Recharge from glacial meltwater is critical for alpine springs and their microbiomes
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LETTER • OPEN ACCESS
Recharge from glacial meltwater is critical for alpine springs and their
microbiomes
To cite this article: Jordyn B Miller et al 2021 Environ. Res. Lett. 16 064012
View the article online for updates and enhancements.
This content was downloaded from IP address 46.4.80.155 on 20/09/2021 at 03:10
Environ. Res. Lett. 16 (2021) 064012 https://doi.org/10.1088/1748-9326/abf06b
LETTER
Recharge from glacial meltwater is critical for alpine springs and
OPEN ACCESS
their microbiomes
RECEIVED
16 October 2020 Jordyn B Miller1,∗, Marty D Frisbee1, Trinity L Hamilton2 and Senthil K Murugapiran3
REVISED 1
26 February 2021 Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, United States of America
2
Department of Plant & Microbial Biology and The BioTechnology Institute, College of Biological Sciences, University of Minnesota,
ACCEPTED FOR PUBLICATION
Twin Cities, MN, United States of America
19 March 2021 3
Current address: Diversigen, 600 County Road D, West, Suite 8, New Brighton, MN, United States of America
PUBLISHED ∗
Author to whom any correspondence should be addressed.
20 May 2021
E-mail: jorbmiller@purdue.edu
Original content from Keywords: hydrogeology, groundwater, microbe, glacial melt, alpine glacier
this work may be used
under the terms of the Supplementary material for this article is available online
Creative Commons
Attribution 4.0 licence.
Any further distribution
of this work must Abstract
maintain attribution to
the author(s) and the title The importance of glacier meltwater as a source of mountain-block recharge remains poorly
of the work, journal quantified, yet it may be essential to the integrity of alpine aquatic ecosystems by maintaining
citation and DOI.
baseflow in streams and perennial flow in springs. We test the hypothesis that meltwater from
alpine glaciers is a critical source of recharge for mountain groundwater systems using traditional
stable isotopic source-identification techniques combined with a novel application of microbial
DNA. We find that not only is alpine glacier meltwater a critical source of water for many springs,
but that alpine springs primarily supported by glacial meltwater contain microbial taxa that are
unique from springs primarily supported by seasonal recharge. Thus, recharge from glacial
meltwater is vital in maintaining flow in alpine springs and it supports their distinct
microbiomes.
1. Introduction exacerbated by geographical and geological variabil-
ity in recharge processes.
Mountain-block recharge (MBR) occurs when rain, We contend that glacial meltwater can infilt-
snowmelt, or possibly meltwater from glacial ice, rate and recharge both shallow aquifers occurring
infiltrates and enters the saturated media beneath the within glacial moraines and sediments as well as
water table in the high elevations of a mountain- deep mountain-block aquifers hosted within bed-
ous watershed. There are considerable uncertainties rock. Thus, the water balance of a glaciated alpine
regarding the partitioning of meltwater released from catchment should include recharge to both shallow
alpine glaciers, especially how much, if any, contrib- and deep aquifers (figure 1). Alpine glaciers form in
utes to MBR. Traditionally, meltwater from alpine diverse lithologies around the world and the intrinsic
glaciers is assumed to leave catchments by (a) surface controls on groundwater flow within these litholo-
runoff as streams, (b) infiltrating and flowing through gies can be highly variable (i.e. different types of
moraine sediment or alluvium, and/or (c) rechar- rocks have different porosities and permeabilities). In
ging shallow alluvial aquifers (figure 1). The bedrock addition, the processes that create mountain ranges
of glaciated mountain blocks is commonly treated (orogenic processes) affect the porosity of the moun-
as an impermeable barrier due to glacial polishing, tain block. Bedrock may have primary porosity, or
thereby limiting infiltration and eventual recharge porosity present when the rock was formed, although
into the mountain groundwater system. However, some crystalline rocks such as plutonic and meta-
there is anecdotal evidence that alpine catchments can morphic rocks may have very low primary porosity.
receive groundwater recharge specifically from gla- Tectonic processes can create fault and fracture net-
cial melt [1–3]. The magnitude of recharge from gla- works called secondary porosity, or porosity formed
cial meltwater and its role in mountain groundwater after the rock was formed. Secondary porosity in
processes remain poorly quantified. This problem is alpine catchments also forms due to processes such
© 2021 The Author(s). Published by IOP Publishing Ltd
Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Figure 1. Conceptual diagram of major hydrological components in a glaciated alpine catchment. Shallow or local flowpaths are
denoted by small, short arrows while intermediate length flowpaths are shown with longer arrows. Groundwater can flow via
primary porosity (pore spaces between grains), as well as through secondary porosity (fractures). The porosity of a mountain
groundwater system varies with lithology and tectonic history.
