discharge and river runoff are two important ways for allochthonous dissolved organic matter (DOM) to enter the ocean, but they vary spatially. Currently, the source, composition, and efflux of groundwater and riverine dissolved organic matter to the tropical northern South China Sea remain poorly understood, preventing an accurate estimate of coastal carbon budgets. In July 2021, nine groundwater and six river water samples were collected along the coast of western Guangdong and were characterized using dissolved organic carbon (DOC) analysis, UV-visible, and fluorescence spectroscopy techniques. Atypical absorption spectra were strongly observed in groundwaters, which were mainly attributed to the high-level nitrates. An exponential-Gaussian equation showed that the average contribution of nitrate to total groundwater absorbances was up to 36% at ∼ 300 nm. Groundwater showed lower levels of DOC, colored dissolved organic matter (CDOM), and fluorescent dissolved organic matter (FDOM) as compared to river water, whereas the humic-like components dominated both groundwater and river water FDOM pools. Higher values of spectral slope S 350–400, fluorescence index, biological index, and the ratio of two humic-like fluorescence jointly reveal that groundwater DOM mainly originated from microbial activities, whereas the river water DOM had considerable contribution from terrestrial soil. High precipitation, natural or agricultural land types, and a lack of seawater intrusion are expected to cause low levels of groundwater DOM in western Guangdong. Among 52 rivers from across the globe, high watershed productivity and precipitation may lead to high–moderate DOM levels in river water in western Guangdong. The DOM fluxes via fresh groundwater discharge in western Guangdong are comparable in magnitude to those by river runoff, accounting for 16%–18% of the Pearl River DOM fluxes. Based on the slopes of linear correlations between CDOM and DOC obtained in groundwater samples and 52 rivers, as well as the data on water fluxes and DOC levels, the global CDOM fluxes via groundwater discharge and river runoff are estimated to be 3.5–12.2 × 10¹² m² yr⁻¹ and 4.3 ± 0.3 × 10¹⁴ m² yr⁻¹, respectively, highlighting the importance of considering groundwater discharge and river runoff in coastal carbon budgets.
... Although 95 % of liquid freshwater on Earth is comprised of groundwater, at the time of writing only 12 % of publications on freshwater DOM or DOC referred to groundwater in their keywords (Web of Science database). An important step towards a better understanding of DOM transformations in aquifers is to connect large-scale environmental contexts with prominent DOM components and microbial parameters in groundwater (Baker et al., 2000;Hofmann et al., 2020;Hofmann and Griebler, 2018;Jiao et al., 2010;Retter et al., 2021). However, such studies are currently missing. ...
Dissolved organic matter (DOM) in terrestrial groundwater is generally low in concentration compared to inland surface waters. However, the overall amount of groundwater DOM is huge, as there is 100 times more fresh groundwater than fresh surface water. To date, research on groundwater DOM has merely focused on specific threats to humans such as e.g. DOM and heavy metal complexations and DOM from hydrocarbon contamination. A comprehensive, large-scale study of groundwater is still missing. Here, we examine DOM properties in a large-scale approach with regards to surface characteristics such as land use and altitude, aquifer characteristics as well as microbial features. We analyzed 1600 water samples from 100 groundwater bodies all over Austria with regards to their DOM quantity, quality and bacterial abundance (BA). DOM quality was evaluated with self-organizing maps on fluorescence excitation-emission-matrices (EEMs) combined with Ward clustering and subsequent parallel factor analysis to describe DOM properties of each cluster. We evaluated how these clusters differed among each other, based on DOC and nitrate concentrations, BA and selected environmental characteristics. Our results show that fluorescence components in groundwater resemble components found in other groundwater studies, in studies from forest streams, the dark ocean, agricultural catchments and wastewater treatment plants. The latter fluorescence components were associated with a cluster that is characterized by agricultural and urban land use, as well as high nitrate concentrations. Clusters with a higher abundance of high-molecular weight and humic components, commonly associated with vascular plant and soil origin, exhibited a higher bacterial abundance. This observation provides evidence that suspended bacteria at elevated concentrations mainly originate from the surface. Our study shows that DOM fluorescence can be a fast monitoring tool to identify aquifers under anthropogenic stress and delineate sensitive recharge areas with high surface-groundwater interaction.
... The import and mixing of electron donors (i.e., organic matter) and electron acceptors (e.g., oxygen, nitrate, etc.) is tightly coupled to the sediment matrix and hydrological dynamics (i.e., seasonal recharge and groundwater flow paths). Subsequently, the flux of organic matter and nutrients within the microbial food web potentially controls the microbial abundance, the productivity and community composition as well as species interactions (Baker et al., 2000;Longnecker et al., 2009;Foulquier et al., 2010). ...
Groundwater ecosystems are typically poor in organic carbon and productivity sustaining a low standing stock of microbial biomass. In consequence, microbial food webs in oligotrophic groundwater are hypothesized to be bottom-up controlled. To date, quantitative information on groundwater microbial communities, food web interactions, and carbon flow is relatively lacking in comparison to that of surface waters. Studying a shallow, porous alpine aquifer we collected data on the numbers of prokaryotes, virus-like particles and heterotrophic nanoflagellates (HNFs), the concentration of dissolved (DOC) and assimilable organic carbon (AOC), bacterial carbon production (BCP), and physical-chemical conditions for a 1 year hydrological cycle. The potential effects of protozoan grazing and viral lysis onto the prokaryotic biomass was tested. Flow of organic carbon through the microbial food web was estimated based on data from the literature. The abundance of prokaryotes in groundwater was low with 6.1 ± 6.9 × 10⁴ cells mL–1, seasonally influenced by the hydrological dynamics, with higher densities coinciding with a lower groundwater table. Overall, the variability in cell numbers was moderate, and so it was for HNFs (179 ± 103 HNFs mL–1) and virus-like particles (9.6 ± 5.7 × 10⁵ VLPs mL–1). The virus to prokaryotes and prokaryote to HNF ratios ranged between 2–230 and 33–2,084, respectively. We found no evidence for a viral control of prokaryotic biomass, and the biomass of HNFs being bottom-up controlled. First estimations point at carbon use efficiencies of 0.2–4.2% with prokaryotic production, and carbon consumed and recycled by HNFs and phages to be of minor importance. This first groundwater microbial food web analysis strongly hints at a bottom-up control on productivity and standing stock in oligotrophic groundwater ecosystems. However, direct measurement of protozoan grazing and phage mediated lysis rates of prokaryotic cells are urgently needed to deepen our mechanistic understanding. The effect of microbial diversity on the population dynamics still needs to be addressed.
... Species richness and biodiversity, on the other hand, are somehow connected to the energy available. There is conclusive evidence from several studies conducted in different aquatic environments that changes in organic matter supply in terms of quantity and quality steers shifts in microbial community composition (Shi et al., 1999;Baker et al., 2000;Findlay et al., 2003;Carlson et al., 2004;Judd et al., 2006;Kritzberg et al., 2006;Li et al., 2012;Wu et al., 2018). In particular, if the altered DOM supply lasts for longer than just a couple of hours or days (Herzyk et al., 2014;Grösbacher et al., 2016). ...
Climate change has a massive impact on the global water cycle. Subsurface ecosystems, the earth largest reservoir of liquid freshwater, currently experience a significant increase in temperature and serious consequences from extreme hydrological events. Extended droughts as well as heavy rains and floods have measurable impacts on groundwater quality and availability. In addition, the growing water demand puts increasing pressure on the already vulnerable groundwater ecosystems. Global change induces undesired dynamics in the typically nutrient and energy poor aquifers that are home to a diverse and specialized microbiome and fauna. Current and future changes in subsurface environmental conditions, without doubt, alter the composition of communities, as well as important ecosystem functions, for instance the cycling of elements such as carbon and nitrogen. A key role is played by the microbes. Understanding the interplay of biotic and abiotic drivers in subterranean ecosystems is required to anticipate future effects of climate change on groundwater resources and habitats. This chapter summarizes potential threats to groundwater ecosystems with emphasis on climate change and the microbial world down below our feet in the water saturated subsurface.
... Such connectivity has important effects on the types of dietary carbon resources that fuel the hyporheic food web. In groundwater ecosystems, primary production is absent, and thus microbial biofilms and/or buried organic matter comprise the base of the food web (Baker et al., 2000;Craft et al., 2002). If high hydrological connectivity allows down-welling transport of organic matter from the surface zone, hyporheic organisms can also have access to food resources such as algae-derived carbon provided by surface zones with access to sunlight (Jones et al., 1995;Malard et al., 2002; Fig. 6A). ...
Bio-assessment protocols for the subsurface domain of river channels (i.e. hyporheic zone) are scarce despite of the known importance of its ecological function in river ecosystems. The larvae of the Alloperla ishikariana Plecoptera species inhabit the hyporheic zone. Therefore, we examined whether emergent adult A. ishikariana could serve as proxy indicators of the effects of pollution in the hyporheic food web. The study site comprised a 15-km stretch of a gravel bed river in Hokkaido, Japan, from which all invertebrates were collected from the riverbed at a depth of 30–50 cm, and emergent A. ishikariana adults were sampled in riparian zones. Water quality changed gradually along the river owing to the effects of a wastewater treatment plant and surrounding land uses. Nitrogen and carbon stable isotope ratios (SIRs) of epilithic biofilms, hyporheic particulate organic matter (POM), benthic POM, and invertebrates were used to determine the major basal carbon sources, the effects of water pollution on the hyporheic zone, and the trophic positions of dominant invertebrates. Generalized linear (mixed) models were used for statistical testing and modeling. Amphipoda and the larvae of A. ishikariana were considered as top predators and secondary consumers in the hyporheic food web, respectively. The observed similarity in water quality between the hyporheic and surface water suggested a large degree of hydrological exchange between the two zones, and resulted in positive increases in the nitrogen SIRs of hyporheic invertebrates, including A. ishikariana larvae, in proportion to nitrate levels in the surface water. The nitrogen SIRs of A. ishikariana adults were significantly correlated with the nitrogen SIRs of their larvae within the same location (adjusted R² = 0.78), indicating that emerged adults can provide information on the longitudinal variability of the effects of synthetic nitrogen. In contrast, the carbon SIRs of adults did not predict those of their larvae, possibly reflecting the diverse feeding habitats of larvae. Overall, we demonstrated that adult aquatic insects emerging from the hyporheic zone can be used as indirect indicators of pollution-associated nutrient assimilation as well as the spatial heterogeneity of dietary carbon resources in hyporheic food web.