as glacial isostatic rebound and basal water pres- Range, U.S. [9]. This observation suggested seismi-
sure. Both of these processes can therefore facilit- city was caused by groundwater recharge, but seismi-
ate groundwater recharge and enhance deep circu- city alone cannot distinguish between the recharge of
lation both within the mountain-block and beyond annual snowpack and glacial meltwater. Other studies
to adjacent valleys [4]. Therefore, the geologic and in the Cascade Range have shown that stable isotopic
tectonic history of a mountain system must be con- values of water (δ 2 H, δ 18 O) from some low-elevation
sidered when investigating the contribution of glacial springs require a high-elevation recharge source, sug-
melt to groundwater recharge in these mountain- gesting that recharge is sourced from snow or perhaps
ous systems. To address this knowledge gap, we spe- glacial melt [10, 11].
cifically investigate the presence and significance of Stable isotopes (δ 18 O and δ 2 H) have been used
MBR from glacial meltwater in dissimilar lithologies extensively to identify the sources of recharge to
(figure S1 (available online at stacks.iop.org/ERL/16/ aquifers in mountainous watersheds where rain and
064012/mmedia)). snow are the only potential sources of recharge
Stable isotopic analyses of deep, regional aquifers [12–15]. While glacial ice is effectively composed of
and ice-sheet/groundwater models indicate substan- compounded seasonal snowpack over the past sev-
tial groundwater recharge occurred from melting eral hundred years, glacier ice and modern-day snow
continental ice sheets during the Pleistocene [5–7]. are commonly isotopically distinct since there have
Seasonal snowmelt has supported MBR in mountain- not been substantial isotopic excursions in precip-
ous regions around the globe since the Last Glacial itation over that same time interval. In fact, on a
Maximum (LGM). Over longer timescales, snowpack local meteoric water line (LMWL) plot, the iso-
is transformed into glacial ice only to be released topic composition of glacier ice falls in line with
back to the active hydrological systems during gla- seasonal snow and rain (but is isotopically lighter
cial retreat. If and to what extent melting mountain than snow), thereby making it difficult to partition
glaciers contribute to MBR in modern alpine catch- the meltwater using traditional isotopic separation
ments remains unknown. Despite the importance of techniques. Because alpine glacier ecosystems host
the world’s water towers [8], it is extremely difficult extremophile microbial communities that can be isol-
to decouple the magnitude of recharge from alpine ated and unique to specific alpine and coastal polar
glacier meltwater from that of seasonal snowpack in regions [16–22], we propose that DNA sequence
mountain groundwater systems. For example, tim- information endemic to glacier microbes can be
ing of seasonal snowmelt and increased seismicity used as an additional indicator of recharge from
were correlated on a glaciated volcano in the Cascade glacial melt.
2
Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Providing estimates on how much meltwater con- containing Mesoproterozoic and sedimentary form-
tributes to recharge and how much MBR from gla- ations with Cretaceous shales underlying them [31].
cial meltwater supports perennial flow in springs and The laminated, low porosity formations are highly
baseflow in alpine streams will significantly improve fractured and some contain dolomitic units which
our understanding of the spatial and temporal extent allow fracture flow, as well as flow along contacts
of ecological responses to glacial retreat [16–19, 23]. A and through karstic conduit systems. GNP represents
recent study specifically highlights that the ecohydro- a severe case of retreat, as there are only 26 of 150
geology of springs and their riparian habitats hold not named glaciers in GNP remaining and model projec-
only immense biodiversity and socio-cultural signi- tions predict they will disappear between 2030 and
ficance, but also enhances our hydrogeological under- 2080 [32–34].
standing of groundwater systems [24]. As such, the
incorporation of DNA as an environmental tracer for 2.3. Spring and stream water collection
aquatic macroinvertebrates and fish populations is Springs samples collected for this study were selec-
increasing in popularity due to cost-effectiveness and ted to achieve a broad spatial distribution with
increases in availability. Similar applications using respect to elevation, geologic setting, and aspect.
DNA to assess the microbial assemblages have been Most spring locations were identified by finding
applied to track rainfall-recharge on Mount Fuji labelled spring locations on published topographic
[25, 26], but this approach has not been used as a maps, while other locations were identified by satel-
tracer of glacier meltwater. Given the likelihood that lite imagery, suggestions from National Forest Ser-
many alpine glaciers will disappear within this cen- vice and National Park Service scientists and staff, as
tury, it is imperative that we quantify the role of well as previously published work [10, 11]. Samples
meltwater from alpine glaciers on mountain ground- of water for stable isotopic analyses were collected
water systems and the significance of their aquatic during the following sampling campaigns: July 2016,
ecosystems. October 2016, July–August 2017, July–August 2018,
and September 2019. While spring samples were pre-
2. Methods ferred for this study, stream samples were collected
when the region containing the spring source could
2.1. Overview not be physically accessed due to rugged terrain, safety
In this study, we combine a Bayesian isotopic end- protocols, or physical limits of the team regarding
member mixing model with 16S rRNA amplicon carrying weight. Differentiation of springs vs streams
sequencing to identify the magnitude of recharge is noted in supplementary dataset S1: table 2. Water
from glacial meltwater in alpine springs and streams. samples from springs and streams were collected
We target rapidly retreating glaciers in Mount Hood using a 3D printed, portable peristaltic pump and
National Forest (MH) of Oregon, USA and Glacier Masterflex silicon tubing. The tubing was placed in
National Park (GNP) of Montana, USA (figures 2 and the spring emergence and a filter was attached to the
S1, supplementary dataset S1: table 2). These sites opposite end (see Microbial DNA methods section
vary by bedrock lithology and meteorological condi- below for more information on filtering apparatus).
tions, thus allowing us to examine the role of bed- Care was taken to ensure spring source areas were not
rock and seasonal precipitation on MBR. We analyzed disturbed. Stream samples were collected by placing
samples of glacier ice, snow, rain, stream, and spring the tubing into the stream. Water was pumped dir-
water collected in MH and GNP from 2016 to 2019. ectly into new Nalgene bottles which were field rinsed
three times with the water being collected at each site.
2.2. Site description Water samples were collected in 250 ml, 500 ml, or 1 l
MH is a stratovolcano in the Cascades with underly- Nalgene bottles and the lids were sealed with electrical
ing geology consisting of andesitic lavas with depos- tape. Samples were stored in our vehicle in coolers at
its from pyroclastic and lahar flows from late-glacial roughly 12 ◦ C–15 ◦ C while completing the fieldwork.
through the Holocene [10, 27]. Very high discharge Samples were stored in the coolers from a few days up
springs in the area (upwards of 283 l s−1 at one loc- to 3 weeks, depending on when samples were collec-
ation), in combination with published isotope and ted during the sampling campaigns. Once returned to
water chemistry data, reveal that springs cannot be the lab, the water samples were kept refrigerated until
topographically defined by watersheds, as they appear the time of analysis.
to be discharging water from a larger area encom- Water was pipetted from the Nalgene bottles
passing recharge at higher elevation area [10, 28, 29]. to 2 ml glass autosampler vials with plastic screw
The glaciers on MH have decreased in area by approx- caps for isotopic analysis. Maximum storage time
imately 34% since the beginning of the 20th century, from the time of collection in the field to pipetting
but the sizeable glaciers still present, and easy access and analysis was up to 2 months. Decontamination
to them, makes this an ideal study site [30]. GNP, protocol for MH water sampling included using a
Montana is located in the Livingston and Lewis Range 91% ethanol (C2 H5 OH) solution to decontaminate
3Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Figure 2. Sampling locations categorized by sample type for MH (A)–(E) and GNP (F)–(H) and surrounding regions. Inset boxes
are denoted by corresponding border color. Spring and stream samples are shown in white, and those having SSU rRNA
sequencing data are shown with an ‘X’. See supplementary dataset S1: table 2 for sample location details.
tubing, shoes and clothes of the research team, 2.4. Isotopic analysis
and other equipment that came in contact with The ice in MH and GNP is estimated to have been
the water between sampling sites. GNP decontam- deposited a maximum of 7000 years ago, but peaked
ination protocol involved following aquatic invas- in the mid-1800s during the Little Ice Age [30, 35–37].
ive species procedures, specifically GNP Aquatic Therefore, the ice remaining today is likely a few
Disinfection Guidelines, in addition to using the hundred years old and is isotopically distinct from
ethanol solution between sampling locations in modern day precipitation. The isotopic end-member
the park. values for this study were selected based on represent-
In MH, 88 total water samples were collected ative samples of collected glacial ice, snowpack, and
including samples of ice, snow, and rain. Of the 88 rain for MH and glacial ice and snowpack for GNP,
samples, 54 samples were spring/stream samples and as we assume these are the predominant sources of
34 were non-spring/stream samples (either glacial ice, recharge in each respective region (table 1).
glacial melt, snow, snow algae, or rain). In total, 38 Analysis of δ 18 O and δ 2 H were measured by the
spring/stream locations were visited in MH with 14 of Purdue Stable Isotope Laboratory (PSI Lab) using
the sites being visited multiple times. In GNP, 44 total a Los Gatos Research, Inc. Triple Water Vapor Iso-
water samples were collected with 11 being samples tope Analyzer (model: 911-0034). The reported pre-
of either glacial ice, glacial melt, snow, or snow cision including correction for water vapor depend-
algae. The remaining 33 samples were spring/stream ence, memory, and drift is 0.2‰ for δ 18 O and 2.0‰
samples with 11 sites being repeat sample sites. Thus, for δ 2 H. The values reported are relative to the Vienna
a total of 22 spring/stream locations were visited. This Standard Mean Ocean Water–Standard Light Antarc-
information can be viewed in table format in supple- tic Precipitation scale (VSMOW-SLAP). The data is
mentary dataset S1. presented in figure 3.
4Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Table 1. Isotopic end-member values and number of samples (n). End-member values are represented by the mean of the data. Site is
abbreviated as MH for Mount Hood National Forest or GNP for Glacier National Park.