... Even though analyses of DOM composition and age have a long history in CZ research, holistic studies of DOM evolution in the CZ and the environmental signals that are integrated are still missing (e.g., Wassenaar et al., 1991;Purdy et al., 1992;Aravena and Wassenaar, 1993;Artinger et al., 2000). Time series of the composition of DOM can be used to identify sources, interactions and flow paths within the CZ (Jin et al., 2014;Shen et al., 2015), that all vary, for instance following groundwater recharge events, causing a high connectivity of surface and subsurface compartments (Baker et al., 2000;Kaiser and Guggenberger, 2005). Surface-derived inputs that enter an aquifer system have been shown to contain signals linking to the decay of plant biomass (Shen et al., 2015). ...
Surface ecosystems are rapidly changing on a global scale and it is important to understand how this influences aquifers in the subsurface, as groundwater quality is a major concern for future generations. Dissolved organic matter (DOM) contains molecular and isotopic signals from surface-derived inputs as well as from the biotic and abiotic subsurface environment and is therefore ideal to study the connectivity between both environments. We evaluated a 3-year time series of DOM composition using ultrahigh resolution mass spectrometry and age using ¹⁴C accelerator mass spectrometry along a hillslope well transect in the fractured bedrock of the Hainich Critical Zone Exploratory, Germany. We found a wide range of DOM ¹⁴C depletion, from Δ¹⁴C = −47.9 to Δ¹⁴C = −782.4, within different zones of the shallow groundwater. The ¹⁴C content of DOM mirrored the connectivity of the aquifers to the surface. The composition of DOM was highly interrelated with its ¹⁴C age. The proportions of surface-derived DOM components decreased with DOM age, whereas microorganism-derived DOM components increased. The intensity of surface-sourced DOM signals differed between the wells and likely reflected the hydrological complexity of fractured-rock environments. During recharge, DOM was more enriched in Δ¹⁴C, contained more surface-derived molecular components and was more diverse. As a potential response to the varying DOM substrate, bacterial 16S rRNA gene analysis revealed community evolution and increased bacterial diversity during recharge. The influx of diverse, surface-derived DOM potentially fueled evolution within the autochthonous bacterial communities, as in contrast to DOM, the bacterial community did not retreat to the initial diversity and community composition during the recession period. Our results demonstrate on the one hand that combined analyses of the composition and age of groundwater DOM strongly contribute to the understanding of interconnections, community evolution and the functioning of subsurface ecosystems and on the other hand that changes in surface ecosystems have an imprint on subsurface ecosystems.
... The nature and availability of carbon (C) in the subsurface are regarded as the most important factors that control the functioning of the subsurface biosphere (Baker et al., 2000;Jones & Holmes, 1996). Separated from the energy of the Sun, the energy for life comes from the oxidation of inorganic or organic C sources. ...
Determining the biogeochemical pathways utilized by microbes living in groundwater is essential for understanding the subsurface C cycle and the fate of organic compounds, including pollutants. The radiocarbon signature (Δ ¹⁴ C) of fatty acid methyl esters derived from microbial phospholipids (PLFA) provides useful information for differentiating microbial C sources and infering microbial metabolism. However, in subsurface environments, those analyses remain challenging. Here we present a method combining large volume groundwater filtration (up to 10,000 L) and PLFA purification for subsequent compound-specific radiocarbon analyses. The analytical method involves conventional chemical extraction of PLFA followed by purification of individual compounds by semipreparative high-performance liquid chromatography. Different saturated PLFA in amounts of up to 10 μg each can be simultaneously separated on a C 18 high-load column using a mixture of MeOH/water and acetonitrile as the mobile phase. Our procedure introduced dead-C ext contaminations of 0.57 ± 0.29 and 0.35 ± 0.18 μg for the high-performance liquid chromatography and combustion/graphitization steps of the sample preparation, respectively. However, tests on different high-performance liquid chromatography C 18 columns revealed a large difference in dead C ext associated with column bleed. Modern C ext in the amount of 0.40 ± 0.20 μg was introduced by the combustion/graphitization step of the sample preparation, but other steps did not add modern C ext. The entire method recovered ∼50% of the purified compounds on average, but this did not affect their ¹⁴ C content. This method will allow routine analysis of the Δ ¹⁴ C of PLFA isolated from groundwaters or other sample types, revealing the relationships between microbial and soil-derived C, sedimentary or dissolved C sources.
... This transport limitation on denitrification is likely an important control on the fate of the excess N pollution leaving both ponds through the SWI to the regional aquifer. The potential denitrification rates in these groundwater flow-through ponds will depend upon NO 3 − and DOC availability as those are prerequisites for the onset of anoxic conditions as seen in Experiment 4 and other freshwater SWI studies of riparian and hyporheic zones in river corridors (Baker et al., 2000;Quick et al., 2016;Zarnetske et al., 2011b). In the lake SWI, the O 2 removal rates systematically increased with subsequent acetate additions, suggesting increased aerobic processing of DOC that depleted dissolved O 2 . ...
For many glacial lakes with highly permeable sediments, water exchange rates control hydrologic residence times within the sediment‐water interface (SWI) and the removal of reactive compounds such as nitrate, a common pollutant in lakes and groundwater. Here we conducted a series of focused tracer injection experiments in the upper 20 cm of the naturally downwelling SWI in a flow‐through lake on Cape Cod, MA. We systematically varied residence time and reactant controls on nitrate processing, using isotopically labeled ¹⁵N nitrate to monitor the effect of these changes on nitrate removal via denitrification. The addition of acetate, a labile carbon compound, triggered the lake SWI to switch from net production to net removal of nitrate. When acetate was combined with increased residence time created by controlled reductions in water flux, we observed a fivefold increase in nitrate removal, a 26‐fold increase in N2 production, and a 42‐fold increase in N2O production. We demonstrate that water residence time is an important control on the fate of nitrate in these lake SWIs and illustrate that seasonal conditions that alter lake exchange rates and variability in lake carbon may predict dynamic nitrate removal across the SWI. Additionally, observed N2O production during the oxic pore water experiments paired with geophysical characterization of the sediment porosity revealed that the lake SWI has less mobile pores occupying upward of 50% of the total porosity volume, which function as reactive microzones for nitrate processing.
... DOC contributes to the formation and accumulation of SOM (Kahl et al. 2012;Bantle et al. 2014), it facilitates metal and organic pollutant mobilization (Hafner et al. 2005) and changes soil microbial communities (Cooper et al. 2016). DOC is the major source of carbon and energy to sustain heterotrophic microbial communities (Baker et al. 2000). Spears et al. (2003) noted a higher concentration of DOC derived from deadwood logs than from the layer of the litter. ...
The aim of study was to estimate how the deadwood of different tree species in various stages of decomposition releases dissolved organic carbon (DOC) and total nitrogen concentration. The deadwood of seven species (Common alder, Common aspen, Common ash, Silver fir, Norway spruce, Common hornbeam, and Silver birch) were selected. Three logs from each species in the third, fourth, and fifth decay classes were chosen for analysis. The investigation was carried out in Czarna Rózga Reserve in Central Poland. The leachate from deadwood was collected in lysimeters. Concentrations of total carbon (TC), inorganic carbon (IC), total organic carbon (TOC), and total nitrogen (TN) were determined. In our investigations, the concentration of TOC measure in filtered water samples was equal to DOC. Additionally, pH and electrical conductivity (EC) values were determined in water samples. The results confirmed the importance of the wood species and the stage of decomposition on the concentration of DOC and TN. Significant differences in the concentration of DOC and TN in leachates released between coniferous and deciduous species were noted. Most DOC and TN were released from decomposing hornbeam and aspen wood, the least DOC, and TN were released into the soil from coniferous species, especially the spruce.
... The interaction of SW-GW is not only reflected in hydraulic connection modes, but also in hydrogeochemical responses (Baker et al. 2000;Whiting and Pomeranets 1997). Hydrogeochemical response involves the variable processes affecting hydrogeochemical composition caused by natural processes or human activity. ...
Investigation of surface water and groundwater interaction (SW–GW interaction) provides basic information for regional water-resource protection, management, and development. In this survey of a 10-km-wide area along both sides of the Songhua River, northeast China, the hydrogeochemical responses to different SW–GW interactions were studied. Three types of SW–GW interactions were identified—“recharge”, “discharge”, and “flow-through”—according to the hydraulic connection between the surface water and groundwater. The single factor index, principal component analysis, and hierarchical cluster analysis of the hydrogeochemistry and pollutant data illuminated the hydrogeochemical response to the various SW–GW interactions. Clear SW–GW interactions along the Songhua River were revealed: (1) upstream in the study area, groundwater usually discharges into the surface water, (2) groundwater is recharged by surface water downstream, and (3) discharge and flow-through coexist in between. Statistical analysis indicated that the degree of hydrogeochemical response in different types of hydraulic connection varied, being clear in recharge and flow-through modes, and less obvious in discharge mode. During the interaction process, dilution, adsorption, redox reactions, nitrification, denitrification, and biodegradation contributed to the pollutant concentration and affected hydrogeochemical response in the hyporheic zone.
Submarine groundwater drainage (SGD) changes the elemental composition of the neighboring coastal ocean and impacts the biogeochemical cycles. To examine the seasonal and spatial variability in dissolved organic carbon (DOC) and labile organic compound biochemical compounds like dissolved carbohydrates (TDCHO), dissolved proteins (TDPRO), and dissolved free amino acid (TDFAA) concentrations during the dry and wet periods, groundwater samples were taken at 90 locations (180 samples) along the Indian coast. The mean DOC contents in Indian coastal groundwaters were more significant than the global mean values. DOC, TDCHO, TDPRO, and TDFAA concentrations are higher during wet than dry periods. The DOC and labile organic compound showed a substantial positive association with soil organic carbon, and respective labile compounds in soil, population, and land usage and poor relation with woodland territories, implying that soil organic compounds leaching is a source of DOC and other labile organic compounds into the groundwater. DOC and other labile compounds concentrations were linearly associated with population density, land usage, and sewage production, demonstrating that anthropogenic activities tightly regulate the formation of DOC in groundwater. During the wet and dry periods, total labile organic compounds (TDCHO, TDFAA, and TDPRO) constituted 21% and 10.5% of DOC, respectively. Compared to the wet time, more aromatic compounds accumulated during the dry season but were less bioavailable. SGD DOC flux contributed 2–7% of riverine DOC flux to the coastal ocean. The SGD flux from the Indian subcontinent to the nearby northern Indian Ocean accounts for approximately 2% of the worldwide SGD flux. The effect of DOC flux via SGD on coastal bacterial activity, the plankton food web, and the oxygen minimum zone must be studied.