End-member δ 18 O (‰) σ δ 18 O δ 2 H (‰) σ δ2 H n
MH
Glacier ice −14.15 0.36 −103.18 4.29 3
Snow −11.72 0.75 −84.60 5.61 14
Rain −10.18 0.85 −70.06 7.51 6
GNP
Glacier ice −18.07 0.31 −135.47 1.51 2
Snow −15.37 0.70 −116.89 7.06 8
2.5. Isotopic end member mixing model between predictions and data [39]. The subsequent
The Bayesian Monte Carlo mixing model is a mod- rejection or acceptance of the prior samples retained
ified version of Arendt et al [39]. The strategy used are relative to the most likely model. Samples of
is that samples of a prior probability density function the prior are accepted if the likelihood is 1, accep-
(PDF) are either accepted or rejected in proportion to ted half of the time if likelihood is 0.5, and never
the likelihood of the data, and yield samples of a pos- accepted if the likelihood is 0 [39]. Roughly 107
terior PDF [39]. The form of Bayes’ theorem applied prior samples were tested and as described above,
here is: the posterior samples retained were in proportion to
the relative likelihood of the associated prediction.
p (fi ) ∝ p ′ (fi & li ) L (oj | fi & li ) (1) The output of the model provides the best estim-
ates of the fractional contribution of each end mem-
where fi are the fractional contributions, oj are the iso-
ber along with mean values and standard deviations
topic measurements, li are the isotopic composition
of the posteriors. Samples that were found to have
of the end-member components, p(fi ) and p′ (fi & li )
a fractional contribution greater than 0.60 of gla-
are the prior and posterior PDFs respectively, and
cial ice were identified as being ‘glacially influenced’
L(oj |fi & li ) is the likelihood function [39]. The pri-
or having substantial glacial recharge. This threshold
ors of the fractional contributions are that of ice (fi ),
value was selected after performing sensitivity tests
snow (fs ), and rain (fr ) and use the assumption that
on the model using the endmembers from this study.
these are the three possible sources of recharge. The
Section 2.6 outlines additional information regard-
prior PDF includes variance in the isotopic compos-
ing this sensitivity study. If a location was sampled
itions because each end member is not represented
multiple times, the isotopic value providing the max-
by a single value. Additionally, we assumed that the
imum glacial influence was selected for that site to
end member compositions are normally distributed
locate all spring/stream locations containing poten-
with standard deviation given by the measurement
tial glacial melt contribution.
uncertainties. We also assume the uncertainties of the
isotopic compositions of the end members are uncor-
related. For MH, the fractional constraint is represen- 2.6. Isotopic model sensitivity study
ted by fi + fs + fr = 1, while GNP is represented by Isotopic samples that had a fractional contribution
fi + fs = 1. We assume that all combinations between from glacial ice of 60% or greater are defined as
0 and 1 are equally likely that the uncertainties of the being glacially influenced. This threshold was determ-
measured isotopic compositions are purely Gaussian ined by creating known fractions of ice, snow, and
and uncorrelated. Therefore, the data likelihood func- rain representing 50 theoretical spring samples. The
tion is given by fractional values were created using a random uni-
form distribution. The known fractional values of ice,
( )2 ( )2
δ 18 Op − δ 18 Oo δ 2 Hp − δ 2 Ho snow, and rain were used to calculate the theoretical
L ∝ exp × exp δ 18 O and δ 2 H values that were then run through the
2σδ18 O 2σδ2 H
(2) models. To see how well the models estimate the frac-
tional contribution from glacial ice, the same end-
where δ 18 Op and δ 18 Oo are the predicted and member zones that were used in the analysis of the
observed measurements of δ 18 O, δ 2 Hp and δ 2 Ho are field-collected MH and GNP samples were also used
the predicted and observed measurements of δ 2 H, for this sensitivity study.
and σδ18 O and σδ2 H are the measurement uncer- The fraction of ice, snow, and rain were calcu-
tainties. The predicted isotopic values are calculated lated for the theoretical MH samples. Likewise, the
using a standard linear mixing model. A Monte Carlo fraction of ice and snow were calculated the theoret-
sampling scheme is used where random samples of ical GNP samples. The standard deviation of the of
the prior are retained as samples of the posterior the fractional values predicted by the model act as
proportional to the likelihood based on the misfit upper and lower bounds for the estimated fraction
5Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Figure 3. Dual stable isotopic plot for MH (A) and GNP (B). The δ 18 O and δ 2 H values for precipitation trend predictably, falling
along the global meteoric water line (GMWL) [38]. The endmember zones used in the mixing model are shown in color. Values
for the end members are shown in table 1 and isotopic values of the data are listed in supplementary dataset S1: table 2. Spring
and stream samples are shown by black outlined circles, and those having SSU rRNA sequencing data are shown with an ‘X’.
Samples shaded in teal represent the samples that receive significant contribution from glacial ice according to the isotopic model.