Current methods for tracing decades-old groundwaters rely on isotope geochemistry, to determine groundwater age and altitude at the point of infiltration. Temporal and spatial variability in atmospheric conditions, and water-rock interactions, can make the interpretation of isotopes uncertain. Here, we propose a new method of groundwater tracing based on fingerprinting of natural dissolved organics. We present our initial findings from the Grimsel Test Site in Switzerland, located within a fractured granite. Using two-dimensional gas chromatography, we derive detailed organic fingerprints from surface soils at several locations and show that different soils produce distinctly different dissolved organic signatures. We then compare the soils with groundwater and lake water using a non-targeted approach using principal component analysis and hierarchical cluster analysis. Our analysis finds three statistically significant clusters. Most groundwaters are clustered with the lakewater samples, but two are clustered with soil from the highest altitude surface sampling location. We hypothesize that for samples to form a significant cluster, they must have derived from a common environment, with a unique combination of organic compounds. For groundwaters to cluster with soil samples or lakewater, we theorize there must be a hydraulic connection between the type of infiltration environment and the groundwater sampling locations within each cluster. Our research demonstrates that organic molecules derived from the surface environment can be used to discriminate near-surface environment(s) through which meteoric groundwater has infiltrated. Organic fingerprinting could prove a powerful tool for improved understanding of groundwater flow systems, particularly when combined with other complementary techniques.
Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive
Supplementary material: https://doi.org/10.6084/m9.figshare.c.712998
With the substantial increase in karst groundwater pollution, the pollution caused by heavy metal migration has become one of the hottest topics. The migration characteristics of heavy metals in karst groundwater are closely related to the geological environment in which they are found. Therefore, this review focuses on the migration characteristics of heavy metals in karst groundwater in southern and northern China and highlights the effect of different environmental contexts such as atmosphere (precipitation), vegetation, soil, rock, and aquifers on the behavior of heavy metals. It also summarizes existing research methods on heavy metal migration in karst groundwater. Meanwhile, current advances and the future perspectives on karst groundwater heavy metal migration will be presented. It is hoped that this review may shed light on the study of heavy metal migration in karst areas.
Organic matter (OM) regulates biogeochemical cycling and provides a fundamental thermodynamic stability to freshwater ecosystems. In this chapter we highlight key knowledge of freshwater OM composition, sources, and cycling. OM consists of a heterogeneous mixture of plant, microbial, and animal products, either alive or as fragments in various stages of decomposition (i.e., detritus). Freshwater OM is either autochthonous, meaning produced within the ecosystem, or allochthonous, meaning of an external, often terrestrial, source from the catchment. OM includes carbon (C) as well as varying quantities of other elements and can be in dissolved or particulate form (DOM, POM; DOC, POC). Gross primary production (GPP) and ecosystem respiration (ER) govern OC fixation and breakdown and can thus be used to characterize organic carbon (OC) cycling at the whole-ecosystem scale. Most ecosystems are heterotrophic (ER > GPP), meaning metabolism and food webs are subsidized by allochthonous OC. Once OC enters ecosystems from internal GPP or allochthonous sources, it can be buried, emitted to the atmosphere after respiration, or exported. OC fluxes, composition, and cycling are changing in response to anthropogenic alterations of land cover, hydrology, chemistry, and climate—how multiple stressors alter OM fluxes and metabolism remains a critical knowledge gap in our understanding of carbon cycling in inland waters.
Coupled abiotic and biotic processes in the hyporheic zone, where surface water and groundwater mix, play a critical role in the biogeochemical cycling of carbon, nutrients, and trace elements in streams and wetlands. Dynamic hydrologic conditions and anthropogenic pollution can impact redox gradients and biogeochemical response, although few studies examine the resulting hydrobiogeochemical interactions generated within the hyporheic zone. This study examines the effect of hyporheic flux dynamics and anthropogenic sulfate loading on the biogeochemistry of a riparian wetland and stream system. The hydrologic gradient as well as sediment, surface water, and porewater geochemistry chemistry was characterized at multiple points throughout the 2017 spring-summer-fall season at a sulfate-impacted stream flanked by wetlands in northern Minnesota. Results show that organic-rich sediments largely buffer the geochemical responses to brief or low magnitude changes in hydrologic gradient, but sustained or higher magnitude fluxes may variably alter the redox regime and, ultimately, the environmental geochemistry. This has implications for a changing climate that is expected to dramatically alter the hydrological cycle. Further, increased sulfate loading and dissolved or adsorbed ferric iron complexes in the hyporheic zone may induce a cryptic sulfur cycle linked to iron and carbon cycling, as indicated by the abundance of intermediate valence sulfur compounds (e.g., polysulfide, elemental sulfur, thiosulfate) throughout the anoxic wetland and stream-channel sediment column. The observed deviation from a classical redox tower coupled with potential changes in hydraulic gradient in these organic-rich wetland and stream hyporheic zones has implications for nutrient, trace element, and greenhouse gas fluxes into surface water and groundwater, ultimately influencing water quality and global climate.
Small waters, like ponds, are the most abundant freshwater environments, and are increasingly recognized for their function in ecosystem service delivery. In agricultural watershed, artificial ponds play an essential role in reducing nitrogen pollution. However, until now artificial ponds remain the least investigated part of water environments. The importance of microbial activities has seldom been discussed, which makes the microbial pathways and processes rates in nitrogen removal poorly understood. To illustrate the role of artificial ponds in microbial nitrogen removal in agricultural watersheds, 21 pond sediments and 11 soils are collected in an agricultural watershed of China. Results show that surface sediments in ponds carry significantly higher dissolved inorganic nitrogen (9.1–21.9 mg/kg) and total organic matter (64.8–113.0 g/kg) compared to the surrounding agricultural soils. High rates of microbial nitrogen removal in ponds (12.4–25.5 nmol N g⁻¹ h⁻¹) are observed, which are 2–9 times higher than those in dryland soils. In pond sediments, denitrification dominates (> 90% N-loss) the microbial nitrogen removal process with only a minor contribution of anaerobic ammonium oxidation. A high potential of N2O production (up to 9.4 nmol N g⁻¹ h⁻¹) occurs in ponds along with the rapid nitrogen removal. For denitrifier genes, nir gene are always more abundant than nosZ gene. Additionally, the nirS gene is more abundant under flooded conditions, while nirK gene prefers higher dissolved oxygen and NO3⁻ in drylands. These findings highlight the ecosystem function of ponds in agricultural watersheds, and provide new ideas on pollution control and global nitrogen cycling.
Aim: The aim of this chapter is to critically highlight existing knowledge gaps, obstacles, and research frontiers in groundwater microbial ecology.
Main concepts covered: We have identified and discuss below 10 topics where there is an urgent need for microbiological research in groundwater.
Main methods covered: We consider the challenges in groundwater microbiology research, including the limited accessibility of aquifers, methods to robustly characterize communities, the importance of microbial interactions, and the current lack of underpinning ecological theory and the testing of basic concepts in groundwater.
Conclusion/Outlook: There is enormous opportunity to progress fundamental ecological understanding through further research of microbiology in the terrestrial subsurface.
Groundwater as the primary source of fresh water particularly in semi-arid regions is heavily threatened by various pollutants such as dissolved organic matter (DOM) and heavy metals due to anthropogenic activities. In this study, 113 shallow groundwater samples were collected from Guanzhong basin of China to explore spatial distributions and interactions of DOM and heavy metals (Fe, Mn and Cu). Fluorescence excitation-emission spectrophotometry with parallel factor analysis showed that DOM in groundwater mainly contained three humic-like and two protein-like substances with an average dissolved organic carbon (DOC) concentration of 12.85 mg L⁻¹. Average Mn and Cu concentrations in groundwater were 19.92 μg L⁻¹ and 7.05 μg L⁻¹ with an increasing trend from west to east, whereas Fe concentration in central regions was much higher (34.23 μg L⁻¹). Structural equation modeling analysis indicated that DOM in groundwater could be significantly affected by surface water, and heavy metals were influenced by urbanization. Moreover, DOM could strongly influence the bioavailability, migration, and transformation of Mn in groundwater. These findings would be beneficial for the effective utilization and protection of groundwater resources.
It has been well documented that agricultural activities lead to significant alterations in surface water dissolved organic matter (DOM), yet their impacts on groundwater DOM remain poorly constrained. The quantity, source, and composition of DOM play a pivotal role in a range of groundwater ecosystem services that are of important ecological and societal values. We assessed the impact of irrigation on the source and compositional characteristics of groundwater DOM in a large river basin supporting intensive agriculture in arid northwestern China. We sampled five water types along a river reach of approximately 40 km, including groundwater, river water, irrigation canal water, hyporheic water, and soil leachates. The excitation-emission matrix (EEM) measurements coupled with parallel factor analysis (PARAFAC) identified two terrestrial-derived, humic-like fluorescent components (C1 and C2) and one protein-like autochthonous component (C3). DOM composition and dissolved organic carbon (DOC) concentration varied as a function of water type, with subsurface waters showing relatively lower DOC and terrestrial humic fluorescence than surface waters. Combining nitrate, electrical conductivity, dissolved inorganic carbon (DIC), and δ¹³C-DIC, irrigation-influenced samples were identified, and the influence of irrigation on groundwater DOM appeared only in shallow aquifers (<50 m). Irrigation-influenced groundwater exhibited higher DOC and terrestrial fluorescence than unimpacted groundwater, suggesting that irrigation return flows accelerated the downward movement of terrestrial humic compounds and led to their accumulation in aquifers. This effect was propagated via surface water-groundwater interaction to upwelling hyporheic water, which also showed enrichment in terrestrial fluorescence. Our findings demonstrate that irrigation can accelerate the biogeochemical cycling of organic compounds via a subsurface pathway of soil to aquifer to hyporheic zone. The enrichment of soil-derived compounds in subsurface waters may have important ecological consequences, such as altering the transport of nutrients and pollutants and changing carbon and energy flows across the surface-subsurface boundary.
Dissolved organic carbon (DOC) is the smallest amount of organic carbon present in aquifer systems and is typically dwarfed by amounts of particulate organic carbon (POC) and adsorbed organic carbon (AOC). Research conducted over the last half century, however, has shown that these dissolved, particulate, and adsorbed compartments interact dynamically with each other. That suggests the hypothesis that the bioavailability of DOC in groundwater may indicate the bioavailability of the associated POC and AOC compartments as well. If that proves to be the case, it would greatly simplify the process of evaluating the bioavailability of total organic carbon present in groundwater systems. That hypothesis was examined by (1) comparing DOC bioavailability between two aquifers receiving modern atmospheric recharge, but with the recharge passing through POC/AOC sources of substantially different geologic ages, and (2) measuring POC/AOC bioavailability in sediments in from two aquifers before and after injection with bioavailable DOC consisting of dissolved sugars and emulsified vegetable oil. The results of both comparisons are consistent with the hypothesis that DOC bioavailability in groundwater reflects the bioavailability of the associated POC and AOC compartments and vice versa. Thus, DOC bioavailability may be a useful indicator of an aquifer's potential to drive reduction/oxidation processes that affect the chemical quality of groundwater.
Freshwater ecosystems play a key role in shaping the global carbon cycle and maintaining the ecological balance that sustains biodiversity worldwide. Surficial water bodies are often interconnected with groundwater, forming a physical continuum, and their interaction has been reported as a crucial driver for organic matter (OM) inputs in groundwater systems. However, despite the growing concerns related to increasing anthropogenic pressure and effects of global change to groundwater environments, our understanding of the dynamics regulating subterranean carbon flows is still sparse. We traced carbon composition and transformations in an arid zone calcrete aquifer using a novel multidisciplinary approach that combined isotopic analyses of dissolved organic carbon (DOC) and inorganic carbon (DIC) (δ¹³CDOC, δ¹³CDIC, ¹⁴CDOC and ¹⁴CDIC) with fluorescence spectroscopy (Chromophoric Dissolved OM (CDOM) characterisation) and metabarcoding analyses (taxonomic and functional genomics on bacterial 16S rRNA). To compare dynamics linked to potential aquifer recharge processes, water samples were collected from two boreholes under contrasting rainfall: low rainfall ((LR), dry season) and high rainfall ((HR), wet season). Our isotopic results indicate limited changes and dominance of modern terrestrial carbon in the upper part (northeast) of the bore field, but correlation between HR and increased old and ¹³C-enriched DOC in the lower area (southwest). CDOM results show a shift from terrestrially to microbially derived compounds after rainfall in the same lower field bore, which was also sampled for microbial genetics. Functional genomic results showed increased genes coding for degradative pathways—dominated by those related to aromatic compound metabolisms—during HR. Our results indicate that rainfall leads to different responses in different parts of the bore field, with an increase in old carbon sources and microbial processing in the lower part of the field. We hypothesise that this may be due to increasing salinity, either due to mobilisation of Cl⁻ from the soil, or infiltration from the downstream salt lake during HR. This study is the first to use a multi-technique assessment using stable and radioactive isotopes together with functional genomics to probe the principal organic biogeochemical pathways regulating an arid zone calcrete system. Further investigations involving extensive sampling from diverse groundwater ecosystems will allow better understanding of the microbiological pathways sustaining the ecological functioning of subterranean biota.
Hyporheic zones (HZ) are hotspots for biogeochemical reactions where groundwater and surface water mix. River dam buildings and other hydrologic controls alter the sediment grain size distribution and modify the downstream hyporheic exchange, with cascading effects on geochemical and microbial processes in river corridors. In this lab-scale column experiment, the N transformations in HZ filled with sediments in different grain sizes were investigated with a focus on understanding the interplay among variational hydraulic connectivity, microbial community structure, functional potential under frequent groundwater−surface water exchange. Porosity was identified as the main driver determining bacterial community assembly in HZ sediments. Significant microbial zonation was observed along the columns and the degree of co-occurrence of bacterial communities in the Fine column was lower than that in the Coarse and Mix columns. The Coarse column allowed for almost 2.47 times the exchange flux relative to the Fine column, and generates the fastest DO consumption rate (−6.52 μg O2/L·s). The enrichment of nitrifiers, i.e., Cytophagaceae and Bacillaceae and nitrification functional genes, i.e., amoA_AOA and amoA_AOB revealed the higher nitrification potential in column filled with coarse sediments. In comparison, the highest NH4⁺ production rates (2.4 × 10⁻³ μg N/L·s) took place in Fine column. The higher abundancies of denitrifiers such as Comamonadaceae and Lysobacter and enrichment of functional genes of nirK and nirS interestingly suggested the elevated denitrification potential in Fine column in a more anaerobic environment. The results implied that variations in microbial functional potential and associated nitrogen transformation may occur in size-fractioned HZ to dynamic hyporheic exchange, which added new knowledge to the underlying biogeochemical and ecological processes in regulated river corridors.
Recharge of aquifers by urban stormwater may trigger significant ecological changes that can be detrimental to the biodiversity and functioning of groundwater ecosystems. Here, the effects of aquifer recharge (AR) on three levels of parameters were investigated: dissolved organic carbon (DOC) quantity and quality, global biofilm characteristics, and diversity changes of bacterial communities. As DOC enrichment by AR can be mitigated by vadose zone (VZ) thickness, three AR sites with thin VZ (< 3 m) and three sites with thick VZ (> 10 m) were selected. For each AR site, clay beads were incubated over a 10-day-long rainy period through wells in recharged and non-recharged groundwaters. Total proteins, dehydrogenase, and hydrolytic activities were monitored from clay beads to assess biofilm development. Bacterial richness on beads was estimated by 16S rRNA-based metabarcoding. AR was found to significantly increase DOC and biodegradable DOC (BDOC) concentrations, biofilm development, and bacterial richness especially in sites with thin VZ. VZ thickness was inversely related to microbial growth indicators and bacterial richness in groundwater, through a control of DOC availability. The proportion of Bacteroidetes 16S rRNA gene reads was higher in recharged groundwater than in non-recharged groundwater, suggesting that this phylum could be used as an indicator of DOC enrichment associated with AR. Quantitative PCR assays for Bacteroides DNA confirmed these trends and showed an enrichment of this bacterial group in DOC-rich aquifer waters. The positive linear relationships between BDOC concentrations and biofilm variables highlighted a strong C-limitation of groundwater impacting bacterial species sorting and activity.
Building on the synthesis of carbon reservoirs in Earth's subsurface, this chapter focuses on the forms, cycling, and fate of the carbon supporting microbial life in the terrestrial and marine subsurface. As the subsurface is estimated to host a vast reservoir of life on Earth, identifying the carbon compounds that life uses for energy and growth is key to understanding ecosystem functioning in the past and at present, and also for extrapolating these findings to the search for life in the universe. This chapter highlights advances in quantifying small carbon compounds, measuring rates of carbon turnover, and the fate of carbon in the deep biosphere.
Stream CO2 emissions contribute significantly to atmospheric climate forcing. While there are strong indications that groundwater inputs sustain these emissions, the specific biogeochemical pathways and timescales involved in this lateral CO2 export are still obscure. Here, via an extensive radiocarbon (¹⁴C) characterisation of CO2 and DOC in stream water and its groundwater sources in an old-growth boreal forest, we demonstrate that the ¹⁴C-CO2 is consistently in tune with the current atmospheric ¹⁴C-CO2 level and shows little association with the ¹⁴C-DOC in the same waters. Our findings thus indicate that stream CO2 emissions act as a shortcut that returns CO2 recently fixed by the forest vegetation to the atmosphere. Our results expose a positive feedback mechanism within the C budget of forested catchments, where stream CO2 emissions will be highly sensitive to changes in forest C allocation patterns associated with climate and land-use changes.
A laboratory flume simulating a riverbed sand dune containing a low-permeability clay lens was constructed to investigate its influence on the quality and quantity of hyporheic exchange. By varying the depths and spatial locations of the clay lens, 24 scenarios and one blank control experiment were created. Dye tracers were applied to visualize patterns of hyporheic exchange and the extent of the hyporheic zone, while NaCl tracers were used to calculate hyporheic fluxes. The results revealed that the clay lens reduces hyporheic exchange and that the reduction depends on its spatial location. In general, the effect was stronger when the lens was in the center of the sand dune. The effect weakened when the lens was moved near the boundary of the sand dune. A change in horizontal location had a stronger influence on the extent of the hyporheic zone compared with a change in depth. The size of the hyporheic zone changed with the depth and position of the clay lens. There was a maximum of hyporheic extent with the lens at a depth of 0.1 m caused by changes of water flow paths.
We studied the export of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from forested shallow groundwater to first-order streams, based on groundwater and surface water sampling and hydrological data. The selected watershed was particularly convenient for such study, with a very low slope, with pine forest growing on sandy permeable podzol and with hydrology occurring exclusively through drainage of shallow groundwater (no surface runoff). A forest plot was instrumented for continuous eddy covariance measurements of precipitation, evapotranspiration, and net ecosystem exchanges of sensible and latent heat fluxes as well as CO2 fluxes. Shallow groundwater was sampled with three piezometers located in different plots, and surface waters were sampled in six first-order streams; river discharge and drainage were modeled based on four gauging stations. On a monthly basis and on the plot scale, we found a good consistency between precipitation on the one hand and the sum of evapotranspiration, shallow groundwater storage and drainage on the other hand. DOC and DIC stocks in groundwater and exports to first-order streams varied drastically during the hydrological cycle, in relation with water table depth and amplitude. In the groundwater, DOC concentrations were maximal in winter when the water table reached the superficial organic-rich layer of the soil. In contrast, DIC (in majority excess CO2) in groundwater showed maximum concentrations at low water table during late summer, concomitant with heterotrophic conditions of the forest plot. Our data also suggest that a large part of the DOC mobilized at high water table was mineralized to DIC during the following months within the groundwater itself. In first-order streams, DOC and DIC followed an opposed seasonal trend similar to groundwater but with lower concentrations. On an annual basis, leaching of carbon to streams occurred as DIC and DOC in similar proportion, but DOC export occurred in majority during short periods of the highest water table, whereas DIC export was more constant throughout the year. Leaching of forest carbon to first-order streams represented a small portion (approximately 2 %) of the net land CO2 sink at the plot. In addition, approximately 75 % of the DIC exported from groundwater was not found in streams, as it returned very fast to the atmosphere through CO2 degassing.