Error bars shown include water vapor dependence, memory, drift, as well as normalization of the samples to the VSMOW/SLAP
scale. The GMWL is presented on both plots. A local meteoric water line (LMWL) was created for MH by fitting a linear
regression through the precipitation (snow and rain) samples collected. A previously published LMWL for MH is also shown
[10]. The LMWL for GNP was created by applying a linear regression through previously published USGS data. Additional
information regarding the GNP LMWL can be found in figure S2 and supplementary dataset S1: table 1.
contributions. Therefore, if the model is performing value receive a significant amount of flow from glacial
well, the assigned theoretical fractional values should ice. This means that additional springs in this study
fall within the upper and lower standard deviation may also be composed partly of glacial melt, but at
bounds. lower fractions, because although this model works it
For this study, we were particularly interested in does have limitations. The threshold value chosen was
identifying, with certainty, which springs receive a 60% and was determined by assessing above sensitiv-
large fraction of water from glacial ice. This is accom- ity study’s results. Specifically this was done by identi-
plished by assigning a threshold value for the frac- fying at what percentage the model could no longer
tion from ice, where springs having greater than this estimate the fractional contribution of glacial ice in
6Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
the sample. In laymen’s terms, this means that below Community composition was visualized using R soft-
60% ice, the assigned fractional value of ice was no ware packages. Specific package citations are listed in
longer consistently falling within the standard devi- the supplementary information.
ation bounds provided by the model. Often, above the
60% threshold the model underestimated the con- 3. Results
tribution, providing a conservative estimation. The
sensitivity study is presented in the supplementary 3.1. Isotopic model results
material. The stable isotopic data for this study plots along
the Global Meteoric Water Line (GMWL) and the
2.7. Microbial DNA collection, extraction, and end member ranges provide appropriate separation
rRNA gene sequencing for a mixing model (table 1, figures 3 and S2, sup-
Filtering was completed by either filtering the water in plementary dataset S1: table 2). The isotopic ana-
a sterilized, negative pressure hood using a vacuum lysis shows that numerous springs in MH and GNP
apparatus and (12 h, 450 ◦ C) GF/F sterilized fil- are supported by recharge from glacial melt. In
ters (0.22 µm and 0.45 µm pore size; Sterlitech Cor- MH, 88 total samples including end members and
poration) after sample return to the lab, or filtered spring/stream samples were collected over the course
in the field using individually sealed, gamma irra- of the sampling campaigns, with 38 of these being
diated, sterile, polyethersulfone pressure filter unit individual spring/stream locations. Our isotopic ana-
membranes (0.22 µm pore size; Sterivex). Between lyses show that 26% of the locations contained more
250 ml and 3 l of water was filtered per site. Snow than 60% glacial melt contribution to total flow. In
and ice samples were first melted and then filtered. GNP, 44 total samples were collected with 22 being
Field-filtered samples were filled with RNALater and individual spring/stream locations. Of these, 54%
capped on both ends for preservation and stored contained more than 60% glacial melt. The results
at −20 ◦ C after transport until nucleic acid extrac- from MH and GNP provide support for our hypo-
tion. These filters were stored in falcon tubes for thesis that alpine glacier meltwater is an important
transport purposes. Prior to extraction, the samples source of recharge for alpine springs. These results are
were washed with 18.2 MΩ cm−1 water to remove plotted graphically in figure S3, spatially in figures S4
RNAlater. DNA was extracted from filters using and S5, and in table format in supplementary dataset
a Qiagen PowerLyzer PowerSoil kit according to S1: table 4.
the manufacturer’s instructions. An unused filter or The importance of glacial-melt recharge is not
18.2 MΩ cm−1 water served as a negative control dur- confined to springs located close to glaciers. For
ing each round of DNA extractions. No DNA was instance, MH springs supported by glacial-melt
detected in any negative controls. Concentration and recharge were more than 10 km away from glacial
quality of DNA were assessed using a Qubit fluor- ice. In contrast, in GNP, low permeability rock and
imeter and 1% agarose gels, respectively. We inter- structural and stratigraphic controls on groundwater
preted high quality DNA as high molecular weight flow restrict the influence of glacial-melt recharge to
whereas low quality/degraded DNA was apparent as springs closer to glacial ice. In both locations, ground-
low molecular weight bands. Using relative propor- water does not always follow topographic watershed
tions, we assigned a quality score to each DNA sample divides and the south-facing glaciers are retreating
between high, medium, and degraded. This quality faster than the northern and eastern slopes. There-
check will serve as a relatively quick and inexpensive fore, in MH, high-elevation springs in the north-
test to check for the viability of DNA across sample east discharge higher proportions of glacial melt than
types and residence times. Samples that did not those towards the southwest. Springs discharging
yield detectable amounts of DNA (using the Qubit™ groundwater from deep, regional flowpaths (regional
dsDNA HS Assay Kit) were not sent for sequencing. at the mountain-block scale) supported by glacial-
The detention limit for the Qubit™ dsDNA HS Assay melt recharge emerge at the lower flanks of the vol-
Kit is around 10 pg µl−1 . Despite not detecting DNA cano perhaps due to large-scale faults in the area
in our negative controls, these were submitted for (figure S4, towards both the northeast and southwest
sequencing. All negative controls failed to pass qual- corners of inset B). The glacially influenced springs in
ity control (performed by the University of Min- the southwest corner are also warm springs and previ-
nesota Genomics Center (UMGC)) and no sequence ous studies inferred that they are discharging recharge
information was obtained. from high-elevation snow or glacial melt [10, 11].