Transformation and retention of nitrogen and other biologically reactive solutes in the hyporheic zones of running water contribute to an essential ecosystem service. However, the synoptic impact of intense agricultural or urban land-uses, elevated nutrient loading, flow alterations, riparian clear-cutting and channelization on the source-sink behavior of solutes in hyporheic zones remains largely uncharacterized and unquantified. Therefore, we studied nutrient dynamics in a hydro-morphologically and chemically modified stream reach using a new monitoring approach allowing the simultaneous measurement of nutrient and water flux through a screened area in the subsurface of rivers (Hyporheic Passive Flux Meter, HPFM). With HPFMs we directly assessed time-integrated lateral hyporheic nitrate fluxes during early spring and mid-summer covering different temperature and discharge regimes. Contrary to our expectations, higher stream discharge coincided with substantially lower hyporheic exchange rates. While in streams featuring a natural morphology, bed-form induced exchange commonly increases with surface flow, the influence of groundwater level was dominant in this reach. Furthermore, in contrast to less impacted environments, where progressive substrate depletion with depths reduces metabolic rates in the subsurface, we identified not the upper, but the intermediate layer of the hyporheic zone as hotspot of nutrient turnover. Overall, the hyporheic zone at the study site functioned partly as nitrate source, partly as a sink. Neither of the commonly used determinants redox state and residence time could explain this source or sink function. Our results give clear evidence to carefully transfer the knowledge of hyporheic zone processes from “natural” systems to anthropologically modified streams.
Groundwater ecosystems are low productivity environments due to the small amount and poor quality of organic carbon, and nutrient limitation (N, P). However, field data show the overall presence of degradable DOC. Studies exploring the relationship between carbon properties, nutrient availability and microbial production, as well as possible priming in groundwater are missing. In multi-factorial groundwater batch experiments, we investigated the effect of various organic carbon sources (acetate, organic fertilizer, fulvic acids, humic acids) and nutrients (N, P) to DOC degradation, bacterial production and growth efficiency. Focusing on DOC concentrations, total cell counts, ATP, and bacterial carbon production, our study revealed several important findings. The type of organic carbon offered influenced bacterial growth pattern with lower assimilation efficiencies for the labile but energetically least favorable compound (acetate). Growth on acetate was soon P-limited, while more complex DOM (humic acids) sustained slow but long-term growth. Active but non-growing cells maintained continuous DOC turnover at nutrient limitation. The overall bacterial carbon production and growth efficiency ranged from <1 ng to >1 µg C l⁻¹ h⁻¹ and <0.1 to 28%, respectively. None of the experiments revealed evidence for priming in groundwater. This study is a first attempt to unravel the multiple limitations and energetic constraints facing microbial communities in oligotrophic groundwater. Future studies should evaluate these findings by including the sediment matrix which carries the major fraction of microbial biomass and by looking in more detail at the structural characteristics and availability of DOM and P species.
Streams are significant sources of CO2 to the atmosphere. Estimates of CO2 evasion fluxes (fCO2) from
streams typically relate to the free flowing water but exclude geomorphological structures within
the stream corridor. We found that gravel bars (GBs) are important sources of CO2 to the atmosphere,
with on average more than twice as high fCO2 as those from the streamwater, affecting fCO2 at the level
of entire headwater networks. Vertical temperature gradients resulting from the interplay between
advective heat transfer and mixing with groundwater within GBs explained the observed variation in
fCO2 from the GBs reasonably well. We propose that increased temperatures and their gradients within
GBs exposed to solar radiation stimulate heterotrophic metabolism therein and facilitate the venting
of CO2 from external sources (e.g. downwelling streamwater, groundwater) within GBs. Our study
shows that GB fCO2 increased fCO2 from stream corridors by [median, (95% confidence interval)] 16.69%,
(15.85–18.49%); 30.44%, (30.40–34.68%) and 2.92%, (2.90–3.0%), for 3rd, 4th and 5th order streams,
respectively. These findings shed new light on regional estimates of fCO2 from streams, and are relevant
given that streamwater thermal regimes change owing to global warming and human alteration of
stream corridors.
http://www.sciencedirect.com/science/article/pii/S0043135417309028
Abstract:
Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.
In this chapter, we explore how the dynamics of key physicochemical parameters (temperature, light, salinity, pH, dissolved oxygen, and turbidity) differ in intermittent rivers and ephemeral streams (IRES) compared to their perennial counterparts. Disruption of longitudinal, lateral, and vertical hydrological connectivity through intermittence explains many of the differences in the physicochemistry between IRES and perennial systems. Physicochemical conditions affect the structure and functioning of all aquatic ecosystems; however, because of the greater variability (e.g., substantial daily, seasonal, and supraseasonal variation in parameters like dissolved oxygen, temperature, and pH) in IRES compared to perennial systems, changes in the physicochemistry are particularly important in defining the ecological conditions of IRES. IRES are especially vulnerable to anthropogenic inputs because their low water volume during flow cessation limits their dilution capacity. Understanding the processes responsible for the natural temporal and spatial variability in the physicochemistry in IRES informs the design and implementation of effective conservation and management plans in IRES.
The hydrological regimes of most intermittent rivers and ephemeral streams (IRES) include the alternation of wet and dry phases in the stream channel and highly dynamic lateral, vertical, and longitudinal connections with their adjacent ecosystems. Consequently, IRES show a unique ‘biogeochemical heartbeat’ with pulsed temporal and spatial variation in nutrient and organic matter inputs, in-stream processing, and downstream transport. Given that IRES are widespread, their improper consideration may cause inaccurate estimation of nutrient and carbon fluxes in river networks. Here, we review existing knowledge on nutrient and organic matter dynamics in IRES considering in-stream processes, interactions with subsurface and lateral compartments, and linkages between terrestrial ecosystems and IRES networks at the catchment scale. Moreover, knowledge gaps and research needs are discussed. Our purpose is to contribute to the flourishing knowledge and research on the biogeochemistry of IRES by providing a comprehensive view of nutrient and organic matter dynamics in these ecosystems.
Physical heterogeneity determines interstitial fluxes in porous media. Nutrients and organic matter distribution in depth influence physicochemical and microbial processes occurring in subsurface. Columns 50 cm long were filled with sterile silica sand following 5 different setups combining fine and coarse sands or a mixture of both mimicking potential water treatment barriers. Water was supplied continuously to all columns during 33 days. Hydraulic conductivity, nutrients and organic matter, biofilm biomass and activity were analysed in order to study the effect of spatial grain size heterogeneity on physicochemical and microbial processes and their mutual interaction. Coarse sediments showed higher biomass and activity in deeper areas compared to the others; however, they resulted in incomplete denitrification, large proportion of dead bacteria in depth, and low functional diversity. Treatments with fine sediment in the upper 20 cm of the columns showed high phosphorous retention. However, low hydraulic conductivity values reported in these sediments seemed to constraint biofilm activity and biomass. On the other hand, sudden transition from coarse-to-fine grain sizes promoted a hot-spot of organic matter degradation and biomass growth at the interface. Our results reinforce the idea that grain-size disposition in subsurface sandy sediments drives the interstitial fluxes, influencing microbial processes.
The groundwater-surface water (GW-SW) ecotone, or hyporheic zone, is an active component of stream ecosystems that influences whole-system metabolism and nutrient retention. Because hydrologic fluxes affect the supply of carbon, nutrients, and oxygen to the GW-SW ecotone, the biogeochemical structure of the ecotone (i.e., nutrient content) and the role of the ecotone in nutrient retention are expected to vary under differing hydrologic conditions. In this paper, we employ an inter-basin comparison of headwater streams to assess the influence of ecosystem hydrology on the structure and functioning of GW-SW ecotones. Specifically, we address how differing rate and extent of GW-SW interaction influences heterogeneity in interstitial nutrient content and how variation in GW-SW interaction alters the role of the ecotone in whole-system nutrient retention. A multiple regression model derived from 6 solute-injection experiments identified the extent and rate of hydrologic exchange between the stream and its aquifer as critical variables that determine the retention of biologically important solutes. This approach emphasizes that the nature of GW-SW interaction is established by catchment geology (i.e., alluvial hydrogeologic properties), is modified by changing discharge within a catchment, and is a strong determinant of nutrient retention. At the landscape scale, identifying catchment geologic composition may be a starting point for comparative studies of GW-SW ecotones that could contribute to a more robust model of lotic ecosystem functioning.
1. The surface water/groundwater (SW/GW) interface is a crucial control point for lateral nutrient fluxes between uplands and aquatic ecosystems and for upstream/downstream (longitudinal) processes in lotic ecosystems.
2. Hydrological and biogeochemical dynamics of the SW/GW ecotone are linked to the degree of channel constraint and the sediment characteristics of the floodplain and stream bed.
3. The availability of specific chemical forms of electron donors and electron acceptors affects the spatial distribution of biogeochemical processes at the SW/GW interface. Temporal change in discharge is also a major factor affecting the rate and extent of these processes.
4. The magnitude of SW/GW interactions in lotic ecosystems is predicted to be a major determinant of solute retention. Channel morphology, stream bed composition and discharge are predicted to be important controls on SW/GW interactions.
5. Interdisciplinary research involving hydrologists, geomorphologists, aquatic ecologists, microbial ecologists and landscape ecologists is needed to further our present understanding of this critical interface linking terrestrial and aquatic ecosystems.
The concentration, composition, and flux of dissolved organic carbon (DOC) were measured in the Hubbard Brook Valley, New Hampshire. Data on precipitation, throughfall, soil solution, streamside seeps, stream water, and lake water are presented for 1976-1980. Characterization of DOC included analysis of phenolics, monomeric and polymeric carbohydrates, carboxylic acids, primary amines, and aldehydes. DOC concentrations increased with passage of water through the forest canopy and forest floor, decreased due to abiotic sorption in the mineral soil, and remained relatively low in most downstream ecosystems (streamside seeps, streams, and Mirror Lake). Average flux of DOC is estimated as 17, 47, 263, 54, 23, and 20 kg{center dot}haâ»Â¹{center dot}yrâ»Â¹ in precipitation, throughfall, soil solution (E, upper B, and B horizons), and streamflow, respectively. The composition of DOC, in particular the relative proportions of carbohydrates, appears to be related to the degree of biological activity at a given site in the landscape. Carbohydrates are particularly high in both absolute and relative terms in throughfall and lake water, which are the areas of highest photosynthetic activity within the terrestrial and aquatic ecosystems, respectively.