To evaluate microbial community composition, Our isotopic results indicate that they are dischar-
the V4 region of the 16S rRNA gene was sequenced ging a large proportion of recharge from glacial melt
at the UMGC using a dual-indexing approach mod- (figure S5).
ified from the Earth Microbiome Project protocols The groundwater systems in GNP are very dif-
[40] and MiSeq Illumina 2 × 300 bp chemistry gen- ferent than MH; they are dominated by local-to-
erating 25 000–30 000 reads/sample. Post sequence intermediate flow at higher elevations due to the
processing was performed with mothur [41, 42]. stratigraphic and structural controls which effectively
7Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Figure 4. The log relative abundance of the top 20 most abundant operational taxonomic units (OTUs) present in MH and GNP
springs and streams. Bar plots of the top 20 most abundant OTUs are colored based on the genus-level taxonomy. The top two
quadrants show the taxa of springs identified as having snow/rain as their dominant recharge, while the bottom two quadrants
show the taxa of springs identified as having glacial melt as their dominant recharge. Bars with a log relative abundances of zero or
below have extremely low abundances.
truncate groundwater circulation depths. Most of range for snow (see supplementary dataset: table 1).
the remaining glaciers in GNP are within north- Rain samples were collected in MH over two days in
northeast facing cirques and their surficial catchment September 2019 to provide an end-member estima-
drainages are oriented mostly towards the northeast. tion. Glacial ice in both locations was sampled where
However, we observe high-discharge glacially influ- accessible at the toe of the glacier and represents an
enced springs emerging on the opposite side of the estimate of the oldest glacial ice and therefore offers
topographic divide containing glaciers and their run- the most isotopically light values, distinct from mod-
off (figure S1 part B). We attribute this discharge ern precipitation. Additionally, for this study, the
to groundwater flow along fractures, geologic con- majority of samples collected are from springs. How-
tacts, and/or karst-like conduits through the moun- ever, in cases where either the spring (or catchment
tain bedrock to the spring emergence following the containing the spring) could not be accessed, or there
attitude of the geologic units which dip to the south- was interest in the isotopic composition of the stream,
west (figures S1 and S5). The glacially influenced a sample from the stream was collected.
springs originating at low elevations are likely receiv-
ing the glacial water via fracture flow over much 3.2. Microbial results
longer timescales. We used the results from the isotopic model to cat-
Please note that all samples were collected in egorize the springs as ‘largely glacially recharged’ and
July, August, and September of 2017, 2018, and 2019 ‘largely snow/rain recharged’ and coupled these data
at various elevations and aspects in MH and GNP to DNA sequencing of 16S rRNA genes (in bac-
in an attempt to include representative, well-mixed teria and archaea) in 60 of the 132 total samples of
examples of winter precipitation from snow and rain, spring/stream, rain, snow, and glacier ice samples
and summer precipitation in the form of rain. We (figures 2 and 3). Springs with more than 60% gla-
recognize that there can be considerable variability in cial melt contribution contain taxa that are unique
isotopic values both between storms and seasonally; compared to springs receiving recharge mainly from
however, published data in MH and GNP either do modern day snowmelt and rain, proving additional
not exist or are very spatially and temporally sparse support for our hypothesis (figure 4; figures S6–S10,
prohibiting spatiotemporal comparison. MH has no and supplementary dataset S1: tables S5–S7). Many
published modern precipitation data to our know- taxa are abundant in both springs supported by
ledge, with the exception of a three samples of snow recharge from glacial melt and in snow or rain
without latitude or longitude data [10] and three recharged springs, but the distinguishing character-
snow samples with only δ 18 O being reported [43]. istic is the taxa unique to glacially recharged springs
GNP only has single snowpack values taken in March or snow/rain recharged springs. Furthermore, springs
during the early 2000s that support our end-member supported by recharge from glacial meltwater in MH
8Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
have microbial communities overall that are distinct streams supported by snow/rain. Regardless, there are
from glacial melt-recharged springs in GNP. Col- taxa present in the glacier ice that are not present
lectively, our Bayesian isotopic end-member mixing in the snow/rain recharge, and likewise there are
model with 16S rRNA amplicon sequencing demon- snow/rain taxa that do not appear in the glacial
strate that (a) MBR from alpine glacier meltwater recharge (figures S6–S10 and supplementary dataset
is appreciable, and (b) springs that are supported S1: tables 5–7). This work highlights the microbial
primarily by recharge from glacier meltwater con- similarities and differences in glacial melt recharged
tain microbial communities unique to the glacially springs between two geographically and geologically
recharged grouping. distinct study sites. When cross comparing the most
abundant Phyla of the two field locations, MH and
4. Discussion and conclusions GNP have commonalities such as Proteobacteria and
Bacteroidetes, but each also contain taxa unique to
We have shown that the proportion of MBR from the locale. Distinctions in biogeography are likely a
alpine-glacial meltwater is comparable to recharge combination of bedrock lithology, precipitation, and
from modern day precipitation in some springs, with season which constrain the distribution and compos-
approximately 26% of the sites in MH and 54% in ition of snow and ice microbiota [45–48].
GNP being primarily supported by glacial meltwater. There are several limitations to our study: we lack
In stark contrast to previous studies, our data show widespread samples of glacial ice due to the hazards
that a substantial proportion of glacial meltwater associated with sampling near retreating alpine gla-
may remain in the catchment (i.e. in the mountain ciers (e.g. rock fall/slides, glacial dam failure, etc), our
groundwater system). This finding cannot be over- data set is limited spatiotemporally, and the glacier
emphasized since this recharge ultimately supports volume and subsequent melt is poorly constrained,
perennial flow from mountain springs and baseflow as is the englacial and subglacial hydrology. We pos-
in mountain streams, processes that are vital to the tulate that the glacial melt must recharge beneath
future integrity of alpine ecosystems. Although alpine the glacier and/or through communication between
glaciers have been present since the LGM, this finding shallow aquifers in glacial sediments and bedrock
also suggests that these springs are dependent upon aquifers. Whether this process is occurring along the
a temporary source of recharge. The permanence entire subsurface area of the ice-bedrock/alluvium/till
(and vulnerability) of these springs depends on the contact, in localized pockets, and/or at the toe of the
volume of water stored in the mountain aquifer and glacier is still unknown (and may remain unknown
the response time of the aquifers to future changes in due to the logistical difficulty in sampling beneath
recharge. Many springs, like those discussed in this glaciers), but may play an important role with regard
study, may disappear or experience decreased dis- to the isotopic signal that is recharging. The sampling
charge in the future. The time lag between the dis- window is still relatively narrow in high-alpine catch-
appearance of the alpine glacier and the desiccation ments due to seasonal inaccessibility and blockages to
of the alpine spring is controlled by the groundwater roads and trails during the melt season, therefore the
response time. A recent study showed that ground- spring/stream samples collected in this study repres-
water recharge predictions in the Columbia River ent a snapshot in time and do not take into account
Plateau Aquifer remain uncertain in future projec- the possible seasonal change in isotopic values of
tions [44]. If there is a modest increase in recharge the springs. As such, throughout time the fractional
this could yield an increase in total recharge in this contribution of glacial ice may increase or decrease,
region, however there is an equal possibility that but we predict that an overall decreasing trend is
future increases in precipitation might not be able expected as the input from melting ice lessens. The
to overcome future increases in evapotranspiration end-member samples collected also represent a single
(as vegetation encroaches on high elevations) and point in space and time, and glacial ice, for example,
decreases in snow water equivalent [44]. However, in contains an amalgamation of years of isotopic values.
regional-scale predictions like these, it is even more However, our sampling strategy has provided a robust
difficult to make predictions for MBR because prior range of possible isotopic values without the ability
knowledge of groundwater routing in mountain sys- to collect ice cores at depth. Additionally, there are
tems is limited and further complicated in glaciated spatial limitations to the methods used here, as iso-
regions [44]. topes in glacial ice, snow, and rain vary as moisture
Springs receiving more than 60% of recharge moves across continents and mountainous terrain. If
from glacial melt contain unique microbes com- these methods are applied to a wider spatial extent
pared to springs receiving most recharge from mod- than what is reasonable, erroneous conclusions may
ern day snow and rain. This could be due to trans- result. For example, a spring in MH and another in
port of microbes originating in glacial ice. Glacial GNP appear to be glacially influenced, but these res-
melt-supported spring water may also have different ults should be taken with caution. In MH, the spring
geochemical and/or thermal conditions which sup- site in the bottom right-hand corner of the study area
ports distinct microbial communities compared to (figure 2(A)) was chosen as a test site because it is
9Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
located near the top of a local mafic stratovolcano that including freshwater species biodiversity. Glacier car-
was thought to have its own local groundwater sys- bon supports downstream ecosystems and our data
tem separate from the MH system, however the model indicate glacier-fed springs/streams support distinct
predicted it was glacially influenced. In GNP, a spring microbiota, but we do not know if the differences
located >20 km from a glacier (figure 2(G)) in a sep- in microbial community structure observed here
arate thrust fault near the southwest end of a large have impacts for invertebrate (and in extension, ver-
northeast-southwest trending lake (Lake McDonald) tebrate) communities in alpine aquatic ecosystems
is also glacially influenced according to the model. [51, 52]. By emphasizing the importance and pres-
Additional studies on a seasonal scale as well as long- ence of groundwater dependent ecosystems and their
term, continuous studies are needed to assess the tem- connection to recharge sources, surface waters, and
poral impact of glacial melt on mountain aquifers. riparian zones, the emerging discipline of ecohydro-
This topic involves the intersection of hydrology, gla- geology has potential to shed light many unresolved
ciology, and microbiology and has countless oppor- groundwater debates [24]. This study shows that gla-
tunities for further research. cial melt is critical to mountain aquifers and holds
Given the current rate of glacier recession and vital importance from both an ecological and water
predictions for the future, many alpine glaciers will resource perspective, thereby highlighting the intric-
likely disappear within the century. This study shows ate connections between the cryosphere and deep
that some of the meltwater stays in the moun- hydrosphere.