The influence of hydrologic linkage between hyporheic and surface subsystems was investigated in sand-bottomed reaches of a desert stream. Direction of hydrologic exchange was measured as vertical hydraulic gradient (VHG) using mini-piezometers. Maps of VHG indicated upwelling (discharge from the interstitial regions into surface water) at the bases of riffles and heads of runs; downwelling (infiltration of surface water into the hyporheic zone) occurred at the bases of runs. Dissolved NO3-N in surface water was higher over or immediately downstream from upwelling zones. Loss of continued supply from the hyporheic zone and intense assimilatory demand by surface autotrophs generated longitudinal declines in NO3-N and lower nutrient concentrations in downwelling zones. Algal standing crop (as chlorophyll a) was significantly higher in upwelling zones than in areas without positive VHG. Postflood trajectories of chlorophyll a indicated that algae at upwelling zones recovered from disturbance significantly faster than those at downwelling zones. Recovery rate was related to supply of NO3-N from enriched interstitial water in the hyporheic zone. Hydrologic linkage integrates surface and hyporheic subsystems and increases ecosystem stability by enhancing resilience of primary producers following flash flood disturbance.
Conservative solute injections were conducted in three first-order montane streams of different geological composition to assess the influence of parent lithology and alluvial characteristics on the hydrological retention of nutrients. Three study sites were established: (1) Aspen Creek, in a sandstone–siltstone catchment with a fine-grained alluvium of low hydraulic conductivity (1·3×10⁻⁴ cm/s), (2) Rio Calaveras, which flows through volcanic tuff with alluvium of intermediate grain size and hydraulic conductivity (1·2×10⁻³ cm/s), and (3) Gallina Creek, located in a granite/gneiss catchment of coarse, poorly sorted alluvium with high hydraulic conductivity (4·1×10⁻³ cm/s). All sites were instrumented with networks of shallow groundwater wells to monitor interstitial solute transport. The rate and extent of groundwater–surface water exchange, determined by the solute response in wells, increased with increasing hydraulic conductivity. The direction of surface water–groundwater interaction within a stream was related to local variation in vertical and horizontal hydraulic gradients. Experimental tracer responses in the surface stream were simulated with a one-dimensional solute transport model with inflow and storage components (OTIS). Model-derived measures of hydrological retention showed a corresponding increase with increasing hydraulic conductivity.
1. This review focuses on the connectivity between river and groundwater ecosystems, viewing them as linked components of a hydrological continuum. Ecological processes that maintain the integrity of both systems and those that are mediated by their ecotones are evaluated.
2. The hyporheic zone, as the connecting ecotone, shows diverse gradients. Thus it can be characterized by hydrological, chemical, zoological and metabolic criteria. However, the characteristics of the hyporheic zone tend to vary widely in space and time as well as from system to system. The exact limits are difficult to designate and the construction of static concepts is inadequate for the representation of ecological processes. The hyporheic interstices are functionally a part of both the fluvial and groundwater ecosystems.
3. The permeability of the ecotone depends on the hydraulic conductivity of the sediment layers which, because of their heterogeneity, form many flowpath connections between the stream and the catchment, from the small scale of a single microhabitat to the large scale of an entire alluvial aquifer. Local up- and downwellings are determined by geomorphologic features such as streambed topography, whereas large-scale exchange processes are determined mainly by the geological properties of the catchment. Colmation—clogging of the top layer of the channel sediments—includes all processes leading to a reduction of pore volume, consolidation of the sediment matrix, and decreased permeability of the stream bed. Consequently, colmation can hinder exchange processes between surface water and groundwater.
4. Physicochemical gradients in the interstices result from several processes: (i) hyporheic flow pattern and the different properties of surface and groundwaters; (ii) retention, caused by the filtering effect of pore size and lithologic sorption as well as the transient storage of solutes caused by diminished water velocities; (iii) biogeochemical transformations in conjunction with local residence time. Each physicochemical parameter may develop its own vertical dynamics laterally from the active channel into the banks as well as longitudinally because of geomorphologic changes.
5. The river–groundwater interface can act as a source or sink for dissolved organic matter, depending on the volume and direction of flow, dissolved organic carbon concentrations and biotic activity. Interstitial storage of particulate organic matter is influenced mainly by grain size distribution and by spates involving bedload movement that may import or release matter, depending on the season. After initial transient and abiotic storage, hyporheic organic matter is mobilized and transformed by the biota. Micro-organisms account for over 90% of the community respiration. In subterranean waters most bacteria are attached to surfaces and remain in a biofilm.
6. Hyporheic interstices are functionally significant for phreatic and riverine metazoans because they act as a refuge against adverse conditions. The net flow direction exerts a dominant influence on interstitial colonization, but many other factors also seem to be important in structuring the hyporheos.
7. The hyporheic corridor concept emphasizes connectivity and interactions between subterranean and surface flow on an ecosystem level for floodplain rivers. It is a complementary concept to others which focus on surficial processes in the lateral and longitudinal dimensions.
8. The ecological integrity of groundwater and fluvial systems is often threatened by human activities: (i) by reducing connectivity; (ii) by altering exchange processes; and (iii) by toxic or organic contamination.
A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants.
In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.
We used two-dimensional unconfined transient groundwater flow models to investigate the interface between stream and groundwater flow systems, or hyporheic zone, of two first-order streams that drain catchments with distinctly different alluvial sediments and bedrock lithology. Particle tracking showed that lateral hyporheic area (planimetric area of flow paths lateral to the stream that are recharged by and return to the stream with travel times of 10 days or less) differed between the two study streams and varied with discharge within each system. At the Rio Calaveras (welded tuff), lateral hyporheic area ranged from 1.7 to 4 m2 over the annual cycle. In the Aspen Creek system (sandstone), lateral hyporheic area (1–1.5 m2) was restricted to roughly half of that observed at Rio Calaveras. The size of the hyporheic zone lateral to the streams at both sites decreased by approximately 50% during high flows. Sensitivity analyses indicated that changes in the hydraulic conductivity of alluvial and streambed sediments and variation in recharge rates have greatest impact on the magnitude, direction, and spatial distribution of stream-groundwater exchange.
Rates of microbial metabolism in deep anaerobic aquifers of the Atlantic coastal plain of South Carolina were investigated by both microbiological and geochemical techniques. Rates of [2-C]acetate and [U-C]glucose oxidation as well as geochemical evidence indicated that metabolic rates were faster in the sandy sediments composing the aquifers than in the clayey sediments of the confining layers. In the sandy aquifer sediments, estimates of the rates of CO(2) production (millimoles of CO(2) per liter per year) based on the oxidation of [2-C] acetate were 9.4 x 10 to 2.4 x 10 for the Black Creek aquifer, 1.1 x 10 for the Middendorf aquifer, and <7 x 10 for the Cape Fear aquifer. These estimates were at least 2 orders of magnitude lower than previously published estimates that were based on the accumulation of CO(2) in laboratory incubations of similar deep subsurface sediments. In contrast, geochemical modeling of groundwater chemistry changes along aquifer flowpaths gave rate estimates that ranged from 10 to 10 mmol of CO(2) per liter per year. The age of these sediments (ca. 80 million years) and their organic carbon content suggest that average rates of CO(2) production could have been no more than 10 mmol per liter per year. Thus, laboratory incubations may greatly overestimate the in situ rates of microbial metabolism in deep subsurface environments. This has important implications for the use of laboratory incubations in attempts to estimate biorestoration capacities of deep aquifers. The rate estimates from geochemical modeling indicate that deep aquifers are among the most oligotrophic aquatic environments in which there is ongoing microbial metabolism.
Hyporheic metabolism in a Sonoran Desert stream was examined, focusing on the sources of detritus supporting hyporheic respiration. Two alternative hypotheses were specifically addressed: (1) organic matter derived from the surface stream supports hyporheic respiration, and (2) detritus buried during flash floods supports hyporheic respiration. As predicted for the surface-derived organic matter hypothesis, respiration was lowest immediately following flash floods and increased significantly with time after flood (P < 0.001). Hyporheic respiration ranged from 0.05 mgO(2) . L sediments(-1). h(-1) immediately following a flash flood to as high as 4.41 mgO(2) . L sediments(-1). h(-1) late in algal succession. Respiration was significantly correlated with surface algal biomass during two spring/summer successional sequences (P < 0.05; partial correlation coefficients 0.58 and 0.88). Respiration was also consistently higher in downwelling than upwelling zones with overall mean rates of 1.12 and 0.46 mgO(2) . L sediments(-1). h(-1), respectively. Respiration exhibited a distinct diel pattern with highest rate coinciding with time of maximum photosynthesis and was also significantly correlated with dissolved organic carbon concentration (P < 0.05), further supporting the hypothesis of hyporheic dependence on algal production. Flash floods bury organic matter that is also respired in the hyporheic zone; however, based upon storage of organic carbon immediately following floods, an average of only 15% of the observed respiration could be supported. We conclude that hyporheic respiration in Sycamore Creek is tightly linked to surface production. It is spatially distributed in biotic ''hot spots'' where surface waters enter hyporheic sediments and is most likely supported by organic matter that is supplied as dissolved organic carbon, perhaps from algal production.
Floodplains of large alluvial rivers are often expansive and characterized by high volume hyporheic flow through lattice-like substrata, probably formed by glacial outwash or lateral migration of the river channel over long time periods. River water downwells into the floodplain at the upstream end; and, depending on bedrock geomorphology and other factors, groundwater from the unconfined aquifer upwells directly into the channel or into floodplain springbrooks at rates determined by head pressure of the water mass moving through the floodplain hydrologic system. These large scale (km3) hyporheic zones contain speciose food webs, including specialized insects with hypogean and epigean life history stages (amphibionts) and obligate groundwater species (stygobionts). Biogeochemical processes in the hyporheic zone may naturally load groundwaters with bioavailable solutes that appear to exert proximal controls on production and biodiversity of surface benthos and riparian vegetation. The effect is especially evident in floodplain springbrooks. Dynamic convergence of aquifer-riverine components adds physical heterogeneity and functional complexity to floodplain landscapes. Because reaches of aggraded alluvium and attendant ecotonal processes occur serially, like beads on a string, along the river continuum, we propose the concept of a hyporheic corridor in alluvial rivers. We expect predictable zonation of groundwater communities and other aquifer-riverine convergence properties within the corridor from headwaters to river mouth. The landscape-level significance and connectivity of processes along the hyporheic corridor must be better understood if river ecosystems, especially those involving large floodplain components, are to be protected and/or rehabilitated.