tain groundwater system; however, this does not
imply that the mountain groundwater system has Data availability statement
intrinsic or ‘built-in’ long-term sustainability. These
findings indicate quite the opposite, that perennial The SSU rRNA raw sequence read data have been
flow from alpine springs and baseflow in headwa- deposited with links to BioProject accession num-
ter alpine streams is supported, at least partially in ber PRJNA629965 in the NCBI BioProject database
most cases and dominantly in others, by a tran- (www.ncbi.nlm.nih.gov/bioproject/). Sampling site
sient source of recharge. Globally, nearly 670 mil- information, stable isotopic data, and mixing model
lion people live in high mountain regions, includ- results have been archived using Purdue PURR: Miller
ing indigenous peoples [49], with at least 1/6 of the et al (2021). Isotopic data and mixing model results of
world’s population depending on glaciers and sea- springs and streams in Mount Hood National Forest
sonal snowpack for water [50]. Glacial runoff deliv- and Glacier National Park, USA. Purdue University
ers vital water for communities and ecosystems, but if Research Repository. DOI: https://doi.org/10.4231/
some of this water is being recharged and temporarily PBEN-FT39. Photos and videos of springs sampled
stored in the mountain block, then there is a lag time have been archived also using Purdue PURR: Miller
in delivery that is not being accurately considered (2021). Photos and videos documenting spring emer-
in regional water studies or management strategies. gences and sampling locations in and around Mount
Quantification of MBR could facilitate water con- Hood National Forest and Glacier National Park.
servation strategies and inform estimates of seasonal Purdue University Research Repository. DOI: https:/
water supply as glaciers recede. Groundwater dis- /doi.org/10.4231/1R4N-RP33. Raw code for produ-
charged as springs has been recognized as an import- cing the mixing model results is available upon reas-
ant water source in mountain communities for cen- onable request from the corresponding author.
turies, but the long-term dependability of this source The data that support the findings of this study are
is now in question. Long-term monitoring of a few openly available at the following URL/DOI: https://
of the springs’ discharge or water level, in addition doi.org/10.4231/PBEN-FT39.
to seasonal isotopic and ecologic sampling would
yield invaluable water resource data not only in MH Acknowledgments
and GNP, but in alpine, glaciated regions around the
globe. Implementing level loggers (for hydrograph We thank Mary Ellen Fitzgerald of Mount Hood
purposes), autosamplers (for geochemistry), or other National Forest and Tara Carolin of the Crown of
passive sampling would likely be the easiest monitor- the Continent Research Learning Center (CCRLC) in
ing strategies, as these spring locations are typically Glacier National Park for granting permits to con-
difficult to access on a regular basis. Springs that have duct field sampling. We are grateful to Greg Wanner
more defined, rather than diffuse, discharge sources of the Mount Hood Forest Service, Joe Giersch of the
and have a depth of at least a few centimeters are USGS in Glacier National Park, and Ranger Elizabeth
recommended. For future studies, it would be espe- Gerrits of Glacier National Park for their assistance
cially interesting to target high discharge springs or in identifying spring locations. We thank the Crys-
springs forming headwaters of societally important tal Springs Water District for allowing us to sample
rivers and streams. their protected springs. We thank Deb Glosser, Amine
Changing climate and glacier recession over the Chater, Kalsey Graner, and Marc Rouleau for collect-
past century is disturbing natural ecosystem function ing high elevation glacier ice samples from Mount
10Environ. Res. Lett. 16 (2021) 064012 J B Miller et al
Hood. We thank Bob and Sally Havig, and Kris [10] Nathenson M 2004 Springs on and in the vicinity of
Nerczuk for receiving packages and their hospitality. Mount Hood volcano, Oregon US Geological Survey
Open-File Report 2004-1298 (available at: https://pubs.
Last, we thank Zach Meyers, Noah Stewart-Maddox,
usgs.gov/of/2004/1298) (Accessed 8 September
James Haydock, Kyle Kube, Rene Paul Acosta, Pete 2020)
Siqueiros, Mariah Romero, and Caelum Mroczek for [11] Wollenberg H A, Bowen R E, Bowman H R and Strisower B
their assistance in the field. The authors acknowledge 1979 Geochemical studies of rocks, water, and gases at
Mt. Hood, Oregon State of Oregon DOGAMI Open-File
the Minnesota Supercomputing Institute (MSI) at the
Report 0-79-2 (available at: www.oregongeology.org/pubs/
University of Minnesota for providing resources that ofr/O-79-02.pdf) (Accessed 8 September 2020)
contributed to the research results reported within [12] Tolley D G, Frisbee M D and Campbell A R 2015
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Funding for this research was provided by the
using stable isotopes New Mexico Geological Society
National Science Foundation Grant (EAR 1904075 Streamflow in the Sangre De Cristo Mountains Using Stable
and 1904159). Additional support was provided by Isotopes (Socorro, NM: NM Bureau of Geology & Mineral
The Consortium of Universities for the Advancement Resources) pp 303–12
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