An annual energy budget is presented for Bear Brook, a small undisturbed second-order stream in northeastern United States. The ecosystem approach, in which all input and output fluxes of potential energy as organic matter are considered, is used to describe the dynamics of energy flow in a 1,700-m segment of the stream. The annual input of energy to the system is 6,039 Kcal/m2. Over 99% of this is allochthonous, from the surrounding forested watershed or from upstream areas. Autochthonous primary production by mosses accounts for less than 1% of the total energy available to the ecosystem. Algae and vascular hydrophytes are absent from the stream. Meteorologic inputs (litter and throughfall) from the adjacent forest account for 44% of annual energy input. Most of this is in particulate form. The remaining 56% of input enters by geologic vectors (inflowing surface and subsurface waters). Eighty-three per cent of the geologic input and 47% of the total input of energy occur as dissolved organic matter. Approximately 4,730 Kcal/m2 of organic detritus, nearly equally divided between leaves and branches, is stored within the system. The size of this detritus reservoir is stable from year to year. The turnover time of the branch compartment is about 4.2 years; of the leaf compartment, about 1 year. Although much of the annual input of energy is in a dissolved state, dissolved organic matter does not tend to accumulate in the system and displays a very rapid rate of turnover. Sixty-six per cent of annual energy input is exported to downstream areas in stream water. The remaining 34% is lost as heat through consumer activity. Bear Brook is a strongly heterotrophic steady-state system in which import and export of organic matter play a significant role. A conceptual scheme is presented by which import, export, photosynthesis, and respiration may be used to describe the functional dynamics and developmental processes of ecosystems.
A stream solute workshop was held February 1-5, 1989, at The University of Mississippi with the goals of 1) suggesting a conceptual model for stream solute studies that integrates physical, chemical, and biological processes, and 2) identifying advantages and limitations of various methods for studying solute transport and exchanges. Solute dynamics refers to the spatial and temporal patterns of transport and transfers of materials that are chemically dissolved in water. Solute transport and exchange processes can be described by solute transport equations that relate solute concentration to advection, dispersion, groundwater and tributary inputs, transient storage zones, and biotic and abiotic transformations. Studies can be based on these model equations even if a full simulation of a particular system is not attempted. Although no common methodological approach can serve every investigation of solute dynamics, experimental approaches represent a range from greatest control and least realism to least control and greatest realism. The model parameters describe processes that can be investigated in laboratory, chamber, and flume experiments designed to reduce confounding experimental variables. Whole-stream studies, particularly solute injection experiments, provide estimates of solute transfer to and from the water column and can be used to calibrate the simulation models. Transport and transfer models can link experimental results obtained at different scales and increase the opportunity for inter-site comparisons and the extrapolation of results between laboratory, chamber, flume, and whole-stream studies.
Analyses of flow data for the Gila (60 years) and Pecos (68 years) Rivers in New Mexico showed that spring flows during snowmelt were significantly increased during El Niño years (periods of elevated sea surface temperature and reduced barometric pressure in the eastern tropical Pacific) and significantly decreased during La Niña years (periods of reduced sea surface temperature and elevated barometric pressure). Over the period of record for these two rivers, mean spring runoff during El Niño years was 2.3-3.2× higher than during medial years and 6.0-7.4× higher than during La Niña years. The results of this study indicate a strong correspondence between El Niño-Southern Oscillation (ENSO) phenomena and stream flow in New Mexico. These results also suggest that, in this region, the increasingly accurate, and remote forecasting of ENSO phenomena, often months in advance of the event, could be used to place future studies of biotic responses to variation in flow on a more predictive basis. Since it has been shown that the ENSO phenomenon affects the weather of large portions of the North American continent and tropical and subtropical regions worldwide, a similar potential for improved study design exists for other regions.
To understand the efficiency of interstitial habitats in the elimination of organic matter as it moves from surface water to groundwater (bank filtration), we studied spatial and temporal variations of sediment organic matter concentration, biodegradable (BDOC) and refractory (RDOC) fractions of the dissolved organic carbon, bacterial abundances, and microbial enzymatic activity in the first metre of sediment of the Rhône River immediately downstream of a large city. The study area was fed most of the year by the surface water inflow (downwelling area), because of groundwater pumping wells located ∼ 80 m from the river. Decreasing gradients from surface water to deep sediments and from the river to the shore were observed in most of the cases for the four variables. The decrease in RDOC concentrations did not vary seasonally (this decrease is probably due to physical process, such as adsorption on fine mineral particles), whereas decreases in BDOC concentrations only occurred when microbial enzymatic activities were high; BDOC is rapidly assimilated by microbial communities. Physical and biological processes together make this first metre of sediment an efficient filter for organic matter.
Chloride and nitrate were coinjected into the surface waters of a third-order stream for 20 d to examine solute retention, and the fate of nitrate during subsurface transport. A series of wells (shallow pits) 0.5-10 m from the adjacent channel were sampled to estimate the lateral interflow of water. Two subsurface return flows beneath the wetted channel were also examined. The conservative tracer (chloride) was hydrologically transported to all wells. Stream water was >88% of flow in wells <4 m from the wetted channel. The lowest percentage of stream water was 47% at a well 10 m perpendicular to the stream. Retention of solutes was greater in the hyporheic zone than in the channel under summer low-flow conditions. Nominal travel time (the interval required for chloride concentration to reach 50% of the plateau concentration) was variable by well location, indicating different flow paths and presumably permeability differences in subsurface gravels. Nominal travel time was M 24 h for wells <5 m from the wetted channel. Coinjected nitrate was not conservative. Two wells were significantly (P < .05) higher in nitrate-N than would be predicted from chloride, while four were significantly lower. Wells 2.0-4.0 m from the wetted channel tended to have higher nitrate concentration than predicted, whereas nitrate sink locations tended to have transport distances >4.3 m. The capacity of the hyporheic zone for transient solute storage and as potential biological habitat varies with channel morphology, bed roughness, and permeability. A conceptual model that considers the groundwater-stream water interface as the fluvial boundary is proposed. Emerging paradigms of the riverine network should consider the hyporheic zone and associated nutrient cycling as an integral component of fluvial structure and function.
Rates of primary production, algal biomass accumulation, and dominance by chlorophytes generally increased with higher irradiance, although these patterns were modified by herbivores. Ungrazed periphyton at low irradiance (photon flux density: 20μmol.m-2.s-1) accumulated little biomass, which was further reduced by grazing snails Juga silicula. At intermediate (100 μmol.m-2.s-1) and high (400 μmol.m-2.s-1) irradiance, snails delayed accumulation of algal biomass but did not affect final biomass attained. After 43 d, net primary production (NPP) at high irradiance was unaffected by grazing, whereas grazing increased NPP at both low and intermediate irradiance. Algal export increased with both irradiance and the presence of grazers and constituted a significant loss of plant biomass from the streams. Grazing by Juga delayed algal succession and altered algal taxonomic structure and assemblage physiognomy by reducing the relative abundance of erect and non-attached algae, while increasing the abundance of adnate diatoms. Snails grew slowly at low irradiance, due to scant food resources, but had high growth rates at intermediate and high irradiance. Assimilation efficiencies for snails generally varied from 40-70% and were highest at low irradiance. At low irradiance, 90% of benthic production was harvested by grazers; only 10% accumulated as attached biomass or was exported. At higher irradiances, <15% of primary production was harvested by grazers, and >85% persisted as attached algae or was exported. Biomass and production of grazers were influenced by abiotic constraints placed on algal productive capacity (the ability of a plant assemblage to generate biomass). Structure and metabolism of algal assemblages were affected by consumptive demand of herbivores. -from Authors
Carbon, nitrogen or phosphorus limitation of microbial growth in a forest and a field soil was evaluated from measurements of respiration and protozoan counts after nutrient addition. In both soils simultaneous addition of C (glucose) and N (NH4NO3) resulted in microbial growth as indicated by a gradual increase in respiration rate whereas a single addition of C did not induce microbial growth. Addition of P (KH2PO4 + Na2HPO4) to C-amended soil stimulated activity slightly in forest soil but not in field soil whereas addition of P to soil amended with C and N increased growth markedly in both soils. The stimulation of microbial growth indicated by respiration profiles following C and N addition was supported by enumeration of bacterivorous protozoa. The numbers of soil protozoa increased above the control only when C and N were added simultaneously. These observations lend support to the proposal that respiration patterns may be valuable in an analysis of nutrient limitation of microbial growth in soils.
We address the ecological ramifications of variation in hydrologic interaction between streams and alluvial aquifers in catchments with alluvium derived from parent materials of contrasting geologic composition. We present a conceptual model in which solute retention in streams results from hydrologic retention (increased water residence time resulting from surface-groundwater exchange), biological nutrient cycling, and chemical processes. Solute injection experiments were done in study catchments comprised of sandstone- siltstone (site l), volcanic tuff (site 2), and granite-gneiss (site 3). Distribution of an injected conservative tracer (Br) illustrated that rate and extent of surface-water penetration into the alluvial aquifer increased across study catchments as was predicted from increasing alluvial hydraulic conductivity. Concurrently, groundwater inputs at baseflow represented between 13 and 57% of aboveground discharge at upstream transects, indicating bidirectional hydrologic exchange along the study reaches. N : P ratios in surface water ranged from 4 to 16, suggesting strong biotic demand for inorganic N. Coinjection of NaBr and NaNO, revealed longest nitrate uptake length (S,) at site 1, intermediate S, at site 2, and shortest uptake length at site 3. Modeling of transient hydrologic solute storage revealed that S, correlated with hydraulic storage, suggesting an important role for subsurface processes in total nitrate retention.
Distinct seasonal patterns in streamwater nutrient and dissolved organic carbon (DOC) concentrations are evident in the data record from 7 years of weekly sampling in the West Fork of Walker Branch (WB) and 4 years of weekly sampling in a nearby stream, upper White Oak Creek (WOC), both first-order streams in the Ridge and Valley Province of eastern Tennessee. Concentrations of NO3 and soluble reactive phosphorus (SRP) in both streams showed a repeated pattern of annual maxima in summer and biannual minima in autumn and spring. Concentrations of DOC in WB exhibited distinct autumn maxima. To determine whether temporal variations in catchment hydrological processes could explain the seasonal nutrient and DOC variations in WB, we used an end-member mixing analysis involving Ca and SO4 concentrations to separate stream discharge into three catchment flow paths of differing nutrient concentrations. Stream NO3, SRP, and DOC concentrations were predicted solely on the basis of temporal variation in the importance of these flow paths and measurements of nutrient concentrations for the different flow paths. Ratios of observed/predicted concentrations in stream water near 1.0 suggested that catchment effects alone explained streamwater concentrations, whereas ratios substantially different from 1.0 suggested that in-stream processes were important determinants of streamwater concentrations. Observed/predicted NO3 and SRP concentration ratios showed repeated annual patterns with values closer to 1.0 during winter and summer (generally 0.8-1.2) and minima (=1 but did show consistent autumn maxima (>2.5), indicating substantial in-stream DOC generation at this time. Observed/predicted ratios for all nutrients were generally less variable and were closer to 1.0 at high flow compared to low flow, suggesting that in-stream controls on streamwater chemistry are less important at high discharge than at low discharge. Our results indicate two general modes of control of stream nutrient concentrations: (1) catchment control via seasonal variation in the dominant hydrologic pathway (greater proportion of deep groundwater in summer), which produces lower winter and higher summer concentrations, and (2) in-stream control via high rates of net nutrient uptake during the spring (primarily by autotrophs) and autumn (primarily by heterotrophs).
An in situ acetate injection was used to determine the influence of labile dissolved organic carbon (DOC) avail- ability on microbial respiration in the hyporheic zone of a headwater stream. We added bromide as a conservative tracer and acetate as an organic substrate to the hyporheic zone of Rio Calaveras, New Mexico, via an injection well. Tracer was observed in four of eight capture wells. Three of the four wells showed increases in bromide without concurrent increases in acetate concentration, suggesting 100% acetate retention. One well had 38% acetate retention. Pore velocity and acetate retention were negatively correlated, suggesting hydrologic control of acetate retention. Acetate did not significantly sorb to the sandy hyporheic sediments at this site, indicating biological consumption of acetate. Acetate addition stimulated total CO2 production along monitored flowpaths and led to changes in solutes associated with microbial terminal electron-accepting processes (TEAPs). Dissolved oxygen (DO), nitrate, and sulfate significantly decreased, and ferrous iron and methane significantly increased compared to background concentrations in most wells. These results support the hypothesis that microbial respiration in the hyporheic zone is limited by labile DOC availability. Furthermore, we have shown that a suite of metabolic pro- cesses, from aerobic respiration to methanogenesis, cooccur and that anaerobic processes dominate heterotrophic metabolism in the hyporheic zone of Rio Calaveras.
Though limited in extent, seasonal snowpacks in the Earth's mountain
ranges account for the major source of the runoff for stream flow and
groundwater recharge over wide areas of the mid-latitudes. In most
alpine regions, snowmelt runoff is responsible for the annual maximum
instantaneous discharge and most of the annual flow. It is therefore
important to understand the processes controlling snowmelt runoff and
its chemical composition for both water resources as well as ecological
reasons. Progress in snow hydrology over the past quadrennium has been
heavily influenced by the availability and anticipated future
availability of remote sensing data at times and spatial scales
appropriate for addressing local, regional and global hydrologic issues
[Dozier, 1992]. Recent research has examined topics contributing to
better estimation of snowpack properties, snow distribution, snowmelt,
and related chemical and biological questions. This review focuses
mainly on seasonally snow-covered alpine areas, but includes other
research potentially applicable to alpine areas of the Earth.
Respiration, photosynthetic production, and diffusion interact to produce the daily curve of oxygen change in a segment of flowing water. Conversely, the observed curves of oxygen in streams can be used to calculate the component rates of production, respiration, and diffusion. New production values obtained with these analyses of oxygen curves from various sources, as well as a few previously existing estimates of primary production, indicate a generally higher rate of production in flowing waters than in other types of aquatic environments.
The ratio of total primary production to total community respiration is used to classify communities quantitatively according to their predominantly heterotrophic or autotrophic characteristics. Longitudinal succession within a stream tends to modify the ratio towards unity from higher values for autotrophic and from lower values for heterotrophic communities. The behavior of this ratio is described for the annual cycle in a stream, for the sequence of pollution recovery, and for diverse types of communities.
Interactions between surface waters and groundwaters were analysed by studying degrees of similarity between a surface stream and groundwater in a karstic system.
Exchange of water and solutes (‘abiotic interactions’) and living organisms (‘biotic interactions’) were quantified by characteristic parameters measured at the outlet of the karstic system during hydrological changes across the year as well as during a flood.
The ratio of bicarbonate to calcium described abiotic interactions. In the seasonal study, biotic interactions were estimated from the proportion of epigean organisms in the communities sampled at the outlet. These interactions during the flood were estimated by comparing numbers of Cladocera, epigean organisms, with numbers of Niphargus virei , hypogean organisms.
The location of the ecotone created by interactions between the surface stream and the karstic system showed marked spatial fluctuations according to the prevailing hydrology. Interactions were strong during high‐water periods whereas they were negligible during low‐water. Similar fluctuations occurred within a few hours during the flood.
Results are compared with those from fluvial systems to discuss the importance of the dynamics of these surface/underground ecotones in the functioning of lotie ecosystems.
A method is described for the rapid determination of dissolved organic carbon in seawater in concentrations between 0.1 and 20 mg/liter. The oxidation is carried out in sealed glass ampoules using K 2 S 2 O 8 as an oxidizing agent after the sample has been freed of inorganic carbon. The resulting CO 2 is passed through a nondispersive infrared analyzer using nitrogen as a carrier and the signal output of the analyzer recorded. Using appropriate calibration curves, the carbon content is determined from the height of the peak. Approximately 100 samples can be analyzed in a single day with a precision of ± 0.1 mg/liter using a sample volume of 5 ml.
A companion method is described for the determination of particulate carbon by high‐temperature combustion after concentration of the sample on a glass‐fiber filter. The precision of this method is ± 10 µ g C in a range of 0–500 µ g C. Approximately six samples can be analyzed in an hour.
Hydrological mechanisms controlling the variation of dissolved organic carbon (DOC) were investigated in the Deer Creek catchment located near Montezuma, CO. Patterns of DOC in streamflow suggested that increased flows through the upper soil horizon during snowmelt are responsible for flushing this DOC-enriched interstitial water to the streams. We examined possible hydrological mechanisms to explain the observed variability of DOC in Deer Creek by first simulating the hydrological response of the catchment using TOPMODEL and then routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model can be taken to represent a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out when infiltrating meltwaters cause the water table to rise into this “reservoir”. Concentrations of DOC measured in the upper soil and in streamflow were compared to model simulations. The simulated DOC response provides a reasonable reproduction of the observed dynamics of DOC in the stream at Deer Creek.
The spatial distribution of source areas and associated residence times of water in the catchment are significant factors controlling the annual cycles of dissolved organic carbon (DOC) concentration in Deer Creek (Summit County, Colorado). During spring snowmelt (April–August 1992), stream DOC concentrations increased with the rising limb of the hydrograph, peaked before maximum discharge, then declined rapidly as melting continued. We investigated catchment sources of DOC to streamflow, measuring DOC in tension lysimeters, groundwater wells, snow and streamflow. Lysimeter data indicate that near-surface soil horizons are a primary contributor of DOC to streamflow during spring snowmelt. Concentrations of DOC in the lysimeters decrease rapidly during the melt period, supporting the hypothesis that hydrological flushing of catchment soils is the primary mechanism affecting the temporal variation of DOC in Deer Creek. Time constants of DOC flushing, characterizing the exponential decay of DOC concentration in the upper soil horizon, ranged from 10 to 30 days for the 10 lysimeter sites. Differences in the rate of flushing are influenced by topographical position, with near-stream riparian soils flushed more quickly than soils located further upslope. Variation in the amount of distribution of accumulated snow, and asynchronous melting of the snowpack across the landscape, staggered the onset of the spring flush throughout the catchment, prolonging the period of increased concentrations of DOC in the stream. Streamflow integrates the catchment-scale flushing responses, yielding a time constant associated with the recession of DOC in the stream channel (84 days) that is significantly longer than the time constants observed for particular locations in the upper soil. © 1997 John Wiley & Sons, Ltd.
The following topics are discussed: the advantages of an interdisciplinary approach to ground water studies; the porous aquifer as an open system; the problem of the terminology used in ground water ecology; the interest of ecologists in hydrogeological terminology; biological activity in the unsaturated zone of porous aquifers; the distribution of ground water fauna in the saturated zone of porous aquifers; ground water animals as environmental indicators; the manipulation of ground water ecosystems; basic lines for the further development of ground water ecological studies.
Minipiezometers installed at different vertical levels within the streambed (20–140 cm) were used to study temporal and spatial variation in the dissolved organic carbon (DOC) content of streamwater and groundwater in three southern Ontario streams. Groundwater, as represented by our streambed samples, contained considerable quantities of DOC but variation between replicate samples was high. Diel fluctuations in DOC content of streamwater were consistent with daytime autochthonous production and night-time uptake by heterotrophs. Water from the streambed neither consistently diluted nor enhanced streamwater levels of DOC. At some stations, DOC variation with depth, including streamwater, seemed to be largely random. At other stations, DOC concentrations from the deepest piezometers were consistently higher than concentrations at intermediate depths, suggesting a loss of DOC from deeper waters to overlying sediments. However, at these stations DOC concentrations were highest at 20 cm and at the surface. Interflow delivery of DOC to the shallow layers of the streambed may be a significant source of carbon for a stream ecosystem, especially in agricultural areas. Late summer diel fluctuations at one station may be related to changing patterns of intermixing of stream and groundwater in the upper layers of the streambed as governed by velocity heads, convective currents and evapotranspiration.
Dissolved organic carbon (DOC) concentrations and export were studied in two small catchments in central Ontario to examine DOC sources and to assess the hypothesis that organic matter adjacent to the stream is a significant contributor of DOC during storms. Different DOC dynamics and exports were observed according to the depth of the riparian water table. In Harp 4-21, riparian flowpaths were predominantly through A and upper B soil horizons and riparian soils contributed between 73 and 84% of the stream DOC export during an autumn storm. In Harp 3A, riparian flowpaths were predominantly through lower B horizons. Consequently, riparian soils were less important and hillslopes contributed more than 50% of the stream DOC export in subcatchments without wetlands during storms. Wetlands and adjacent soils contributed significantly to DOC export in Harp 3A; 8% of the total catchment area exported 32 to 46% of the storm runoff DOC. DOC export dynamics in wetlands and riparian soils were distinctly different. In wetlands, transport was affected by leaching and flushing of DOC at the wetland surface leading to lower DOC concentrations with successive storms. In riparian soils, groundwater flowpaths were more important and stronger positive relationships between discharge and DOC concentration were observed. Precipitation, throughfall and stemflow were minor sources of stream DOC during storms and contributed less than 20% of the total export.
The importance of groundwater in the dissolved organic matter (DOM) budget of small upland streams is not well understood. This paper is concerned with the